Re: [PATCH] Expand PIC calls without PLT with -fno-plt

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Wed, May 6, 2015 at 12:17 PM, Rich Felker <dalias@libc.org> wrote:
> On Wed, May 06, 2015 at 12:05:20PM -0700, H.J. Lu wrote:
>> >> -Bsymbolic will bind all references to local definitions in shared libraries,
>> >> with and without visibility, weak or non-weak.  Compiler can use it
>> >> in binds_tls_local_p and we can generate much better codes in shared
>> >> libraries.
>> >
>> > Yes, I'm aware of what it does. But at compile-time the compiler can't
>> > know whether the referenced symbol will be defined in the same DSO
>> > unless this is visibility annotation telling it. Even when linking a
>> > shared library using -Bsymbolic, the library code can still make calls
>> > (or data references) to symbols in other DSOs.
>>
>> Even without LTO, -fsymbolic -fPIC will generate better codes for
>>
>> ---
>> int glob_a = 1;
>>
>> int foo ()
>> {
>>   return glob_a;
>> }
>> ---
>
> I see how this case is improved, but it depends on the dubious (and
> undocumented?) behavior of -Bsymbolic breaking copy relocations.

-Bsymbolic breaks copy relocations, independent of compiler.
However, we can pass -fsymbolic when building PIE to avoid
copy relocation.  With -fsymbolic -fPIE -pie -flto, we can generate
direct reference for locally defined symbol.


>> and
>>
>> ---
>> int glob_a (void)
>> {
>>   return -1;
>> }
>>
>> int foo ()
>> {
>>   return glob_a ();
>> }
>> ---
>
> I don't see how this case is improved unless GCC is failing to
> consider strong definitions in the same TU as locally-binding. If this
> is the case, is there a reason for that behavior? IMO it's wrong.

glob_a is a strong definition.  If you have another strong definition,
you will get a linker error.


-- 
H.J.