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From: "ktkachov at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/110039] New: FAIL: gcc.target/aarch64/rev16_2.c
 scan-assembler-times rev16\\tw[0-9]+ 2
Date: Tue, 30 May 2023 12:49:21 +0000
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X-Bugzilla-Keywords: missed-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110039

            Bug ID: 110039
           Summary: FAIL: gcc.target/aarch64/rev16_2.c
                    scan-assembler-times rev16\\tw[0-9]+ 2
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ktkachov at gcc dot gnu.org
  Target Milestone: ---
            Target: aarch64

I think after g:d8545fb2c71683f407bfd96706103297d4d6e27b the test regresses=
 on
aarch64.
We now generate:
__rev16_32_alt:
        rev     w0, w0
        ror     w0, w0, 16
        ret

__rev16_32:
        rev     w0, w0
        ror     w0, w0, 16
        ret

whereas before it was:
__rev16_32_alt:
        rev16   w0, w0
        ret

__rev16_32:
        rev16   w0, w0
        ret

I think the GIMPLE at expand time is better and the RTL that it tries to ma=
tch
is simpler:
Failed to match this instruction:
(set (reg:SI 95)
    (rotate:SI (bswap:SI (reg:SI 96))
        (const_int 16 [0x10])))

So maybe it's simply a matter of adding that pattern to aarch64.md.

Anyway, filing this here to track the regression=