[Bug rtl-optimization/14483] More aggressive compare insn elimination

[Bug rtl-optimization/14483] More aggressive compare insn elimination

[steven at gcc dot gnu.org]

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

Steven Bosscher <steven at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|                            |4.8.0

--- Comment #4 from Steven Bosscher <steven at gcc dot gnu.org> 2012-10-25 23:36:13 UTC ---
(In reply to comment #2)
> The problem with extending cse_condition_code_reg() in cse.c is that
> we have to determine the result of "testl", (reg 17) dies at the conditional
> jump insn, but cse_condition_code_reg() is run before the flow analysis.
> So we don't have liveness information.  If this is to be implemented,
> we may have to do this after the combine or something.

This is not a problem anymore since the DF merge. Liveness is now
available (and used) in all CSE passes.

[Bug tree-optimization/14483] More aggressive compare insn elimination

[pinskia at gcc dot gnu.org]

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

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|rtl-optimization            |tree-optimization
                 CC|                            |pinskia at gcc dot gnu.org

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
What we could do in isel pass:
find a == 0 (in gimple_cond or in gimple_assign).
for gimple_cond see if either branch only has a == 1 in it. and change `a == 0`
to `(unsigned)a < 1`

This can be a generic part of isel even.
And then the RTL passes should be able to remove the comparison.

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

[kazu at cs dot umass dot edu]

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

[Bug rtl-optimization/14483] More aggressive compare insn elimination

[kazu at cs dot umass dot edu]

------- Additional Comments From kazu at cs dot umass dot edu  2005-01-31 14:28 -------
*** Bug 19702 has been marked as a duplicate of this bug. ***

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |yuri at tsoft dot com


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