For golang#15756. This still depends on Curfn, but it’s progress.

Change-Id: Ic32fe56f44fcfbc023e7668d4dee07f8b47bf3a4

This still depends on Curfn, but it's progress.

Updates #15756

Change-Id: Ic32fe56f44fcfbc023e7668d4dee07f8b47bf3a4
Reviewed-on: https://go-review.googlesource.com/26661
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

fn == Curfn in this context. Prefer the local variable.

Passes toolstash -cmp. Updates #15756.

Change-Id: I75b589c682d0c1b524cac2bbf2bba368a6027b06
Reviewed-on: https://go-review.googlesource.com/38151
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

There were a surprising number of places
in the tree that used yyerror for failed internal
consistency checks. Switch them to Fatalf.

Updates #15756
Updates #19250

Change-Id: Ie4278148185795a28ff3c27dacffc211cda5bbdd
Reviewed-on: https://go-review.googlesource.com/38153
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

Bad pragmas should never make it to the backend.
I've confirmed manually that the error position is unchanged.

Updates #15756
Updates #19250

Change-Id: If14f7ce868334f809e337edc270a49680b26f48e
Reviewed-on: https://go-review.googlesource.com/38152
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

The fmtmode and fmtpkgpfx globals stand in the
way of making the compiler more concurrent (#15756).
This CL removes them.

The natural way to eliminate a global is to explicitly
thread it as a parameter through all function calls.
However, most of the functions in gc/fmt.go
get called indirectly, by way of fmt format strings,
so there's nowhere natural to add a parameter.

Since there are only a few fmtmode modes,
use named types to distinguish between modes.
For example, fmtNodeErr, fmtNodeDbg, and fmtNodeTypeId
are all gc.Node, but they print in different modes.
Varying the type allows us to thread mode through fmt.
Handle fmtpkgpfx by converting it to a printing mode,
FTypeIdName, and using the same type-based approach.

To avoid a loss of readability and danger of bugs
from introducing conversions at all call sites,
instead add a helper that systematically modifies the args.

The only remaining gc/fmt.go global is dumpdepth.
Since that is used for debugging only,
it that can be handled with a global mutex,
or some similarly basic, if inefficient, protection.

Passes toolstash -cmp. No compiler performance impact.

For future reference, other options for threading state
that were considered and rejected:

* Wrapping values in structs, such as:

  type fmtNode struct {
  	n *Node
  	mode fmtMode
  }

  This reduces the proliferation of types, and supports
  easily adding extra local parameters.
  However, putting such a struct into an interface{} allocates.
  This is unacceptable in this particular area of code.

* Passing state via precision, such as:

  fmt.Fprintf("%*v", mode, n)

  where mode is the state encoded as an integer.
  This avoids extra allocations, but it is out of keeping
  with the intended semantics of precision, and is less readable.

* Modify the fmt package to support setting/getting context
  via fmt.State. Unavailable due to Go 1 compatibility,
  and probably the wrong solution anyway.

* Give up on package fmt. This would be a huge readability
  regression and cause high code churn.

* Attempt a de-novo rewrite that circumvents these problems.
  Too high a risk of bugs, with insufficient reward for the effort,
  particularly since long term plans call for elimination
  of gc.Node.


Change-Id: Iea2440d5a34a938e64273707de27e3a897cb41d1
Reviewed-on: https://go-review.googlesource.com/38147
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

This CL introduces yet another compiler pass,
which checks for correct control flow constructs
prior to converting from AST to SSA form.

It cannot be integrated with walk, since walk rewrites
switch and select statements on the fly.

To reduce code duplication, this CL also does some
minor refactoring.

With this pass in place, the AST to SSA converter
can now stop generating SSA for any known-dead code.
This minor savings pays for the minor cost of the new pass.

Performance is almost a wash:

name       old time/op     new time/op     delta
Template       206ms ± 4%      205ms ± 4%   ~     (p=0.108 n=43+43)
Unicode       84.0ms ± 4%     84.0ms ± 4%   ~     (p=0.979 n=43+43)
GoTypes        550ms ± 3%      553ms ± 3%   ~     (p=0.065 n=40+41)
Compiler       2.57s ± 4%      2.58s ± 2%   ~     (p=0.103 n=44+41)
SSA            3.94s ± 3%      3.93s ± 2%   ~     (p=0.833 n=44+42)
Flate          126ms ± 6%      125ms ± 4%   ~     (p=0.941 n=43+39)
GoParser       147ms ± 4%      148ms ± 3%   ~     (p=0.164 n=42+39)
Reflect        359ms ± 3%      357ms ± 5%   ~     (p=0.241 n=43+44)
Tar            106ms ± 5%      106ms ± 7%   ~     (p=0.853 n=40+43)
XML            202ms ± 3%      203ms ± 3%   ~     (p=0.488 n=42+41)

name       old user-ns/op  new user-ns/op  delta
Template        240M ± 4%       239M ± 4%   ~     (p=0.844 n=42+43)
Unicode         107M ± 5%       107M ± 4%   ~     (p=0.332 n=40+43)
GoTypes         735M ± 3%       731M ± 4%   ~     (p=0.141 n=43+44)
Compiler       3.51G ± 3%      3.52G ± 3%   ~     (p=0.208 n=42+43)
SSA            5.72G ± 4%      5.72G ± 3%   ~     (p=0.928 n=44+42)
Flate           151M ± 7%       150M ± 8%   ~     (p=0.662 n=44+43)
GoParser        181M ± 5%       181M ± 4%   ~     (p=0.379 n=41+44)
Reflect         447M ± 4%       445M ± 4%   ~     (p=0.344 n=43+43)
Tar             125M ± 7%       124M ± 6%   ~     (p=0.353 n=43+43)
XML             248M ± 4%       250M ± 6%   ~     (p=0.158 n=44+44)

name       old alloc/op    new alloc/op    delta
Template      40.3MB ± 0%     40.2MB ± 0%  -0.27%  (p=0.000 n=10+10)
Unicode       30.3MB ± 0%     30.2MB ± 0%  -0.10%  (p=0.015 n=10+10)
GoTypes        114MB ± 0%      114MB ± 0%  -0.06%  (p=0.000 n=7+9)
Compiler       480MB ± 0%      481MB ± 0%  +0.07%  (p=0.000 n=10+10)
SSA            864MB ± 0%      862MB ± 0%  -0.25%  (p=0.000 n=9+10)
Flate         25.9MB ± 0%     25.9MB ± 0%    ~     (p=0.123 n=10+10)
GoParser      32.1MB ± 0%     32.1MB ± 0%    ~     (p=0.631 n=10+10)
Reflect       79.9MB ± 0%     79.6MB ± 0%  -0.39%  (p=0.000 n=10+9)
Tar           27.1MB ± 0%     27.0MB ± 0%  -0.18%  (p=0.003 n=10+10)
XML           42.6MB ± 0%     42.6MB ± 0%    ~     (p=0.143 n=10+10)

name       old allocs/op   new allocs/op   delta
Template        401k ± 0%       401k ± 1%    ~     (p=0.353 n=10+10)
Unicode         322k ± 0%       322k ± 0%    ~     (p=0.739 n=10+10)
GoTypes        1.18M ± 0%      1.18M ± 0%  +0.25%  (p=0.001 n=7+8)
Compiler       4.51M ± 0%      4.53M ± 0%  +0.37%  (p=0.000 n=10+10)
SSA            7.91M ± 0%      7.93M ± 0%  +0.20%  (p=0.000 n=9+10)
Flate           244k ± 0%       245k ± 0%    ~     (p=0.123 n=10+10)
GoParser        323k ± 1%       324k ± 1%  +0.40%  (p=0.035 n=10+10)
Reflect        1.01M ± 0%      1.02M ± 0%  +0.37%  (p=0.000 n=10+9)
Tar             258k ± 1%       258k ± 1%    ~     (p=0.661 n=10+9)
XML             403k ± 0%       405k ± 0%  +0.47%  (p=0.004 n=10+10)

Updates #15756
Updates #19250

Change-Id: I647bfbb745c35630447eb79dfcaa994b490ce942
Reviewed-on: https://go-review.googlesource.com/38159
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

Passes toolstash -cmp.

Updates #15756

Change-Id: Ia071dbbd7f2ee0f8433d8c37af4f7b588016244e
Reviewed-on: https://go-review.googlesource.com/38231
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

Suggested by mdempsky in CL 38232.
This allows us to use the Frontend field
to associate frontend state and information
with a function.
See the following CL in the series for examples.

This is a giant CL, but it is almost entirely routine refactoring.

The ssa test API is starting to feel a bit unwieldy.
I will clean it up separately, once the dust has settled.

Passes toolstash -cmp.

Updates #15756

Change-Id: I71c573bd96ff7251935fce1391b06b1f133c3caf
Reviewed-on: https://go-review.googlesource.com/38327
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

This is a first step towards eliminating the
Curfn global in the backend.
There's more to do.

Passes toolstash -cmp. No compiler performance impact.

Updates #15756

Change-Id: Ib09f550a001e279a5aeeed0f85698290f890939c
Reviewed-on: https://go-review.googlesource.com/38232
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

Preparation for de-globalizing Stksize and MaxArg.

Passes toolstash -cmp. No compiler performance impact.

Updates #15756

Change-Id: I312f0bbd15587a6aebf472cd66c8e62b89e55c8a
Reviewed-on: https://go-review.googlesource.com/38328
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

Passes toolstash -cmp. No compiler performance impact.

Updates #15756

Change-Id: I1294058716d83dd1be495d399ed7ab2277754dc6
Reviewed-on: https://go-review.googlesource.com/38329
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

Though LSym.Version is an int, it can only have the value 0 or 1.
Using that, split Link.hash into two maps, one for version 0
(which is far more common) and one for version 1.
This lets use just the name for lookups,
which is both faster and more compact.
This matters because Link.hash map lookups are frequent,
and will be contended once the backend is concurrent.

name        old time/op       new time/op       delta
Template          194ms ± 3%        192ms ± 5%  -1.46%  (p=0.000 n=47+49)
Unicode          84.5ms ± 3%       83.8ms ± 3%  -0.81%  (p=0.011 n=50+49)
GoTypes           543ms ± 2%        545ms ± 4%    ~     (p=0.566 n=46+49)
Compiler          2.48s ± 2%        2.48s ± 3%    ~     (p=0.706 n=47+50)
SSA               5.94s ± 3%        5.98s ± 2%  +0.55%  (p=0.040 n=49+50)
Flate             119ms ± 6%        119ms ± 4%    ~     (p=0.681 n=48+47)
GoParser          145ms ± 4%        145ms ± 3%    ~     (p=0.662 n=47+49)
Reflect           348ms ± 3%        344ms ± 3%  -1.17%  (p=0.000 n=47+47)
Tar               105ms ± 4%        104ms ± 3%    ~     (p=0.155 n=50+47)
XML               197ms ± 2%        197ms ± 3%    ~     (p=0.666 n=49+49)
[Geo mean]        332ms             331ms       -0.37%

name        old user-time/op  new user-time/op  delta
Template          230ms ±10%        226ms ±10%  -1.85%  (p=0.041 n=50+50)
Unicode           104ms ± 6%        103ms ± 5%    ~     (p=0.076 n=49+49)
GoTypes           707ms ± 4%        705ms ± 5%    ~     (p=0.521 n=50+50)
Compiler          3.30s ± 3%        3.33s ± 4%  +0.76%  (p=0.003 n=50+49)
SSA               8.17s ± 4%        8.23s ± 3%  +0.66%  (p=0.030 n=50+49)
Flate             139ms ± 6%        138ms ± 8%    ~     (p=0.184 n=49+48)
GoParser          174ms ± 5%        172ms ± 6%    ~     (p=0.107 n=48+49)
Reflect           431ms ± 8%        420ms ± 5%  -2.57%  (p=0.000 n=50+46)
Tar               119ms ± 6%        118ms ± 7%  -0.95%  (p=0.033 n=50+49)
XML               236ms ± 4%        236ms ± 4%    ~     (p=0.935 n=50+48)
[Geo mean]        410ms             407ms       -0.67%

name        old alloc/op      new alloc/op      delta
Template         38.7MB ± 0%       38.6MB ± 0%  -0.29%  (p=0.008 n=5+5)
Unicode          29.8MB ± 0%       29.7MB ± 0%  -0.24%  (p=0.008 n=5+5)
GoTypes           113MB ± 0%        113MB ± 0%  -0.29%  (p=0.008 n=5+5)
Compiler          462MB ± 0%        462MB ± 0%  -0.12%  (p=0.008 n=5+5)
SSA              1.27GB ± 0%       1.27GB ± 0%  -0.05%  (p=0.008 n=5+5)
Flate            25.2MB ± 0%       25.1MB ± 0%  -0.37%  (p=0.008 n=5+5)
GoParser         31.7MB ± 0%       31.6MB ± 0%    ~     (p=0.056 n=5+5)
Reflect          77.5MB ± 0%       77.2MB ± 0%  -0.38%  (p=0.008 n=5+5)
Tar              26.4MB ± 0%       26.3MB ± 0%    ~     (p=0.151 n=5+5)
XML              41.9MB ± 0%       41.9MB ± 0%  -0.20%  (p=0.032 n=5+5)
[Geo mean]       74.5MB            74.3MB       -0.23%

name        old allocs/op     new allocs/op     delta
Template           378k ± 1%         377k ± 1%    ~     (p=0.690 n=5+5)
Unicode            321k ± 0%         322k ± 0%    ~     (p=0.595 n=5+5)
GoTypes           1.14M ± 0%        1.14M ± 0%    ~     (p=0.310 n=5+5)
Compiler          4.25M ± 0%        4.25M ± 0%    ~     (p=0.151 n=5+5)
SSA               9.84M ± 0%        9.84M ± 0%    ~     (p=0.841 n=5+5)
Flate              232k ± 1%         232k ± 0%    ~     (p=0.690 n=5+5)
GoParser           315k ± 1%         315k ± 1%    ~     (p=0.841 n=5+5)
Reflect            970k ± 0%         970k ± 0%    ~     (p=0.841 n=5+5)
Tar                248k ± 0%         248k ± 1%    ~     (p=0.841 n=5+5)
XML                389k ± 0%         389k ± 0%    ~     (p=1.000 n=5+5)
[Geo mean]         724k              724k       +0.01%

Updates golang#15756

Change-Id: I2646332e89f0444ca9d5a41d7172537d904ed636

This CL adds initial support for concurrent backend compilation.


CAVEATS

I suspect it's going to end up on Twitter.
If you're coming here from the internet:

* Don't believe the hype.
* If you're going to try it out, please also try with
  the race detector enabled and report an issue
  at golang.org/issue/new if you see a race report.
  Just run 'go install -race cmd/compile' and
  'go build -a yourpackages'. And then run make.bash
  again to return a reasonable compiler.


BACKGROUND

The compiler current consists (very roughly) of a handful of phases:

1. Initialization.
2. Lexing and parsing into the cmd/compile/internal/syntax AST.
3. Translation into the cmd/compile/internal/gc AST.
4. Some gc AST passes: typechecking, escape analysis, inlining,
   closure handling, expression evaluation ordering (order.go),
   and some lowering and optimization (walk.go).
5. Translation into the cmd/compile/internal/ssa SSA form.
6. Optimization and lowering of SSA form.
7. Translation from SSA form to assembler instructions.
8. Translation from assembler instructions to machine code.
9. Writing lots of output: machine code, DWARF symbols,
   type and reflection info, export data.

Phase 2 was already concurrent as of Go 1.8.

Phase 3 is planned for eventual removal;
we hope to go straight from syntax AST to SSA.

Phases 5–8 are per-function; this CL adds support for
processing multiple functions concurrently.
The slowest phases in the compiler are 5 and 6,
so this offers the opportunity for some good speed-ups.

Unfortunately, it's not quite that straightforward.
In the current compiler, the latter parts of phase 4
(order, walk) are done function-at-a-time as needed.
Making order and walk concurrency-safe proved hard,
and they're not particularly slow, so there wasn't much reward.
To enable phases 5–8 to be done concurrently,
when concurrent backend compilation is requested,
we complete phase 4 for all functions
before starting later phases for any functions.

Also, in reality, we automatically generate new
functions in phase 9, such as method wrappers
and equality and has routines.
Those new functions then go through phases 4–8.
This CL disables concurrent backend compilation
after the first, big, user-provided batch of
functions has been compiled.
This is done to keep things simple,
and because the autogenerated functions
tend to be small, few, simple, and fast to compile.


USAGE

Concurrent backend compilation still defaults to off.
To set the number of functions that may be backend-compiled
concurrently, use the compiler flag -c.
In future work, cmd/go will automatically set -c.

Furthermore, this CL has been intentionally written
so that the c=1 path has no concurrency whatsoever,
not even spawning any goroutines.
This helps ensure that, should problems arise
let in the development cycle,
we can simply have cmd/go set -c=1 always,
and revert to the original compiler behavior.


MUTEXES

Most of the work required to make concurrent backend
compilation safe has occurred over the past month.
This CL adds a handful of mutexes to get the rest of the way there;
they are the mutexes that I didn't see a clean way to avoid.
Some of them may still be eliminable in future work.

In no particular order:

* gc.funcsymsmu. The global funcsyms slice is populated
  lazily when we need function symbols for closures.
  This occurs during gc AST to SSA translation.
  This mutex is low priority: it adds a single global,
  it is in an infrequently used code path, and it is low contention.
  It requires additional sorting to preserve reproducible builds.

* gc.largeStackFramesMu. We don't discover until after SSA compilation
  that a function's stack frame is gigantic.
  Recording that error happens basically never,
  but it does happen concurrently.
  Fix with a low priority mutex and sorting.

* obj.Link.Hashmu. ctxt.Hash stores the mapping from
  types.Syms (compiler symbols) to obj.LSyms (linker symbols).
  It is accessed fairly heavily through all the phases.
  This is easily the most heavily contended mutex.
  I hope that the syncmap proposed in golang#18177 may
  provide some free speed-ups here.

* gc.signatlistmu. The global signatlist map is
  populated with types through several of the concurrent phases,
  including notably via ngotype during DWARF generation.
  It is low priority for removal, aside from some mild
  awkwardness to avoid deadlocks due to recursive calls.

* types.Pkg.Symsmu. Looking up symbols in a package
  happens a fair amount during backend compilation,
  including the construction of gcargs/gclocals symbols,
  and types, particularly via ngotype.
  It has low-to-moderate contention.

* types.internedStringsmu. gc AST to SSA conversion and
  some SSA work introduce new autotmps.
  Those autotmps have their names interned to reduce allocations.
  That interning requires protecting types.internedStrings.
  The autotmp names are heavily re-used, and the mutex
  overhead and contention here are low, so it is probably
  a worthwhile performance optimization to keep this mutex.

* types.Sym.Lsymmu. Syms keep a cache of their associated LSym,
  to reduce lookups in ctxt.Hash. This cache itself
  needs concurrency protection. This mutex adds to the size
  of a moderately important data structure, but the alloc benchmarks
  below show that this doesn't hurt much in practice.
  It is moderately contended, mostly because when lookups fail,
  the lock is held while vying for the contended ctxt.Hash mutex.
  The fact that it keeps load off the ctxt.Hash mutex, though,
  makes this mutex worth keeping.


TESTING

I have been testing this code locally by running
'go install -race cmd/compile'
and then doing
'go build -a -gcflags=-c=128 std cmd'
for all architectures and a variety of compiler flags.
This obviously needs to be made part of the builders,
but it is too expensive to make part of all.bash.
I have filed golang#19962 for this.


REPRODUCIBLE BUILDS

This version of the compiler generates reproducible builds.
Testing reproducible builds also needs automation, however,
and is also too expensive for all.bash.
This is golang#19961.

Also of note is that some of the compiler flags used by 'toolstash -cmp'
are currently incompatible with concurrent backend compilation.
They still work fine with c=1.
Time will tell whether this is a problem.


NEXT STEPS

* Continue to find and fix races and bugs,
  using a combination of code inspection, fuzzing,
  and hopefully some community experimentation.
  I do not know of any outstanding races,
  but there probably are some.
* Improve testing.
* Improve performance, for many values of c.
* Integrate with cmd/go and fine tune.
* Support concurrent compilation with the -race flag.
  It is a sad irony that it does not yet work.
* Minor code cleanup that has been deferred during
  the last month due to uncertainty about the
  ultimate shape of this CL.


PERFORMANCE

Here's the buried lede, at last. :)

All benchmarks are from my 8 core 2.9 GHz Intel Core i7 darwin/amd64 laptop.

