[Bug pending/55987] New: Redundant constant emitted

["tilman at code-monkey dot de" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55987

             Bug #: 55987
           Summary: Redundant constant emitted
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: pending
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: tilman@code-monkey.de
            Target: arm


For this code

  uint32_t add (uint32_t x, uint32_t y)
  {
      uint32_t a, b;

      a = (x & 0x7f7f7f7f) + (y & 0x7f7f7f7f);
      b = (x ^ y) & ~0x7f7f7f7f;

      return a ^ b;
  }

gcc 4.7.2 generates the following machine code for ARMv5:

$ gcc -Wall -O2 -mtune=arm9tdmi -march=armv5te -S add.c

    ldr    r2, .L2     @ .word  2139062143
    ldr    r3, .L2+4   @ .word -2139062144
    eor    ip, r1, r0
    and    r1, r1, r2
    and    r2, r0, r2
    and    r3, ip, r3
    add    r0, r1, r2
    eor    r0, r3, r0
    bx    lr

gcc missed that we can use bic to clear the low bits, which means
we don't need that 2nd constant:

    ldr    r2, .L2     @ .word 2139062143
    eor    ip, r1, r0
    bic    ip, ip, r2  @ use bic instead of and
    and    r1, r1, r2
    and    r0, r0, r2
    add    r0, r0, r1
    eor    r0, r0, ip
    bx    lr