--as-needed change wrt undefined weak symbols

[H.J. Lu]

On Thu, Apr 4, 2013 at 3:39 PM, Alan Modra <amodra@gmail.com> wrote:
> On Thu, Apr 04, 2013 at 10:15:58AM -0700, H.J. Lu wrote:
>> On Sun, Mar 17, 2013 at 7:41 PM, Alan Modra <amodra@gmail.com> wrote:
>> >         PR ld/12549
>> >         elflink.c (elf_link_add_object_symbols): Exclude weak refs when
>> >         considering whether an --as-needed library is needed.
>>
>> Just for the record, this patch may change the behavior of
>> the resulting executables for
>>
>> extern void bar () __attribute__((weak));
>>
>>   if (bar)
>>     bar ();
>>
>> if bar is defined in the DT_NEEDED library.  Binutils 2.22
>> will resolve bar and add a DT_NEEDED entry.  The new
>> linker will resolve bar to 0.  We will see if it causes any
>> problems.
>
> Your testcase in http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56431
> shows that gold resolves bar to zero, but on powerpc and powerpc64 ld
> keeps bar dynamic.
>
> Hmm, I see x86_64 ld resolves bar to 0.  Isn't this just an x86_64
> backend ld bug?

For executable:

0000000000000000 <main>:
   0:	b8 00 00 00 00       	mov    $0x0,%eax	1: R_X86_64_32	bar
   5:	48 85 c0             	test   %rax,%rax
   8:	74 18                	je     22 <main+0x22>
   a:	48 83 ec 08          	sub    $0x8,%rsp
   e:	b8 00 00 00 00       	mov    $0x0,%eax
  13:	e8 00 00 00 00       	callq  18 <main+0x18>	14: R_X86_64_PC32	bar-0x4
  18:	b8 00 00 00 00       	mov    $0x0,%eax
  1d:	48 83 c4 08          	add    $0x8,%rsp
  21:	c3                   	retq
  22:	b8 00 00 00 00       	mov    $0x0,%eax
  27:	c3                   	retq

It is resolved to either 0,  if it is undefined,  or its PLT entry, if
it is defined.  Once it is resolved to 0 at link-time, change to
defined at run-time won't affect executable.  If it is resolved
to defined at link-time, change it to undefined at run-time
will lead to seg-fault.

If the executable is compiled with PIC/PIE, it works
fine.

-- 
H.J.