gccgo: zero padding rule in runtime.structToFFI is out of sync

[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:

package main

import "fmt"
import "reflect"

type Empty struct {
}

type StillEmpty struct {
  x Empty
}

type LastBlankEmpty struct {
  a int64
  b int64
  _ Empty
}

//go:noinline
func check1(a StillEmpty, b int64) bool {
	return b == 1
}

//go:noinline
func check2(x LastBlankEmpty) bool {
	return x.a == 1 && x.b == 2
}

func main() {
  empty := StillEmpty{}
  r1 := check1(empty, 1)
  r2 := reflect.ValueOf(check1).Call([]reflect.Value{reflect.ValueOf(empty), reflect.ValueOf(int64(1))})[0].Interface().(bool)
  fmt.Println(r1, r2)
  x := LastBlankEmpty{a: 1, b: 2}
  r3 := check2(x)
  r4 := reflect.ValueOf(check2).Call([]reflect.Value{reflect.ValueOf(x)})[0].Interface().(bool)
  fmt.Println(r3, r4)
}

What did you expect to see?

true true
true true

What did you see instead?

true false
true false

A fix has been proposed at https://go-review.googlesource.com/c/gofrontend/+/431735.

[gopherbot]

Change https://go.dev/cl/431735 mentions this issue: gofrontend: synchronize empty struct handling

[melonedo]

Fixed, thanks!