[Bug target/11825] New: [ARM] Redundant move in some cases (dead code before return)

[alga at rgai dot hu]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11825

           Summary: [ARM] Redundant move in some cases (dead code before
                    return)
           Product: gcc
           Version: 3.4
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: alga at rgai dot hu
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: arm-unknown-elf

In some cases before the function return GCC generates a redundant mov defining
r0 just before another instruction defines r0 again. The first (dead) mov is
unnecessary (see the example).

--- c example ---
int foo (int a, int x)
{
  long long b,c;
  b = a << 16;
  c = x << 8;
  b += c;
  return (int)(b >> 32);
}

--- arm code ---
foo:
  mov	r1, r1, asl #8
  mov	r2, r1, asr #31
  mov	r0, r0, asl #16
  stmfd	sp!, {r4, lr}
  adds	r3, r1, r0
  adc	r4, r2, r0, asr #31
  mov	r1, r4
  mov	r0, r3  <- OLD
  mov	r0, r1
  ldmfd	sp!, {r4, pc}

--- possible solution ---
foo:
  mov	r1, r1, asl #8
  mov	r2, r1, asr #31
  mov	r0, r0, asl #16
  stmfd	sp!, {r4, lr}
  adds	r3, r1, r0
  adc	r4, r2, r0, asr #31
  mov	r1, r4
  mov	r0, r1
  ldmfd	sp!, {r4, pc}