First, going from tip to this CL with c=1
costs about 3% CPU and has almost no memory impact.

name       old time/op       new time/op       delta
Template         194ms ± 4%        195ms ± 4%  +0.91%  (p=0.002 n=49+47)
Unicode         82.9ms ± 4%       85.2ms ± 3%  +2.68%  (p=0.000 n=47+48)
GoTypes          518ms ± 3%        527ms ± 2%  +1.81%  (p=0.000 n=46+46)
SSA              5.59s ± 2%        5.77s ± 2%  +3.12%  (p=0.000 n=48+50)
Flate            120ms ± 3%        122ms ± 3%  +1.54%  (p=0.000 n=49+48)
GoParser         140ms ± 3%        143ms ± 4%  +2.24%  (p=0.000 n=47+49)
Reflect          333ms ± 3%        342ms ± 3%  +2.67%  (p=0.000 n=47+49)
Tar              102ms ± 6%        104ms ± 4%  +2.37%  (p=0.000 n=50+47)
XML              202ms ±13%        196ms ± 4%  -2.63%  (p=0.036 n=50+48)

name       old user-time/op  new user-time/op  delta
Template         236ms ± 9%        237ms ±10%    ~     (p=0.750 n=50+50)
Unicode          104ms ± 7%        107ms ± 4%  +2.09%  (p=0.000 n=49+47)
GoTypes          691ms ± 3%        701ms ± 3%  +1.40%  (p=0.000 n=50+49)
SSA              7.91s ± 3%        8.07s ± 4%  +1.98%  (p=0.000 n=48+49)
Flate            142ms ± 4%        145ms ± 5%  +2.12%  (p=0.000 n=47+48)
GoParser         172ms ± 6%        175ms ± 5%  +1.76%  (p=0.002 n=50+49)
Reflect          425ms ± 9%        436ms ± 8%  +2.55%  (p=0.001 n=50+50)
Tar              119ms ± 6%        121ms ± 5%  +2.52%  (p=0.000 n=49+49)
XML              242ms ± 8%        239ms ± 6%  -1.54%  (p=0.039 n=49+49)

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       38.8MB ± 0%    ~     (p=0.247 n=10+10)
Unicode         29.8MB ± 0%       29.8MB ± 0%    ~     (p=0.631 n=10+10)
GoTypes          113MB ± 0%        113MB ± 0%    ~     (p=0.218 n=10+10)
SSA             1.25GB ± 0%       1.25GB ± 0%  +0.02%  (p=0.000 n=10+10)
Flate           25.3MB ± 0%       25.3MB ± 0%    ~     (p=0.315 n=9+10)
GoParser        31.7MB ± 0%       31.7MB ± 0%    ~     (p=0.089 n=10+10)
Reflect         78.2MB ± 0%       78.2MB ± 0%  +0.07%  (p=0.019 n=10+10)
Tar             26.5MB ± 0%       26.6MB ± 0%    ~     (p=0.165 n=10+10)
XML             42.4MB ± 0%       42.4MB ± 0%    ~     (p=0.497 n=9+10)

name       old allocs/op     new allocs/op     delta
Template          378k ± 1%         379k ± 1%    ~     (p=0.353 n=10+10)
Unicode           321k ± 0%         321k ± 1%    ~     (p=0.684 n=10+10)
GoTypes          1.14M ± 0%        1.14M ± 0%    ~     (p=0.247 n=10+10)
SSA              9.71M ± 0%        9.72M ± 0%  +0.08%  (p=0.000 n=10+10)
Flate             234k ± 1%         234k ± 1%    ~     (p=0.280 n=10+10)
GoParser          316k ± 0%         315k ± 1%  -0.26%  (p=0.040 n=9+9)
Reflect           980k ± 0%         981k ± 0%    ~     (p=0.052 n=10+10)
Tar               249k ± 1%         250k ± 1%    ~     (p=0.190 n=10+10)
XML               391k ± 1%         391k ± 1%    ~     (p=0.829 n=8+10)


Comparing this CL to itself, from c=1 to c=2
improves real times 20-30%, costs 5-10% more CPU time,
and adds about 2% alloc.
The allocation increase comes from allocating more ssa.Caches.

name       old time/op       new time/op       delta
Template         202ms ± 3%        149ms ± 3%  -26.15%  (p=0.000 n=49+49)
Unicode         87.4ms ± 4%       84.2ms ± 3%   -3.68%  (p=0.000 n=48+48)
GoTypes          560ms ± 2%        398ms ± 2%  -28.96%  (p=0.000 n=49+49)
Compiler         2.46s ± 3%        1.76s ± 2%  -28.61%  (p=0.000 n=48+46)
SSA              6.17s ± 2%        4.04s ± 1%  -34.52%  (p=0.000 n=49+49)
Flate            126ms ± 3%         92ms ± 2%  -26.81%  (p=0.000 n=49+48)
GoParser         148ms ± 4%        107ms ± 2%  -27.78%  (p=0.000 n=49+48)
Reflect          361ms ± 3%        281ms ± 3%  -22.10%  (p=0.000 n=49+49)
Tar              109ms ± 4%         86ms ± 3%  -20.81%  (p=0.000 n=49+47)
XML              204ms ± 3%        144ms ± 2%  -29.53%  (p=0.000 n=48+45)

name       old user-time/op  new user-time/op  delta
Template         246ms ± 9%        246ms ± 4%     ~     (p=0.401 n=50+48)
Unicode          109ms ± 4%        111ms ± 4%   +1.47%  (p=0.000 n=44+50)
GoTypes          728ms ± 3%        765ms ± 3%   +5.04%  (p=0.000 n=46+50)
Compiler         3.33s ± 3%        3.41s ± 2%   +2.31%  (p=0.000 n=49+48)
SSA              8.52s ± 2%        9.11s ± 2%   +6.93%  (p=0.000 n=49+47)
Flate            149ms ± 4%        161ms ± 3%   +8.13%  (p=0.000 n=50+47)
GoParser         181ms ± 5%        192ms ± 2%   +6.40%  (p=0.000 n=49+46)
Reflect          452ms ± 9%        474ms ± 2%   +4.99%  (p=0.000 n=50+48)
Tar              126ms ± 6%        136ms ± 4%   +7.95%  (p=0.000 n=50+49)
XML              247ms ± 5%        264ms ± 3%   +6.94%  (p=0.000 n=48+50)

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       39.3MB ± 0%   +1.48%  (p=0.008 n=5+5)
Unicode         29.8MB ± 0%       30.2MB ± 0%   +1.19%  (p=0.008 n=5+5)
GoTypes          113MB ± 0%        114MB ± 0%   +0.69%  (p=0.008 n=5+5)
Compiler         443MB ± 0%        447MB ± 0%   +0.95%  (p=0.008 n=5+5)
SSA             1.25GB ± 0%       1.26GB ± 0%   +0.89%  (p=0.008 n=5+5)
Flate           25.3MB ± 0%       25.9MB ± 1%   +2.35%  (p=0.008 n=5+5)
GoParser        31.7MB ± 0%       32.2MB ± 0%   +1.59%  (p=0.008 n=5+5)
Reflect         78.2MB ± 0%       78.9MB ± 0%   +0.91%  (p=0.008 n=5+5)
Tar             26.6MB ± 0%       27.0MB ± 0%   +1.80%  (p=0.008 n=5+5)
XML             42.4MB ± 0%       43.4MB ± 0%   +2.35%  (p=0.008 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          379k ± 0%         378k ± 0%     ~     (p=0.421 n=5+5)
Unicode           322k ± 0%         321k ± 0%     ~     (p=0.222 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%     ~     (p=0.548 n=5+5)
Compiler         4.12M ± 0%        4.11M ± 0%   -0.14%  (p=0.032 n=5+5)
SSA              9.72M ± 0%        9.72M ± 0%     ~     (p=0.421 n=5+5)
Flate             234k ± 1%         234k ± 0%     ~     (p=0.421 n=5+5)
GoParser          316k ± 1%         315k ± 0%     ~     (p=0.222 n=5+5)
Reflect           980k ± 0%         979k ± 0%     ~     (p=0.095 n=5+5)
Tar               249k ± 1%         249k ± 1%     ~     (p=0.841 n=5+5)
XML               392k ± 0%         391k ± 0%     ~     (p=0.095 n=5+5)


From c=1 to c=4, real time is down ~40%, CPU usage up 10-20%, alloc up ~5%:

name       old time/op       new time/op       delta
Template         203ms ± 3%        131ms ± 5%  -35.45%  (p=0.000 n=50+50)
Unicode         87.2ms ± 4%       84.1ms ± 2%   -3.61%  (p=0.000 n=48+47)
GoTypes          560ms ± 4%        310ms ± 2%  -44.65%  (p=0.000 n=50+49)
Compiler         2.47s ± 3%        1.41s ± 2%  -43.10%  (p=0.000 n=50+46)
SSA              6.17s ± 2%        3.20s ± 2%  -48.06%  (p=0.000 n=49+49)
Flate            126ms ± 4%         74ms ± 2%  -41.06%  (p=0.000 n=49+48)
GoParser         148ms ± 4%         89ms ± 3%  -39.97%  (p=0.000 n=49+50)
Reflect          360ms ± 3%        242ms ± 3%  -32.81%  (p=0.000 n=49+49)
Tar              108ms ± 4%         73ms ± 4%  -32.48%  (p=0.000 n=50+49)
XML              203ms ± 3%        119ms ± 3%  -41.56%  (p=0.000 n=49+48)

name       old user-time/op  new user-time/op  delta
Template         246ms ± 9%        287ms ± 9%  +16.98%  (p=0.000 n=50+50)
Unicode          109ms ± 4%        118ms ± 5%   +7.56%  (p=0.000 n=46+50)
GoTypes          735ms ± 4%        806ms ± 2%   +9.62%  (p=0.000 n=50+50)
Compiler         3.34s ± 4%        3.56s ± 2%   +6.78%  (p=0.000 n=49+49)
SSA              8.54s ± 3%       10.04s ± 3%  +17.55%  (p=0.000 n=50+50)
Flate            149ms ± 6%        176ms ± 3%  +17.82%  (p=0.000 n=50+48)
GoParser         181ms ± 5%        213ms ± 3%  +17.47%  (p=0.000 n=50+50)
Reflect          453ms ± 6%        499ms ± 2%  +10.11%  (p=0.000 n=50+48)
Tar              126ms ± 5%        149ms ±11%  +18.76%  (p=0.000 n=50+50)
XML              246ms ± 5%        287ms ± 4%  +16.53%  (p=0.000 n=49+50)

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       40.4MB ± 0%   +4.21%  (p=0.008 n=5+5)
Unicode         29.8MB ± 0%       30.9MB ± 0%   +3.68%  (p=0.008 n=5+5)
GoTypes          113MB ± 0%        116MB ± 0%   +2.71%  (p=0.008 n=5+5)
Compiler         443MB ± 0%        455MB ± 0%   +2.75%  (p=0.008 n=5+5)
SSA             1.25GB ± 0%       1.27GB ± 0%   +1.84%  (p=0.008 n=5+5)
Flate           25.3MB ± 0%       26.9MB ± 1%   +6.31%  (p=0.008 n=5+5)
GoParser        31.7MB ± 0%       33.2MB ± 0%   +4.61%  (p=0.008 n=5+5)
Reflect         78.2MB ± 0%       80.2MB ± 0%   +2.53%  (p=0.008 n=5+5)
Tar             26.6MB ± 0%       27.9MB ± 0%   +5.19%  (p=0.008 n=5+5)
XML             42.4MB ± 0%       44.6MB ± 0%   +5.20%  (p=0.008 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          380k ± 0%         379k ± 0%   -0.39%  (p=0.032 n=5+5)
Unicode           321k ± 0%         321k ± 0%     ~     (p=0.841 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%     ~     (p=0.421 n=5+5)
Compiler         4.12M ± 0%        4.14M ± 0%   +0.52%  (p=0.008 n=5+5)
SSA              9.72M ± 0%        9.76M ± 0%   +0.37%  (p=0.008 n=5+5)
Flate             234k ± 1%         234k ± 1%     ~     (p=0.690 n=5+5)
GoParser          316k ± 0%         317k ± 1%     ~     (p=0.841 n=5+5)
Reflect           981k ± 0%         981k ± 0%     ~     (p=1.000 n=5+5)
Tar               250k ± 0%         249k ± 1%     ~     (p=0.151 n=5+5)
XML               393k ± 0%         392k ± 0%     ~     (p=0.056 n=5+5)


Going beyond c=4 on my machine tends to increase CPU time and allocs
without impacting real time.

The CPU time numbers matter, because when there are many concurrent
compilation processes, that will impact the overall throughput.

The numbers above are in many ways the best case scenario;
we can take full advantage of all cores.
Fortunately, the most common compilation scenario is incremental
re-compilation of a single package during a build/test cycle.


Updates golang#15756

Change-Id: I6725558ca2069edec0ac5b0d1683105a9fff6bea

Prior to this CL, flags such as NOSPLIT
on ATEXT Progs were stored in From3.Offset.
Some but not all of those flags were also
duplicated into From.Sym.Attribute.

This CL migrates all of those flags into
From.Sym.Attribute and stops creating a From3.

A side-effect of this is that printing an
ATEXT Prog can no longer simply dump From3.Offset.
That's kind of good, since the raw flag value
wasn't very informative anyway, but it did
necessitate a bunch of updates to the cmd/asm tests.

The reason I'm doing this work now is that
avoiding storing flags in both From.Sym and From3.Offset
simplifies some other changes to fix the data
race first described in CL 40254.

This CL almost passes toolstash-check -all.
The only changes are in cases where the assembler
has decided that a function's flags may be altered,
e.g. to make a function with no calls in it NOSPLIT.
Prior to this CL, that information was not printed.

Sample before:

"".Ctz64 t=1 size=63 args=0x10 locals=0x0
	0x0000 00000 (/Users/josh/go/tip/src/runtime/internal/sys/intrinsics.go:35)	TEXT	"".Ctz64(SB), $0-16
	0x0000 00000 (/Users/josh/go/tip/src/runtime/internal/sys/intrinsics.go:35)	FUNCDATA	$0, gclocals·f207267fbf96a0178e8758c6e3e0ce28(SB)

Sample after:

"".Ctz64 t=1 nosplit size=63 args=0x10 locals=0x0
	0x0000 00000 (/Users/josh/go/tip/src/runtime/internal/sys/intrinsics.go:35)	TEXT	"".Ctz64(SB), NOSPLIT, $0-16
	0x0000 00000 (/Users/josh/go/tip/src/runtime/internal/sys/intrinsics.go:35)	FUNCDATA	$0, gclocals·f207267fbf96a0178e8758c6e3e0ce28(SB)

Observe the additional "nosplit" in the first line
and the additional "NOSPLIT" in the second line.

Updates golang#15756

Change-Id: I5c59bd8f3bdc7c780361f801d94a261f0aef3d13

The initialization of an ATEXT Prog's From.Sym
can race with the assemblers in a concurrent compiler.
CL 40254 contains an initial, failed attempt to
fix that race.

This CL takes a different approach: Rather than
expose an API to initialize the Prog,
expose an API to initialize the Sym.

The initialization of the Sym can then be
moved earlier in the compiler, avoiding the race.

The growth of gc.Func has negligible
performance impact; see below.

Passes toolstash -cmp.

Updates golang#15756

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       38.8MB ± 0%    ~     (p=0.968 n=9+10)
Unicode         29.8MB ± 0%       29.8MB ± 0%    ~     (p=0.684 n=10+10)
GoTypes          113MB ± 0%        113MB ± 0%    ~     (p=0.912 n=10+10)
SSA             1.25GB ± 0%       1.25GB ± 0%    ~     (p=0.481 n=10+10)
Flate           25.3MB ± 0%       25.3MB ± 0%    ~     (p=0.105 n=10+10)
GoParser        31.7MB ± 0%       31.8MB ± 0%  +0.09%  (p=0.016 n=8+10)
Reflect         78.3MB ± 0%       78.2MB ± 0%    ~     (p=0.190 n=10+10)
Tar             26.5MB ± 0%       26.6MB ± 0%  +0.13%  (p=0.011 n=10+10)
XML             42.4MB ± 0%       42.4MB ± 0%    ~     (p=0.971 n=10+10)

name       old allocs/op     new allocs/op     delta
Template          378k ± 1%         378k ± 0%    ~     (p=0.315 n=10+9)
Unicode           321k ± 1%         321k ± 0%    ~     (p=0.436 n=10+10)
GoTypes          1.14M ± 0%        1.14M ± 0%    ~     (p=0.079 n=10+9)
SSA              9.70M ± 0%        9.70M ± 0%  -0.04%  (p=0.035 n=10+10)
Flate             233k ± 1%         234k ± 1%    ~     (p=0.529 n=10+10)
GoParser          315k ± 0%         316k ± 0%    ~     (p=0.095 n=9+10)
Reflect           980k ± 0%         980k ± 0%    ~     (p=0.436 n=10+10)
Tar               249k ± 1%         250k ± 0%    ~     (p=0.280 n=10+10)
XML               391k ± 1%         391k ± 1%    ~     (p=0.481 n=10+10)

Change-Id: I3c93033dddd2e1df8cc54a106a6e615d27859e71

To preserve reproducible builds, the text entries
during compilation will be sorted before being printed.
TestAssembly currently assumes that function init
comes after all user-defined functions.
Remove that assumption.
Instead of looking for "TEXT" to tell you where
a function ends--which may now yield lots of
non-function-code junk--look for a line beginning
with non-whitespace.

Updates golang#15756

Change-Id: Ibc82dba6143d769ef4c391afc360e523b1a51348

Add concurrency protection to Ctxt.Text.
This is a low-contention mutex;
it is only touched once per function compilation.
To preserve reproducible builds,
sort Ctxt.Text when writing.

TODO: benchmarks

Updates golang#15756

Change-Id: Ibde43aa156cae98130e5f50d8581ff0413bcd236

Though LSym.Version is an int, it can only have the value 0 or 1.
Using that, split Link.hash into two maps, one for version 0
(which is far more common) and one for version 1.
This lets use just the name for lookups,
which is both faster and more compact.
This matters because Link.hash map lookups are frequent,
and will be contended once the backend is concurrent.

name        old time/op       new time/op       delta
Template          194ms ± 3%        192ms ± 5%  -1.46%  (p=0.000 n=47+49)
Unicode          84.5ms ± 3%       83.8ms ± 3%  -0.81%  (p=0.011 n=50+49)
GoTypes           543ms ± 2%        545ms ± 4%    ~     (p=0.566 n=46+49)
Compiler          2.48s ± 2%        2.48s ± 3%    ~     (p=0.706 n=47+50)
SSA               5.94s ± 3%        5.98s ± 2%  +0.55%  (p=0.040 n=49+50)
Flate             119ms ± 6%        119ms ± 4%    ~     (p=0.681 n=48+47)
GoParser          145ms ± 4%        145ms ± 3%    ~     (p=0.662 n=47+49)
Reflect           348ms ± 3%        344ms ± 3%  -1.17%  (p=0.000 n=47+47)
Tar               105ms ± 4%        104ms ± 3%    ~     (p=0.155 n=50+47)
XML               197ms ± 2%        197ms ± 3%    ~     (p=0.666 n=49+49)
[Geo mean]        332ms             331ms       -0.37%

name        old user-time/op  new user-time/op  delta
Template          230ms ±10%        226ms ±10%  -1.85%  (p=0.041 n=50+50)
Unicode           104ms ± 6%        103ms ± 5%    ~     (p=0.076 n=49+49)
GoTypes           707ms ± 4%        705ms ± 5%    ~     (p=0.521 n=50+50)
Compiler          3.30s ± 3%        3.33s ± 4%  +0.76%  (p=0.003 n=50+49)
SSA               8.17s ± 4%        8.23s ± 3%  +0.66%  (p=0.030 n=50+49)
Flate             139ms ± 6%        138ms ± 8%    ~     (p=0.184 n=49+48)
GoParser          174ms ± 5%        172ms ± 6%    ~     (p=0.107 n=48+49)
Reflect           431ms ± 8%        420ms ± 5%  -2.57%  (p=0.000 n=50+46)
Tar               119ms ± 6%        118ms ± 7%  -0.95%  (p=0.033 n=50+49)
XML               236ms ± 4%        236ms ± 4%    ~     (p=0.935 n=50+48)
[Geo mean]        410ms             407ms       -0.67%

name        old alloc/op      new alloc/op      delta
Template         38.7MB ± 0%       38.6MB ± 0%  -0.29%  (p=0.008 n=5+5)
Unicode          29.8MB ± 0%       29.7MB ± 0%  -0.24%  (p=0.008 n=5+5)
GoTypes           113MB ± 0%        113MB ± 0%  -0.29%  (p=0.008 n=5+5)
Compiler          462MB ± 0%        462MB ± 0%  -0.12%  (p=0.008 n=5+5)
SSA              1.27GB ± 0%       1.27GB ± 0%  -0.05%  (p=0.008 n=5+5)
Flate            25.2MB ± 0%       25.1MB ± 0%  -0.37%  (p=0.008 n=5+5)
GoParser         31.7MB ± 0%       31.6MB ± 0%    ~     (p=0.056 n=5+5)
Reflect          77.5MB ± 0%       77.2MB ± 0%  -0.38%  (p=0.008 n=5+5)
Tar              26.4MB ± 0%       26.3MB ± 0%    ~     (p=0.151 n=5+5)
XML              41.9MB ± 0%       41.9MB ± 0%  -0.20%  (p=0.032 n=5+5)
[Geo mean]       74.5MB            74.3MB       -0.23%

name        old allocs/op     new allocs/op     delta
Template           378k ± 1%         377k ± 1%    ~     (p=0.690 n=5+5)
Unicode            321k ± 0%         322k ± 0%    ~     (p=0.595 n=5+5)
GoTypes           1.14M ± 0%        1.14M ± 0%    ~     (p=0.310 n=5+5)
Compiler          4.25M ± 0%        4.25M ± 0%    ~     (p=0.151 n=5+5)
SSA               9.84M ± 0%        9.84M ± 0%    ~     (p=0.841 n=5+5)
Flate              232k ± 1%         232k ± 0%    ~     (p=0.690 n=5+5)
GoParser           315k ± 1%         315k ± 1%    ~     (p=0.841 n=5+5)
Reflect            970k ± 0%         970k ± 0%    ~     (p=0.841 n=5+5)
Tar                248k ± 0%         248k ± 1%    ~     (p=0.841 n=5+5)
XML                389k ± 0%         389k ± 0%    ~     (p=1.000 n=5+5)
[Geo mean]         724k              724k       +0.01%

Updates #15756

Change-Id: I2646332e89f0444ca9d5a41d7172537d904ed636
Reviewed-on: https://go-review.googlesource.com/41050
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

This CL adds initial support for concurrent backend compilation.

CAVEATS

I suspect it's going to end up on Twitter.
If you're coming here from the internet:

* Don't believe the hype.
* If you're going to try it out, please also try with
  the race detector enabled and report an issue
  at golang.org/issue/new if you see a race report.
  Just run 'go install -race cmd/compile' and
  'go build -a yourpackages'. And then run make.bash
  again to return a reasonable compiler.

BACKGROUND

The compiler currently consists (very roughly) of the following phases:

1. Initialization.
2. Lexing and parsing into the cmd/compile/internal/syntax AST.
3. Translation into the cmd/compile/internal/gc AST.
4. Some gc AST passes: typechecking, escape analysis, inlining,
   closure handling, expression evaluation ordering (order.go),
   and some lowering and optimization (walk.go).
5. Translation into the cmd/compile/internal/ssa SSA form.
6. Optimization and lowering of SSA form.
7. Translation from SSA form to assembler instructions.
8. Translation from assembler instructions to machine code.
9. Writing lots of output: machine code, DWARF symbols,
   type and reflection info, export data.

Phase 2 was already concurrent as of Go 1.8.

Phase 3 is planned for eventual removal;
we hope to go straight from syntax AST to SSA.

Phases 5–8 are per-function; this CL adds support for
processing multiple functions concurrently.
The slowest phases in the compiler are 5 and 6,
so this offers the opportunity for some good speed-ups.

Unfortunately, it's not quite that straightforward.
In the current compiler, the latter parts of phase 4
(order, walk) are done function-at-a-time as needed.
Making order and walk concurrency-safe proved hard,
and they're not particularly slow, so there wasn't much reward.
To enable phases 5–8 to be done concurrently,
when concurrent backend compilation is requested,
we complete phase 4 for all functions
before starting later phases for any functions.

Also, in reality, we automatically generate new
functions in phase 9, such as method wrappers
and equality and has routines.
Those new functions then go through phases 4–8.
This CL disables concurrent backend compilation
after the first, big, user-provided batch of
functions has been compiled.
This is done to keep things simple,
and because the autogenerated functions
tend to be small, few, simple, and fast to compile.

USAGE

Concurrent backend compilation still defaults to off.
To set the number of functions that may be backend-compiled
concurrently, use the compiler flag -c.
In future work, cmd/go will automatically set -c.

Furthermore, this CL has been intentionally written
so that the c=1 path has no backend concurrency whatsoever,
not even spawning any goroutines.
This helps ensure that, should problems arise
let in the development cycle,
we can simply have cmd/go set -c=1 always,
and revert to the original compiler behavior.

MUTEXES

Most of the work required to make concurrent backend
compilation safe has occurred over the past month.
This CL adds a handful of mutexes to get the rest of the way there;
they are the mutexes that I didn't see a clean way to avoid.
Some of them may still be eliminable in future work.

In no particular order:

* gc.funcsymsmu. The global funcsyms slice is populated
  lazily when we need function symbols for closures.
  This occurs during gc AST to SSA translation.
  The function funcsym also does a package lookup,
  which is a source of races on types.Pkg.Syms;
  funcsymsmu also covers that package lookup.
  This mutex is low priority: it adds a single global,
  it is in an infrequently used code path, and it is low contention.
  It requires additional sorting to preserve reproducible builds.

* gc.largeStackFramesMu. We don't discover until after SSA compilation
  that a function's stack frame is gigantic.
  Recording that error happens basically never,
  but it does happen concurrently.
  Fix with a low priority mutex and sorting.

* obj.Link.hashmu. ctxt.hash stores the mapping from
  types.Syms (compiler symbols) to obj.LSyms (linker symbols).
  It is accessed fairly heavily through all the phases.
  This is easily the most heavily contended mutex.
  I hope that the syncmap proposed in golang#18177 may
  provide some free speed-ups here.
  Some lookups may also be removable.

