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Re: [PATCH] PR gold/18609
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Andrew Senkevich <andrew dot n dot senkevich at gmail dot com>
- Cc: Cary Coutant <ccoutant at gmail dot com>, Binutils <binutils at sourceware dot org>
- Date: Mon, 20 Jul 2015 08:30:24 -0700
- Subject: Re: [PATCH] PR gold/18609
- Authentication-results: sourceware.org; auth=none
- References: <CAMXFM3ufsJmH8trBQ6GJ5ZWBRxEpXbTTr-cDY_X=hxrkEsK=pQ at mail dot gmail dot com> <CAJimCsGgpjpAZdmwZF+kviPANMWnS_Fi6+hPtKftN82nJOLT7A at mail dot gmail dot com> <CAMXFM3tDV0tELGu6iiLop6gBO2c0S48GYTDaMet-wHXbQxGF6g at mail dot gmail dot com> <CAMe9rOrGp-KGrRyqH6TNsMJfsAOS3ze=e=O1SAOgd0GQhQqBMA at mail dot gmail dot com> <CAMXFM3vmaJNcg_Kz5tMUoR7jsJABiF2qVYRrXqg1p=qok_g5aQ at mail dot gmail dot com>
On Mon, Jul 20, 2015 at 8:24 AM, Andrew Senkevich
<andrew.n.senkevich@gmail.com> wrote:
>>> diff --git a/gold/x86_64.cc b/gold/x86_64.cc
>>> index 007af1d..8f53bfc 100644
>>> --- a/gold/x86_64.cc
>>> +++ b/gold/x86_64.cc
>>> @@ -2480,23 +2480,6 @@ Target_x86_64<size>::Scan::local(Symbol_table* symtab,
>>> // The symbol requires a GOT section.
>>> Output_data_got<64, false>* got = target->got_section(symtab, layout);
>>>
>>> - // If the relocation symbol isn't IFUNC,
>>> - // and is local, then we will convert
>>> - // mov foo@GOTPCREL(%rip), %reg
>>> - // to lea foo(%rip), %reg.
>>> - // in Relocate::relocate.
>>> - if (r_type == elfcpp::R_X86_64_GOTPCREL
>>> - && reloc.get_r_offset() >= 2
>>> - && !is_ifunc)
>>> - {
>>> - section_size_type stype;
>>> - const unsigned char* view = object->section_contents(data_shndx,
>>> - &stype, true);
>>> - if (view[reloc.get_r_offset() - 2] == 0x8b)
>>> - break;
>>> - }
>>> -
>>> -
>>> // The symbol requires a GOT entry.
>>> unsigned int r_sym = elfcpp::elf_r_sym<size>(reloc.get_r_info());
>>>
>>> @@ -2906,21 +2889,6 @@ Target_x86_64<size>::Scan::global(Symbol_table* symtab,
>>> // The symbol requires a GOT entry.
>>> Output_data_got<64, false>* got = target->got_section(symtab, layout);
>>>
>>> - // If we convert this from
>>> - // mov foo@GOTPCREL(%rip), %reg
>>> - // to lea foo(%rip), %reg.
>>> - // in Relocate::relocate, then there is nothing to do here.
>>> - if (r_type == elfcpp::R_X86_64_GOTPCREL
>>> - && reloc.get_r_offset() >= 2
>>> - && Target_x86_64<size>::can_convert_mov_to_lea(gsym))
>>> - {
>>> - section_size_type stype;
>>> - const unsigned char* view = object->section_contents(data_shndx,
>>> - &stype, true);
>>> - if (view[reloc.get_r_offset() - 2] == 0x8b)
>>> - break;
>>> - }
>>> -
>>
>> If you remove those changes, won't it generate an unused GOT slot
>> when GOTPCREL relocation is converted to PC-relative relocation?
>
> Yes, it can generate unused GOT slots.
>
I think we should add a testcase to check for the unused GOT slot.
Please check if you can implement similar heuristic in gold:
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commitdiff;h=59cab532835904f368b0aa99267afba5fda5ded2
--
H.J.