cmd/cgo: build warning from freebsd/clang: DWARF2 only supports one section per compilation unit

[wheelcomplex]

Please answer these questions before submitting your issue. Thanks!

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

go version devel +beabd87 Tue Mar 8 06:02:15 2016 +0000 freebsd/amd64

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

GOARCH="amd64"
GOBIN="/home/rhinofly/home/ds/sandspace/go/bin.freebsd-amd64"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="freebsd"
GOOS="freebsd"
GOPATH="/home/rhinofly/home/ds/sandspace/external/:/home/rhinofly/home/ds/sandspace/golibs/"
GORACE=""
GOROOT="/home/rhinofly/home/ds/sandspace/go"
GOTOOLDIR="/home/rhinofly/home/ds/sandspace/go/pkg/tool/freebsd_amd64"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -gno-record-gcc-switches"
CXX="clang++"
CGO_ENABLED="1"

$ ifconfig -a
em0: flags=8843 metric 0 mtu 1500
    options=209b
    ether 66:30:38:38:64:30
    inet 10.236.12.201 netmask 0xffffff00 broadcast 10.236.12.255 
    nd6 options=29
    media: Ethernet autoselect (1000baseT )
    status: active
lo0: flags=8049 metric 0 mtu 16384
    options=600003
    inet6 ::1 prefixlen 128 
    inet6 fe80::1%lo0 prefixlen 64 scopeid 0x2 
    inet 127.0.0.1 netmask 0xff000000 
    nd6 options=21
    groups: lo 

$ uname -a
FreeBSD rhinofly-devel-800201.localdomain 11.0-CURRENT FreeBSD 11.0-CURRENT #0 r295683: Wed Feb 17 02:07:17 UTC 2016     root@releng2.nyi.freebsd.org:/usr/obj/usr/src/sys/GENERIC  amd64

1. What did you do?
   (Use play.golang.org to provide a runnable example, if possible.)

git clone https://github.com/golang/go.git tips-go && cd tips-go/src && ./all.bash

1. What did you expect to see?

build done without error and warning.

1. What did you see instead?

   # runtime/cgo
   runtime/cgo/gcc_amd64.S:47:1: warning: DWARF2 only supports one section per compilation unit
   .section .note.GNU-stack,"",@progbits
   

   (all others build is ok)

[ianlancetaylor]

What does clang --version print?

This kind of sounds like a bug in clang. What do you see if you compile a simple C program with the -gdwarf-2 option?

[wheelcomplex]

clang --version

FreeBSD clang version 3.7.1 (tags/RELEASE_371/final 255217) 20151225
Target: x86_64-unknown-freebsd11.0
Thread model: posix

hello.c

// Hello world
#include 
int main(void){
    printf("Hello, world!\n");
    return 0;
}

clang -gdwarf-2 hello.c
[NOTE: nothing printed]

[rsc]

@wheelcomplex, has there been a clang update on FreeBSD since the report? Did it fix this?

[rsc]

Also, does this stop the build or just print a warning?
If it's just a warning (not an error stopping the build), we can easily postpone it to Go 1.8.
Thanks.

[wheelcomplex]

@rsc

Re-try in a new snapshot version of FreeBSD 11(r297692) and most recent tip(+ cdcb827), the warning generated as before.

It only warning when CGO_ENABLED="1" and not an error stopping the build, postpone it to Go 1.8 would be OK.

clang -version:

FreeBSD clang version 3.8.0 (tags/RELEASE_380/final 262564) (based on LLVM 3.8.0)
Target: x86_64-unknown-freebsd11.0
Thread model: posix
InstalledDir: /usr/bin

uname -a:

FreeBSD n550jk.localdomain 11.0-CURRENT FreeBSD 11.0-CURRENT #0 r297692: Fri Apr  8 03:07:13 UTC 2016     root@releng2.nyi.freebsd.org:/usr/obj/usr/src/sys/GENERIC  amd64

[ianlancetaylor]

I can recreate the warning with clang 3.6:

foo:
    ret

.section .note.GNU-stack,"",@progbits

clang -c -gdwarf-2 ~/foo.s
foo.s:4:1: warning: DWARF2 only supports one section per compilation unit
.section .note.GNU-stack,"",@progbits

But what I don't know is where the -gdwarf-2 option is coming from. I don't see it being passed to the assembler myself. Do you have CGO_CFLAGS set in your environment?

[rsc]

@ianlancetaylor, -gdwarf-2 is set by both cmd/cgo and cmd/link. I think the original motivation was to avoid some dwarf extensions we didn't understand. We could try changing it to -gdwarf and see what happens. There's certainly some dwarf 4 code in debug/dwarf now, with a comment:

// New in DWARF 4, but clang can generate them with -gdwarf-2.

:-)

[rsc]

Since this is only a warning, let's leave it for now and address this in Go 1.8.