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From: "ubizjak at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/111698] New: Narrow memory access of compare to byte
 width
Date: Wed, 04 Oct 2023 20:52:34 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: target
X-Bugzilla-Version: 12.3.1
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: ubizjak at gmail dot com
X-Bugzilla-Status: UNCONFIRMED
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111698

            Bug ID: 111698
           Summary: Narrow memory access of compare to byte width
           Product: gcc
           Version: 12.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ubizjak at gmail dot com
  Target Milestone: ---

Following testcase:

--cut here--
int m;

_Bool foo (void)
{
  return m & 0x0f0000;
}
--cut here--

compiles to:

  0:   f7 05 00 00 00 00 00    testl  $0xf0000,0x0(%rip)
  7:   00 0f 00=20

The test instruction can be demoted to byte test from addr+2.

Currently, the demotion works for lowest byte, so the testcase:

--cut here--
int m;

_Bool foo (void)
{
  return m & 0x0f;
}
--cut here--

compiles to:

   0:   f6 05 00 00 00 00 0f    testb  $0xf,0x0(%rip)

which is three bytes shorter.

Any half-way modern Intel and AMD cores will forward any fully contained lo=
ad,
so there is no danger of forwarding stall with recent CPU cores.=