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cmd/compile: escape analysis miss #12588

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davecheney opened this issue Sep 11, 2015 · 4 comments
Closed

cmd/compile: escape analysis miss #12588

davecheney opened this issue Sep 11, 2015 · 4 comments
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Go1.6

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@davecheney
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package main

type A struct {
        b [3]uint64
}

func f(a A) int {
        for i, x := range &a.b { // & to avoid making a copy of a.b
                if x != 0 {
                        return 64*i + int(x)
                }
        }
        return 0
}

func main() {
        var a A
        f(a)
}

% go build -gcflags=-m escape.go
# command-line-arguments
./escape.go:7: moved to heap: a
./escape.go:8: &a.b escapes to heap

Why does taking the address of field of a local value cause the formal parameter to escape ?

/cc @dr2chase
@cznic
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cznic commented Sep 11, 2015

I'd guess int(x) causes that. At least at some (early) stage it's not distinguished from a function call.

@dr2chase dr2chase self-assigned this Sep 11, 2015
@dr2chase dr2chase added this to the Go1.6 milestone Sep 11, 2015
@dr2chase
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My guess is that we don't do the right thing for range and that escapes &a.b, and thus a nail was wanting and the kingdom falls. I'll put it on my list for after SSA coverage is done.

@gopherbot
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CL https://golang.org/cl/14501 mentions this issue.

davecheney added a commit that referenced this issue Sep 12, 2015
@davecheney
cmd/compile/internal/gc: avoid allocation in bnum
ea4e321 
Although bnum was being called with a Bits value, a limitation
of the escape analyser (#12588) meant that taking the
address of the Bits.b array in the range statement caused the
formal parameter to escape to the heap.

Passing the a pointer to a Bits, as with all the other Bits helper
methods avoids the allocation.

Before:
BenchmarkBnum1-4        20000000                69.6 ns/op            32 B/op          1 allocs/op

After:
BenchmarkBnum1-4        100000000               10.1 ns/op             0 B/op          0 allocs/op

Change-Id: I673bd57ddc032ee67d09474156d795fb1ba72018
Reviewed-on: https://go-review.googlesource.com/14501
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
@gopherbot
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CL https://golang.org/cl/15130 mentions this issue.

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@davecheney @cznic @dr2chase @gopherbot