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Date: Sat, 16 May 2015 00:18:00 -0000
From: Rich Felker <dalias@libc.org>
To: "H.J. Lu" <hjl.tools@gmail.com>
Cc: Jan Hubicka <hubicka@ucw.cz>, Alexander Monakov <amonakov@ispras.ru>,	GCC Patches <gcc-patches@gcc.gnu.org>,	Uros Bizjak <ubizjak@gmail.com>
Subject: Re: [PATCH i386] Allow sibcalls in no-PLT PIC
Message-ID: <20150515234900.GH17573@brightrain.aerifal.cx>
References: <20150515194824.GB14415@kam.mff.cuni.cz> <CAMe9rOr3yQv-+cJxwp_YfNY=9G+FnV6=JdYh_T6kPDJtSA0fzg@mail.gmail.com> <20150515202319.GE17573@brightrain.aerifal.cx> <CAMe9rOqRBz7L6Fr1nxDVrTEh3EQ-AVV0dMCC0-xdpq87k=e4EQ@mail.gmail.com> <20150515204237.GF17573@brightrain.aerifal.cx> <CAMe9rOp1ibVzf0KOt+1boOxb5ag85fe2Rv2Rst=CgAtOTGFBxA@mail.gmail.com> <20150515230810.GA73210@kam.mff.cuni.cz> <CAMe9rOr1_r0tvi_JdsCL8w-MMAqhSeVpA6sbksnn1yP224zn_A@mail.gmail.com> <CAMe9rOpvg_PzKWQ5w0X5J0o7=52qNaJVEiKdgGDFpk70HXehbw@mail.gmail.com> <CAMe9rOqUn8iFZo=o=VPGhb=o0aRWmwzzw9Q9X5eTLBKrBOH-qw@mail.gmail.com>
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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.

Rich