diff mbox

Fix PR55152

Message ID alpine.LSU.2.11.1609281614110.26629@t29.fhfr.qr
State New
Headers show

Commit Message

Richard Biener Sept. 28, 2016, 2:14 p.m. UTC 
The following adds a pattern to optimize MAX (a, -a) to abs (a).

Bootstrap / regtest pending on x86_64-unknown-linux-gnu.

Richard.

2016-09-28  Richard Biener  <rguenther@suse.de>

	PR middle-end/55152
	* match.pd: Add max(a,-a) -> abs(a) pattern.

	* gcc.dg/pr55152.c: New testcase.

Comments

Joseph Myers Sept. 28, 2016, 5:32 p.m. UTC | #1 
On Wed, 28 Sep 2016, Richard Biener wrote:

> Index: gcc/testsuite/gcc.dg/pr55152.c
> ===================================================================
> --- gcc/testsuite/gcc.dg/pr55152.c	(revision 0)
> +++ gcc/testsuite/gcc.dg/pr55152.c	(working copy)
> @@ -0,0 +1,13 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O -ffinite-math-only -fstrict-overflow -fdump-tree-optimized" } */
> +
> +double g (double a)
> +{
> +  return (a>=-a)?a:-a;

You should need -fno-signed-zeros for this (that is, for the 
transformation to MAX_EXPR), not -ffinite-math-only.  For a == -0, that 
function should return -0, but abs would produce +0.
Richard Biener Sept. 29, 2016, 7:24 a.m. UTC | #2 
On Wed, 28 Sep 2016, Joseph Myers wrote:

> On Wed, 28 Sep 2016, Richard Biener wrote:
> 
> > Index: gcc/testsuite/gcc.dg/pr55152.c
> > ===================================================================
> > --- gcc/testsuite/gcc.dg/pr55152.c	(revision 0)
> > +++ gcc/testsuite/gcc.dg/pr55152.c	(working copy)
> > @@ -0,0 +1,13 @@
> > +/* { dg-do compile } */
> > +/* { dg-options "-O -ffinite-math-only -fstrict-overflow -fdump-tree-optimized" } */
> > +
> > +double g (double a)
> > +{
> > +  return (a>=-a)?a:-a;
> 
> You should need -fno-signed-zeros for this (that is, for the 
> transformation to MAX_EXPR), not -ffinite-math-only.  For a == -0, that 
> function should return -0, but abs would produce +0.

This means that tree-ssa-phiopt.c has a bug:

static bool
minmax_replacement (basic_block cond_bb, basic_block middle_bb,
                    edge e0, edge e1, gimple *phi,
                    tree arg0, tree arg1)
{
...
  /* The optimization may be unsafe due to NaNs.  */
  if (HONOR_NANS (type))
    return false;

and it should check HONOR_SIGNED_ZEROS as well.

I'll fix that as a followup.

Thanks,
Richard.
diff mbox

Patch

Index: gcc/match.pd
===================================================================
--- gcc/match.pd	(revision 240565)
+++ gcc/match.pd	(working copy)
@@ -1271,6 +1284,13 @@  DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
 (simplify
  (max:c (min:c @0 @1) @1)
  @1)
+/* max(a,-a) -> abs(a).  */
+(simplify
+ (max:c @0 (negate @0))
+ (if (TREE_CODE (type) != COMPLEX_TYPE
+      && (! ANY_INTEGRAL_TYPE_P (type)
+	  || TYPE_OVERFLOW_UNDEFINED (type)))
+  (abs @0)))
 (simplify
  (min @0 @1)
  (switch
Index: gcc/testsuite/gcc.dg/pr55152.c
===================================================================
--- gcc/testsuite/gcc.dg/pr55152.c	(revision 0)
+++ gcc/testsuite/gcc.dg/pr55152.c	(working copy)
@@ -0,0 +1,13 @@ 
+/* { dg-do compile } */
+/* { dg-options "-O -ffinite-math-only -fstrict-overflow -fdump-tree-optimized" } */
+
+double g (double a)
+{
+  return (a>=-a)?a:-a;
+}
+int f(int a)
+{
+  return (a>=-a)?a:-a;
+}
+
+/* { dg-final { scan-tree-dump-times "ABS_EXPR" 2 "optimized" } } */