[11/13] Use vec_perm_builder::series_p in shift_amt_for_vec_perm_mask

[Richard Sandiford <richard dot sandiford at linaro dot org>]

This patch makes shift_amt_for_vec_perm_mask use series_p to check
for the simple case of a natural linear series before falling back
to testing each element individually.  The series_p test works with
variable-length vectors but testing every individual element doesn't.


2017-12-09  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* optabs.c (shift_amt_for_vec_perm_mask): Try using series_p
	before testing each element individually.

Index: gcc/optabs.c
===================================================================
--- gcc/optabs.c	2017-12-09 22:48:52.266015836 +0000
+++ gcc/optabs.c	2017-12-09 22:48:56.257154317 +0000
@@ -5375,20 +5375,20 @@ vector_compare_rtx (machine_mode cmp_mod
 static rtx
 shift_amt_for_vec_perm_mask (machine_mode mode, const vec_perm_indices &sel)
 {
-  unsigned int i, first, nelt = GET_MODE_NUNITS (mode);
+  unsigned int nelt = GET_MODE_NUNITS (mode);
   unsigned int bitsize = GET_MODE_UNIT_BITSIZE (mode);
-
-  first = sel[0];
+  unsigned int first = sel[0];
   if (first >= nelt)
     return NULL_RTX;
-  for (i = 1; i < nelt; i++)
-    {
-      int idx = sel[i];
-      unsigned int expected = i + first;
-      /* Indices into the second vector are all equivalent.  */
-      if (idx < 0 || (MIN (nelt, (unsigned) idx) != MIN (nelt, expected)))
-	return NULL_RTX;
-    }
+
+  if (!sel.series_p (0, 1, first, 1))
+    for (unsigned int i = 1; i < nelt; i++)
+      {
+	unsigned int expected = i + first;
+	/* Indices into the second vector are all equivalent.  */
+	if (MIN (nelt, sel[i]) != MIN (nelt, expected))
+	  return NULL_RTX;
+      }
 
   return GEN_INT (first * bitsize);
 }