[Bug optimization/14483] New: More aggressive compare insn elimination

["kazu at cs dot umass dot edu" <gcc-bugzilla@gcc.gnu.org>]

Consider:

void bar_0 (void);
void bar_1 (void);

void
foo (int a)
{
  if (a == 1)
    goto L0;
  if (a == 0)
    goto L1;
  return;

 L0:
  bar_0 ();
  return;

 L1:
  bar_1 ();
  return;
}

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

foo:
	movl	4(%esp), %eax
	cmpl	$1, %eax
	je	.L2
	testl	%eax, %eax      <- sort of redundant
	je	.L4
	ret
	.p2align 2,,3
.L4:
	jmp	bar_1
	.p2align 2,,3
.L2:
	jmp	bar_0

At point where "testl" is, we still have the result of the last cmpl.
So we could do:

foo:
	movl	4(%esp), %eax
	cmpl	$1, %eax
	je	.L2
	jb	.L4          <- Notice, no "testl"!
	ret
	.p2align 2,,3
.L4:
	jmp	bar_1
	.p2align 2,,3
.L2:
	jmp	bar_0

-- 
           Summary: More aggressive compare insn elimination
           Product: gcc
           Version: tree-ssa
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P2
         Component: optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: kazu at cs dot umass dot edu
                CC: gcc-bugs at gcc dot gnu dot org
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14483