go/compile: corrupted function return value

[cznic]

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

go version go1.9.4 linux/amd64

Does this issue reproduce with the latest release?

Yes (go version devel +2010189407 Tue Feb 13 16:34:46 2018 +0000 linux/amd64)

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/jnml"
GORACE=""
GOROOT="/home/jnml/go"
GOTOOLDIR="/home/jnml/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build334481324=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

// Minimized reproducer for the (incorrect) translation of github.com/gcc-mirror/gcc/gcc/testsuite/gcc.c-torture/execute/pr70429.c

package main

func main() {
	if foo(1) != int32(0x3edae8) {
		panic(42)
	}
}

func foo(_a int32) (r int32) {
	//defer func() { println("r ", r) }() // <- Uncommenting this defer statement makes the panic go away.

	return shr1(int32(shr2(int64(0x14ff6e2207db5d1f), int(_a))), 4)
}

func shr1(n int32, m int) int32 { return n >> uint(m) }

func shr2(n int64, m int) int64 {
	if m < 0 { // <- Commenting out this if statement makes the panic go away.
		m = -m
	}
	if m >= 64 { // <- Commenting out this if statement makes the panic go away.
		return n
	}

	return n >> uint(m)
}

Playground

What did you expect to see?

No panic.

What did you see instead?

Panic.

[bradfitz]

To @randall77 for codegen.

[randall77]

Probably introduced here: https://go-review.googlesource.com/c/go/+/13641
@brtzsnr
Fix coming in a sec.

[randall77]

Possibly worth backporting to 1.9 and 1.10.
Pretty obscure though.

[gopherbot]

Change https://golang.org/cl/93716 mentions this issue: cmd/compile: fix constant folding of right shifts

[randall77]

Reopening for possible 1.10 final and 1.9.5.

[gopherbot]

Change https://golang.org/cl/94028 mentions this issue: cmd/compile: fix constant folding of right shifts on s390x

[gopherbot]

Change https://golang.org/cl/94215 mentions this issue: [release-branch.go1.10] cmd/compile: fix constant folding of right shifts

[gopherbot]

Change https://golang.org/cl/94216 mentions this issue: [release-branch.go1.10] cmd/compile: fix constant folding of right shifts on s390x

[andybons]

CL 93716 OK for Go 1.9.5
CL 94028 OK for Go 1.9.5

[gopherbot]

Change https://golang.org/cl/102775 mentions this issue: [release-branch.go1.9] cmd/compile: fix constant folding of right shifts

[gopherbot]

Change https://golang.org/cl/102777 mentions this issue: [release-branch.go1.9] cmd/compile: fix constant folding of right shifts on s390x