* gc.signatlistmu. The global signatlist map is
  populated with types through several of the concurrent phases,
  including notably via ngotype during DWARF generation.
  It is low priority for removal, aside from some mild
  awkwardness to avoid deadlocks due to recursive calls.

* gc.typepkgmu. Looking up symbols in the types package
  happens a fair amount during backend compilation
  and DWARF generation, particularly via ngotype.
  This mutex helps us to avoid a broader mutex on types.Pkg.Syms.
  It has low-to-moderate contention.

* types.internedStringsmu. gc AST to SSA conversion and
  some SSA work introduce new autotmps.
  Those autotmps have their names interned to reduce allocations.
  That interning requires protecting types.internedStrings.
  The autotmp names are heavily re-used, and the mutex
  overhead and contention here are low, so it is probably
  a worthwhile performance optimization to keep this mutex.

* types.Sym.Lsymmu. Syms keep a cache of their associated LSym,
  to reduce lookups in ctxt.Hash. This cache itself
  needs concurrency protection. This mutex adds to the size
  of a moderately important data structure, but the alloc benchmarks
  below show that this doesn't hurt much in practice.
  It is moderately contended, mostly because when lookups fail,
  the lock is held while vying for the contended ctxt.Hash mutex.
  The fact that it keeps load off the ctxt.Hash mutex, though,
  makes this mutex worth keeping.
  Also, a fair number of calls to Linksym could be avoided
  by judicious addition of a local variable.

TESTING

I have been testing this code locally by running
'go install -race cmd/compile'
and then doing
'go build -a -gcflags=-c=128 std cmd'
for all architectures and a variety of compiler flags.
This obviously needs to be made part of the builders,
but it is too expensive to make part of all.bash.
I have filed golang#19962 for this.

REPRODUCIBLE BUILDS

This version of the compiler generates reproducible builds.
Testing reproducible builds also needs automation, however,
and is also too expensive for all.bash.
This is golang#19961.

Also of note is that some of the compiler flags used by 'toolstash -cmp'
are currently incompatible with concurrent backend compilation.
They still work fine with c=1.
Time will tell whether this is a problem.

NEXT STEPS

* Continue to find and fix races and bugs,
  using a combination of code inspection, fuzzing,
  and hopefully some community experimentation.
  I do not know of any outstanding races,
  but there probably are some.
* Improve testing.
* Improve performance, for many values of c.
* Integrate with cmd/go and fine tune.
* Support concurrent compilation with the -race flag.
  It is a sad irony that it does not yet work.
* Minor code cleanup that has been deferred during
  the last month due to uncertainty about the
  ultimate shape of this CL.

PERFORMANCE

Here's the buried lede, at last. :)

All benchmarks are from my 8 core 2.9 GHz Intel Core i7 darwin/amd64 laptop.

First, going from tip to this CL with c=1
costs about 1-2% CPU and has almost no memory impact.

name       old time/op       new time/op       delta
Template         193ms ± 4%        195ms ± 5%  +0.99%  (p=0.007 n=50+50)
Unicode         82.0ms ± 3%       84.7ms ± 4%  +3.26%  (p=0.000 n=47+48)
GoTypes          539ms ± 4%        549ms ± 3%  +1.90%  (p=0.000 n=46+46)
SSA              5.92s ± 2%        5.95s ± 3%  +0.49%  (p=0.018 n=42+48)
Flate            121ms ± 4%        122ms ± 3%    ~     (p=0.783 n=48+46)
GoParser         143ms ± 6%        144ms ± 3%  +0.90%  (p=0.001 n=49+47)
Reflect          342ms ± 4%        347ms ± 3%  +1.47%  (p=0.000 n=48+49)
Tar              104ms ± 5%        105ms ± 4%  +1.06%  (p=0.003 n=47+47)
XML              197ms ± 4%        199ms ± 5%  +0.85%  (p=0.014 n=48+49)

name       old user-time/op  new user-time/op  delta
Template         238ms ±10%        241ms ±10%    ~     (p=0.066 n=50+50)
Unicode          104ms ± 4%        106ms ± 5%  +2.23%  (p=0.000 n=46+49)
GoTypes          706ms ± 3%        714ms ± 5%  +1.15%  (p=0.002 n=45+49)
SSA              8.21s ± 3%        8.27s ± 2%  +0.78%  (p=0.002 n=43+47)
Flate            144ms ± 7%        145ms ± 5%    ~     (p=0.098 n=49+49)
GoParser         175ms ± 4%        177ms ± 4%  +1.35%  (p=0.003 n=47+50)
Reflect          435ms ± 6%        433ms ± 7%    ~     (p=0.852 n=50+50)
Tar              121ms ± 6%        122ms ± 7%    ~     (p=0.143 n=48+50)
XML              240ms ± 4%        241ms ± 4%    ~     (p=0.231 n=50+49)

name       old alloc/op      new alloc/op      delta
Template        38.7MB ± 0%       38.7MB ± 0%    ~     (p=0.690 n=5+5)
Unicode         29.8MB ± 0%       29.8MB ± 0%    ~     (p=0.222 n=5+5)
GoTypes          113MB ± 0%        113MB ± 0%    ~     (p=0.310 n=5+5)
SSA             1.24GB ± 0%       1.24GB ± 0%  +0.04%  (p=0.008 n=5+5)
Flate           25.2MB ± 0%       25.2MB ± 0%    ~     (p=0.841 n=5+5)
GoParser        31.7MB ± 0%       31.7MB ± 0%    ~     (p=0.151 n=5+5)
Reflect         77.5MB ± 0%       77.5MB ± 0%    ~     (p=0.690 n=5+5)
Tar             26.4MB ± 0%       26.4MB ± 0%    ~     (p=0.690 n=5+5)
XML             42.0MB ± 0%       42.0MB ± 0%    ~     (p=0.690 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          378k ± 0%         377k ± 1%    ~     (p=0.841 n=5+5)
Unicode           321k ± 0%         322k ± 0%    ~     (p=0.310 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%    ~     (p=0.310 n=5+5)
SSA              9.67M ± 0%        9.69M ± 0%  +0.20%  (p=0.008 n=5+5)
Flate             233k ± 0%         233k ± 1%    ~     (p=1.000 n=5+5)
GoParser          315k ± 0%         314k ± 1%    ~     (p=0.151 n=5+5)
Reflect           971k ± 0%         970k ± 0%    ~     (p=0.690 n=5+5)
Tar               249k ± 0%         249k ± 0%    ~     (p=0.310 n=5+5)
XML               391k ± 0%         390k ± 0%    ~     (p=0.841 n=5+5)

Comparing this CL to itself, from c=1 to c=2
improves real times 20-30%, costs 5-10% more CPU time,
and adds about 2% alloc.
The allocation increase comes from allocating more ssa.Caches.

name       old time/op       new time/op       delta
Template         202ms ± 3%        149ms ± 3%  -26.15%  (p=0.000 n=49+49)
Unicode         87.4ms ± 4%       84.2ms ± 3%   -3.68%  (p=0.000 n=48+48)
GoTypes          560ms ± 2%        398ms ± 2%  -28.96%  (p=0.000 n=49+49)
Compiler         2.46s ± 3%        1.76s ± 2%  -28.61%  (p=0.000 n=48+46)
SSA              6.17s ± 2%        4.04s ± 1%  -34.52%  (p=0.000 n=49+49)
Flate            126ms ± 3%         92ms ± 2%  -26.81%  (p=0.000 n=49+48)
GoParser         148ms ± 4%        107ms ± 2%  -27.78%  (p=0.000 n=49+48)
Reflect          361ms ± 3%        281ms ± 3%  -22.10%  (p=0.000 n=49+49)
Tar              109ms ± 4%         86ms ± 3%  -20.81%  (p=0.000 n=49+47)
XML              204ms ± 3%        144ms ± 2%  -29.53%  (p=0.000 n=48+45)

name       old user-time/op  new user-time/op  delta
Template         246ms ± 9%        246ms ± 4%     ~     (p=0.401 n=50+48)
Unicode          109ms ± 4%        111ms ± 4%   +1.47%  (p=0.000 n=44+50)
GoTypes          728ms ± 3%        765ms ± 3%   +5.04%  (p=0.000 n=46+50)
Compiler         3.33s ± 3%        3.41s ± 2%   +2.31%  (p=0.000 n=49+48)
SSA              8.52s ± 2%        9.11s ± 2%   +6.93%  (p=0.000 n=49+47)
Flate            149ms ± 4%        161ms ± 3%   +8.13%  (p=0.000 n=50+47)
GoParser         181ms ± 5%        192ms ± 2%   +6.40%  (p=0.000 n=49+46)
Reflect          452ms ± 9%        474ms ± 2%   +4.99%  (p=0.000 n=50+48)
Tar              126ms ± 6%        136ms ± 4%   +7.95%  (p=0.000 n=50+49)
XML              247ms ± 5%        264ms ± 3%   +6.94%  (p=0.000 n=48+50)

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       39.3MB ± 0%   +1.48%  (p=0.008 n=5+5)
Unicode         29.8MB ± 0%       30.2MB ± 0%   +1.19%  (p=0.008 n=5+5)
GoTypes          113MB ± 0%        114MB ± 0%   +0.69%  (p=0.008 n=5+5)
Compiler         443MB ± 0%        447MB ± 0%   +0.95%  (p=0.008 n=5+5)
SSA             1.25GB ± 0%       1.26GB ± 0%   +0.89%  (p=0.008 n=5+5)
Flate           25.3MB ± 0%       25.9MB ± 1%   +2.35%  (p=0.008 n=5+5)
GoParser        31.7MB ± 0%       32.2MB ± 0%   +1.59%  (p=0.008 n=5+5)
Reflect         78.2MB ± 0%       78.9MB ± 0%   +0.91%  (p=0.008 n=5+5)
Tar             26.6MB ± 0%       27.0MB ± 0%   +1.80%  (p=0.008 n=5+5)
XML             42.4MB ± 0%       43.4MB ± 0%   +2.35%  (p=0.008 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          379k ± 0%         378k ± 0%     ~     (p=0.421 n=5+5)
Unicode           322k ± 0%         321k ± 0%     ~     (p=0.222 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%     ~     (p=0.548 n=5+5)
Compiler         4.12M ± 0%        4.11M ± 0%   -0.14%  (p=0.032 n=5+5)
SSA              9.72M ± 0%        9.72M ± 0%     ~     (p=0.421 n=5+5)
Flate             234k ± 1%         234k ± 0%     ~     (p=0.421 n=5+5)
GoParser          316k ± 1%         315k ± 0%     ~     (p=0.222 n=5+5)
Reflect           980k ± 0%         979k ± 0%     ~     (p=0.095 n=5+5)
Tar               249k ± 1%         249k ± 1%     ~     (p=0.841 n=5+5)
XML               392k ± 0%         391k ± 0%     ~     (p=0.095 n=5+5)

From c=1 to c=4, real time is down ~40%, CPU usage up 10-20%, alloc up ~5%:

name       old time/op       new time/op       delta
Template         203ms ± 3%        131ms ± 5%  -35.45%  (p=0.000 n=50+50)
Unicode         87.2ms ± 4%       84.1ms ± 2%   -3.61%  (p=0.000 n=48+47)
GoTypes          560ms ± 4%        310ms ± 2%  -44.65%  (p=0.000 n=50+49)
Compiler         2.47s ± 3%        1.41s ± 2%  -43.10%  (p=0.000 n=50+46)
SSA              6.17s ± 2%        3.20s ± 2%  -48.06%  (p=0.000 n=49+49)
Flate            126ms ± 4%         74ms ± 2%  -41.06%  (p=0.000 n=49+48)
GoParser         148ms ± 4%         89ms ± 3%  -39.97%  (p=0.000 n=49+50)
Reflect          360ms ± 3%        242ms ± 3%  -32.81%  (p=0.000 n=49+49)
Tar              108ms ± 4%         73ms ± 4%  -32.48%  (p=0.000 n=50+49)
XML              203ms ± 3%        119ms ± 3%  -41.56%  (p=0.000 n=49+48)

name       old user-time/op  new user-time/op  delta
Template         246ms ± 9%        287ms ± 9%  +16.98%  (p=0.000 n=50+50)
Unicode          109ms ± 4%        118ms ± 5%   +7.56%  (p=0.000 n=46+50)
GoTypes          735ms ± 4%        806ms ± 2%   +9.62%  (p=0.000 n=50+50)
Compiler         3.34s ± 4%        3.56s ± 2%   +6.78%  (p=0.000 n=49+49)
SSA              8.54s ± 3%       10.04s ± 3%  +17.55%  (p=0.000 n=50+50)
Flate            149ms ± 6%        176ms ± 3%  +17.82%  (p=0.000 n=50+48)
GoParser         181ms ± 5%        213ms ± 3%  +17.47%  (p=0.000 n=50+50)
Reflect          453ms ± 6%        499ms ± 2%  +10.11%  (p=0.000 n=50+48)
Tar              126ms ± 5%        149ms ±11%  +18.76%  (p=0.000 n=50+50)
XML              246ms ± 5%        287ms ± 4%  +16.53%  (p=0.000 n=49+50)

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       40.4MB ± 0%   +4.21%  (p=0.008 n=5+5)
Unicode         29.8MB ± 0%       30.9MB ± 0%   +3.68%  (p=0.008 n=5+5)
GoTypes          113MB ± 0%        116MB ± 0%   +2.71%  (p=0.008 n=5+5)
Compiler         443MB ± 0%        455MB ± 0%   +2.75%  (p=0.008 n=5+5)
SSA             1.25GB ± 0%       1.27GB ± 0%   +1.84%  (p=0.008 n=5+5)
Flate           25.3MB ± 0%       26.9MB ± 1%   +6.31%  (p=0.008 n=5+5)
GoParser        31.7MB ± 0%       33.2MB ± 0%   +4.61%  (p=0.008 n=5+5)
Reflect         78.2MB ± 0%       80.2MB ± 0%   +2.53%  (p=0.008 n=5+5)
Tar             26.6MB ± 0%       27.9MB ± 0%   +5.19%  (p=0.008 n=5+5)
XML             42.4MB ± 0%       44.6MB ± 0%   +5.20%  (p=0.008 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          380k ± 0%         379k ± 0%   -0.39%  (p=0.032 n=5+5)
Unicode           321k ± 0%         321k ± 0%     ~     (p=0.841 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%     ~     (p=0.421 n=5+5)
Compiler         4.12M ± 0%        4.14M ± 0%   +0.52%  (p=0.008 n=5+5)
SSA              9.72M ± 0%        9.76M ± 0%   +0.37%  (p=0.008 n=5+5)
Flate             234k ± 1%         234k ± 1%     ~     (p=0.690 n=5+5)
GoParser          316k ± 0%         317k ± 1%     ~     (p=0.841 n=5+5)
Reflect           981k ± 0%         981k ± 0%     ~     (p=1.000 n=5+5)
Tar               250k ± 0%         249k ± 1%     ~     (p=0.151 n=5+5)
XML               393k ± 0%         392k ± 0%     ~     (p=0.056 n=5+5)

Going beyond c=4 on my machine tends to increase CPU time and allocs
without impacting real time.

The CPU time numbers matter, because when there are many concurrent
compilation processes, that will impact the overall throughput.

The numbers above are in many ways the best case scenario;
we can take full advantage of all cores.
Fortunately, the most common compilation scenario is incremental
re-compilation of a single package during a build/test cycle.

Updates golang#15756

Change-Id: I6725558ca2069edec0ac5b0d1683105a9fff6bea

This CL adds initial support for concurrent backend compilation.

CAVEATS

I suspect it's going to end up on Twitter.
If you're coming here from the internet:

* Don't believe the hype.
* If you're going to try it out, please also try with
  the race detector enabled and report an issue
  at golang.org/issue/new if you see a race report.
  Just run 'go install -race cmd/compile' and
  'go build -a yourpackages'. And then run make.bash
  again to return a reasonable compiler.

BACKGROUND

The compiler currently consists (very roughly) of the following phases:

1. Initialization.
2. Lexing and parsing into the cmd/compile/internal/syntax AST.
3. Translation into the cmd/compile/internal/gc AST.
4. Some gc AST passes: typechecking, escape analysis, inlining,
   closure handling, expression evaluation ordering (order.go),
   and some lowering and optimization (walk.go).
5. Translation into the cmd/compile/internal/ssa SSA form.
6. Optimization and lowering of SSA form.
7. Translation from SSA form to assembler instructions.
8. Translation from assembler instructions to machine code.
9. Writing lots of output: machine code, DWARF symbols,
   type and reflection info, export data.

Phase 2 was already concurrent as of Go 1.8.

Phase 3 is planned for eventual removal;
we hope to go straight from syntax AST to SSA.

Phases 5–8 are per-function; this CL adds support for
processing multiple functions concurrently.
The slowest phases in the compiler are 5 and 6,
so this offers the opportunity for some good speed-ups.

Unfortunately, it's not quite that straightforward.
In the current compiler, the latter parts of phase 4
(order, walk) are done function-at-a-time as needed.
Making order and walk concurrency-safe proved hard,
and they're not particularly slow, so there wasn't much reward.
To enable phases 5–8 to be done concurrently,
when concurrent backend compilation is requested,
we complete phase 4 for all functions
before starting later phases for any functions.

Also, in reality, we automatically generate new
functions in phase 9, such as method wrappers
and equality and has routines.
Those new functions then go through phases 4–8.
This CL disables concurrent backend compilation
after the first, big, user-provided batch of
functions has been compiled.
This is done to keep things simple,
and because the autogenerated functions
tend to be small, few, simple, and fast to compile.

USAGE

Concurrent backend compilation still defaults to off.
To set the number of functions that may be backend-compiled
concurrently, use the compiler flag -c.
In future work, cmd/go will automatically set -c.

Furthermore, this CL has been intentionally written
so that the c=1 path has no backend concurrency whatsoever,
not even spawning any goroutines.
This helps ensure that, should problems arise
let in the development cycle,
we can simply have cmd/go set -c=1 always,
and revert to the original compiler behavior.

MUTEXES

Most of the work required to make concurrent backend
compilation safe has occurred over the past month.
This CL adds a handful of mutexes to get the rest of the way there;
they are the mutexes that I didn't see a clean way to avoid.
Some of them may still be eliminable in future work.

In no particular order:

* gc.funcsymsmu. The global funcsyms slice is populated
  lazily when we need function symbols for closures.
  This occurs during gc AST to SSA translation.
  The function funcsym also does a package lookup,
  which is a source of races on types.Pkg.Syms;
  funcsymsmu also covers that package lookup.
  This mutex is low priority: it adds a single global,
  it is in an infrequently used code path, and it is low contention.
  It requires additional sorting to preserve reproducible builds.

* gc.largeStackFramesMu. We don't discover until after SSA compilation
  that a function's stack frame is gigantic.
  Recording that error happens basically never,
  but it does happen concurrently.
  Fix with a low priority mutex and sorting.

* obj.Link.hashmu. ctxt.hash stores the mapping from
  types.Syms (compiler symbols) to obj.LSyms (linker symbols).
  It is accessed fairly heavily through all the phases.
  This is easily the most heavily contended mutex.
  I hope that the syncmap proposed in golang#18177 may
  provide some free speed-ups here.
  Some lookups may also be removable.

* gc.signatlistmu. The global signatlist map is
  populated with types through several of the concurrent phases,
  including notably via ngotype during DWARF generation.
  It is low priority for removal, aside from some mild
  awkwardness to avoid deadlocks due to recursive calls.

* gc.typepkgmu. Looking up symbols in the types package
  happens a fair amount during backend compilation
  and DWARF generation, particularly via ngotype.
  This mutex helps us to avoid a broader mutex on types.Pkg.Syms.
  It has low-to-moderate contention.

* types.internedStringsmu. gc AST to SSA conversion and
  some SSA work introduce new autotmps.
  Those autotmps have their names interned to reduce allocations.
  That interning requires protecting types.internedStrings.
  The autotmp names are heavily re-used, and the mutex
  overhead and contention here are low, so it is probably
  a worthwhile performance optimization to keep this mutex.

* types.Sym.Lsymmu. Syms keep a cache of their associated LSym,
  to reduce lookups in ctxt.Hash. This cache itself
  needs concurrency protection. This mutex adds to the size
  of a moderately important data structure, but the alloc benchmarks
  below show that this doesn't hurt much in practice.
  It is moderately contended, mostly because when lookups fail,
  the lock is held while vying for the contended ctxt.Hash mutex.
  The fact that it keeps load off the ctxt.Hash mutex, though,
  makes this mutex worth keeping.
  Also, a fair number of calls to Linksym could be avoided
  by judicious addition of a local variable.

TESTING

I have been testing this code locally by running
'go install -race cmd/compile'
and then doing
'go build -a -gcflags=-c=128 std cmd'
for all architectures and a variety of compiler flags.
This obviously needs to be made part of the builders,
but it is too expensive to make part of all.bash.
I have filed golang#19962 for this.

REPRODUCIBLE BUILDS

This version of the compiler generates reproducible builds.
Testing reproducible builds also needs automation, however,
and is also too expensive for all.bash.
This is golang#19961.

Also of note is that some of the compiler flags used by 'toolstash -cmp'
are currently incompatible with concurrent backend compilation.
They still work fine with c=1.
Time will tell whether this is a problem.

NEXT STEPS

* Continue to find and fix races and bugs,
  using a combination of code inspection, fuzzing,
  and hopefully some community experimentation.
  I do not know of any outstanding races,
  but there probably are some.
* Improve testing.
* Improve performance, for many values of c.
* Integrate with cmd/go and fine tune.
* Support concurrent compilation with the -race flag.
  It is a sad irony that it does not yet work.
* Minor code cleanup that has been deferred during
  the last month due to uncertainty about the
  ultimate shape of this CL.

PERFORMANCE

Here's the buried lede, at last. :)

All benchmarks are from my 8 core 2.9 GHz Intel Core i7 darwin/amd64 laptop.

