[Bug target/25112] New: [m68k] Inefficient equality comparison with small integers

[kazu at gcc dot gnu dot org]

Consider:

int bar (void);

void
foo (void)
{
  int a = bar ();
  if (a == 1)
    bar ();
}

./cc1 -quiet -O2 -m5200 -fomit-frame-pointer generates

foo:
        move.l %a2,-(%sp)
        lea bar,%a2
        jbsr (%a2)
        moveq #1,%d1
        cmp.l %d0,%d1
        jbne .L4
        jbsr (%a2)
.L4:
        move.l (%sp)+,%a2
        rts

Note that

        moveq #1,%d1
        cmp.l %d0,%d1

can be replaced with

        subq.l #1,%d1

as long as the comparison is either == or !=, and %d1 dies at the comparison.

A similar optimization can be done for an equality comparison with
a negative numbers between -8 and -1, inclusive.


-- 
           Summary: [m68k] Inefficient equality comparison with small
                    integers
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: kazu at gcc dot gnu dot org
GCC target triplet: m68k-elf


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25112