cmd/compile: miscompilation (missing BSWAPL) in go1.8

[flyingmutant]

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

go version go1.8 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/user/go"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build043674809=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

What did you do?

https://play.golang.org/p/O9a63D2aU4:

package main

import (
        "encoding/binary"
        "fmt"
)

var (
        ch1 = make(chan int)
        ch2 = make(chan int)

        bin  = []byte("a\000\000\001")
        want = binary.BigEndian.Uint32(bin)

        c consumer = noopConsumer{}
)

type msg struct {
        code uint32
}

type consumer interface {
        consume(msg)
}

type noopConsumer struct{}

func (noopConsumer) consume(msg) {}

func init() {
        close(ch1)
}

func main() {
        var m msg
        m.code = binary.BigEndian.Uint32(bin)

        select {
        case <-ch1:
                c.consume(m)
                if m.code != want {
                        // can not use m.code here, or it will work
                        fmt.Printf("wrong msg %#v, want code 0x%x\n", m, want)
                } 
        case <-ch2:
        }
}

What did you expect to see?

Nothing.

What did you see instead?

wrong msg main.msg{code:0x1000061}, want code 0x61000001

[ALTree]

Go1.8 regression, it's not in go1.7.5.

Git bisect says: introduced in 10f7574.

[ALTree]

cc @TocarIP @randall77

[TocarIP]

Stepping through instructions in gdb shows that bswap + store are executed twice. bswap(bswap(x)) == x, so this case fails. First execution of bswap is on normal path and the second is after runtime.selectgo sets pc to the basic block containing bswap. I suspect fuse pass, which puts shits/ors into the same block as newselect/selectrecv

[gopherbot]

CL https://golang.org/cl/37376 mentions this issue.

[aclements]

Need to decide whether to move CL 37376 to the release branch or cherry pick CL 37661.

[aclements]

Created https://go-review.googlesource.com/c/38587 for the change to the release branch. @TocarIP, would you mind paring down the one for master (https://go-review.googlesource.com/c/37376) to just the test?

[gopherbot]

CL https://golang.org/cl/38587 mentions this issue.