First, going from tip to this CL with c=1
costs about 1-2% CPU and has almost no memory impact.

name       old time/op       new time/op       delta
Template         193ms ± 4%        195ms ± 5%  +0.99%  (p=0.007 n=50+50)
Unicode         82.0ms ± 3%       84.7ms ± 4%  +3.26%  (p=0.000 n=47+48)
GoTypes          539ms ± 4%        549ms ± 3%  +1.90%  (p=0.000 n=46+46)
SSA              5.92s ± 2%        5.95s ± 3%  +0.49%  (p=0.018 n=42+48)
Flate            121ms ± 4%        122ms ± 3%    ~     (p=0.783 n=48+46)
GoParser         143ms ± 6%        144ms ± 3%  +0.90%  (p=0.001 n=49+47)
Reflect          342ms ± 4%        347ms ± 3%  +1.47%  (p=0.000 n=48+49)
Tar              104ms ± 5%        105ms ± 4%  +1.06%  (p=0.003 n=47+47)
XML              197ms ± 4%        199ms ± 5%  +0.85%  (p=0.014 n=48+49)

name       old user-time/op  new user-time/op  delta
Template         238ms ±10%        241ms ±10%    ~     (p=0.066 n=50+50)
Unicode          104ms ± 4%        106ms ± 5%  +2.23%  (p=0.000 n=46+49)
GoTypes          706ms ± 3%        714ms ± 5%  +1.15%  (p=0.002 n=45+49)
SSA              8.21s ± 3%        8.27s ± 2%  +0.78%  (p=0.002 n=43+47)
Flate            144ms ± 7%        145ms ± 5%    ~     (p=0.098 n=49+49)
GoParser         175ms ± 4%        177ms ± 4%  +1.35%  (p=0.003 n=47+50)
Reflect          435ms ± 6%        433ms ± 7%    ~     (p=0.852 n=50+50)
Tar              121ms ± 6%        122ms ± 7%    ~     (p=0.143 n=48+50)
XML              240ms ± 4%        241ms ± 4%    ~     (p=0.231 n=50+49)

name       old alloc/op      new alloc/op      delta
Template        38.7MB ± 0%       38.7MB ± 0%    ~     (p=0.690 n=5+5)
Unicode         29.8MB ± 0%       29.8MB ± 0%    ~     (p=0.222 n=5+5)
GoTypes          113MB ± 0%        113MB ± 0%    ~     (p=0.310 n=5+5)
SSA             1.24GB ± 0%       1.24GB ± 0%  +0.04%  (p=0.008 n=5+5)
Flate           25.2MB ± 0%       25.2MB ± 0%    ~     (p=0.841 n=5+5)
GoParser        31.7MB ± 0%       31.7MB ± 0%    ~     (p=0.151 n=5+5)
Reflect         77.5MB ± 0%       77.5MB ± 0%    ~     (p=0.690 n=5+5)
Tar             26.4MB ± 0%       26.4MB ± 0%    ~     (p=0.690 n=5+5)
XML             42.0MB ± 0%       42.0MB ± 0%    ~     (p=0.690 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          378k ± 0%         377k ± 1%    ~     (p=0.841 n=5+5)
Unicode           321k ± 0%         322k ± 0%    ~     (p=0.310 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%    ~     (p=0.310 n=5+5)
SSA              9.67M ± 0%        9.69M ± 0%  +0.20%  (p=0.008 n=5+5)
Flate             233k ± 0%         233k ± 1%    ~     (p=1.000 n=5+5)
GoParser          315k ± 0%         314k ± 1%    ~     (p=0.151 n=5+5)
Reflect           971k ± 0%         970k ± 0%    ~     (p=0.690 n=5+5)
Tar               249k ± 0%         249k ± 0%    ~     (p=0.310 n=5+5)
XML               391k ± 0%         390k ± 0%    ~     (p=0.841 n=5+5)

Comparing this CL to itself, from c=1 to c=2
improves real times 20-30%, costs 5-10% more CPU time,
and adds about 2% alloc.
The allocation increase comes from allocating more ssa.Caches.

name       old time/op       new time/op       delta
Template         202ms ± 3%        149ms ± 3%  -26.15%  (p=0.000 n=49+49)
Unicode         87.4ms ± 4%       84.2ms ± 3%   -3.68%  (p=0.000 n=48+48)
GoTypes          560ms ± 2%        398ms ± 2%  -28.96%  (p=0.000 n=49+49)
Compiler         2.46s ± 3%        1.76s ± 2%  -28.61%  (p=0.000 n=48+46)
SSA              6.17s ± 2%        4.04s ± 1%  -34.52%  (p=0.000 n=49+49)
Flate            126ms ± 3%         92ms ± 2%  -26.81%  (p=0.000 n=49+48)
GoParser         148ms ± 4%        107ms ± 2%  -27.78%  (p=0.000 n=49+48)
Reflect          361ms ± 3%        281ms ± 3%  -22.10%  (p=0.000 n=49+49)
Tar              109ms ± 4%         86ms ± 3%  -20.81%  (p=0.000 n=49+47)
XML              204ms ± 3%        144ms ± 2%  -29.53%  (p=0.000 n=48+45)

name       old user-time/op  new user-time/op  delta
Template         246ms ± 9%        246ms ± 4%     ~     (p=0.401 n=50+48)
Unicode          109ms ± 4%        111ms ± 4%   +1.47%  (p=0.000 n=44+50)
GoTypes          728ms ± 3%        765ms ± 3%   +5.04%  (p=0.000 n=46+50)
Compiler         3.33s ± 3%        3.41s ± 2%   +2.31%  (p=0.000 n=49+48)
SSA              8.52s ± 2%        9.11s ± 2%   +6.93%  (p=0.000 n=49+47)
Flate            149ms ± 4%        161ms ± 3%   +8.13%  (p=0.000 n=50+47)
GoParser         181ms ± 5%        192ms ± 2%   +6.40%  (p=0.000 n=49+46)
Reflect          452ms ± 9%        474ms ± 2%   +4.99%  (p=0.000 n=50+48)
Tar              126ms ± 6%        136ms ± 4%   +7.95%  (p=0.000 n=50+49)
XML              247ms ± 5%        264ms ± 3%   +6.94%  (p=0.000 n=48+50)

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       39.3MB ± 0%   +1.48%  (p=0.008 n=5+5)
Unicode         29.8MB ± 0%       30.2MB ± 0%   +1.19%  (p=0.008 n=5+5)
GoTypes          113MB ± 0%        114MB ± 0%   +0.69%  (p=0.008 n=5+5)
Compiler         443MB ± 0%        447MB ± 0%   +0.95%  (p=0.008 n=5+5)
SSA             1.25GB ± 0%       1.26GB ± 0%   +0.89%  (p=0.008 n=5+5)
Flate           25.3MB ± 0%       25.9MB ± 1%   +2.35%  (p=0.008 n=5+5)
GoParser        31.7MB ± 0%       32.2MB ± 0%   +1.59%  (p=0.008 n=5+5)
Reflect         78.2MB ± 0%       78.9MB ± 0%   +0.91%  (p=0.008 n=5+5)
Tar             26.6MB ± 0%       27.0MB ± 0%   +1.80%  (p=0.008 n=5+5)
XML             42.4MB ± 0%       43.4MB ± 0%   +2.35%  (p=0.008 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          379k ± 0%         378k ± 0%     ~     (p=0.421 n=5+5)
Unicode           322k ± 0%         321k ± 0%     ~     (p=0.222 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%     ~     (p=0.548 n=5+5)
Compiler         4.12M ± 0%        4.11M ± 0%   -0.14%  (p=0.032 n=5+5)
SSA              9.72M ± 0%        9.72M ± 0%     ~     (p=0.421 n=5+5)
Flate             234k ± 1%         234k ± 0%     ~     (p=0.421 n=5+5)
GoParser          316k ± 1%         315k ± 0%     ~     (p=0.222 n=5+5)
Reflect           980k ± 0%         979k ± 0%     ~     (p=0.095 n=5+5)
Tar               249k ± 1%         249k ± 1%     ~     (p=0.841 n=5+5)
XML               392k ± 0%         391k ± 0%     ~     (p=0.095 n=5+5)

From c=1 to c=4, real time is down ~40%, CPU usage up 10-20%, alloc up ~5%:

name       old time/op       new time/op       delta
Template         203ms ± 3%        131ms ± 5%  -35.45%  (p=0.000 n=50+50)
Unicode         87.2ms ± 4%       84.1ms ± 2%   -3.61%  (p=0.000 n=48+47)
GoTypes          560ms ± 4%        310ms ± 2%  -44.65%  (p=0.000 n=50+49)
Compiler         2.47s ± 3%        1.41s ± 2%  -43.10%  (p=0.000 n=50+46)
SSA              6.17s ± 2%        3.20s ± 2%  -48.06%  (p=0.000 n=49+49)
Flate            126ms ± 4%         74ms ± 2%  -41.06%  (p=0.000 n=49+48)
GoParser         148ms ± 4%         89ms ± 3%  -39.97%  (p=0.000 n=49+50)
Reflect          360ms ± 3%        242ms ± 3%  -32.81%  (p=0.000 n=49+49)
Tar              108ms ± 4%         73ms ± 4%  -32.48%  (p=0.000 n=50+49)
XML              203ms ± 3%        119ms ± 3%  -41.56%  (p=0.000 n=49+48)

name       old user-time/op  new user-time/op  delta
Template         246ms ± 9%        287ms ± 9%  +16.98%  (p=0.000 n=50+50)
Unicode          109ms ± 4%        118ms ± 5%   +7.56%  (p=0.000 n=46+50)
GoTypes          735ms ± 4%        806ms ± 2%   +9.62%  (p=0.000 n=50+50)
Compiler         3.34s ± 4%        3.56s ± 2%   +6.78%  (p=0.000 n=49+49)
SSA              8.54s ± 3%       10.04s ± 3%  +17.55%  (p=0.000 n=50+50)
Flate            149ms ± 6%        176ms ± 3%  +17.82%  (p=0.000 n=50+48)
GoParser         181ms ± 5%        213ms ± 3%  +17.47%  (p=0.000 n=50+50)
Reflect          453ms ± 6%        499ms ± 2%  +10.11%  (p=0.000 n=50+48)
Tar              126ms ± 5%        149ms ±11%  +18.76%  (p=0.000 n=50+50)
XML              246ms ± 5%        287ms ± 4%  +16.53%  (p=0.000 n=49+50)

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       40.4MB ± 0%   +4.21%  (p=0.008 n=5+5)
Unicode         29.8MB ± 0%       30.9MB ± 0%   +3.68%  (p=0.008 n=5+5)
GoTypes          113MB ± 0%        116MB ± 0%   +2.71%  (p=0.008 n=5+5)
Compiler         443MB ± 0%        455MB ± 0%   +2.75%  (p=0.008 n=5+5)
SSA             1.25GB ± 0%       1.27GB ± 0%   +1.84%  (p=0.008 n=5+5)
Flate           25.3MB ± 0%       26.9MB ± 1%   +6.31%  (p=0.008 n=5+5)
GoParser        31.7MB ± 0%       33.2MB ± 0%   +4.61%  (p=0.008 n=5+5)
Reflect         78.2MB ± 0%       80.2MB ± 0%   +2.53%  (p=0.008 n=5+5)
Tar             26.6MB ± 0%       27.9MB ± 0%   +5.19%  (p=0.008 n=5+5)
XML             42.4MB ± 0%       44.6MB ± 0%   +5.20%  (p=0.008 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          380k ± 0%         379k ± 0%   -0.39%  (p=0.032 n=5+5)
Unicode           321k ± 0%         321k ± 0%     ~     (p=0.841 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%     ~     (p=0.421 n=5+5)
Compiler         4.12M ± 0%        4.14M ± 0%   +0.52%  (p=0.008 n=5+5)
SSA              9.72M ± 0%        9.76M ± 0%   +0.37%  (p=0.008 n=5+5)
Flate             234k ± 1%         234k ± 1%     ~     (p=0.690 n=5+5)
GoParser          316k ± 0%         317k ± 1%     ~     (p=0.841 n=5+5)
Reflect           981k ± 0%         981k ± 0%     ~     (p=1.000 n=5+5)
Tar               250k ± 0%         249k ± 1%     ~     (p=0.151 n=5+5)
XML               393k ± 0%         392k ± 0%     ~     (p=0.056 n=5+5)

Going beyond c=4 on my machine tends to increase CPU time and allocs
without impacting real time.

The CPU time numbers matter, because when there are many concurrent
compilation processes, that will impact the overall throughput.

The numbers above are in many ways the best case scenario;
we can take full advantage of all cores.
Fortunately, the most common compilation scenario is incremental
re-compilation of a single package during a build/test cycle.

Updates golang#15756

Change-Id: I6725558ca2069edec0ac5b0d1683105a9fff6bea

This CL adds initial support for concurrent backend compilation.

BACKGROUND

The compiler currently consists (very roughly) of the following phases:

1. Initialization.
2. Lexing and parsing into the cmd/compile/internal/syntax AST.
3. Translation into the cmd/compile/internal/gc AST.
4. Some gc AST passes: typechecking, escape analysis, inlining,
   closure handling, expression evaluation ordering (order.go),
   and some lowering and optimization (walk.go).
5. Translation into the cmd/compile/internal/ssa SSA form.
6. Optimization and lowering of SSA form.
7. Translation from SSA form to assembler instructions.
8. Translation from assembler instructions to machine code.
9. Writing lots of output: machine code, DWARF symbols,
   type and reflection info, export data.

Phase 2 was already concurrent as of Go 1.8.

Phase 3 is planned for eventual removal;
we hope to go straight from syntax AST to SSA.

Phases 5–8 are per-function; this CL adds support for
processing multiple functions concurrently.
The slowest phases in the compiler are 5 and 6,
so this offers the opportunity for some good speed-ups.

Unfortunately, it's not quite that straightforward.
In the current compiler, the latter parts of phase 4
(order, walk) are done function-at-a-time as needed.
Making order and walk concurrency-safe proved hard,
and they're not particularly slow, so there wasn't much reward.
To enable phases 5–8 to be done concurrently,
when concurrent backend compilation is requested,
we complete phase 4 for all functions
before starting later phases for any functions.

Also, in reality, we automatically generate new
functions in phase 9, such as method wrappers
and equality and has routines.
Those new functions then go through phases 4–8.
This CL disables concurrent backend compilation
after the first, big, user-provided batch of
functions has been compiled.
This is done to keep things simple,
and because the autogenerated functions
tend to be small, few, simple, and fast to compile.

USAGE

Concurrent backend compilation still defaults to off.
To set the number of functions that may be backend-compiled
concurrently, use the compiler flag -c.
In future work, cmd/go will automatically set -c.

Furthermore, this CL has been intentionally written
so that the c=1 path has no backend concurrency whatsoever,
not even spawning any goroutines.
This helps ensure that, should problems arise
late in the development cycle,
we can simply have cmd/go set -c=1 always,
and revert to the original compiler behavior.

MUTEXES

Most of the work required to make concurrent backend
compilation safe has occurred over the past month.
This CL adds a handful of mutexes to get the rest of the way there;
they are the mutexes that I didn't see a clean way to avoid.
Some of them may still be eliminable in future work.

In no particular order:

* gc.funcsymsmu. The global funcsyms slice is populated
  lazily when we need function symbols for closures.
  This occurs during gc AST to SSA translation.
  The function funcsym also does a package lookup,
  which is a source of races on types.Pkg.Syms;
  funcsymsmu also covers that package lookup.
  This mutex is low priority: it adds a single global,
  it is in an infrequently used code path, and it is low contention.
  Since funcsyms may now be added in any order,
  we must sort them to preserve reproducible builds.

* gc.largeStackFramesMu. We don't discover until after SSA compilation
  that a function's stack frame is gigantic.
  Recording that error happens basically never,
  but it does happen concurrently.
  Fix with a low priority mutex and sorting.

* obj.Link.hashmu. ctxt.hash stores the mapping from
  types.Syms (compiler symbols) to obj.LSyms (linker symbols).
  It is accessed fairly heavily through all the phases.
  This is easily the most heavily contended mutex.
  I hope that the syncmap proposed in golang#18177 may
  provide some free speed-ups here.
  Some lookups may also be removable.

* gc.signatlistmu. The global signatlist map is
  populated with types through several of the concurrent phases,
  including notably via ngotype during DWARF generation.
  It is low priority for removal, aside from some mild
  awkwardness to avoid deadlocks due to recursive calls.

* gc.typepkgmu. Looking up symbols in the types package
  happens a fair amount during backend compilation
  and DWARF generation, particularly via ngotype.
  This mutex helps us to avoid a broader mutex on types.Pkg.Syms.
  It has low-to-moderate contention.

* types.internedStringsmu. gc AST to SSA conversion and
  some SSA work introduce new autotmps.
  Those autotmps have their names interned to reduce allocations.
  That interning requires protecting types.internedStrings.
  The autotmp names are heavily re-used, and the mutex
  overhead and contention here are low, so it is probably
  a worthwhile performance optimization to keep this mutex.

* types.Sym.Lsymmu. Syms keep a cache of their associated LSym,
  to reduce lookups in ctxt.Hash. This cache itself
  needs concurrency protection. This mutex adds to the size
  of a moderately important data structure, but the alloc benchmarks
  below show that this doesn't hurt much in practice.
  It is moderately contended, mostly because when lookups fail,
  the lock is held while vying for the contended ctxt.Hash mutex.
  The fact that it keeps load off the ctxt.Hash mutex, though,
  makes this mutex worth keeping.
  Also, a fair number of calls to Linksym could be avoided
  by judicious addition of a local variable.

TESTING

I have been testing this code locally by running
'go install -race cmd/compile'
and then doing
'go build -a -gcflags=-c=128 std cmd'
for all architectures and a variety of compiler flags.
This obviously needs to be made part of the builders,
but it is too expensive to make part of all.bash.
I have filed golang#19962 for this.

REPRODUCIBLE BUILDS

This version of the compiler generates reproducible builds.
Testing reproducible builds also needs automation, however,
and is also too expensive for all.bash.
This is golang#19961.

Also of note is that some of the compiler flags used by 'toolstash -cmp'
are currently incompatible with concurrent backend compilation.
They still work fine with c=1.
Time will tell whether this is a problem.

NEXT STEPS

* Continue to find and fix races and bugs,
  using a combination of code inspection, fuzzing,
  and hopefully some community experimentation.
  I do not know of any outstanding races,
  but there probably are some.
* Improve testing.
* Improve performance, for many values of c.
* Integrate with cmd/go and fine tune.
* Support concurrent compilation with the -race flag.
  It is a sad irony that it does not yet work.
* Minor code cleanup that has been deferred during
  the last month due to uncertainty about the
  ultimate shape of this CL.

PERFORMANCE

Here's the buried lede, at last. :)

All benchmarks are from my 8 core 2.9 GHz Intel Core i7 darwin/amd64 laptop.

First, going from tip to this CL with c=1 has almost no impact.

name        old time/op       new time/op       delta
Template          195ms ± 3%        194ms ± 5%    ~     (p=0.370 n=30+29)
Unicode          86.6ms ± 3%       87.0ms ± 7%    ~     (p=0.958 n=29+30)
GoTypes           548ms ± 3%        555ms ± 4%  +1.35%  (p=0.001 n=30+28)
Compiler          2.51s ± 2%        2.54s ± 2%  +1.17%  (p=0.000 n=28+30)
SSA               5.16s ± 3%        5.16s ± 2%    ~     (p=0.910 n=30+29)
Flate             124ms ± 5%        124ms ± 4%    ~     (p=0.947 n=30+30)
GoParser          146ms ± 3%        146ms ± 3%    ~     (p=0.150 n=29+28)
Reflect           354ms ± 3%        352ms ± 4%    ~     (p=0.096 n=29+29)
Tar               107ms ± 5%        106ms ± 3%    ~     (p=0.370 n=30+29)
XML               200ms ± 4%        201ms ± 4%    ~     (p=0.313 n=29+28)
[Geo mean]        332ms             333ms       +0.10%

name        old user-time/op  new user-time/op  delta
Template          227ms ± 5%        225ms ± 5%    ~     (p=0.457 n=28+27)
Unicode           109ms ± 4%        109ms ± 5%    ~     (p=0.758 n=29+29)
GoTypes           713ms ± 4%        721ms ± 5%    ~     (p=0.051 n=30+29)
Compiler          3.36s ± 2%        3.38s ± 3%    ~     (p=0.146 n=30+30)
SSA               7.46s ± 3%        7.47s ± 3%    ~     (p=0.804 n=30+29)
Flate             146ms ± 7%        147ms ± 3%    ~     (p=0.833 n=29+27)
GoParser          179ms ± 5%        179ms ± 5%    ~     (p=0.866 n=30+30)
Reflect           431ms ± 4%        429ms ± 4%    ~     (p=0.593 n=29+30)
Tar               124ms ± 5%        123ms ± 5%    ~     (p=0.140 n=29+29)
XML               243ms ± 4%        242ms ± 7%    ~     (p=0.404 n=29+29)
[Geo mean]        415ms             415ms       +0.02%

name        old obj-bytes     new obj-bytes     delta
Template           382k ± 0%         382k ± 0%    ~     (all equal)
Unicode            203k ± 0%         203k ± 0%    ~     (all equal)
GoTypes           1.18M ± 0%        1.18M ± 0%    ~     (all equal)
Compiler          3.98M ± 0%        3.98M ± 0%    ~     (all equal)
SSA               8.28M ± 0%        8.28M ± 0%    ~     (all equal)
Flate              230k ± 0%         230k ± 0%    ~     (all equal)
GoParser           287k ± 0%         287k ± 0%    ~     (all equal)
Reflect           1.00M ± 0%        1.00M ± 0%    ~     (all equal)
Tar                190k ± 0%         190k ± 0%    ~     (all equal)
XML                416k ± 0%         416k ± 0%    ~     (all equal)
[Geo mean]         660k              660k       +0.00%

Comparing this CL to itself, from c=1 to c=2
improves real times 20-30%, costs 5-10% more CPU time,
and adds about 2% alloc.
The allocation increase comes from allocating more ssa.Caches.

name       old time/op       new time/op       delta
Template         202ms ± 3%        149ms ± 3%  -26.15%  (p=0.000 n=49+49)
Unicode         87.4ms ± 4%       84.2ms ± 3%   -3.68%  (p=0.000 n=48+48)
GoTypes          560ms ± 2%        398ms ± 2%  -28.96%  (p=0.000 n=49+49)
Compiler         2.46s ± 3%        1.76s ± 2%  -28.61%  (p=0.000 n=48+46)
SSA              6.17s ± 2%        4.04s ± 1%  -34.52%  (p=0.000 n=49+49)
Flate            126ms ± 3%         92ms ± 2%  -26.81%  (p=0.000 n=49+48)
GoParser         148ms ± 4%        107ms ± 2%  -27.78%  (p=0.000 n=49+48)
Reflect          361ms ± 3%        281ms ± 3%  -22.10%  (p=0.000 n=49+49)
Tar              109ms ± 4%         86ms ± 3%  -20.81%  (p=0.000 n=49+47)
XML              204ms ± 3%        144ms ± 2%  -29.53%  (p=0.000 n=48+45)

name       old user-time/op  new user-time/op  delta
Template         246ms ± 9%        246ms ± 4%     ~     (p=0.401 n=50+48)
Unicode          109ms ± 4%        111ms ± 4%   +1.47%  (p=0.000 n=44+50)
GoTypes          728ms ± 3%        765ms ± 3%   +5.04%  (p=0.000 n=46+50)
Compiler         3.33s ± 3%        3.41s ± 2%   +2.31%  (p=0.000 n=49+48)
SSA              8.52s ± 2%        9.11s ± 2%   +6.93%  (p=0.000 n=49+47)
Flate            149ms ± 4%        161ms ± 3%   +8.13%  (p=0.000 n=50+47)
GoParser         181ms ± 5%        192ms ± 2%   +6.40%  (p=0.000 n=49+46)
Reflect          452ms ± 9%        474ms ± 2%   +4.99%  (p=0.000 n=50+48)
Tar              126ms ± 6%        136ms ± 4%   +7.95%  (p=0.000 n=50+49)
XML              247ms ± 5%        264ms ± 3%   +6.94%  (p=0.000 n=48+50)

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       39.3MB ± 0%   +1.48%  (p=0.008 n=5+5)
Unicode         29.8MB ± 0%       30.2MB ± 0%   +1.19%  (p=0.008 n=5+5)
GoTypes          113MB ± 0%        114MB ± 0%   +0.69%  (p=0.008 n=5+5)
Compiler         443MB ± 0%        447MB ± 0%   +0.95%  (p=0.008 n=5+5)
SSA             1.25GB ± 0%       1.26GB ± 0%   +0.89%  (p=0.008 n=5+5)
Flate           25.3MB ± 0%       25.9MB ± 1%   +2.35%  (p=0.008 n=5+5)
GoParser        31.7MB ± 0%       32.2MB ± 0%   +1.59%  (p=0.008 n=5+5)
Reflect         78.2MB ± 0%       78.9MB ± 0%   +0.91%  (p=0.008 n=5+5)
Tar             26.6MB ± 0%       27.0MB ± 0%   +1.80%  (p=0.008 n=5+5)
XML             42.4MB ± 0%       43.4MB ± 0%   +2.35%  (p=0.008 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          379k ± 0%         378k ± 0%     ~     (p=0.421 n=5+5)
Unicode           322k ± 0%         321k ± 0%     ~     (p=0.222 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%     ~     (p=0.548 n=5+5)
Compiler         4.12M ± 0%        4.11M ± 0%   -0.14%  (p=0.032 n=5+5)
SSA              9.72M ± 0%        9.72M ± 0%     ~     (p=0.421 n=5+5)
Flate             234k ± 1%         234k ± 0%     ~     (p=0.421 n=5+5)
GoParser          316k ± 1%         315k ± 0%     ~     (p=0.222 n=5+5)
Reflect           980k ± 0%         979k ± 0%     ~     (p=0.095 n=5+5)
Tar               249k ± 1%         249k ± 1%     ~     (p=0.841 n=5+5)
XML               392k ± 0%         391k ± 0%     ~     (p=0.095 n=5+5)

