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Re: [x86-64-psABI] RFC: Add R_X86_64_RELAX_PC32 and R_X86_64_RELAX_PLT32
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Jan Beulich <JBeulich at suse dot com>
- Cc: GCC Development <gcc at gcc dot gnu dot org>, "x86-64-abi at googlegroups dot com" <x86-64-abi at googlegroups dot com>, Michael Matz <matz at suse dot de>
- Date: Wed, 13 May 2015 04:02:24 -0700
- Subject: Re: [x86-64-psABI] RFC: Add R_X86_64_RELAX_PC32 and R_X86_64_RELAX_PLT32
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On Tue, May 12, 2015 at 11:22 PM, Jan Beulich <JBeulich@suse.com> wrote:
>>>> On 12.05.15 at 20:42, <hjl.tools@gmail.com> wrote:
>> Here is the updated proposal. I changed nop prefix from 0x48
>> to 0x67 and clarified how foo@GOTPCREL(%rip) should be
>> resolved.
>
> Mind clarifying how 67 is better than 48?
0x67 works for both x86-64 and i386. We can use the same byte
for the "relax" prefix.
>> I am proposing to add 2 new relocations, R_X86_64_RELAX_PC32 and
>> R_X86_64_RELAX_PLT32:
>>
>> 1. They can only be used on 32-bit direct call/jmp instructions.
>> 2. call/jmp instructions must have a 0x67 prefix, which is the address
>> size prefix and is ignored by 32-bit direct call/jmp instructions.
>
> The same could have been said several years ago about segment
> overrides used with conditional branches, yet they obtained a
> meaning (even if only affecting performance, not correctness). Is
> it anywhere publicly stated that the address size override will
> continue to be ignored?
I will ask to put it in Intel SDM.
--
H.J.