[PATCH] Fix PR56118

[Richard Biener <rguenther@suse.de>]

The following fixes PR56118 by adjusting the cost model handling of
basic-block vectorization to favor the vectorized version in case
estimated cost is the same as the estimated cost of the scalar
version.  This makes sense because we over-estimate the vectorized
cost in several places.

Bootstrapped and tested on x86_64-unknown-linux-gnu, applied.

Richard.

2015-11-10  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/56118
	* tree-vect-slp.c (vect_bb_vectorization_profitable_p): Make equal
	cost favor vectorized version.

	* gcc.target/i386/pr56118.c: New testcase.

Index: gcc/tree-vect-slp.c
===================================================================
--- gcc/tree-vect-slp.c	(revision 230020)
+++ gcc/tree-vect-slp.c	(working copy)
@@ -2317,9 +2317,12 @@ vect_bb_vectorization_profitable_p (bb_v
       dump_printf (MSG_NOTE, "  Scalar cost of basic block: %d\n", scalar_cost);
     }
 
-  /* Vectorization is profitable if its cost is less than the cost of scalar
-     version.  */
-  if (vec_outside_cost + vec_inside_cost >= scalar_cost)
+  /* Vectorization is profitable if its cost is more than the cost of scalar
+     version.  Note that we err on the vector side for equal cost because
+     the cost estimate is otherwise quite pessimistic (constant uses are
+     free on the scalar side but cost a load on the vector side for
+     example).  */
+  if (vec_outside_cost + vec_inside_cost > scalar_cost)
     return false;
 
   return true;
Index: gcc/testsuite/gcc.target/i386/pr56118.c
===================================================================
--- gcc/testsuite/gcc.target/i386/pr56118.c	(revision 0)
+++ gcc/testsuite/gcc.target/i386/pr56118.c	(working copy)
@@ -0,0 +1,17 @@
+/* { dg-do compile } */
+/* { dg-options "-O3 -msse2" } */
+
+#include <emmintrin.h>
+
+__m128d f()
+{
+  __m128d r={3,4};
+  r[0]=1;
+  r[1]=2;
+  return r;
+}
+
+/* We want to "vectorize" this to a aligned vector load from the
+   constant pool.  */
+
+/* { dg-final { scan-assembler "movapd" } } */