From c=1 to c=4, real time is down ~40%, CPU usage up 10-20%, alloc up ~5%:

name       old time/op       new time/op       delta
Template         203ms ± 3%        131ms ± 5%  -35.45%  (p=0.000 n=50+50)
Unicode         87.2ms ± 4%       84.1ms ± 2%   -3.61%  (p=0.000 n=48+47)
GoTypes          560ms ± 4%        310ms ± 2%  -44.65%  (p=0.000 n=50+49)
Compiler         2.47s ± 3%        1.41s ± 2%  -43.10%  (p=0.000 n=50+46)
SSA              6.17s ± 2%        3.20s ± 2%  -48.06%  (p=0.000 n=49+49)
Flate            126ms ± 4%         74ms ± 2%  -41.06%  (p=0.000 n=49+48)
GoParser         148ms ± 4%         89ms ± 3%  -39.97%  (p=0.000 n=49+50)
Reflect          360ms ± 3%        242ms ± 3%  -32.81%  (p=0.000 n=49+49)
Tar              108ms ± 4%         73ms ± 4%  -32.48%  (p=0.000 n=50+49)
XML              203ms ± 3%        119ms ± 3%  -41.56%  (p=0.000 n=49+48)

name       old user-time/op  new user-time/op  delta
Template         246ms ± 9%        287ms ± 9%  +16.98%  (p=0.000 n=50+50)
Unicode          109ms ± 4%        118ms ± 5%   +7.56%  (p=0.000 n=46+50)
GoTypes          735ms ± 4%        806ms ± 2%   +9.62%  (p=0.000 n=50+50)
Compiler         3.34s ± 4%        3.56s ± 2%   +6.78%  (p=0.000 n=49+49)
SSA              8.54s ± 3%       10.04s ± 3%  +17.55%  (p=0.000 n=50+50)
Flate            149ms ± 6%        176ms ± 3%  +17.82%  (p=0.000 n=50+48)
GoParser         181ms ± 5%        213ms ± 3%  +17.47%  (p=0.000 n=50+50)
Reflect          453ms ± 6%        499ms ± 2%  +10.11%  (p=0.000 n=50+48)
Tar              126ms ± 5%        149ms ±11%  +18.76%  (p=0.000 n=50+50)
XML              246ms ± 5%        287ms ± 4%  +16.53%  (p=0.000 n=49+50)

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       40.4MB ± 0%   +4.21%  (p=0.008 n=5+5)
Unicode         29.8MB ± 0%       30.9MB ± 0%   +3.68%  (p=0.008 n=5+5)
GoTypes          113MB ± 0%        116MB ± 0%   +2.71%  (p=0.008 n=5+5)
Compiler         443MB ± 0%        455MB ± 0%   +2.75%  (p=0.008 n=5+5)
SSA             1.25GB ± 0%       1.27GB ± 0%   +1.84%  (p=0.008 n=5+5)
Flate           25.3MB ± 0%       26.9MB ± 1%   +6.31%  (p=0.008 n=5+5)
GoParser        31.7MB ± 0%       33.2MB ± 0%   +4.61%  (p=0.008 n=5+5)
Reflect         78.2MB ± 0%       80.2MB ± 0%   +2.53%  (p=0.008 n=5+5)
Tar             26.6MB ± 0%       27.9MB ± 0%   +5.19%  (p=0.008 n=5+5)
XML             42.4MB ± 0%       44.6MB ± 0%   +5.20%  (p=0.008 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          380k ± 0%         379k ± 0%   -0.39%  (p=0.032 n=5+5)
Unicode           321k ± 0%         321k ± 0%     ~     (p=0.841 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%     ~     (p=0.421 n=5+5)
Compiler         4.12M ± 0%        4.14M ± 0%   +0.52%  (p=0.008 n=5+5)
SSA              9.72M ± 0%        9.76M ± 0%   +0.37%  (p=0.008 n=5+5)
Flate             234k ± 1%         234k ± 1%     ~     (p=0.690 n=5+5)
GoParser          316k ± 0%         317k ± 1%     ~     (p=0.841 n=5+5)
Reflect           981k ± 0%         981k ± 0%     ~     (p=1.000 n=5+5)
Tar               250k ± 0%         249k ± 1%     ~     (p=0.151 n=5+5)
XML               393k ± 0%         392k ± 0%     ~     (p=0.056 n=5+5)

Going beyond c=4 on my machine tends to increase CPU time and allocs
without impacting real time.

The CPU time numbers matter, because when there are many concurrent
compilation processes, that will impact the overall throughput.

The numbers above are in many ways the best case scenario;
we can take full advantage of all cores.
Fortunately, the most common compilation scenario is incremental
re-compilation of a single package during a build/test cycle.

Updates golang#15756

Change-Id: I6725558ca2069edec0ac5b0d1683105a9fff6bea

This CL adds initial support for concurrent backend compilation.

BACKGROUND

The compiler currently consists (very roughly) of the following phases:

1. Initialization.
2. Lexing and parsing into the cmd/compile/internal/syntax AST.
3. Translation into the cmd/compile/internal/gc AST.
4. Some gc AST passes: typechecking, escape analysis, inlining,
   closure handling, expression evaluation ordering (order.go),
   and some lowering and optimization (walk.go).
5. Translation into the cmd/compile/internal/ssa SSA form.
6. Optimization and lowering of SSA form.
7. Translation from SSA form to assembler instructions.
8. Translation from assembler instructions to machine code.
9. Writing lots of output: machine code, DWARF symbols,
   type and reflection info, export data.

Phase 2 was already concurrent as of Go 1.8.

Phase 3 is planned for eventual removal;
we hope to go straight from syntax AST to SSA.

Phases 5–8 are per-function; this CL adds support for
processing multiple functions concurrently.
The slowest phases in the compiler are 5 and 6,
so this offers the opportunity for some good speed-ups.

Unfortunately, it's not quite that straightforward.
In the current compiler, the latter parts of phase 4
(order, walk) are done function-at-a-time as needed.
Making order and walk concurrency-safe proved hard,
and they're not particularly slow, so there wasn't much reward.
To enable phases 5–8 to be done concurrently,
when concurrent backend compilation is requested,
we complete phase 4 for all functions
before starting later phases for any functions.

Also, in reality, we automatically generate new
functions in phase 9, such as method wrappers
and equality and has routines.
Those new functions then go through phases 4–8.
This CL disables concurrent backend compilation
after the first, big, user-provided batch of
functions has been compiled.
This is done to keep things simple,
and because the autogenerated functions
tend to be small, few, simple, and fast to compile.

USAGE

Concurrent backend compilation still defaults to off.
To set the number of functions that may be backend-compiled
concurrently, use the compiler flag -c.
In future work, cmd/go will automatically set -c.

Furthermore, this CL has been intentionally written
so that the c=1 path has no backend concurrency whatsoever,
not even spawning any goroutines.
This helps ensure that, should problems arise
late in the development cycle,
we can simply have cmd/go set c=1 always,
and revert to the original compiler behavior.

MUTEXES

Most of the work required to make concurrent backend
compilation safe has occurred over the past month.
This CL adds a handful of mutexes to get the rest of the way there;
they are the mutexes that I didn't see a clean way to avoid.
Some of them may still be eliminable in future work.

In no particular order:

* gc.funcsymsmu. The global funcsyms slice is populated
  lazily when we need function symbols for closures.
  This occurs during gc AST to SSA translation.
  The function funcsym also does a package lookup,
  which is a source of races on types.Pkg.Syms;
  funcsymsmu also covers that package lookup.
  This mutex is low priority: it adds a single global,
  it is in an infrequently used code path, and it is low contention.
  Since funcsyms may now be added in any order,
  we must sort them to preserve reproducible builds.

* gc.largeStackFramesMu. We don't discover until after SSA compilation
  that a function's stack frame is gigantic.
  Recording that error happens basically never,
  but it does happen concurrently.
  Fix with a low priority mutex and sorting.

* obj.Link.hashmu. ctxt.hash stores the mapping from
  types.Syms (compiler symbols) to obj.LSyms (linker symbols).
  It is accessed fairly heavily through all the phases.
  This is the only heavily contended mutex.

* gc.signatlistmu. The global signatlist map is
  populated with types through several of the concurrent phases,
  including notably via ngotype during DWARF generation.
  It is low priority for removal.

* gc.typepkgmu. Looking up symbols in the types package
  happens a fair amount during backend compilation
  and DWARF generation, particularly via ngotype.
  This mutex helps us to avoid a broader mutex on types.Pkg.Syms.
  It has low-to-moderate contention.

* types.internedStringsmu. gc AST to SSA conversion and
  some SSA work introduce new autotmps.
  Those autotmps have their names interned to reduce allocations.
  That interning requires protecting types.internedStrings.
  The autotmp names are heavily re-used, and the mutex
  overhead and contention here are low, so it is probably
  a worthwhile performance optimization to keep this mutex.

TESTING

I have been testing this code locally by running
'go install -race cmd/compile'
and then doing
'go build -a -gcflags=-c=128 std cmd'
for all architectures and a variety of compiler flags.
This obviously needs to be made part of the builders,
but it is too expensive to make part of all.bash.
I have filed golang#19962 for this.

REPRODUCIBLE BUILDS

This version of the compiler generates reproducible builds.
Testing reproducible builds also needs automation, however,
and is also too expensive for all.bash.
This is golang#19961.

Also of note is that some of the compiler flags used by 'toolstash -cmp'
are currently incompatible with concurrent backend compilation.
They still work fine with c=1.
Time will tell whether this is a problem.

NEXT STEPS

* Continue to find and fix races and bugs,
  using a combination of code inspection, fuzzing,
  and hopefully some community experimentation.
  I do not know of any outstanding races,
  but there probably are some.
* Improve testing.
* Improve performance, for many values of c.
* Integrate with cmd/go and fine tune.
* Support concurrent compilation with the -race flag.
  It is a sad irony that it does not yet work.
* Minor code cleanup that has been deferred during
  the last month due to uncertainty about the
  ultimate shape of this CL.

PERFORMANCE

Here's the buried lede, at last. :)

All benchmarks are from my 8 core 2.9 GHz Intel Core i7 darwin/amd64 laptop.

First, going from tip to this CL with c=1 has almost no impact.

name        old time/op       new time/op       delta
Template          195ms ± 3%        194ms ± 5%    ~     (p=0.370 n=30+29)
Unicode          86.6ms ± 3%       87.0ms ± 7%    ~     (p=0.958 n=29+30)
GoTypes           548ms ± 3%        555ms ± 4%  +1.35%  (p=0.001 n=30+28)
Compiler          2.51s ± 2%        2.54s ± 2%  +1.17%  (p=0.000 n=28+30)
SSA               5.16s ± 3%        5.16s ± 2%    ~     (p=0.910 n=30+29)
Flate             124ms ± 5%        124ms ± 4%    ~     (p=0.947 n=30+30)
GoParser          146ms ± 3%        146ms ± 3%    ~     (p=0.150 n=29+28)
Reflect           354ms ± 3%        352ms ± 4%    ~     (p=0.096 n=29+29)
Tar               107ms ± 5%        106ms ± 3%    ~     (p=0.370 n=30+29)
XML               200ms ± 4%        201ms ± 4%    ~     (p=0.313 n=29+28)
[Geo mean]        332ms             333ms       +0.10%

name        old user-time/op  new user-time/op  delta
Template          227ms ± 5%        225ms ± 5%    ~     (p=0.457 n=28+27)
Unicode           109ms ± 4%        109ms ± 5%    ~     (p=0.758 n=29+29)
GoTypes           713ms ± 4%        721ms ± 5%    ~     (p=0.051 n=30+29)
Compiler          3.36s ± 2%        3.38s ± 3%    ~     (p=0.146 n=30+30)
SSA               7.46s ± 3%        7.47s ± 3%    ~     (p=0.804 n=30+29)
Flate             146ms ± 7%        147ms ± 3%    ~     (p=0.833 n=29+27)
GoParser          179ms ± 5%        179ms ± 5%    ~     (p=0.866 n=30+30)
Reflect           431ms ± 4%        429ms ± 4%    ~     (p=0.593 n=29+30)
Tar               124ms ± 5%        123ms ± 5%    ~     (p=0.140 n=29+29)
XML               243ms ± 4%        242ms ± 7%    ~     (p=0.404 n=29+29)
[Geo mean]        415ms             415ms       +0.02%

name        old obj-bytes     new obj-bytes     delta
Template           382k ± 0%         382k ± 0%    ~     (all equal)
Unicode            203k ± 0%         203k ± 0%    ~     (all equal)
GoTypes           1.18M ± 0%        1.18M ± 0%    ~     (all equal)
Compiler          3.98M ± 0%        3.98M ± 0%    ~     (all equal)
SSA               8.28M ± 0%        8.28M ± 0%    ~     (all equal)
Flate              230k ± 0%         230k ± 0%    ~     (all equal)
GoParser           287k ± 0%         287k ± 0%    ~     (all equal)
Reflect           1.00M ± 0%        1.00M ± 0%    ~     (all equal)
Tar                190k ± 0%         190k ± 0%    ~     (all equal)
XML                416k ± 0%         416k ± 0%    ~     (all equal)
[Geo mean]         660k              660k       +0.00%

Comparing this CL to itself, from c=1 to c=2
improves real times 20-30%, costs 5-10% more CPU time,
and adds about 2% alloc.
The allocation increase comes from allocating more ssa.Caches.

name       old time/op       new time/op       delta
Template         202ms ± 3%        149ms ± 3%  -26.15%  (p=0.000 n=49+49)
Unicode         87.4ms ± 4%       84.2ms ± 3%   -3.68%  (p=0.000 n=48+48)
GoTypes          560ms ± 2%        398ms ± 2%  -28.96%  (p=0.000 n=49+49)
Compiler         2.46s ± 3%        1.76s ± 2%  -28.61%  (p=0.000 n=48+46)
SSA              6.17s ± 2%        4.04s ± 1%  -34.52%  (p=0.000 n=49+49)
Flate            126ms ± 3%         92ms ± 2%  -26.81%  (p=0.000 n=49+48)
GoParser         148ms ± 4%        107ms ± 2%  -27.78%  (p=0.000 n=49+48)
Reflect          361ms ± 3%        281ms ± 3%  -22.10%  (p=0.000 n=49+49)
Tar              109ms ± 4%         86ms ± 3%  -20.81%  (p=0.000 n=49+47)
XML              204ms ± 3%        144ms ± 2%  -29.53%  (p=0.000 n=48+45)

name       old user-time/op  new user-time/op  delta
Template         246ms ± 9%        246ms ± 4%     ~     (p=0.401 n=50+48)
Unicode          109ms ± 4%        111ms ± 4%   +1.47%  (p=0.000 n=44+50)
GoTypes          728ms ± 3%        765ms ± 3%   +5.04%  (p=0.000 n=46+50)
Compiler         3.33s ± 3%        3.41s ± 2%   +2.31%  (p=0.000 n=49+48)
SSA              8.52s ± 2%        9.11s ± 2%   +6.93%  (p=0.000 n=49+47)
Flate            149ms ± 4%        161ms ± 3%   +8.13%  (p=0.000 n=50+47)
GoParser         181ms ± 5%        192ms ± 2%   +6.40%  (p=0.000 n=49+46)
Reflect          452ms ± 9%        474ms ± 2%   +4.99%  (p=0.000 n=50+48)
Tar              126ms ± 6%        136ms ± 4%   +7.95%  (p=0.000 n=50+49)
XML              247ms ± 5%        264ms ± 3%   +6.94%  (p=0.000 n=48+50)

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       39.3MB ± 0%   +1.48%  (p=0.008 n=5+5)
Unicode         29.8MB ± 0%       30.2MB ± 0%   +1.19%  (p=0.008 n=5+5)
GoTypes          113MB ± 0%        114MB ± 0%   +0.69%  (p=0.008 n=5+5)
Compiler         443MB ± 0%        447MB ± 0%   +0.95%  (p=0.008 n=5+5)
SSA             1.25GB ± 0%       1.26GB ± 0%   +0.89%  (p=0.008 n=5+5)
Flate           25.3MB ± 0%       25.9MB ± 1%   +2.35%  (p=0.008 n=5+5)
GoParser        31.7MB ± 0%       32.2MB ± 0%   +1.59%  (p=0.008 n=5+5)
Reflect         78.2MB ± 0%       78.9MB ± 0%   +0.91%  (p=0.008 n=5+5)
Tar             26.6MB ± 0%       27.0MB ± 0%   +1.80%  (p=0.008 n=5+5)
XML             42.4MB ± 0%       43.4MB ± 0%   +2.35%  (p=0.008 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          379k ± 0%         378k ± 0%     ~     (p=0.421 n=5+5)
Unicode           322k ± 0%         321k ± 0%     ~     (p=0.222 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%     ~     (p=0.548 n=5+5)
Compiler         4.12M ± 0%        4.11M ± 0%   -0.14%  (p=0.032 n=5+5)
SSA              9.72M ± 0%        9.72M ± 0%     ~     (p=0.421 n=5+5)
Flate             234k ± 1%         234k ± 0%     ~     (p=0.421 n=5+5)
GoParser          316k ± 1%         315k ± 0%     ~     (p=0.222 n=5+5)
Reflect           980k ± 0%         979k ± 0%     ~     (p=0.095 n=5+5)
Tar               249k ± 1%         249k ± 1%     ~     (p=0.841 n=5+5)
XML               392k ± 0%         391k ± 0%     ~     (p=0.095 n=5+5)

From c=1 to c=4, real time is down ~40%, CPU usage up 10-20%, alloc up ~5%:

name       old time/op       new time/op       delta
Template         203ms ± 3%        131ms ± 5%  -35.45%  (p=0.000 n=50+50)
Unicode         87.2ms ± 4%       84.1ms ± 2%   -3.61%  (p=0.000 n=48+47)
GoTypes          560ms ± 4%        310ms ± 2%  -44.65%  (p=0.000 n=50+49)
Compiler         2.47s ± 3%        1.41s ± 2%  -43.10%  (p=0.000 n=50+46)
SSA              6.17s ± 2%        3.20s ± 2%  -48.06%  (p=0.000 n=49+49)
Flate            126ms ± 4%         74ms ± 2%  -41.06%  (p=0.000 n=49+48)
GoParser         148ms ± 4%         89ms ± 3%  -39.97%  (p=0.000 n=49+50)
Reflect          360ms ± 3%        242ms ± 3%  -32.81%  (p=0.000 n=49+49)
Tar              108ms ± 4%         73ms ± 4%  -32.48%  (p=0.000 n=50+49)
XML              203ms ± 3%        119ms ± 3%  -41.56%  (p=0.000 n=49+48)

name       old user-time/op  new user-time/op  delta
Template         246ms ± 9%        287ms ± 9%  +16.98%  (p=0.000 n=50+50)
Unicode          109ms ± 4%        118ms ± 5%   +7.56%  (p=0.000 n=46+50)
GoTypes          735ms ± 4%        806ms ± 2%   +9.62%  (p=0.000 n=50+50)
Compiler         3.34s ± 4%        3.56s ± 2%   +6.78%  (p=0.000 n=49+49)
SSA              8.54s ± 3%       10.04s ± 3%  +17.55%  (p=0.000 n=50+50)
Flate            149ms ± 6%        176ms ± 3%  +17.82%  (p=0.000 n=50+48)
GoParser         181ms ± 5%        213ms ± 3%  +17.47%  (p=0.000 n=50+50)
Reflect          453ms ± 6%        499ms ± 2%  +10.11%  (p=0.000 n=50+48)
Tar              126ms ± 5%        149ms ±11%  +18.76%  (p=0.000 n=50+50)
XML              246ms ± 5%        287ms ± 4%  +16.53%  (p=0.000 n=49+50)

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       40.4MB ± 0%   +4.21%  (p=0.008 n=5+5)
Unicode         29.8MB ± 0%       30.9MB ± 0%   +3.68%  (p=0.008 n=5+5)
GoTypes          113MB ± 0%        116MB ± 0%   +2.71%  (p=0.008 n=5+5)
Compiler         443MB ± 0%        455MB ± 0%   +2.75%  (p=0.008 n=5+5)
SSA             1.25GB ± 0%       1.27GB ± 0%   +1.84%  (p=0.008 n=5+5)
Flate           25.3MB ± 0%       26.9MB ± 1%   +6.31%  (p=0.008 n=5+5)
GoParser        31.7MB ± 0%       33.2MB ± 0%   +4.61%  (p=0.008 n=5+5)
Reflect         78.2MB ± 0%       80.2MB ± 0%   +2.53%  (p=0.008 n=5+5)
Tar             26.6MB ± 0%       27.9MB ± 0%   +5.19%  (p=0.008 n=5+5)
XML             42.4MB ± 0%       44.6MB ± 0%   +5.20%  (p=0.008 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          380k ± 0%         379k ± 0%   -0.39%  (p=0.032 n=5+5)
Unicode           321k ± 0%         321k ± 0%     ~     (p=0.841 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%     ~     (p=0.421 n=5+5)
Compiler         4.12M ± 0%        4.14M ± 0%   +0.52%  (p=0.008 n=5+5)
SSA              9.72M ± 0%        9.76M ± 0%   +0.37%  (p=0.008 n=5+5)
Flate             234k ± 1%         234k ± 1%     ~     (p=0.690 n=5+5)
GoParser          316k ± 0%         317k ± 1%     ~     (p=0.841 n=5+5)
Reflect           981k ± 0%         981k ± 0%     ~     (p=1.000 n=5+5)
Tar               250k ± 0%         249k ± 1%     ~     (p=0.151 n=5+5)
XML               393k ± 0%         392k ± 0%     ~     (p=0.056 n=5+5)

Going beyond c=4 on my machine tends to increase CPU time and allocs
without impacting real time.

The CPU time numbers matter, because when there are many concurrent
compilation processes, that will impact the overall throughput.

The numbers above are in many ways the best case scenario;
we can take full advantage of all cores.
Fortunately, the most common compilation scenario is incremental
re-compilation of a single package during a build/test cycle.

Updates golang#15756

Change-Id: I6725558ca2069edec0ac5b0d1683105a9fff6bea

This CL adds initial support for concurrent backend compilation.

BACKGROUND

The compiler currently consists (very roughly) of the following phases:

1. Initialization.
2. Lexing and parsing into the cmd/compile/internal/syntax AST.
3. Translation into the cmd/compile/internal/gc AST.
4. Some gc AST passes: typechecking, escape analysis, inlining,
   closure handling, expression evaluation ordering (order.go),
   and some lowering and optimization (walk.go).
5. Translation into the cmd/compile/internal/ssa SSA form.
6. Optimization and lowering of SSA form.
7. Translation from SSA form to assembler instructions.
8. Translation from assembler instructions to machine code.
9. Writing lots of output: machine code, DWARF symbols,
   type and reflection info, export data.

