gccgo: struct comparison with blank field could be wrong

[melonedo]

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

$ go version
go version go1.18.2 linux/amd64

Does this issue reproduce with the latest release?

Yes, tested on GCC 12.

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/melonedo/.cache/go-build"
GOENV="/home/melonedo/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/melonedo/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/melonedo/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.18.2"
GCCGO="/usr/bin/gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/melonedo/fuzzTest/go.mod"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build557955963=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Run go run -compiler gccgo issue.go on code:

// MWE from signature fuzzer
package main

import "fmt"

type StructF11S1 struct {
        _ int16
        F1 int16
}

//go:noinline
func compare(a *StructF11S1, b *StructF11S1) bool {
        return *a == *b
}

func main() {
        a := StructF11S1{1, 2}
        fmt.Println(compare(&a, &a))
}

What did you expect to see?

true should be printed.

What did you see instead?

false is printed.

Run gccgo -issue.go -fgo-dump-ast, we can find that *a and *b are not referenced at all:

.main.compare(.main.a (g._2main_0StructF11S1),.main.b (g._2main_0StructF11S1)) (.main.$ret0 (bool)) : (g.func_8_2main_0StructF11S1_3_2main_0StructF11S1_9_8bool_9)
{
  tmp.66002704 (main.StructF11S1) // struct-equal2.go:12
  tmp.66089344 (main.StructF11S1) // struct-equal2.go:12
  {
    {
      tmp.66109760 (bool) = true // struct-equal2.go:12
      if tmp.66109760 // struct-equal2.go:12
      {
        tmp.66109760 = (tmp.66002704.1 == tmp.66089344.1)  // struct-equal2.go:12
      }
    }
    tmp.65273920 = tmp.66109760 // struct-equal2.go:12
    $ret0 = tmp.65273920 // struct-equal2.go:12
    return  // struct-equal2.go:12
  }
}

A fix has been proposed at https://go-review.googlesource.com/c/gofrontend/+/431736.

[gopherbot]

Change https://go.dev/cl/431736 mentions this issue: gofrontend: fix struct comparison with blank field