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.to...@gmail.com> wrote:
> > On Fri, May 15, 2015 at 4:14 PM, H.J. Lu <hjl.to...@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.
Rich