Phase 2 was already concurrent as of Go 1.8.

Phase 3 is planned for eventual removal;
we hope to go straight from syntax AST to SSA.

Phases 5–8 are per-function; this CL adds support for
processing multiple functions concurrently.
The slowest phases in the compiler are 5 and 6,
so this offers the opportunity for some good speed-ups.

Unfortunately, it's not quite that straightforward.
In the current compiler, the latter parts of phase 4
(order, walk) are done function-at-a-time as needed.
Making order and walk concurrency-safe proved hard,
and they're not particularly slow, so there wasn't much reward.
To enable phases 5–8 to be done concurrently,
when concurrent backend compilation is requested,
we complete phase 4 for all functions
before starting later phases for any functions.

Also, in reality, we automatically generate new
functions in phase 9, such as method wrappers
and equality and has routines.
Those new functions then go through phases 4–8.
This CL disables concurrent backend compilation
after the first, big, user-provided batch of
functions has been compiled.
This is done to keep things simple,
and because the autogenerated functions
tend to be small, few, simple, and fast to compile.

USAGE

Concurrent backend compilation still defaults to off.
To set the number of functions that may be backend-compiled
concurrently, use the compiler flag -c.
In future work, cmd/go will automatically set -c.

Furthermore, this CL has been intentionally written
so that the c=1 path has no backend concurrency whatsoever,
not even spawning any goroutines.
This helps ensure that, should problems arise
late in the development cycle,
we can simply have cmd/go set c=1 always,
and revert to the original compiler behavior.

MUTEXES

Most of the work required to make concurrent backend
compilation safe has occurred over the past month.
This CL adds a handful of mutexes to get the rest of the way there;
they are the mutexes that I didn't see a clean way to avoid.
Some of them may still be eliminable in future work.

In no particular order:

* gc.funcsymsmu. The global funcsyms slice is populated
  lazily when we need function symbols for closures.
  This occurs during gc AST to SSA translation.
  The function funcsym also does a package lookup,
  which is a source of races on types.Pkg.Syms;
  funcsymsmu also covers that package lookup.
  This mutex is low priority: it adds a single global,
  it is in an infrequently used code path, and it is low contention.
  Since funcsyms may now be added in any order,
  we must sort them to preserve reproducible builds.

* gc.largeStackFramesMu. We don't discover until after SSA compilation
  that a function's stack frame is gigantic.
  Recording that error happens basically never,
  but it does happen concurrently.
  Fix with a low priority mutex and sorting.

* obj.Link.hashmu. ctxt.hash stores the mapping from
  types.Syms (compiler symbols) to obj.LSyms (linker symbols).
  It is accessed fairly heavily through all the phases.
  This is the only heavily contended mutex.

* gc.signatlistmu. The global signatlist map is
  populated with types through several of the concurrent phases,
  including notably via ngotype during DWARF generation.
  It is low priority for removal.

* gc.typepkgmu. Looking up symbols in the types package
  happens a fair amount during backend compilation
  and DWARF generation, particularly via ngotype.
  This mutex helps us to avoid a broader mutex on types.Pkg.Syms.
  It has low-to-moderate contention.

* types.internedStringsmu. gc AST to SSA conversion and
  some SSA work introduce new autotmps.
  Those autotmps have their names interned to reduce allocations.
  That interning requires protecting types.internedStrings.
  The autotmp names are heavily re-used, and the mutex
  overhead and contention here are low, so it is probably
  a worthwhile performance optimization to keep this mutex.

TESTING

I have been testing this code locally by running
'go install -race cmd/compile'
and then doing
'go build -a -gcflags=-c=128 std cmd'
for all architectures and a variety of compiler flags.
This obviously needs to be made part of the builders,
but it is too expensive to make part of all.bash.
I have filed golang#19962 for this.

REPRODUCIBLE BUILDS

This version of the compiler generates reproducible builds.
Testing reproducible builds also needs automation, however,
and is also too expensive for all.bash.
This is golang#19961.

Also of note is that some of the compiler flags used by 'toolstash -cmp'
are currently incompatible with concurrent backend compilation.
They still work fine with c=1.
Time will tell whether this is a problem.

NEXT STEPS

* Continue to find and fix races and bugs,
  using a combination of code inspection, fuzzing,
  and hopefully some community experimentation.
  I do not know of any outstanding races,
  but there probably are some.
* Improve testing.
* Improve performance, for many values of c.
* Integrate with cmd/go and fine tune.
* Support concurrent compilation with the -race flag.
  It is a sad irony that it does not yet work.
* Minor code cleanup that has been deferred during
  the last month due to uncertainty about the
  ultimate shape of this CL.

PERFORMANCE

Here's the buried lede, at last. :)

All benchmarks are from my 8 core 2.9 GHz Intel Core i7 darwin/amd64 laptop.

First, going from tip to this CL with c=1 has almost no impact.

name        old time/op       new time/op       delta
Template          195ms ± 3%        194ms ± 5%    ~     (p=0.370 n=30+29)
Unicode          86.6ms ± 3%       87.0ms ± 7%    ~     (p=0.958 n=29+30)
GoTypes           548ms ± 3%        555ms ± 4%  +1.35%  (p=0.001 n=30+28)
Compiler          2.51s ± 2%        2.54s ± 2%  +1.17%  (p=0.000 n=28+30)
SSA               5.16s ± 3%        5.16s ± 2%    ~     (p=0.910 n=30+29)
Flate             124ms ± 5%        124ms ± 4%    ~     (p=0.947 n=30+30)
GoParser          146ms ± 3%        146ms ± 3%    ~     (p=0.150 n=29+28)
Reflect           354ms ± 3%        352ms ± 4%    ~     (p=0.096 n=29+29)
Tar               107ms ± 5%        106ms ± 3%    ~     (p=0.370 n=30+29)
XML               200ms ± 4%        201ms ± 4%    ~     (p=0.313 n=29+28)
[Geo mean]        332ms             333ms       +0.10%

name        old user-time/op  new user-time/op  delta
Template          227ms ± 5%        225ms ± 5%    ~     (p=0.457 n=28+27)
Unicode           109ms ± 4%        109ms ± 5%    ~     (p=0.758 n=29+29)
GoTypes           713ms ± 4%        721ms ± 5%    ~     (p=0.051 n=30+29)
Compiler          3.36s ± 2%        3.38s ± 3%    ~     (p=0.146 n=30+30)
SSA               7.46s ± 3%        7.47s ± 3%    ~     (p=0.804 n=30+29)
Flate             146ms ± 7%        147ms ± 3%    ~     (p=0.833 n=29+27)
GoParser          179ms ± 5%        179ms ± 5%    ~     (p=0.866 n=30+30)
Reflect           431ms ± 4%        429ms ± 4%    ~     (p=0.593 n=29+30)
Tar               124ms ± 5%        123ms ± 5%    ~     (p=0.140 n=29+29)
XML               243ms ± 4%        242ms ± 7%    ~     (p=0.404 n=29+29)
[Geo mean]        415ms             415ms       +0.02%

name        old obj-bytes     new obj-bytes     delta
Template           382k ± 0%         382k ± 0%    ~     (all equal)
Unicode            203k ± 0%         203k ± 0%    ~     (all equal)
GoTypes           1.18M ± 0%        1.18M ± 0%    ~     (all equal)
Compiler          3.98M ± 0%        3.98M ± 0%    ~     (all equal)
SSA               8.28M ± 0%        8.28M ± 0%    ~     (all equal)
Flate              230k ± 0%         230k ± 0%    ~     (all equal)
GoParser           287k ± 0%         287k ± 0%    ~     (all equal)
Reflect           1.00M ± 0%        1.00M ± 0%    ~     (all equal)
Tar                190k ± 0%         190k ± 0%    ~     (all equal)
XML                416k ± 0%         416k ± 0%    ~     (all equal)
[Geo mean]         660k              660k       +0.00%

Comparing this CL to itself, from c=1 to c=2
improves real times 20-30%, costs 5-10% more CPU time,
and adds about 2% alloc.
The allocation increase comes from allocating more ssa.Caches.

name       old time/op       new time/op       delta
Template         202ms ± 3%        149ms ± 3%  -26.15%  (p=0.000 n=49+49)
Unicode         87.4ms ± 4%       84.2ms ± 3%   -3.68%  (p=0.000 n=48+48)
GoTypes          560ms ± 2%        398ms ± 2%  -28.96%  (p=0.000 n=49+49)
Compiler         2.46s ± 3%        1.76s ± 2%  -28.61%  (p=0.000 n=48+46)
SSA              6.17s ± 2%        4.04s ± 1%  -34.52%  (p=0.000 n=49+49)
Flate            126ms ± 3%         92ms ± 2%  -26.81%  (p=0.000 n=49+48)
GoParser         148ms ± 4%        107ms ± 2%  -27.78%  (p=0.000 n=49+48)
Reflect          361ms ± 3%        281ms ± 3%  -22.10%  (p=0.000 n=49+49)
Tar              109ms ± 4%         86ms ± 3%  -20.81%  (p=0.000 n=49+47)
XML              204ms ± 3%        144ms ± 2%  -29.53%  (p=0.000 n=48+45)

name       old user-time/op  new user-time/op  delta
Template         246ms ± 9%        246ms ± 4%     ~     (p=0.401 n=50+48)
Unicode          109ms ± 4%        111ms ± 4%   +1.47%  (p=0.000 n=44+50)
GoTypes          728ms ± 3%        765ms ± 3%   +5.04%  (p=0.000 n=46+50)
Compiler         3.33s ± 3%        3.41s ± 2%   +2.31%  (p=0.000 n=49+48)
SSA              8.52s ± 2%        9.11s ± 2%   +6.93%  (p=0.000 n=49+47)
Flate            149ms ± 4%        161ms ± 3%   +8.13%  (p=0.000 n=50+47)
GoParser         181ms ± 5%        192ms ± 2%   +6.40%  (p=0.000 n=49+46)
Reflect          452ms ± 9%        474ms ± 2%   +4.99%  (p=0.000 n=50+48)
Tar              126ms ± 6%        136ms ± 4%   +7.95%  (p=0.000 n=50+49)
XML              247ms ± 5%        264ms ± 3%   +6.94%  (p=0.000 n=48+50)

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       39.3MB ± 0%   +1.48%  (p=0.008 n=5+5)
Unicode         29.8MB ± 0%       30.2MB ± 0%   +1.19%  (p=0.008 n=5+5)
GoTypes          113MB ± 0%        114MB ± 0%   +0.69%  (p=0.008 n=5+5)
Compiler         443MB ± 0%        447MB ± 0%   +0.95%  (p=0.008 n=5+5)
SSA             1.25GB ± 0%       1.26GB ± 0%   +0.89%  (p=0.008 n=5+5)
Flate           25.3MB ± 0%       25.9MB ± 1%   +2.35%  (p=0.008 n=5+5)
GoParser        31.7MB ± 0%       32.2MB ± 0%   +1.59%  (p=0.008 n=5+5)
Reflect         78.2MB ± 0%       78.9MB ± 0%   +0.91%  (p=0.008 n=5+5)
Tar             26.6MB ± 0%       27.0MB ± 0%   +1.80%  (p=0.008 n=5+5)
XML             42.4MB ± 0%       43.4MB ± 0%   +2.35%  (p=0.008 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          379k ± 0%         378k ± 0%     ~     (p=0.421 n=5+5)
Unicode           322k ± 0%         321k ± 0%     ~     (p=0.222 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%     ~     (p=0.548 n=5+5)
Compiler         4.12M ± 0%        4.11M ± 0%   -0.14%  (p=0.032 n=5+5)
SSA              9.72M ± 0%        9.72M ± 0%     ~     (p=0.421 n=5+5)
Flate             234k ± 1%         234k ± 0%     ~     (p=0.421 n=5+5)
GoParser          316k ± 1%         315k ± 0%     ~     (p=0.222 n=5+5)
Reflect           980k ± 0%         979k ± 0%     ~     (p=0.095 n=5+5)
Tar               249k ± 1%         249k ± 1%     ~     (p=0.841 n=5+5)
XML               392k ± 0%         391k ± 0%     ~     (p=0.095 n=5+5)

From c=1 to c=4, real time is down ~40%, CPU usage up 10-20%, alloc up ~5%:

name       old time/op       new time/op       delta
Template         203ms ± 3%        131ms ± 5%  -35.45%  (p=0.000 n=50+50)
Unicode         87.2ms ± 4%       84.1ms ± 2%   -3.61%  (p=0.000 n=48+47)
GoTypes          560ms ± 4%        310ms ± 2%  -44.65%  (p=0.000 n=50+49)
Compiler         2.47s ± 3%        1.41s ± 2%  -43.10%  (p=0.000 n=50+46)
SSA              6.17s ± 2%        3.20s ± 2%  -48.06%  (p=0.000 n=49+49)
Flate            126ms ± 4%         74ms ± 2%  -41.06%  (p=0.000 n=49+48)
GoParser         148ms ± 4%         89ms ± 3%  -39.97%  (p=0.000 n=49+50)
Reflect          360ms ± 3%        242ms ± 3%  -32.81%  (p=0.000 n=49+49)
Tar              108ms ± 4%         73ms ± 4%  -32.48%  (p=0.000 n=50+49)
XML              203ms ± 3%        119ms ± 3%  -41.56%  (p=0.000 n=49+48)

name       old user-time/op  new user-time/op  delta
Template         246ms ± 9%        287ms ± 9%  +16.98%  (p=0.000 n=50+50)
Unicode          109ms ± 4%        118ms ± 5%   +7.56%  (p=0.000 n=46+50)
GoTypes          735ms ± 4%        806ms ± 2%   +9.62%  (p=0.000 n=50+50)
Compiler         3.34s ± 4%        3.56s ± 2%   +6.78%  (p=0.000 n=49+49)
SSA              8.54s ± 3%       10.04s ± 3%  +17.55%  (p=0.000 n=50+50)
Flate            149ms ± 6%        176ms ± 3%  +17.82%  (p=0.000 n=50+48)
GoParser         181ms ± 5%        213ms ± 3%  +17.47%  (p=0.000 n=50+50)
Reflect          453ms ± 6%        499ms ± 2%  +10.11%  (p=0.000 n=50+48)
Tar              126ms ± 5%        149ms ±11%  +18.76%  (p=0.000 n=50+50)
XML              246ms ± 5%        287ms ± 4%  +16.53%  (p=0.000 n=49+50)

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       40.4MB ± 0%   +4.21%  (p=0.008 n=5+5)
Unicode         29.8MB ± 0%       30.9MB ± 0%   +3.68%  (p=0.008 n=5+5)
GoTypes          113MB ± 0%        116MB ± 0%   +2.71%  (p=0.008 n=5+5)
Compiler         443MB ± 0%        455MB ± 0%   +2.75%  (p=0.008 n=5+5)
SSA             1.25GB ± 0%       1.27GB ± 0%   +1.84%  (p=0.008 n=5+5)
Flate           25.3MB ± 0%       26.9MB ± 1%   +6.31%  (p=0.008 n=5+5)
GoParser        31.7MB ± 0%       33.2MB ± 0%   +4.61%  (p=0.008 n=5+5)
Reflect         78.2MB ± 0%       80.2MB ± 0%   +2.53%  (p=0.008 n=5+5)
Tar             26.6MB ± 0%       27.9MB ± 0%   +5.19%  (p=0.008 n=5+5)
XML             42.4MB ± 0%       44.6MB ± 0%   +5.20%  (p=0.008 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          380k ± 0%         379k ± 0%   -0.39%  (p=0.032 n=5+5)
Unicode           321k ± 0%         321k ± 0%     ~     (p=0.841 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%     ~     (p=0.421 n=5+5)
Compiler         4.12M ± 0%        4.14M ± 0%   +0.52%  (p=0.008 n=5+5)
SSA              9.72M ± 0%        9.76M ± 0%   +0.37%  (p=0.008 n=5+5)
Flate             234k ± 1%         234k ± 1%     ~     (p=0.690 n=5+5)
GoParser          316k ± 0%         317k ± 1%     ~     (p=0.841 n=5+5)
Reflect           981k ± 0%         981k ± 0%     ~     (p=1.000 n=5+5)
Tar               250k ± 0%         249k ± 1%     ~     (p=0.151 n=5+5)
XML               393k ± 0%         392k ± 0%     ~     (p=0.056 n=5+5)

Going beyond c=4 on my machine tends to increase CPU time and allocs
without impacting real time.

The CPU time numbers matter, because when there are many concurrent
compilation processes, that will impact the overall throughput.

The numbers above are in many ways the best case scenario;
we can take full advantage of all cores.
Fortunately, the most common compilation scenario is incremental
re-compilation of a single package during a build/test cycle.

Updates golang#15756

Change-Id: I6725558ca2069edec0ac5b0d1683105a9fff6bea

This CL adds initial support for concurrent backend compilation.

BACKGROUND

The compiler currently consists (very roughly) of the following phases:

1. Initialization.
2. Lexing and parsing into the cmd/compile/internal/syntax AST.
3. Translation into the cmd/compile/internal/gc AST.
4. Some gc AST passes: typechecking, escape analysis, inlining,
   closure handling, expression evaluation ordering (order.go),
   and some lowering and optimization (walk.go).
5. Translation into the cmd/compile/internal/ssa SSA form.
6. Optimization and lowering of SSA form.
7. Translation from SSA form to assembler instructions.
8. Translation from assembler instructions to machine code.
9. Writing lots of output: machine code, DWARF symbols,
   type and reflection info, export data.

Phase 2 was already concurrent as of Go 1.8.

Phase 3 is planned for eventual removal;
we hope to go straight from syntax AST to SSA.

Phases 5–8 are per-function; this CL adds support for
processing multiple functions concurrently.
The slowest phases in the compiler are 5 and 6,
so this offers the opportunity for some good speed-ups.

Unfortunately, it's not quite that straightforward.
In the current compiler, the latter parts of phase 4
(order, walk) are done function-at-a-time as needed.
Making order and walk concurrency-safe proved hard,
and they're not particularly slow, so there wasn't much reward.
To enable phases 5–8 to be done concurrently,
when concurrent backend compilation is requested,
we complete phase 4 for all functions
before starting later phases for any functions.

Also, in reality, we automatically generate new
functions in phase 9, such as method wrappers
and equality and has routines.
Those new functions then go through phases 4–8.
This CL disables concurrent backend compilation
after the first, big, user-provided batch of
functions has been compiled.
This is done to keep things simple,
and because the autogenerated functions
tend to be small, few, simple, and fast to compile.

USAGE

Concurrent backend compilation still defaults to off.
To set the number of functions that may be backend-compiled
concurrently, use the compiler flag -c.
In future work, cmd/go will automatically set -c.

Furthermore, this CL has been intentionally written
so that the c=1 path has no backend concurrency whatsoever,
not even spawning any goroutines.
This helps ensure that, should problems arise
late in the development cycle,
we can simply have cmd/go set c=1 always,
and revert to the original compiler behavior.

MUTEXES

Most of the work required to make concurrent backend
compilation safe has occurred over the past month.
This CL adds a handful of mutexes to get the rest of the way there;
they are the mutexes that I didn't see a clean way to avoid.
Some of them may still be eliminable in future work.

In no particular order:

* gc.funcsymsmu. The global funcsyms slice is populated
  lazily when we need function symbols for closures.
  This occurs during gc AST to SSA translation.
  The function funcsym also does a package lookup,
  which is a source of races on types.Pkg.Syms;
  funcsymsmu also covers that package lookup.
  This mutex is low priority: it adds a single global,
  it is in an infrequently used code path, and it is low contention.
  Since funcsyms may now be added in any order,
  we must sort them to preserve reproducible builds.

* gc.largeStackFramesMu. We don't discover until after SSA compilation
  that a function's stack frame is gigantic.
  Recording that error happens basically never,
  but it does happen concurrently.
  Fix with a low priority mutex and sorting.

* obj.Link.hashmu. ctxt.hash stores the mapping from
  types.Syms (compiler symbols) to obj.LSyms (linker symbols).
  It is accessed fairly heavily through all the phases.
  This is the only heavily contended mutex.

* gc.signatlistmu. The global signatlist map is
  populated with types through several of the concurrent phases,
  including notably via ngotype during DWARF generation.
  It is low priority for removal.

* gc.typepkgmu. Looking up symbols in the types package
  happens a fair amount during backend compilation
  and DWARF generation, particularly via ngotype.
  This mutex helps us to avoid a broader mutex on types.Pkg.Syms.
  It has low-to-moderate contention.

* types.internedStringsmu. gc AST to SSA conversion and
  some SSA work introduce new autotmps.
  Those autotmps have their names interned to reduce allocations.
  That interning requires protecting types.internedStrings.
  The autotmp names are heavily re-used, and the mutex
  overhead and contention here are low, so it is probably
  a worthwhile performance optimization to keep this mutex.

TESTING

I have been testing this code locally by running
'go install -race cmd/compile'
and then doing
'go build -a -gcflags=-c=128 std cmd'
for all architectures and a variety of compiler flags.
This obviously needs to be made part of the builders,
but it is too expensive to make part of all.bash.
I have filed golang#19962 for this.

REPRODUCIBLE BUILDS

This version of the compiler generates reproducible builds.
Testing reproducible builds also needs automation, however,
and is also too expensive for all.bash.
This is golang#19961.

Also of note is that some of the compiler flags used by 'toolstash -cmp'
are currently incompatible with concurrent backend compilation.
They still work fine with c=1.
Time will tell whether this is a problem.

NEXT STEPS

* Continue to find and fix races and bugs,
  using a combination of code inspection, fuzzing,
  and hopefully some community experimentation.
  I do not know of any outstanding races,
  but there probably are some.
* Improve testing.
* Improve performance, for many values of c.
* Integrate with cmd/go and fine tune.
* Support concurrent compilation with the -race flag.
  It is a sad irony that it does not yet work.
* Minor code cleanup that has been deferred during
  the last month due to uncertainty about the
  ultimate shape of this CL.

PERFORMANCE

Here's the buried lede, at last. :)

All benchmarks are from my 8 core 2.9 GHz Intel Core i7 darwin/amd64 laptop.

