[Bug tree-optimization/55177] New: Missed optimisation: bswap, mask with constant, bswap back again.

[dwmw2 at infradead dot org]

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: unassig...@gcc.gnu.org

        ReportedBy: dw...@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)