From: Steve Ellcey <sje@cup.hp.com>
To: paul.winalski@intel.com, gcc@gcc.gnu.org, rth@redhat.com
Subject: RE: gcc and the IA64 ABI
Date: Fri, 23 May 2003 20:38:00 -0000 [thread overview]
Message-ID: <200305232036.NAA03842@hpsje.cup.hp.com> (raw)
> > But eliminating caller gp save/restore
> > seems to be promising. Early testing seems to indicate significant
> > improvement on some important programs; we're still in the process of
> > collecting performance data.
>
> I wouldn't have suspected that this would produce that much of a win,
> given that restoring the gp is so cheap; just a single mov instruction.
I wonder if the speed up is you are seeing is due to a side-effect of
how the GP is saved and restored. GCC on IA64 was (is?) using a single
register to save/restore gp and it always used the same register. This
made otherwise loop invariant function calls (like integer division with
loop invariant arguments) look like they were never loop-invariant and
could not be moved out of the loop, especially when there were multiple
divisions (all loop invariant) in one loop. See
http://gcc.gnu.org/ml/gcc-patches/2002-03/msg00179.html for a change I
proposed that allowed loop invariant code motion of calls, this change
was not checked in and I believe this situation still exists in 3.3. It
may have been fixed on the top-of-tree as the way calls are expanded has
been changed and the routine ia64_gp_save_reg no longer exists. See
http://gcc.gnu.org/ml/gcc-patches/2003-03/msg01196.html for where that
was removed by Richard Henderson.
Steve Ellcey
sje@cup.hp.com
next reply other threads:[~2003-05-23 20:36 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-05-23 20:38 Steve Ellcey [this message]
-- strict thread matches above, loose matches on Subject: below --
2003-05-28 15:12 Winalski, Paul
[not found] <A5974D8E5F98D511BB910002A50A6647065013B8@hdsmsx103.hd.intel.com>
2003-05-23 19:09 ` Richard Henderson
2003-05-23 18:41 Winalski, Paul
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