RFC: Add a new relocation to x86-64/i386 psABIs

[H.J. Lu]

On Mon, May 18, 2015 at 6:13 AM, Michael Matz <matz@suse.de> wrote:
> Hi,
>
> On Mon, 18 May 2015, H.J. Lu wrote:
>
>> To avoid indirect branch to internal functions, I am proposing to add a
>> new relocation, R_X86_64_RELAX_GOTPCREL, to x86-64 psABI:
>>
>> 1. When branching to an external function, foo, compiler may generate
>>       call/jmp *foo@GOTRELAX(%rip)
>>    which generates R_X86_64_RELAX_GOTPCREL relocation, instead of
>>       call/jmp foo[@PLT]
>> 2. When function foo is locally defined, linker converts
>>       call/jmp *foo@GOTRELAX(%rip)
>>    to
>>       nop call/jmp foo
>
> For the jmp case the nop can also be added after it, to not even disturb

Yes, we should convert it to

nop call foo/jmp foo nop

I implemented it on users/hjl/relax branch in binutils git repo.

> the insn decoder.  For calls as well of course, but there it might be
> better to have it before the call.
>

I think a nop prefix is better on call.  We won't  mandate "nop call foo"
in psABI and linker is free to use either a nop prefix or a nop suffix.

Should we move forward with it?

Thanks.

-- 
H.J.