Implement constant-folding simplifications of reductions.

This patch addresses a code quality regression in GCC 12 by implementing
some constant folding/simplification transformations for REDUC_PLUS_EXPR
in match.pd.  The motivating example is gcc.dg/vect/pr89440.c which with
-O2 -ffast-math (with vectorization now enabled) gets optimized to:

float f (float x)
{
  vector(4) float vect_x_14.11;
  vector(4) float _2;
  float _32;

  _2 = {x_9(D), 0.0, 0.0, 0.0};
  vect_x_14.11_29 = _2 + { 1.0e+1, 2.6e+1, 4.2e+1, 5.8e+1 };
  _32 = .REDUC_PLUS (vect_x_14.11_29); [tail call]
  return _32;
}

With these proposed new transformations, we can simplify the
above code even further.

float f (float x)
{
  float _32;
  _32 = x_9(D) + 1.36e+2;
  return _32;
}

[which happens to match what we'd produce with -fno-tree-vectorize,
and with GCC 11].

2022-02-22  Roger Sayle  <roger@nextmovesoftware.com>
	    Richard Biener  <rguenther@suse.de>

gcc/ChangeLog
	* fold-const.cc (ctor_single_nonzero_element): New function to
	return the single non-zero element of a (vector) constructor.
	* fold-const.h (ctor_single_nonzero_element): Prototype here.
	* match.pd (reduc (constructor@0)): Simplify reductions of a
	constructor containing a single non-zero element.
	(reduc (@0 op VECTOR_CST) ->  (reduc @0) op CONST): Simplify
	reductions of vector operations of the same operator with
	constant vector operands.

gcc/testsuite/ChangeLog
	* gcc.dg/fold-reduc-1.c: New test case.

1 files changed, 20 insertions, 0 deletions

diff --git a/gcc/fold-const.cc b/gcc/fold-const.cc
index 7b21240..39a5a52 100644
--- a/gcc/fold-const.cc
+++ b/gcc/fold-const.cc
@@ -16792,6 +16792,26 @@ address_compare (tree_code code, tree type, tree op0, tree op1,
   return equal;
 }
 
+/* Return the single non-zero element of a CONSTRUCTOR or NULL_TREE.  */
+tree
+ctor_single_nonzero_element (const_tree t)
+{
+  unsigned HOST_WIDE_INT idx;
+  constructor_elt *ce;
+  tree elt = NULL_TREE;
+
+  if (TREE_CODE (t) != CONSTRUCTOR)
+    return NULL_TREE;
+  for (idx = 0; vec_safe_iterate (CONSTRUCTOR_ELTS (t), idx, &ce); idx++)
+    if (!integer_zerop (ce->value) && !real_zerop (ce->value))
+      {
+	if (elt)
+	  return NULL_TREE;
+	elt = ce->value;
+      }
+  return elt;
+}
+
 #if CHECKING_P
 
 namespace selftest {