code review 156430044: cmd/cgo: disable clang's integrated assembler

cmd/cgo: disable clang's integrated assembler

Fixes issue 8348.

Clang's internal assembler (introduced by default in clang 3.4) understands the .arch directive, but doesn't change the default value of -march. This causes the build to fail when we use BLX (armv5 and above) when clang is compiled for the default armv4t architecture (which appears to be the default on all the distros I've used).

This is probably a clang bug, so work around it for the time being by disabling the integrated assembler when compiling the cgo assembly shim.

This CL also includes a small change to ldelf.c which was required as clang 3.4 and above generate more weird symtab entries.

[dfc, 2014-10-20 11:46:14]

Hello golang-codereviews@googlegroups.com,

I'd like you to review this change to
https://code.google.com/p/go

[iant, 2014-10-20 15:08:21]

https://codereview.appspot.com/156430044/diff/20001/src/cmd/cgo/gcc.go
File src/cmd/cgo/gcc.go (right):

https://codereview.appspot.com/156430044/diff/20001/src/cmd/cgo/gcc.go#newcod...
src/cmd/cgo/gcc.go:750: // The clang integrated assembler understands the .arch
directive
Do you know if this bug has been reported upstream to the clang maintainers?

https://codereview.appspot.com/156430044/diff/20001/src/cmd/ld/ldelf.c
File src/cmd/ld/ldelf.c (right):

https://codereview.appspot.com/156430044/diff/20001/src/cmd/ld/ldelf.c#newcod...
src/cmd/ld/ldelf.c:587: if(strncmp(sym.name, "$d.", 3) == 0) // clang 3.4+ does
this
This looks like an ARM mapping symbol.  See the code in readsym that ignores
these.  Has clang changed the name of the mapping symbol?  Should we tweak the
code in readsym instead?  Or is this something else entirely?

[dfc, 2014-10-20 20:57:20]

On 2014/10/20 15:08:21, iant wrote:
> https://codereview.appspot.com/156430044/diff/20001/src/cmd/cgo/gcc.go
> File src/cmd/cgo/gcc.go (right):
> 
>
https://codereview.appspot.com/156430044/diff/20001/src/cmd/cgo/gcc.go#newcod...
> src/cmd/cgo/gcc.go:750: // The clang integrated assembler understands the
.arch
> directive
> Do you know if this bug has been reported upstream to the clang maintainers?

No, I have not, but I should. I think I can make a very simple repro that

> 
> https://codereview.appspot.com/156430044/diff/20001/src/cmd/ld/ldelf.c
> File src/cmd/ld/ldelf.c (right):
> 
>
https://codereview.appspot.com/156430044/diff/20001/src/cmd/ld/ldelf.c#newcod...
> src/cmd/ld/ldelf.c:587: if(strncmp(sym.name, "$d.", 3) == 0) // clang 3.4+
does
> this
> This looks like an ARM mapping symbol.  See the code in readsym that ignores
> these.  Has clang changed the name of the mapping symbol?  Should we tweak the
> code in readsym instead?  Or is this something else entirely?

Thanks for the hint, this is what readelf tells me for net.a under gcc

[root@alarmcbi ~]# readelf -s go/pkg/linux_arm/net.a | grep NOTYPE | grep LOCAL
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND 
    17: 00000000     0 NOTYPE  LOCAL  DEFAULT    1 $a
    18: 00000010     0 NOTYPE  LOCAL  DEFAULT   15 $d

