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From: "crazylht at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/108938] Missing bswap detection
Date: Mon, 06 Mar 2023 09:52:02 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108938

--- Comment #5 from Hongtao.liu <crazylht at gmail dot com> ---
(In reply to Jakub Jelinek from comment #4)
> And perhaps next to rotate it could try some left or right (logical) shift
> too.

Yes, we have bit mask in bswap detection, left or right (logical) shift can=
 be
reprensented as special bit mask + rotate.

I'm cooking a patch, for shift cases, it looks like gimple doesn't simplify=
 bit
mask + rotate to shift. .i.e.

unsigned
foo (unsigned int a)
{
  unsigned int b =3D a & 0x00FFFFFF;
  unsigned int c =3D ((b & 0x000000FF) << 8
                    | (b & 0x0000FF00) << 8
                    | (b & 0x00FF0000) << 8
                    | (b & 0xFF000000) >> 24);
  return c;
}

gcc generates

foo:
        mov     eax, edi
        and     eax, 16777215
        rol     eax, 8
        ret

But it can be optimized as below

foo:                                    # @foo
        mov     eax, edi
        shl     eax, 8
        ret=