[PATCH] Expand PIC calls without PLT with -fno-plt

[Rich Felker]

On Mon, May 04, 2015 at 11:42:20AM -0600, Jeff Law wrote:
> On 05/04/2015 11:39 AM, Jakub Jelinek wrote:
> >On Mon, May 04, 2015 at 11:34:05AM -0600, Jeff Law wrote:
> >>On 05/04/2015 10:37 AM, Alexander Monakov wrote:
> >>>This patch introduces option -fno-plt that allows to expand calls that would
> >>>go via PLT to load the address of the function immediately at call site (which
> >>>introduces a GOT load).  Cover letter explains the motivation for this patch.
> >>>
> >>>New option documentation for invoke.texi is missing from the patch; if this is
> >>>accepted I'll be happy to send a v2 with documentation added.
> >>>
> >>>	* calls.c (prepare_call_address): Transform PLT call to GOT lookup and
> >>>	indirect call by forcing address into a pseudo with -fno-plt.
> >>>	* common.opt (flag_plt): New option.
> >>OK once you cobble together the invoke.texi changes.
> >
> >Isn't what Michael/Alan suggested better?  I mean as/ld/compiler changes to
> >inline the plt slot's first part, then lazy binding will work fine.
> I must have missed Alan/Michael's message.
> 
> ISTM the win here is that by going through the GOT, you can CSE the
> GOT reference and possibly get some more register allocation
> freedom.  Is that still the case with Alan/Michael's approach?

There are many advantages to 'going through the GOT'. CSE'ing the
reference is just one. The biggest (IMO) is that you can avoid the bad
PLT ABI that most targets have, where making a call to a PLT slot
requires the GOT address to be pre-loaded into a fixed, call-saved
register. This precludes sibcalls and forces many functions which
otherwise would not need their own stack frames to create one for
saving the old value of the GOT register. See my blog entry on the
topic here: http://ewontfix.com/18/

Anyone who really wants lazy binding can use -fplt (which is
presumably still the default; I didn't check) but lazy binding should
largely be considered deprecated anyway since effective use of relro
protection requires -z now too, in which case you're paying all the
costs (which are considerable!) for lazy binding support even though
you won't get it.

Rich