externally_visible and resoultion file

[Richard Guenther]

On Tue, Jun 8, 2010 at 3:01 PM, Bingfeng Mei <bmei@broadcom.com> wrote:
> Hi,
> Sorry for coming back to this issue after a while. I am still puzzled
> by this. The following are two test files:
>
> a.c
>
> #include <string.h>
> #include <stdio.h>
> extern int foo(int);
> void bar()
> {
>  printf("bar\n");
> }
> extern int src[], dst[];
> int vvvvvv;
> int main()
> {
>  int ret;
>  vvvvvv = 12;
>  ret = foo(20);
>  memcpy(dst, src, 100);
>  return ret + 3;
> }
>
> b.c
> #include <stdio.h>
> int src[100];
> int dst[100];
> extern int vvvvvv;
> extern void bar();
> int foo(int c)
> {
>  printf("Hello world: %d\n", c);
>  bar();
>  return 1000 + vvvvvv;
> }
>
> I compiled with following steps
> ~/work/install-x86/bin/gcc -flto a.c -O2 -c
> ~/work/install-x86/bin/gcc b.c -O2 -c
> ar cru libb.a b.o
> ~/work/install-x86/bin/gcc -flto a.o -L. -lb -O2 -fuse-linker-plugin -o f -fwhole-program -save-temps
>
> Since you both mentioned that resolution file is not used to replace externally_visible attribute yet,
> I expect there will be link errors regarding both vvvvvv and bar. Somehow, GCC doesn't complain at all.
> However, looking at generated assembly code, GCC produces wrong code for calling bar, and vvvvvv is
> linked correctly though.
>
> 0000000000400510 <foo>:
>  400510:       48 83 ec 08             sub    $0x8,%rsp
>  400514:       89 fe                   mov    %edi,%esi
>  400516:       31 c0                   xor    %eax,%eax
>  400518:       bf 1c 06 40 00          mov    $0x40061c,%edi
>  40051d:       e8 56 01 00 00          callq  400678 <printf@plt>
>  400522:       31 c0                   xor    %eax,%eax
>  400524:       e8 d7 fa bf ff          callq  0 <__fini_array_end>    <--------  should call bar.
>  400529:       8b 05 a1 17 00 00       mov    0x17a1(%rip),%eax        # 401cd0 <vvvvvv>
>  40052f:       48 83 c4 08             add    $0x8,%rsp
>  400533:       05 e8 03 00 00          add    $0x3e8,%eax
>  400538:       c3                      retq
>  400539:       0f 1f 80 00 00 00 00    nopl   0x0(%rax)
>
> Is this a bug? GCC should issue warning/error here. Why is vvvvvv linked correctly?
> Shouldn't it be treated as static with -fwhile-program and without externally_visible?

bar should be indeed eliminated which is what I see with not
using the linker plugin.

If you use the linker-plugin I see in the .optimized dump that
the were eliminated.  But it seems that gold picks them up
from the original objects which seem to be retained somehow.

I guess this is a bug in the linker-plugin.

Richard.

> My trunk is 160104 by the way.