cmd/go: when building gccgo, GOPKGPATH encoding does not match pkgpath encoding

[paleozogt]

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

$ go version
go version go1.12.2 gccgo (Ubuntu 9.2.1-9ubuntu2) 9.2.1 20191008 linux/amd64

Does this issue reproduce with the latest release?

I'm using gccgo-9, which I think is the latest release.

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

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/apollo/.gocache"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/apollo/Development/weird-gopath"
GOPROXY=""
GORACE=""
GOROOT="/usr"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/gcc/x86_64-linux-gnu/9"
GCCGO="/usr/bin/x86_64-linux-gnu-gccgo-9"
CC="x86_64-linux-gnu-gcc-9"
CXX="x86_64-linux-gnu-g++-9"
CGO_ENABLED="1"
GOMOD=""
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-build492198966=/tmp/go-build -gno-record-gcc-switches -funwind-tables"

What did you do?

Code: https://play.golang.org/p/CwSEukNuiAg

// main.go

package main

import "fmt"
import "golang.org/x/tools/container/intsets"

func main() {
	var foo intsets.Sparse
	fmt.Println(foo)
}

Building with regular go works:

> go version
go version go1.12.2 linux/amd64
> CGO_ENABLED=1 go build -v -i -o foo
> ./foo
{{0 [0 0 0 0] <nil> <nil>}}

while building with gccgo fails:

> go version
go version go1.12.2 gccgo (Ubuntu 9.2.1-9ubuntu2) 9.2.1 20191008 linux/amd64
> CGO_ENABLED=1 go build -v -i -o foo
github.com/paleozogt/golangtoolstest
# github.com/paleozogt/golangtoolstest
/usr/bin/ld: /home/apollo/Development/weird-gopath/pkg/gccgo_linux_amd64/golang.org/x/tools/container/libintsets.a(_go_.o): in function `golang.org..z2fx..z2ftools..z2fcontainer..z2fintsets.block.len':
/home/apollo/Development/weird-gopath/src/golang.org/x/tools/container/intsets/sparse.go:137: undefined reference to `golang.org..z2fx..z2ftools..z2fcontainer..z2fintsets.popcount'
/usr/bin/ld: /home/apollo/Development/weird-gopath/pkg/gccgo_linux_amd64/golang.org/x/tools/container/libintsets.a(_go_.o): in function `intsets.nlz':
/home/apollo/Development/weird-gopath/src/golang.org/x/tools/container/intsets/util.go:51: undefined reference to `golang.org..z2fx..z2ftools..z2fcontainer..z2fintsets.popcount'
collect2: error: ld returned 1 exit status

What did you expect to see?

Build success.

What did you see instead?

Build failure.

[paleozogt]

What's odd is that there's gccgo code in golang.org/x/tools/container/intsets, but it doesn't seem to be getting hooked up.

Running nm shows this

> nm pkg/gccgo_linux_amd64/golang.org/x/tools/container/libintsets.a | grep popcount
                 U golang.org..z2fx..z2ftools..z2fcontainer..z2fintsets.popcount
0000000000000000 T golang_org_x_tools_container_intsets.popcount
                 U __popcountdi2

so the function definition is there.

That naming is coming from an ASM label in popcnt_gccgo_c.c:

extern intptr_t popcount(uintptr_t x) __asm__(GOSYM_PREFIX GOPKGPATH ".popcount");

It appears that gccgo isn't following this naming convention? I'm not sure where this sort of thing is documented.

At any rate, the problem can be fixed by commenting out the ASM labelling and switching to //extern in popcnt_gccgo.go:

//extern popcount
func popcount(x word) int

[ianlancetaylor]

It's a bug in the go tool. The GOPKGPATH macro is supposed to be defined to the encoding of the pkgpath. But when we changed the pkgpath encoding in https://golang.org/cl/135455 and https://golang.org/cl/200838 we didn't update the go tool accordingly.

CC @thanm