[GOLD] Support --icf=safe with -pie for x86_64

[Rahul Chaudhry via binutils]

>> I looked (maybe not carefully enough), but I didn't find anything in
>> this call chain that restricts the check to text sections:
>>
>> gc_process_relocs
>>  -> scan.[global|local]_reloc_may_be_function_pointer
>>  -> possible_function_pointer_reloc
>>
>> You need to check the SHF_EXECINST flag before assuming you're looking
>> at an opcode. (Look in x86_64.cc for "is_executable".)
>
> I'll take a closer look at this, add a check, and send an updated patch.
>
>
>> I think you should also be checking that the target symbol is
>> STT_FUNC; that should rule out most jump table cases. (I see a check
>> for != STT_OBJECT in the local symbol path, but nothing in the global
>> symbol path. It may be the case that STT_NOTYPE is used for some
>> extern function symbols, but you want to be conservative, right?)
>
> Ditto for this one.

Please see the updated patch below. There are two updates:

* Added a check for SHF_EXECINSTR flag before checking the call/jump opcodes.

* Added a check for STT_FUNC in the global symbol path.


Also, to add some background behind this patch, we're linking a large C++
binary (chrome-browser) with -pie.

* With --icf=none, the binary size is 177M.

* With --icf=safe, before this patch, the binary size is 176M.
ICF is able to fold 16730 sections (only ctors and dtors, since
Target_x86_64::do_can_check_for_function_pointers returns
false when using -pie).

* After applying this patch, with --icf=safe, the binary size is 169M.
ICF is able to fold 55623 sections.


        * x86_64.cc (Target_x86_64::do_can_check_for_function_pointers):
        Return true even when building pie binaries.
        (Target_x86_64::possible_function_pointer_reloc): Check opcode
        for R_X86_64_PC32 relocations.
        (Target_x86_64::local_reloc_may_be_function_pointer): Pass
        extra arguments to local_reloc_may_be_function_pointer.
        (Target_x86_64::global_reloc_may_be_function_pointer): Likewise.
        * gc.h (gc_process_relocs): Add check for STT_FUNC.
        * testsuite/Makefile.am (icf_safe_pie_test): New test case.
        * testsuite/Makefile.in: Regenerate.
        * testsuite/icf_safe_pie_test.sh: New shell script.


-- 
Rahul
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