[Bug optimization/11826] New: [ARM] Minor register allocation problem before function return

["alga at rgai dot hu" <gcc-bugzilla at gcc dot gnu dot org>]

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11826

           Summary: [ARM] Minor register allocation problem before function
                    return
           Product: gcc
           Version: 3.4
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: optimization
        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

For C functions with a return value GCC should store the result of the last
instruction just before returning it in r0 (in order to avoid using unnecessary
register moves).
The new register allocation algorithm doesn't solve this problem either.

--- c example ---
int foo (int a, int b)
{
  if(a+b == 0) return a-b;
  return b-a;
}

--- arm code ---
--- arm-elf-gcc -S -g0 -Os -o reg-alloc.s reg-alloc.c ---
foo:
 cmn r0, r1
 rsb r3, r1, r0
 rsbne r3, r0, r1
 mov r0, r3
 mov pc, lr

--- arm code with new-ra ---
--- arm-elf-gcc -S -g0 -Os -fnew-ra -o reg-alloc.s reg-alloc.c ---
foo:
 mov r3, r0
 cmn r3, r1
 rsb r0, r1, r0
 rsbne r0, r3, r1
 mov pc, lr

--- possible solution ---
foo:
   cmn r0, r1
   rsbeq r0, r1, r0
   rsbne r0, r0, r1
   mov pc, lr