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From: "linkw at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/108184] New: rs6000: Use optimize_function_for_speed_p
 too early
Date: Tue, 20 Dec 2022 03:01:59 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108184

            Bug ID: 108184
           Summary: rs6000: Use optimize_function_for_speed_p too early
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: linkw at gcc dot gnu.org
  Target Milestone: ---

In the review of patch [1] for PR105818, Honza pointed out=20

"I think we should generally avoid doing decisions about size/speed
optimizations so early since the setting may change due to attributes or
profile feedback..."

I agreed that the current uses of optimize_function_for_speed_p in function
rs6000_option_override_internal are too early and can be inaccurate. I trie=
d to
make the below test case to demonstrate it.

Compiled with -mdejagnu-tune=3Dpower8 -O3:

__attribute__ ((cold)) int
fusion_short (short *p)
{
  return p[0x12345];
}

Since the function is attributed with cold, it's considered not to optimize=
 for
speed, so we shouldn't break the sign extended loads and fuse it with the
addis, that is it's unexpected to see extsh generated but we have:

        addis 3,3,0x2
        lhz 3,18058(3)
        extsh 3,3

[1] https://gcc.gnu.org/pipermail/gcc-patches/2022-November/607527.html=