First, going from tip to this CL with c=1 has almost no impact.

name        old time/op       new time/op       delta
Template          195ms ± 3%        194ms ± 5%    ~     (p=0.370 n=30+29)
Unicode          86.6ms ± 3%       87.0ms ± 7%    ~     (p=0.958 n=29+30)
GoTypes           548ms ± 3%        555ms ± 4%  +1.35%  (p=0.001 n=30+28)
Compiler          2.51s ± 2%        2.54s ± 2%  +1.17%  (p=0.000 n=28+30)
SSA               5.16s ± 3%        5.16s ± 2%    ~     (p=0.910 n=30+29)
Flate             124ms ± 5%        124ms ± 4%    ~     (p=0.947 n=30+30)
GoParser          146ms ± 3%        146ms ± 3%    ~     (p=0.150 n=29+28)
Reflect           354ms ± 3%        352ms ± 4%    ~     (p=0.096 n=29+29)
Tar               107ms ± 5%        106ms ± 3%    ~     (p=0.370 n=30+29)
XML               200ms ± 4%        201ms ± 4%    ~     (p=0.313 n=29+28)
[Geo mean]        332ms             333ms       +0.10%

name        old user-time/op  new user-time/op  delta
Template          227ms ± 5%        225ms ± 5%    ~     (p=0.457 n=28+27)
Unicode           109ms ± 4%        109ms ± 5%    ~     (p=0.758 n=29+29)
GoTypes           713ms ± 4%        721ms ± 5%    ~     (p=0.051 n=30+29)
Compiler          3.36s ± 2%        3.38s ± 3%    ~     (p=0.146 n=30+30)
SSA               7.46s ± 3%        7.47s ± 3%    ~     (p=0.804 n=30+29)
Flate             146ms ± 7%        147ms ± 3%    ~     (p=0.833 n=29+27)
GoParser          179ms ± 5%        179ms ± 5%    ~     (p=0.866 n=30+30)
Reflect           431ms ± 4%        429ms ± 4%    ~     (p=0.593 n=29+30)
Tar               124ms ± 5%        123ms ± 5%    ~     (p=0.140 n=29+29)
XML               243ms ± 4%        242ms ± 7%    ~     (p=0.404 n=29+29)
[Geo mean]        415ms             415ms       +0.02%

name        old obj-bytes     new obj-bytes     delta
Template           382k ± 0%         382k ± 0%    ~     (all equal)
Unicode            203k ± 0%         203k ± 0%    ~     (all equal)
GoTypes           1.18M ± 0%        1.18M ± 0%    ~     (all equal)
Compiler          3.98M ± 0%        3.98M ± 0%    ~     (all equal)
SSA               8.28M ± 0%        8.28M ± 0%    ~     (all equal)
Flate              230k ± 0%         230k ± 0%    ~     (all equal)
GoParser           287k ± 0%         287k ± 0%    ~     (all equal)
Reflect           1.00M ± 0%        1.00M ± 0%    ~     (all equal)
Tar                190k ± 0%         190k ± 0%    ~     (all equal)
XML                416k ± 0%         416k ± 0%    ~     (all equal)
[Geo mean]         660k              660k       +0.00%

Comparing this CL to itself, from c=1 to c=2
improves real times 20-30%, costs 5-10% more CPU time,
and adds about 2% alloc.
The allocation increase comes from allocating more ssa.Caches.

name       old time/op       new time/op       delta
Template         202ms ± 3%        149ms ± 3%  -26.15%  (p=0.000 n=49+49)
Unicode         87.4ms ± 4%       84.2ms ± 3%   -3.68%  (p=0.000 n=48+48)
GoTypes          560ms ± 2%        398ms ± 2%  -28.96%  (p=0.000 n=49+49)
Compiler         2.46s ± 3%        1.76s ± 2%  -28.61%  (p=0.000 n=48+46)
SSA              6.17s ± 2%        4.04s ± 1%  -34.52%  (p=0.000 n=49+49)
Flate            126ms ± 3%         92ms ± 2%  -26.81%  (p=0.000 n=49+48)
GoParser         148ms ± 4%        107ms ± 2%  -27.78%  (p=0.000 n=49+48)
Reflect          361ms ± 3%        281ms ± 3%  -22.10%  (p=0.000 n=49+49)
Tar              109ms ± 4%         86ms ± 3%  -20.81%  (p=0.000 n=49+47)
XML              204ms ± 3%        144ms ± 2%  -29.53%  (p=0.000 n=48+45)

name       old user-time/op  new user-time/op  delta
Template         246ms ± 9%        246ms ± 4%     ~     (p=0.401 n=50+48)
Unicode          109ms ± 4%        111ms ± 4%   +1.47%  (p=0.000 n=44+50)
GoTypes          728ms ± 3%        765ms ± 3%   +5.04%  (p=0.000 n=46+50)
Compiler         3.33s ± 3%        3.41s ± 2%   +2.31%  (p=0.000 n=49+48)
SSA              8.52s ± 2%        9.11s ± 2%   +6.93%  (p=0.000 n=49+47)
Flate            149ms ± 4%        161ms ± 3%   +8.13%  (p=0.000 n=50+47)
GoParser         181ms ± 5%        192ms ± 2%   +6.40%  (p=0.000 n=49+46)
Reflect          452ms ± 9%        474ms ± 2%   +4.99%  (p=0.000 n=50+48)
Tar              126ms ± 6%        136ms ± 4%   +7.95%  (p=0.000 n=50+49)
XML              247ms ± 5%        264ms ± 3%   +6.94%  (p=0.000 n=48+50)

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       39.3MB ± 0%   +1.48%  (p=0.008 n=5+5)
Unicode         29.8MB ± 0%       30.2MB ± 0%   +1.19%  (p=0.008 n=5+5)
GoTypes          113MB ± 0%        114MB ± 0%   +0.69%  (p=0.008 n=5+5)
Compiler         443MB ± 0%        447MB ± 0%   +0.95%  (p=0.008 n=5+5)
SSA             1.25GB ± 0%       1.26GB ± 0%   +0.89%  (p=0.008 n=5+5)
Flate           25.3MB ± 0%       25.9MB ± 1%   +2.35%  (p=0.008 n=5+5)
GoParser        31.7MB ± 0%       32.2MB ± 0%   +1.59%  (p=0.008 n=5+5)
Reflect         78.2MB ± 0%       78.9MB ± 0%   +0.91%  (p=0.008 n=5+5)
Tar             26.6MB ± 0%       27.0MB ± 0%   +1.80%  (p=0.008 n=5+5)
XML             42.4MB ± 0%       43.4MB ± 0%   +2.35%  (p=0.008 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          379k ± 0%         378k ± 0%     ~     (p=0.421 n=5+5)
Unicode           322k ± 0%         321k ± 0%     ~     (p=0.222 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%     ~     (p=0.548 n=5+5)
Compiler         4.12M ± 0%        4.11M ± 0%   -0.14%  (p=0.032 n=5+5)
SSA              9.72M ± 0%        9.72M ± 0%     ~     (p=0.421 n=5+5)
Flate             234k ± 1%         234k ± 0%     ~     (p=0.421 n=5+5)
GoParser          316k ± 1%         315k ± 0%     ~     (p=0.222 n=5+5)
Reflect           980k ± 0%         979k ± 0%     ~     (p=0.095 n=5+5)
Tar               249k ± 1%         249k ± 1%     ~     (p=0.841 n=5+5)
XML               392k ± 0%         391k ± 0%     ~     (p=0.095 n=5+5)

From c=1 to c=4, real time is down ~40%, CPU usage up 10-20%, alloc up ~5%:

name       old time/op       new time/op       delta
Template         203ms ± 3%        131ms ± 5%  -35.45%  (p=0.000 n=50+50)
Unicode         87.2ms ± 4%       84.1ms ± 2%   -3.61%  (p=0.000 n=48+47)
GoTypes          560ms ± 4%        310ms ± 2%  -44.65%  (p=0.000 n=50+49)
Compiler         2.47s ± 3%        1.41s ± 2%  -43.10%  (p=0.000 n=50+46)
SSA              6.17s ± 2%        3.20s ± 2%  -48.06%  (p=0.000 n=49+49)
Flate            126ms ± 4%         74ms ± 2%  -41.06%  (p=0.000 n=49+48)
GoParser         148ms ± 4%         89ms ± 3%  -39.97%  (p=0.000 n=49+50)
Reflect          360ms ± 3%        242ms ± 3%  -32.81%  (p=0.000 n=49+49)
Tar              108ms ± 4%         73ms ± 4%  -32.48%  (p=0.000 n=50+49)
XML              203ms ± 3%        119ms ± 3%  -41.56%  (p=0.000 n=49+48)

name       old user-time/op  new user-time/op  delta
Template         246ms ± 9%        287ms ± 9%  +16.98%  (p=0.000 n=50+50)
Unicode          109ms ± 4%        118ms ± 5%   +7.56%  (p=0.000 n=46+50)
GoTypes          735ms ± 4%        806ms ± 2%   +9.62%  (p=0.000 n=50+50)
Compiler         3.34s ± 4%        3.56s ± 2%   +6.78%  (p=0.000 n=49+49)
SSA              8.54s ± 3%       10.04s ± 3%  +17.55%  (p=0.000 n=50+50)
Flate            149ms ± 6%        176ms ± 3%  +17.82%  (p=0.000 n=50+48)
GoParser         181ms ± 5%        213ms ± 3%  +17.47%  (p=0.000 n=50+50)
Reflect          453ms ± 6%        499ms ± 2%  +10.11%  (p=0.000 n=50+48)
Tar              126ms ± 5%        149ms ±11%  +18.76%  (p=0.000 n=50+50)
XML              246ms ± 5%        287ms ± 4%  +16.53%  (p=0.000 n=49+50)

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       40.4MB ± 0%   +4.21%  (p=0.008 n=5+5)
Unicode         29.8MB ± 0%       30.9MB ± 0%   +3.68%  (p=0.008 n=5+5)
GoTypes          113MB ± 0%        116MB ± 0%   +2.71%  (p=0.008 n=5+5)
Compiler         443MB ± 0%        455MB ± 0%   +2.75%  (p=0.008 n=5+5)
SSA             1.25GB ± 0%       1.27GB ± 0%   +1.84%  (p=0.008 n=5+5)
Flate           25.3MB ± 0%       26.9MB ± 1%   +6.31%  (p=0.008 n=5+5)
GoParser        31.7MB ± 0%       33.2MB ± 0%   +4.61%  (p=0.008 n=5+5)
Reflect         78.2MB ± 0%       80.2MB ± 0%   +2.53%  (p=0.008 n=5+5)
Tar             26.6MB ± 0%       27.9MB ± 0%   +5.19%  (p=0.008 n=5+5)
XML             42.4MB ± 0%       44.6MB ± 0%   +5.20%  (p=0.008 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          380k ± 0%         379k ± 0%   -0.39%  (p=0.032 n=5+5)
Unicode           321k ± 0%         321k ± 0%     ~     (p=0.841 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%     ~     (p=0.421 n=5+5)
Compiler         4.12M ± 0%        4.14M ± 0%   +0.52%  (p=0.008 n=5+5)
SSA              9.72M ± 0%        9.76M ± 0%   +0.37%  (p=0.008 n=5+5)
Flate             234k ± 1%         234k ± 1%     ~     (p=0.690 n=5+5)
GoParser          316k ± 0%         317k ± 1%     ~     (p=0.841 n=5+5)
Reflect           981k ± 0%         981k ± 0%     ~     (p=1.000 n=5+5)
Tar               250k ± 0%         249k ± 1%     ~     (p=0.151 n=5+5)
XML               393k ± 0%         392k ± 0%     ~     (p=0.056 n=5+5)

Going beyond c=4 on my machine tends to increase CPU time and allocs
without impacting real time.

The CPU time numbers matter, because when there are many concurrent
compilation processes, that will impact the overall throughput.

The numbers above are in many ways the best case scenario;
we can take full advantage of all cores.
Fortunately, the most common compilation scenario is incremental
re-compilation of a single package during a build/test cycle.

Updates golang#15756

Change-Id: I6725558ca2069edec0ac5b0d1683105a9fff6bea

This CL adds initial support for concurrent backend compilation.

BACKGROUND

The compiler currently consists (very roughly) of the following phases:

1. Initialization.
2. Lexing and parsing into the cmd/compile/internal/syntax AST.
3. Translation into the cmd/compile/internal/gc AST.
4. Some gc AST passes: typechecking, escape analysis, inlining,
   closure handling, expression evaluation ordering (order.go),
   and some lowering and optimization (walk.go).
5. Translation into the cmd/compile/internal/ssa SSA form.
6. Optimization and lowering of SSA form.
7. Translation from SSA form to assembler instructions.
8. Translation from assembler instructions to machine code.
9. Writing lots of output: machine code, DWARF symbols,
   type and reflection info, export data.

Phase 2 was already concurrent as of Go 1.8.

Phase 3 is planned for eventual removal;
we hope to go straight from syntax AST to SSA.

Phases 5–8 are per-function; this CL adds support for
processing multiple functions concurrently.
The slowest phases in the compiler are 5 and 6,
so this offers the opportunity for some good speed-ups.

Unfortunately, it's not quite that straightforward.
In the current compiler, the latter parts of phase 4
(order, walk) are done function-at-a-time as needed.
Making order and walk concurrency-safe proved hard,
and they're not particularly slow, so there wasn't much reward.
To enable phases 5–8 to be done concurrently,
when concurrent backend compilation is requested,
we complete phase 4 for all functions
before starting later phases for any functions.

Also, in reality, we automatically generate new
functions in phase 9, such as method wrappers
and equality and has routines.
Those new functions then go through phases 4–8.
This CL disables concurrent backend compilation
after the first, big, user-provided batch of
functions has been compiled.
This is done to keep things simple,
and because the autogenerated functions
tend to be small, few, simple, and fast to compile.

USAGE

Concurrent backend compilation still defaults to off.
To set the number of functions that may be backend-compiled
concurrently, use the compiler flag -c.
In future work, cmd/go will automatically set -c.

Furthermore, this CL has been intentionally written
so that the c=1 path has no backend concurrency whatsoever,
not even spawning any goroutines.
This helps ensure that, should problems arise
late in the development cycle,
we can simply have cmd/go set c=1 always,
and revert to the original compiler behavior.

MUTEXES

Most of the work required to make concurrent backend
compilation safe has occurred over the past month.
This CL adds a handful of mutexes to get the rest of the way there;
they are the mutexes that I didn't see a clean way to avoid.
Some of them may still be eliminable in future work.

In no particular order:

* gc.funcsymsmu. The global funcsyms slice is populated
  lazily when we need function symbols for closures.
  This occurs during gc AST to SSA translation.
  The function funcsym also does a package lookup,
  which is a source of races on types.Pkg.Syms;
  funcsymsmu also covers that package lookup.
  This mutex is low priority: it adds a single global,
  it is in an infrequently used code path, and it is low contention.
  Since funcsyms may now be added in any order,
  we must sort them to preserve reproducible builds.

* gc.largeStackFramesMu. We don't discover until after SSA compilation
  that a function's stack frame is gigantic.
  Recording that error happens basically never,
  but it does happen concurrently.
  Fix with a low priority mutex and sorting.

* obj.Link.hashmu. ctxt.hash stores the mapping from
  types.Syms (compiler symbols) to obj.LSyms (linker symbols).
  It is accessed fairly heavily through all the phases.
  This is the only heavily contended mutex.

* gc.signatlistmu. The global signatlist map is
  populated with types through several of the concurrent phases,
  including notably via ngotype during DWARF generation.
  It is low priority for removal.

* gc.typepkgmu. Looking up symbols in the types package
  happens a fair amount during backend compilation
  and DWARF generation, particularly via ngotype.
  This mutex helps us to avoid a broader mutex on types.Pkg.Syms.
  It has low-to-moderate contention.

* types.internedStringsmu. gc AST to SSA conversion and
  some SSA work introduce new autotmps.
  Those autotmps have their names interned to reduce allocations.
  That interning requires protecting types.internedStrings.
  The autotmp names are heavily re-used, and the mutex
  overhead and contention here are low, so it is probably
  a worthwhile performance optimization to keep this mutex.

TESTING

I have been testing this code locally by running
'go install -race cmd/compile'
and then doing
'go build -a -gcflags=-c=128 std cmd'
for all architectures and a variety of compiler flags.
This obviously needs to be made part of the builders,
but it is too expensive to make part of all.bash.
I have filed #19962 for this.

REPRODUCIBLE BUILDS

This version of the compiler generates reproducible builds.
Testing reproducible builds also needs automation, however,
and is also too expensive for all.bash.
This is #19961.

Also of note is that some of the compiler flags used by 'toolstash -cmp'
are currently incompatible with concurrent backend compilation.
They still work fine with c=1.
Time will tell whether this is a problem.

NEXT STEPS

* Continue to find and fix races and bugs,
  using a combination of code inspection, fuzzing,
  and hopefully some community experimentation.
  I do not know of any outstanding races,
  but there probably are some.
* Improve testing.
* Improve performance, for many values of c.
* Integrate with cmd/go and fine tune.
* Support concurrent compilation with the -race flag.
  It is a sad irony that it does not yet work.
* Minor code cleanup that has been deferred during
  the last month due to uncertainty about the
  ultimate shape of this CL.

PERFORMANCE

Here's the buried lede, at last. :)

All benchmarks are from my 8 core 2.9 GHz Intel Core i7 darwin/amd64 laptop.

First, going from tip to this CL with c=1 has almost no impact.

name        old time/op       new time/op       delta
Template          195ms ± 3%        194ms ± 5%    ~     (p=0.370 n=30+29)
Unicode          86.6ms ± 3%       87.0ms ± 7%    ~     (p=0.958 n=29+30)
GoTypes           548ms ± 3%        555ms ± 4%  +1.35%  (p=0.001 n=30+28)
Compiler          2.51s ± 2%        2.54s ± 2%  +1.17%  (p=0.000 n=28+30)
SSA               5.16s ± 3%        5.16s ± 2%    ~     (p=0.910 n=30+29)
Flate             124ms ± 5%        124ms ± 4%    ~     (p=0.947 n=30+30)
GoParser          146ms ± 3%        146ms ± 3%    ~     (p=0.150 n=29+28)
Reflect           354ms ± 3%        352ms ± 4%    ~     (p=0.096 n=29+29)
Tar               107ms ± 5%        106ms ± 3%    ~     (p=0.370 n=30+29)
XML               200ms ± 4%        201ms ± 4%    ~     (p=0.313 n=29+28)
[Geo mean]        332ms             333ms       +0.10%

name        old user-time/op  new user-time/op  delta
Template          227ms ± 5%        225ms ± 5%    ~     (p=0.457 n=28+27)
Unicode           109ms ± 4%        109ms ± 5%    ~     (p=0.758 n=29+29)
GoTypes           713ms ± 4%        721ms ± 5%    ~     (p=0.051 n=30+29)
Compiler          3.36s ± 2%        3.38s ± 3%    ~     (p=0.146 n=30+30)
SSA               7.46s ± 3%        7.47s ± 3%    ~     (p=0.804 n=30+29)
Flate             146ms ± 7%        147ms ± 3%    ~     (p=0.833 n=29+27)
GoParser          179ms ± 5%        179ms ± 5%    ~     (p=0.866 n=30+30)
Reflect           431ms ± 4%        429ms ± 4%    ~     (p=0.593 n=29+30)
Tar               124ms ± 5%        123ms ± 5%    ~     (p=0.140 n=29+29)
XML               243ms ± 4%        242ms ± 7%    ~     (p=0.404 n=29+29)
[Geo mean]        415ms             415ms       +0.02%

name        old obj-bytes     new obj-bytes     delta
Template           382k ± 0%         382k ± 0%    ~     (all equal)
Unicode            203k ± 0%         203k ± 0%    ~     (all equal)
GoTypes           1.18M ± 0%        1.18M ± 0%    ~     (all equal)
Compiler          3.98M ± 0%        3.98M ± 0%    ~     (all equal)
SSA               8.28M ± 0%        8.28M ± 0%    ~     (all equal)
Flate              230k ± 0%         230k ± 0%    ~     (all equal)
GoParser           287k ± 0%         287k ± 0%    ~     (all equal)
Reflect           1.00M ± 0%        1.00M ± 0%    ~     (all equal)
Tar                190k ± 0%         190k ± 0%    ~     (all equal)
XML                416k ± 0%         416k ± 0%    ~     (all equal)
[Geo mean]         660k              660k       +0.00%

Comparing this CL to itself, from c=1 to c=2
improves real times 20-30%, costs 5-10% more CPU time,
and adds about 2% alloc.
The allocation increase comes from allocating more ssa.Caches.

name       old time/op       new time/op       delta
Template         202ms ± 3%        149ms ± 3%  -26.15%  (p=0.000 n=49+49)
Unicode         87.4ms ± 4%       84.2ms ± 3%   -3.68%  (p=0.000 n=48+48)
GoTypes          560ms ± 2%        398ms ± 2%  -28.96%  (p=0.000 n=49+49)
Compiler         2.46s ± 3%        1.76s ± 2%  -28.61%  (p=0.000 n=48+46)
SSA              6.17s ± 2%        4.04s ± 1%  -34.52%  (p=0.000 n=49+49)
Flate            126ms ± 3%         92ms ± 2%  -26.81%  (p=0.000 n=49+48)
GoParser         148ms ± 4%        107ms ± 2%  -27.78%  (p=0.000 n=49+48)
Reflect          361ms ± 3%        281ms ± 3%  -22.10%  (p=0.000 n=49+49)
Tar              109ms ± 4%         86ms ± 3%  -20.81%  (p=0.000 n=49+47)
XML              204ms ± 3%        144ms ± 2%  -29.53%  (p=0.000 n=48+45)

name       old user-time/op  new user-time/op  delta
Template         246ms ± 9%        246ms ± 4%     ~     (p=0.401 n=50+48)
Unicode          109ms ± 4%        111ms ± 4%   +1.47%  (p=0.000 n=44+50)
GoTypes          728ms ± 3%        765ms ± 3%   +5.04%  (p=0.000 n=46+50)
Compiler         3.33s ± 3%        3.41s ± 2%   +2.31%  (p=0.000 n=49+48)
SSA              8.52s ± 2%        9.11s ± 2%   +6.93%  (p=0.000 n=49+47)
Flate            149ms ± 4%        161ms ± 3%   +8.13%  (p=0.000 n=50+47)
GoParser         181ms ± 5%        192ms ± 2%   +6.40%  (p=0.000 n=49+46)
Reflect          452ms ± 9%        474ms ± 2%   +4.99%  (p=0.000 n=50+48)
Tar              126ms ± 6%        136ms ± 4%   +7.95%  (p=0.000 n=50+49)
XML              247ms ± 5%        264ms ± 3%   +6.94%  (p=0.000 n=48+50)

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       39.3MB ± 0%   +1.48%  (p=0.008 n=5+5)
Unicode         29.8MB ± 0%       30.2MB ± 0%   +1.19%  (p=0.008 n=5+5)
GoTypes          113MB ± 0%        114MB ± 0%   +0.69%  (p=0.008 n=5+5)
Compiler         443MB ± 0%        447MB ± 0%   +0.95%  (p=0.008 n=5+5)
SSA             1.25GB ± 0%       1.26GB ± 0%   +0.89%  (p=0.008 n=5+5)
Flate           25.3MB ± 0%       25.9MB ± 1%   +2.35%  (p=0.008 n=5+5)
GoParser        31.7MB ± 0%       32.2MB ± 0%   +1.59%  (p=0.008 n=5+5)
Reflect         78.2MB ± 0%       78.9MB ± 0%   +0.91%  (p=0.008 n=5+5)
Tar             26.6MB ± 0%       27.0MB ± 0%   +1.80%  (p=0.008 n=5+5)
XML             42.4MB ± 0%       43.4MB ± 0%   +2.35%  (p=0.008 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          379k ± 0%         378k ± 0%     ~     (p=0.421 n=5+5)
Unicode           322k ± 0%         321k ± 0%     ~     (p=0.222 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%     ~     (p=0.548 n=5+5)
Compiler         4.12M ± 0%        4.11M ± 0%   -0.14%  (p=0.032 n=5+5)
SSA              9.72M ± 0%        9.72M ± 0%     ~     (p=0.421 n=5+5)
Flate             234k ± 1%         234k ± 0%     ~     (p=0.421 n=5+5)
GoParser          316k ± 1%         315k ± 0%     ~     (p=0.222 n=5+5)
Reflect           980k ± 0%         979k ± 0%     ~     (p=0.095 n=5+5)
Tar               249k ± 1%         249k ± 1%     ~     (p=0.841 n=5+5)
XML               392k ± 0%         391k ± 0%     ~     (p=0.095 n=5+5)

