RE: externally_visible and resoultion file

["Bingfeng Mei" <bmei at broadcom dot com>]

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? 

My trunk is 160104 by the way.

Thanks,
Bingfeng

> -----Original Message-----
> From: Jan Hubicka [mailto:hubicka@ucw.cz]
> Sent: 27 May 2010 09:04
> To: Richard Guenther
> Cc: Bingfeng Mei; gcc@gcc.gnu.org
> Subject: Re: externally_visible and resoultion file
> 
> > On Wed, May 26, 2010 at 5:53 PM, Bingfeng Mei <bmei@broadcom.com>
> wrote:
> > > Hi, Richard,
> > > With resolution file generated by GOLD (or I am going to hack gnu
> LD), ?is
> > > externally_visible attribute still needed to annotate those symbols
> accessed
> > > from non-LTO objects when compiling with -fwhole-program.
> >
> > Yes it is.  We do not parse the complete resolution file but only the
> > entries we'll end up reading .o files with LTO information for.
> >
> > > In theory, it shouldn't be needed since LTO has all information.
> But what
> > > about current implementation. I checked relevant source files and
> can't
> > > get immediate clue.
> >
> > The current implementation has no idea of the external references
> > (though it's probably not too hard to implement).
> 
> Yep, this is also one of my TODO list items...
> 
> Honza
> >
> > Richard.