gccgo: bogus warning "control reaches end of non-void function"

[xry111]

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

go version go1.9 gccgo (GCC) 8.0.0 20171114 (experimental) linux/amd64

Does this issue reproduce with the latest release?

Yes. This is gccgo in gcc trunk.

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/xry111/go"
GORACE=""
GOROOT="/opt/go"
GOTOOLDIR="/usr/lib64/gcc/x86_64-pc-linux-gnu/8.0.0"
GCCGO="/usr/bin/gccgo-8.0"
CC="gcc-8.0"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build037067240=/tmp/go-build -gno-record-gcc-switches -funwind-tables"
CXX="g++-8.0"
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?

package main

import (
	"fmt"
	"os"
)

func foo() int {
	switch os.Args[0] {
	case "cat":
		return 0
	case "ls":
		return 1
	}
	panic("wtf")
}

func main() {
	fmt.Println(foo())
}

And build it with LC_ALL=C go-8.0 build.

What did you expect to see?

No warnings and no errors.

What did you see instead?

# hw
./main.go: In function 'main.foo':
go1: warning: control reaches end of non-void function [-Wreturn-type]

I think panic() should have something like __attribute__((noreturn)) in GNU C, to suppress the warning.

[xry111]

By the way, this bogus warning spamed my terminal while GCC building system was compiling Go standard library with newly built gccgo. And even

func foo(x int) int {
    panic("not implemented")
}

could trigger this.

[xry111]

With -gccgoflags "-O" the warning gone.

[ianlancetaylor]

Thanks, I have seen this too. Seems to be an annoying change in the GCC middle-end. Since the Go frontend provides its own "missing return" warning anyhow, the simplest approach may be for the frontend to disable the middle-end warning.

[gopherbot]

Change https://golang.org/cl/80416 mentions this issue: compiler: avoid GCC middle-end control warnings