From c=1 to c=4, real time is down ~40%, CPU usage up 10-20%, alloc up ~5%:

name       old time/op       new time/op       delta
Template         203ms ± 3%        131ms ± 5%  -35.45%  (p=0.000 n=50+50)
Unicode         87.2ms ± 4%       84.1ms ± 2%   -3.61%  (p=0.000 n=48+47)
GoTypes          560ms ± 4%        310ms ± 2%  -44.65%  (p=0.000 n=50+49)
Compiler         2.47s ± 3%        1.41s ± 2%  -43.10%  (p=0.000 n=50+46)
SSA              6.17s ± 2%        3.20s ± 2%  -48.06%  (p=0.000 n=49+49)
Flate            126ms ± 4%         74ms ± 2%  -41.06%  (p=0.000 n=49+48)
GoParser         148ms ± 4%         89ms ± 3%  -39.97%  (p=0.000 n=49+50)
Reflect          360ms ± 3%        242ms ± 3%  -32.81%  (p=0.000 n=49+49)
Tar              108ms ± 4%         73ms ± 4%  -32.48%  (p=0.000 n=50+49)
XML              203ms ± 3%        119ms ± 3%  -41.56%  (p=0.000 n=49+48)

name       old user-time/op  new user-time/op  delta
Template         246ms ± 9%        287ms ± 9%  +16.98%  (p=0.000 n=50+50)
Unicode          109ms ± 4%        118ms ± 5%   +7.56%  (p=0.000 n=46+50)
GoTypes          735ms ± 4%        806ms ± 2%   +9.62%  (p=0.000 n=50+50)
Compiler         3.34s ± 4%        3.56s ± 2%   +6.78%  (p=0.000 n=49+49)
SSA              8.54s ± 3%       10.04s ± 3%  +17.55%  (p=0.000 n=50+50)
Flate            149ms ± 6%        176ms ± 3%  +17.82%  (p=0.000 n=50+48)
GoParser         181ms ± 5%        213ms ± 3%  +17.47%  (p=0.000 n=50+50)
Reflect          453ms ± 6%        499ms ± 2%  +10.11%  (p=0.000 n=50+48)
Tar              126ms ± 5%        149ms ±11%  +18.76%  (p=0.000 n=50+50)
XML              246ms ± 5%        287ms ± 4%  +16.53%  (p=0.000 n=49+50)

name       old alloc/op      new alloc/op      delta
Template        38.8MB ± 0%       40.4MB ± 0%   +4.21%  (p=0.008 n=5+5)
Unicode         29.8MB ± 0%       30.9MB ± 0%   +3.68%  (p=0.008 n=5+5)
GoTypes          113MB ± 0%        116MB ± 0%   +2.71%  (p=0.008 n=5+5)
Compiler         443MB ± 0%        455MB ± 0%   +2.75%  (p=0.008 n=5+5)
SSA             1.25GB ± 0%       1.27GB ± 0%   +1.84%  (p=0.008 n=5+5)
Flate           25.3MB ± 0%       26.9MB ± 1%   +6.31%  (p=0.008 n=5+5)
GoParser        31.7MB ± 0%       33.2MB ± 0%   +4.61%  (p=0.008 n=5+5)
Reflect         78.2MB ± 0%       80.2MB ± 0%   +2.53%  (p=0.008 n=5+5)
Tar             26.6MB ± 0%       27.9MB ± 0%   +5.19%  (p=0.008 n=5+5)
XML             42.4MB ± 0%       44.6MB ± 0%   +5.20%  (p=0.008 n=5+5)

name       old allocs/op     new allocs/op     delta
Template          380k ± 0%         379k ± 0%   -0.39%  (p=0.032 n=5+5)
Unicode           321k ± 0%         321k ± 0%     ~     (p=0.841 n=5+5)
GoTypes          1.14M ± 0%        1.14M ± 0%     ~     (p=0.421 n=5+5)
Compiler         4.12M ± 0%        4.14M ± 0%   +0.52%  (p=0.008 n=5+5)
SSA              9.72M ± 0%        9.76M ± 0%   +0.37%  (p=0.008 n=5+5)
Flate             234k ± 1%         234k ± 1%     ~     (p=0.690 n=5+5)
GoParser          316k ± 0%         317k ± 1%     ~     (p=0.841 n=5+5)
Reflect           981k ± 0%         981k ± 0%     ~     (p=1.000 n=5+5)
Tar               250k ± 0%         249k ± 1%     ~     (p=0.151 n=5+5)
XML               393k ± 0%         392k ± 0%     ~     (p=0.056 n=5+5)

Going beyond c=4 on my machine tends to increase CPU time and allocs
without impacting real time.

The CPU time numbers matter, because when there are many concurrent
compilation processes, that will impact the overall throughput.

The numbers above are in many ways the best case scenario;
we can take full advantage of all cores.
Fortunately, the most common compilation scenario is incremental
re-compilation of a single package during a build/test cycle.

Updates #15756

Change-Id: I6725558ca2069edec0ac5b0d1683105a9fff6bea
Reviewed-on: https://go-review.googlesource.com/40693
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

When using a concurrent backend,
the overall compilation time is bounded
in part by the slowest function to compile.
The number of top-level statements in a function
is an easily calculated and fairly reliable
proxy for compilation time.

Here's a standard compilecmp output for -c=8 with this CL:

name       old time/op       new time/op       delta
Template         127ms ± 4%        125ms ± 6%   -1.33%  (p=0.000 n=47+50)
Unicode         84.8ms ± 4%       84.5ms ± 4%     ~     (p=0.217 n=49+49)
GoTypes          289ms ± 3%        287ms ± 3%   -0.78%  (p=0.002 n=48+50)
Compiler         1.36s ± 3%        1.34s ± 2%   -1.29%  (p=0.000 n=49+47)
SSA              2.95s ± 3%        2.77s ± 4%   -6.23%  (p=0.000 n=50+49)
Flate           70.7ms ± 3%       70.9ms ± 2%     ~     (p=0.112 n=50+49)
GoParser        85.0ms ± 3%       83.0ms ± 4%   -2.31%  (p=0.000 n=48+49)
Reflect          229ms ± 3%        225ms ± 4%   -1.83%  (p=0.000 n=49+49)
Tar             70.2ms ± 3%       69.4ms ± 3%   -1.17%  (p=0.000 n=49+49)
XML              115ms ± 7%        114ms ± 6%     ~     (p=0.158 n=49+47)

name       old user-time/op  new user-time/op  delta
Template         352ms ± 5%        342ms ± 8%   -2.74%  (p=0.000 n=49+50)
Unicode          117ms ± 5%        118ms ± 4%   +0.88%  (p=0.005 n=46+48)
GoTypes          986ms ± 3%        980ms ± 4%     ~     (p=0.110 n=46+48)
Compiler         4.39s ± 2%        4.43s ± 4%   +0.97%  (p=0.002 n=50+50)
SSA              12.0s ± 2%        13.3s ± 3%  +11.33%  (p=0.000 n=49+49)
Flate            222ms ± 5%        219ms ± 6%   -1.56%  (p=0.002 n=50+50)
GoParser         271ms ± 5%        268ms ± 4%   -0.83%  (p=0.036 n=49+48)
Reflect          560ms ± 4%        571ms ± 3%   +1.90%  (p=0.000 n=50+49)
Tar              183ms ± 3%        183ms ± 3%     ~     (p=0.903 n=45+50)
XML              364ms ±13%        391ms ± 4%   +7.16%  (p=0.000 n=50+40)

A more interesting way of viewing the data is by
looking at the ratio of the time taken to compile
the slowest-to-compile function to the overall
time spent compiling functions.

If this ratio is small (near 0), then increased concurrency might help.
If this ratio is big (near 1), then we're bounded by that single function.

I instrumented the compiler to emit this ratio per-package,
ran 'go build -a -gcflags=-c=C -p=P std cmd' three times,
for varying values of C and P,
and collected the ratios encountered into an ASCII histogram.

Here's c=1 p=1, which is a non-concurrent backend, single process at a time:

 90%|
 80%|
 70%|
 60%|
 50%|
 40%|
 30%|
 20%|**
 10%|***
  0%|*********
----+----------
    |0123456789

The x-axis is floor(10*ratio), so the first column indicates the percent of
ratios that fell in the 0% to 9.9999% range.
We can see in this histogram that more concurrency will help;
in most cases, the ratio is small.

Here's c=8 p=1, before this CL:

 90%|
 80%|
 70%|
 60%|
 50%|
 40%|
 30%|         *
 20%|         *
 10%|*   *    *
  0%|**********
----+----------
    |0123456789

In 30-40% of cases, we're mostly bound by the compilation time
of a single function.

Here's c=8 p=1, after this CL:

 90%|
 80%|
 70%|
 60%|
 50%|         *
 40%|         *
 30%|         *
 20%|         *
 10%|         *
  0%|**********
----+----------
    |0123456789

The sorting pays off; we are bound by the
compilation time of a single function in over half of packages.
The single * in the histogram indicates 0-10%.
The actual values for this chart are:
0: 5%, 1: 1%, 2: 1%, 3: 4%, 4: 5%, 5: 7%, 6: 7%, 7: 7%, 8: 9%, 9: 55%

This indicates that efforts to increase or enable more concurrency,
e.g. by optimizing mutexes or increasing the value of c,
will probably not yield fruit.
That matches what compilecmp tells us.

Further optimization efforts should thus focus instead on one of:

(1) making more functions compile concurrently
(2) improving the compilation time of the slowest functions
(3) speeding up the remaining serial parts of the compiler
(4) automatically splitting up some large autogenerated functions
    into small ones, as discussed in #19751

I hope to spend more time on (1) before the freeze.

Adding process parallelism doesn't change the story much.
For example, here's c=8 p=8, after this CL:

 90%|
 80%|
 70%|
 60%|
 50%|
 40%|         *
 30%|         *
 20%|         *
 10%|       ***
  0%|**********
----+----------
    |0123456789

Since we don't need to worry much about p,
these histograms can help us select a good
general value of c to use as a default,
assuming we're not bounded by GOMAXPROCS.

Here are some charts after this CL, for c from 1 to 8:

c=1 p=1

 90%|
 80%|
 70%|
 60%|
 50%|
 40%|
 30%|
 20%|**
 10%|***
  0%|*********
----+----------
    |0123456789

c=2 p=1

 90%|
 80%|
 70%|
 60%|
 50%|
 40%|
 30%|
 20%|
 10%| ****    *
  0%|**********
----+----------
    |0123456789

c=3 p=1

 90%|
 80%|
 70%|
 60%|
 50%|
 40%|
 30%|
 20%|         *
 10%|  ** *   *
  0%|**********
----+----------
    |0123456789

c=4 p=1

 90%|
 80%|
 70%|
 60%|
 50%|
 40%|
 30%|         *
 20%|         *
 10%|     *   *
  0%|**********
----+----------
    |0123456789

c=5 p=1

 90%|
 80%|
 70%|
 60%|
 50%|
 40%|
 30%|         *
 20%|         *
 10%|     *   *
  0%|**********
----+----------
    |0123456789

c=6 p=1

 90%|
 80%|
 70%|
 60%|
 50%|
 40%|         *
 30%|         *
 20%|         *
 10%|         *
  0%|**********
----+----------
    |0123456789

c=7 p=1

 90%|
 80%|
 70%|
 60%|
 50%|         *
 40%|         *
 30%|         *
 20%|         *
 10%|        **
  0%|**********
----+----------
    |0123456789

c=8 p=1

 90%|
 80%|
 70%|
 60%|
 50%|         *
 40%|         *
 30%|         *
 20%|         *
 10%|         *
  0%|**********
----+----------
    |0123456789

Given the increased user-CPU costs as
c increases, it looks like c=4 is probably
the sweet spot, at least for now.

Pleasingly, this matches (and explains)
the results of the standard benchmarking
that I have done.

Updates #15756

Change-Id: I82b606c06efd34a5dbd1afdbcf66a605905b2aeb
Reviewed-on: https://go-review.googlesource.com/41192
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

CL 40693 added concurrent backend compilation support,
and used it for user-provided functions.
Autogenerated functions were still compiled serially.
This CL brings them into the fold.
As of this CL, when requested,
no functions are compiled serially.

There generally aren't many autogenerated functions.
When there are, this CL can help a lot,
because autogenerated functions are usually short.
Many short functions is the best case scenario
for concurrent compilation; see CL 41192.

One example of such a package comes from Dave Cheney's benchjuju:
github.com/juju/govmomi/vim25/types.
It has thousands of autogenerated functions.
This CL improves performance on the entire benchmark
by around a second on my machine at c=8, or about ~5%.

Updates golang#15756

Change-Id: Ia21e302b2469a9ed743df02244ec7ebde55b32f3

It is disabled by default.
It can be enabled by setting the environment variable
GO19CONCURRENTCOMPILATION=1.

Benchmarking results are presented in a grid.
Columns are different values of c (compiler backend concurrency);
rows are different values of p (process concurrency).

'go build -a std cmd', a 4 core raspberry pi 3:

            c=1        c=2        c=4
StdCmd/p=1  504s ± 2%  413s ± 4%  367s ± 3%
StdCmd/p=2  314s ± 3%  266s ± 4%  267s ± 4%
StdCmd/p=4  254s ± 5%  241s ± 5%  238s ± 6%

'go build -a std cmd', an 8 core darwin/amd64 laptop:

            c=1         c=2         c=4         c=6         c=8
StdCmd/p=1  40.4s ± 7%  31.0s ± 1%  27.3s ± 1%  27.8s ± 0%  27.7s ± 0%
StdCmd/p=2  21.9s ± 1%  17.9s ± 1%  16.9s ± 1%  17.0s ± 1%  17.2s ± 0%
StdCmd/p=4  17.4s ± 2%  14.5s ± 2%  13.3s ± 2%  13.5s ± 2%  13.6s ± 2%
StdCmd/p=6  16.9s ± 1%  14.2s ± 2%  13.1s ± 2%  13.2s ± 2%  13.3s ± 2%
StdCmd/p=8  16.7s ± 2%  14.2s ± 2%  13.2s ± 3%  13.2s ± 2%  13.4s ± 2%

'go build -a std cmd', a 96 core arm64 server:

             c=1         c=2         c=4         c=6         c=8         c=16        c=32        c=64        c=96
StdCmd/p=1    173s ± 1%   133s ± 1%   114s ± 1%   109s ± 1%   106s ± 0%   106s ± 1%   107s ± 1%   110s ± 1%   113s ± 1%
StdCmd/p=2   94.2s ± 2%  71.5s ± 1%  61.7s ± 1%  58.7s ± 1%  57.5s ± 2%  56.9s ± 1%  58.0s ± 1%  59.6s ± 1%  61.0s ± 1%
StdCmd/p=4   74.1s ± 2%  53.5s ± 1%  43.7s ± 2%  40.5s ± 1%  39.2s ± 2%  38.9s ± 2%  39.5s ± 3%  40.3s ± 2%  40.8s ± 1%
StdCmd/p=6   69.3s ± 1%  50.2s ± 2%  40.3s ± 2%  37.3s ± 3%  36.0s ± 3%  35.3s ± 2%  36.0s ± 2%  36.8s ± 2%  37.5s ± 2%
StdCmd/p=8   66.1s ± 2%  47.7s ± 2%  38.6s ± 2%  35.7s ± 2%  34.4s ± 1%  33.6s ± 2%  34.2s ± 2%  34.6s ± 1%  35.0s ± 1%
StdCmd/p=16  63.4s ± 2%  45.3s ± 2%  36.3s ± 2%  33.3s ± 2%  32.0s ± 3%  31.6s ± 2%  32.1s ± 2%  32.5s ± 2%  32.7s ± 2%
StdCmd/p=32  62.2s ± 1%  44.2s ± 2%  35.3s ± 2%  32.4s ± 2%  31.2s ± 2%  30.9s ± 2%  31.1s ± 2%  31.7s ± 2%  32.0s ± 2%
StdCmd/p=64  62.2s ± 1%  44.3s ± 2%  35.4s ± 2%  32.4s ± 2%  31.2s ± 2%  30.9s ± 2%  31.2s ± 2%  31.8s ± 3%  32.2s ± 3%
StdCmd/p=96  62.2s ± 2%  44.4s ± 2%  35.3s ± 2%  32.3s ± 2%  31.1s ± 2%  30.9s ± 3%  31.3s ± 2%  31.7s ± 1%  32.1s ± 2%

benchjuju, an 8 core darwin/amd64 laptop:

               c=1         c=2         c=4         c=6         c=8
BuildJuju/p=1  55.3s ± 0%  46.3s ± 0%  41.9s ± 0%  41.4s ± 1%  41.3s ± 0%
BuildJuju/p=2  33.7s ± 1%  28.4s ± 1%  26.7s ± 1%  26.6s ± 1%  26.8s ± 1%
BuildJuju/p=4  24.7s ± 1%  22.3s ± 1%  21.4s ± 1%  21.7s ± 1%  21.8s ± 1%
BuildJuju/p=6  20.6s ± 1%  19.3s ± 2%  19.4s ± 1%  19.7s ± 1%  19.9s ± 1%
BuildJuju/p=8  20.6s ± 2%  19.5s ± 2%  19.3s ± 2%  19.6s ± 1%  19.8s ± 2%

Updates golang#15756

Change-Id: I8a56e88953071a05eee764002024c54cd888a56c

It can be disabled by setting the environment variable
GO19CONCURRENTCOMPILATION=0, or with -gcflags=-c=1.

Fixes golang#15756.

Change-Id: I7acbf16330512b62ee14ecbab1f46b53ec5a67b6

It is disabled by default.
It can be enabled by setting the environment variable
GO19CONCURRENTCOMPILATION=1.

Benchmarking results are presented in a grid.
Columns are different values of c (compiler backend concurrency);
rows are different values of p (process concurrency).

'go build -a std cmd', a 4 core raspberry pi 3:

            c=1        c=2        c=4
StdCmd/p=1  504s ± 2%  413s ± 4%  367s ± 3%
StdCmd/p=2  314s ± 3%  266s ± 4%  267s ± 4%
StdCmd/p=4  254s ± 5%  241s ± 5%  238s ± 6%

'go build -a std cmd', an 8 core darwin/amd64 laptop:

            c=1         c=2         c=4         c=6         c=8
StdCmd/p=1  40.4s ± 7%  31.0s ± 1%  27.3s ± 1%  27.8s ± 0%  27.7s ± 0%
StdCmd/p=2  21.9s ± 1%  17.9s ± 1%  16.9s ± 1%  17.0s ± 1%  17.2s ± 0%
StdCmd/p=4  17.4s ± 2%  14.5s ± 2%  13.3s ± 2%  13.5s ± 2%  13.6s ± 2%
StdCmd/p=6  16.9s ± 1%  14.2s ± 2%  13.1s ± 2%  13.2s ± 2%  13.3s ± 2%
StdCmd/p=8  16.7s ± 2%  14.2s ± 2%  13.2s ± 3%  13.2s ± 2%  13.4s ± 2%

'go build -a std cmd', a 96 core arm64 server:

             c=1         c=2         c=4         c=6         c=8         c=16        c=32        c=64        c=96
StdCmd/p=1    173s ± 1%   133s ± 1%   114s ± 1%   109s ± 1%   106s ± 0%   106s ± 1%   107s ± 1%   110s ± 1%   113s ± 1%
StdCmd/p=2   94.2s ± 2%  71.5s ± 1%  61.7s ± 1%  58.7s ± 1%  57.5s ± 2%  56.9s ± 1%  58.0s ± 1%  59.6s ± 1%  61.0s ± 1%
StdCmd/p=4   74.1s ± 2%  53.5s ± 1%  43.7s ± 2%  40.5s ± 1%  39.2s ± 2%  38.9s ± 2%  39.5s ± 3%  40.3s ± 2%  40.8s ± 1%
StdCmd/p=6   69.3s ± 1%  50.2s ± 2%  40.3s ± 2%  37.3s ± 3%  36.0s ± 3%  35.3s ± 2%  36.0s ± 2%  36.8s ± 2%  37.5s ± 2%
StdCmd/p=8   66.1s ± 2%  47.7s ± 2%  38.6s ± 2%  35.7s ± 2%  34.4s ± 1%  33.6s ± 2%  34.2s ± 2%  34.6s ± 1%  35.0s ± 1%
StdCmd/p=16  63.4s ± 2%  45.3s ± 2%  36.3s ± 2%  33.3s ± 2%  32.0s ± 3%  31.6s ± 2%  32.1s ± 2%  32.5s ± 2%  32.7s ± 2%
StdCmd/p=32  62.2s ± 1%  44.2s ± 2%  35.3s ± 2%  32.4s ± 2%  31.2s ± 2%  30.9s ± 2%  31.1s ± 2%  31.7s ± 2%  32.0s ± 2%
StdCmd/p=64  62.2s ± 1%  44.3s ± 2%  35.4s ± 2%  32.4s ± 2%  31.2s ± 2%  30.9s ± 2%  31.2s ± 2%  31.8s ± 3%  32.2s ± 3%
StdCmd/p=96  62.2s ± 2%  44.4s ± 2%  35.3s ± 2%  32.3s ± 2%  31.1s ± 2%  30.9s ± 3%  31.3s ± 2%  31.7s ± 1%  32.1s ± 2%

benchjuju, an 8 core darwin/amd64 laptop:

               c=1         c=2         c=4         c=6         c=8
BuildJuju/p=1  55.3s ± 0%  46.3s ± 0%  41.9s ± 0%  41.4s ± 1%  41.3s ± 0%
BuildJuju/p=2  33.7s ± 1%  28.4s ± 1%  26.7s ± 1%  26.6s ± 1%  26.8s ± 1%
BuildJuju/p=4  24.7s ± 1%  22.3s ± 1%  21.4s ± 1%  21.7s ± 1%  21.8s ± 1%
BuildJuju/p=6  20.6s ± 1%  19.3s ± 2%  19.4s ± 1%  19.7s ± 1%  19.9s ± 1%
BuildJuju/p=8  20.6s ± 2%  19.5s ± 2%  19.3s ± 2%  19.6s ± 1%  19.8s ± 2%

Updates #15756

Change-Id: I8a56e88953071a05eee764002024c54cd888a56c
Reviewed-on: https://go-review.googlesource.com/41819
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

CL 40693 added concurrent backend compilation support,
and used it for user-provided functions.
Autogenerated functions were still compiled serially.
This CL brings them into the fold.
As of this CL, when requested,
no functions are compiled serially.

There generally aren't many autogenerated functions.
When there are, this CL can help a lot,
because autogenerated functions are usually short.
Many short functions is the best case scenario
for concurrent compilation; see CL 41192.

One example of such a package comes from Dave Cheney's benchjuju:
github.com/juju/govmomi/vim25/types.
It has thousands of autogenerated functions.
This CL improves performance on the entire benchmark
by around a second on my machine at c=8, or about ~5%.

Updates golang#15756

Change-Id: Ia21e302b2469a9ed743df02244ec7ebde55b32f3

CL 40693 added concurrent backend compilation support,
and used it for user-provided functions.
Autogenerated functions were still compiled serially.
This CL brings them into the fold.
As of this CL, when requested,
no functions are compiled serially.

There generally aren't many autogenerated functions.
When there are, this CL can help a lot,
because autogenerated functions are usually short.
Many short functions is the best case scenario
for concurrent compilation; see CL 41192.

One example of such a package comes from Dave Cheney's benchjuju:
github.com/juju/govmomi/vim25/types.
It has thousands of autogenerated functions.
This CL improves performance on the entire benchmark
by around a second on my machine at c=8, or about ~5%.

Updates #15756

Change-Id: Ia21e302b2469a9ed743df02244ec7ebde55b32f3
Reviewed-on: https://go-review.googlesource.com/41503
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>