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cmd/compile/internal/ssa: incorrect deadcode elim #47594

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wdvxdr1123 opened this issue Aug 8, 2021 · 1 comment
Closed

cmd/compile/internal/ssa: incorrect deadcode elim #47594

wdvxdr1123 opened this issue Aug 8, 2021 · 1 comment

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@wdvxdr1123
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@wdvxdr1123 wdvxdr1123 commented Aug 8, 2021

What version of Go are you using (go version)?

$ go version
go version go1.17rc2 windows/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output
$ go env
set GO111MODULE=on
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\wdvxdr\AppData\Local\go-build
set GOENV=C:\Users\wdvxdr\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\wdvxdr\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\wdvxdr\go
set GOPRIVATE=
set GOPROXY=https://goproxy.cn,direct
set GOROOT=C:\Users\wdvxdr\sdk\go1.17rc2
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\Users\wdvxdr\sdk\go1.17rc2\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.17rc2
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=xxxxxxxxxx
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\wdvxdr\AppData\Local\Temp\go-build3667860290=/tmp/go-build -gno-record-gcc-switches

What did you do?

package main

import (
	"fmt"
	"time"
)

func main() {
	x := 0
	go func() {
		for {
			x++
		}
	}()
	time.Sleep(time.Second)
	fmt.Println(x)
}

What did you expect to see?

output > 0

What did you see instead?

output: 0

function main.func1 SSA:

b1:-
v1 (?) = InitMem <mem>
v2 (?) = SP <uintptr>
v3 (?) = SB <uintptr>
v4 (?) = LocalAddr <**int> {&x} v2 v1
v5 (10) = Arg <*int> {&x} (&x[*int])
v9 (?) = Const64 <int> [1]
Plain → b2 (+11)
b2: ← b1 b4-
v13 (12) = Phi <mem> v1 v11
v14 (12) = Copy <*int> v5
Plain → b3
b3: ← b2-
v6 (12) = Copy <*int> v5
v7 (12) = Copy <mem> v13
v8 (+12) = Load <int> v5 v13
v10 (12) = Add64 <int> v8 v9
v11 (12) = Store <mem> {int} v5 v10 v13
Plain → b4 (12)
b4: ← b3
Plain → b2 (12)
b5:-
v12 (14) = Unknown <mem>
Ret v12
name &x[*int]: v5 v5 v5

After deadcode elim:

b1:
Plain → b2 (+11)
b2: ← b1 b4
Plain → b3
b3: ← b2
Plain → b4 (+12)
b4: ← b3
Plain → b2 (12)
@randall77
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@randall77 randall77 commented Aug 8, 2021

This is expected. There is no synchronization in your code. It has a data race and the compiler is allowed to optimize such accesses away.
See, e.g. #45826 or #30339 .

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@randall77 randall77 closed this Aug 8, 2021
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