forwprop: Tweak choice of VEC_PERM_EXPR filler [PR92822]

For the 2s failures in the PR, we have a V4SF VEC_PERM_EXPR in
which the first two elements are duplicates of one element and
the other two are don't-care:

    v4sf_b = VEC_PERM_EXPR <v4sf_a, v4sf_a, { 1, 1, ?, ? }>;

The heuristic was to extend this with a blend:

    v4sf_b = VEC_PERM_EXPR <v4sf_a, v4sf_a, { 1, 1, 2, 3 }>;

but it seems better to extend a partial duplicate to a full duplicate:

    v4sf_b = VEC_PERM_EXPR <v4sf_a, v4sf_a, { 1, 1, 1, 1 }>;

Obviously this is still just a heuristic though.

I wondered whether to restrict this to two elements or more
but couldn't find any examples in which it made a difference.
Either way should be fine for the purposes of fixing this PR.

2020-01-28  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/92822
	* tree-ssa-forwprop.c (simplify_vector_constructor): When filling
	out the don't-care elements of a vector whose significant elements
	are duplicates, make the don't-care elements duplicates too.

2 files changed, 23 insertions, 6 deletions

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 8507c2e..8f5ec70 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,5 +1,12 @@
 2020-01-28  Richard Sandiford  <richard.sandiford@arm.com>
 
+	PR tree-optimization/92822
+	* tree-ssa-forwprop.c (simplify_vector_constructor): When filling
+	out the don't-care elements of a vector whose significant elements
+	are duplicates, make the don't-care elements duplicates too.
+
+2020-01-28  Richard Sandiford  <richard.sandiford@arm.com>
+
 	PR tree-optimization/93434
 	* tree-predcom.c (split_data_refs_to_components): Record which
 	components have had aliasing loads removed.  Prevent store-store
diff --git a/gcc/tree-ssa-forwprop.c b/gcc/tree-ssa-forwprop.c
index d63e87c..5203891 100644
--- a/gcc/tree-ssa-forwprop.c
+++ b/gcc/tree-ssa-forwprop.c
@@ -2455,16 +2455,26 @@ simplify_vector_constructor (gimple_stmt_iterator *gsi)
 	 it and its source indexes to make the permutation supported.
 	 For now it mimics a blend.  */
       vec_perm_builder sel (refnelts, refnelts, 1);
+      bool all_same_p = true;
       for (i = 0; i < elts.length (); ++i)
-	sel.quick_push (elts[i].second + elts[i].first * refnelts);
+	{
+	  sel.quick_push (elts[i].second + elts[i].first * refnelts);
+	  all_same_p &= known_eq (sel[i], sel[0]);
+	}
       /* And fill the tail with "something".  It's really don't care,
          and ideally we'd allow VEC_PERM to have a smaller destination
-	 vector.  As heuristic try to preserve a uniform orig[0] which
-	 facilitates later pattern-matching VEC_PERM_EXPR to a
-	 BIT_INSERT_EXPR.  */
+	 vector.  As a heuristic:
+
+	 (a) if what we have so far duplicates a single element, make the
+	     tail do the same
+
+	 (b) otherwise preserve a uniform orig[0].  This facilitates
+	     later pattern-matching of VEC_PERM_EXPR to a BIT_INSERT_EXPR.  */
       for (; i < refnelts; ++i)
-	sel.quick_push ((elts[0].second == 0 && elts[0].first == 0
-			 ? 0 : refnelts) + i);
+	sel.quick_push (all_same_p
+			? sel[0]
+			: (elts[0].second == 0 && elts[0].first == 0
+			   ? 0 : refnelts) + i);
       vec_perm_indices indices (sel, orig[1] ? 2 : 1, refnelts);
       if (!can_vec_perm_const_p (TYPE_MODE (perm_type), indices))
 	return false;