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Re: [PATCH i386] Allow sibcalls in no-PLT PIC


On Fri, May 15, 2015 at 4:49 PM, Rich Felker <dalias@libc.org> wrote:
> On Fri, May 15, 2015 at 04:34:57PM -0700, H.J. Lu wrote:
>> On Fri, May 15, 2015 at 4:30 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> > On Fri, May 15, 2015 at 4:14 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> >> My relax branch proposal works even without LTO.
>> >>
>> >
>> > I will borrow GOTPCREL from x86-64 and do
>> >
>> > [hjl@gnu-6 relax-4]$ cat b.S
>> > call *foo@GOTPCREL(%eax)
>>
>> call *foo@GOTPLT(%eax)
>>
>> is a better choice.
>
> foo@GOTPCREL is preferable (but does not yet exist for ia32, so the
> reloc type would have to be added) since it saves a useless add.
> Instead of:
>
>         call __x86.get_pc_thunk.ax
>         addl $_GLOBAL_OFFSET_TABLE_, %eax
>         call *foo@GOTPLT(%eax)
>
> you can just do:
>
>         call __x86.get_pc_thunk.ax
>         call *foo@GOTPCREL(%eax)
>
> Note that it also works to have extra instructions between:
>
>         call __x86.get_pc_thunk.ax
> 1:      ...
>         call *foo@GOTPCREL+(1b-.)(%eax)
>
> I may not have gotten the syntax quite right, but hopefully yoy get
> the idea. This same approach (with GOTPCREL) can be used for _all_ GOT
> accesses, including global data, to eliminate the useless add.
>

This is a good idea.  But I'd like to use something for both i386 and
x86-64.  I am proposing

call/jmp *foo@GOTPCRELAX+addend(%reg)

It is similar to @GOTPCREL, but with a new relax relocation.  Before
I can do that, I need to fix

https://sourceware.org/bugzilla/show_bug.cgi?id=18423

first.

-- 
H.J.


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