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From: "dwmw2 at infradead dot org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/55177] New: Missed optimisation: bswap, mask with constant, bswap back again.
Date: Fri, 02 Nov 2012 09:59:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55177

             Bug #: 55177
           Summary: Missed optimisation: bswap, mask with constant, bswap
                    back again.
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: dwmw2@infradead.org


extern int x;

void foo(void)
{
  int a = __builtin_bswap32(x);
  a &= 0x5a5b5c5d;
  x = __builtin_bswap32(a);
}

With GCC 4.7.2 (x86_64 Fedora) this compiles to:
foo:
.LFB0:
    .cfi_startproc
    movl    x(%rip), %eax
    bswap    %eax
    andl    $-1515936861, %eax
    bswap    %eax
    movl    %eax, x(%rip)
    ret
    .cfi_endproc


Surely the actual swap should be optimised out, and the 0x5a5b5c5d constant
mask should be swapped instead so we just load, mask with 0x5d5c5b5a and store
without any runtime swapping?

(See also PR42586 which may be related)