under clang-3.5 I see, after filtering out the .Linfo.* 's

[root@alarmcbi src]# readelf -s $HOME/go/pkg/linux_arm/net.a | grep NOTYPE |
grep LOCAL | grep -v Linfo
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND 
    17: 00000000     0 NOTYPE  LOCAL  DEFAULT    8 $d.0
    18: 00000000     0 NOTYPE  LOCAL  DEFAULT   18 $d.1
    19: 00000000     0 NOTYPE  LOCAL  DEFAULT   11 $d.2
    20: 00000000     0 NOTYPE  LOCAL  DEFAULT   13 $d.3
    21: 00000000     0 NOTYPE  LOCAL  DEFAULT    7 $d.4
    22: 00000000     0 NOTYPE  LOCAL  DEFAULT   14 $d.5
    27: 0000003f     0 NOTYPE  LOCAL  DEFAULT    8 $d.0
    28: 0000006c     0 NOTYPE  LOCAL  DEFAULT   18 $d.1
    29: 0000001e     0 NOTYPE  LOCAL  DEFAULT   11 $d.2
    30: 00000013     0 NOTYPE  LOCAL  DEFAULT   13 $d.3
    31: 0000002e     0 NOTYPE  LOCAL  DEFAULT    7 $d.4
    32: 0000001d     0 NOTYPE  LOCAL  DEFAULT   14 $d.5
    37: 00000000     0 NOTYPE  LOCAL  DEFAULT    1 $a.1
    38: 0000007e     0 NOTYPE  LOCAL  DEFAULT    8 $d.0
    39: 00000000     0 NOTYPE  LOCAL  DEFAULT   16 $d.10
    40: 0000003a     0 NOTYPE  LOCAL  DEFAULT   14 $d.11
    41: 00000000     0 NOTYPE  LOCAL  DEFAULT    3 $d.2
    42: 000000db     0 NOTYPE  LOCAL  DEFAULT   18 $d.3
    43: 0000003c     0 NOTYPE  LOCAL  DEFAULT   11 $d.4
    44: 00000026     0 NOTYPE  LOCAL  DEFAULT   13 $d.5
    45: 00000000     0 NOTYPE  LOCAL  DEFAULT   19 $d.6
    46: 00000000     0 NOTYPE  LOCAL  DEFAULT    9 $d.7
    47: 00000000     0 NOTYPE  LOCAL  DEFAULT   20 $d.8
    48: 0000005c     0 NOTYPE  LOCAL  DEFAULT    7 $d.9

[dfc, 2014-10-20 21:26:26]

Hello golang-codereviews@googlegroups.com, iant@golang.org (cc:
golang-codereviews@googlegroups.com),

Please take another look.

[iant, 2014-10-20 23:14:51]

LGTM

[dfc, 2014-10-20 23:16:03]

Thanks Ian

On Tue, Oct 21, 2014 at 10:14 AM,  <iant@golang.org> wrote:
> LGTM
>
> https://codereview.appspot.com/156430044/

[dfc, 2014-10-20 23:30:46]

*** Submitted as https://code.google.com/p/go/source/detail?r=5d69cad4faaf ***

cmd/cgo: disable clang's integrated assembler

Fixes issue 8348.

Clang's internal assembler (introduced by default in clang 3.4) understands the
.arch directive, but doesn't change the default value of -march. This causes the
build to fail when we use BLX (armv5 and above) when clang is compiled for the
default armv4t architecture (which appears to be the default on all the distros
I've used).

This is probably a clang bug, so work around it for the time being by disabling
the integrated assembler when compiling the cgo assembly shim.

This CL also includes a small change to ldelf.c which was required as clang 3.4
and above generate more weird symtab entries.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://codereview.appspot.com/156430044

[dfc, 2014-10-20 23:42:34]

http://llvm.org/bugs/show_bug.cgi?id=21319 FWIW

On Tue, Oct 21, 2014 at 10:30 AM,  <dave@cheney.net> wrote:
> *** Submitted as
> https://code.google.com/p/go/source/detail?r=5d69cad4faaf ***
>
> cmd/cgo: disable clang's integrated assembler
>
> Fixes issue 8348.
>
> Clang's internal assembler (introduced by default in clang 3.4)
> understands the .arch directive, but doesn't change the default value of
> -march. This causes the build to fail when we use BLX (armv5 and above)
> when clang is compiled for the default armv4t architecture (which
> appears to be the default on all the distros I've used).
>
> This is probably a clang bug, so work around it for the time being by
> disabling the integrated assembler when compiling the cgo assembly shim.
>
> This CL also includes a small change to ldelf.c which was required as
> clang 3.4 and above generate more weird symtab entries.
>
> LGTM=iant
> R=golang-codereviews, iant
> CC=golang-codereviews
> https://codereview.appspot.com/156430044
>
>
> https://codereview.appspot.com/156430044/

[gobot, 2014-10-21 04:31:01]

This CL appears to have broken the nacl-arm-cheney builder.
See http://build.golang.org/log/9ddd9d040b70b050da1c799df711908b0cc0b446

[dfc, 2014-10-21 04:42:17]

Not really, this builder crashed half way through the build.

On 21 Oct 2014 15:31, <gobot@golang.org> wrote:
>
> This CL appears to have broken the nacl-arm-cheney builder.
> See http://build.golang.org/log/9ddd9d040b70b050da1c799df711908b0cc0b446
>
> https://codereview.appspot.com/156430044/