[Bug target/25128] New: [m68k] Suboptimal comparisons against 65536

[kazu at gcc dot gnu dot org]

Consider:

unsigned int bar (void);

void
foo (void)
{
  unsigned int a = bar ();
  if (65536 > a)
    bar ();
}

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

foo:
        move.l %a2,-(%sp)
        lea bar,%a2
        jbsr (%a2)
        cmp.l #65535,%d0
        jbhi .L10
        jbsr (%a2)
.L10:
        move.l (%sp)+,%a2
        rts

Note that

        cmp.l #65535,%d0
        jbhi .L10

can be replaced with

        swap %d0
        tst.w %d0
        jbne .L10

A similar trick can be applied to signed comparisons as well.


-- 
           Summary: [m68k] Suboptimal comparisons against 65536
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          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=25128