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cmd/compile: xdcl_list / testdclstack slow when there are lots of decls #14781

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josharian opened this issue Mar 11, 2016 · 3 comments
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cmd/compile: xdcl_list / testdclstack slow when there are lots of decls #14781

josharian opened this issue Mar 11, 2016 · 3 comments
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Go1.7

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@josharian
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$ go get github.com/davecheney/genpkg
$ genpkg -n 50000 > x.go
$ go tool compile -cpuprofile=cpu.prof -d=ssa/check/off x.go  # slow!
$ go tool pprof -pdf `go tool -n compile` cpu.prof > cpu.pdf

testdclstack shows up as ~20% of the cpu time, with xdcl_list one frame up.

I quick look suggests that there may be an O(n^2) interaction between xdcl_list and testdclstack--it looks like xdcl_list adds decls one at a time and calls testdclstack for the entire stack each time. (Could testdclstack be called after the loop instead?)

I think that @mdempsky or @griesemer may be looking into this (and I am not); just filing an issue so we don't forget about it.

cc @davecheney -- thanks for the test case :)
@josharian josharian added this to the Go1.7 milestone Mar 11, 2016
@griesemer
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testdclstack is a consistency check only - fine to disable (and only run in a special debug mode in the compiler)

In the long run we should replace this sequential list and use a proper block structure. I've started working on this yesterday but hit some roadblocks.

@mdempsky
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I'm curious if the problem is that declare is adding PFUNC declarations (i.e., global function declarations) to dclstack, rather than adding them to externdecl like it already does for PEXTERN declarations (i.e., global variable, const, type declarations). There shouldn't be any need for us to track package scoped symbols in dclstack.

@mdempsky mdempsky self-assigned this Mar 11, 2016
@bradfitz bradfitz mentioned this issue Mar 15, 2016
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@josharian josharian mentioned this issue Jan 11, 2017
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CL https://golang.org/cl/35113 mentions this issue.

gopherbot pushed a commit that referenced this issue Jan 11, 2017
@josharian
cmd/compile: hide testdclstack behind debug flag
f65abf6 
This reduces compilation time for the program
in #18602 from 7 hours to 30 min.

Updates #14781
Updates #18602

Change-Id: I3c4af878a08920e6373d3b3b0c4453ee002e32eb
Reviewed-on: https://go-review.googlesource.com/35113
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
@golang golang locked and limited conversation to collaborators Jan 11, 2018
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