Use vec_perm_builder::series_p in shift_amt_for_vec_perm_mask

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.

2018-01-02  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* optabs.c (shift_amt_for_vec_perm_mask): Try using series_p
	before testing each element individually.
	* tree-vect-generic.c (lower_vec_perm): Likewise.

From-SVN: r256099

gcc/ChangeLog

 2018-01-02  Richard Sandiford  <richard.sandiford@linaro.org>
 
+	* optabs.c (shift_amt_for_vec_perm_mask): Try using series_p
+	before testing each element individually.
+	* tree-vect-generic.c (lower_vec_perm): Likewise.
+
+2018-01-02  Richard Sandiford  <richard.sandiford@linaro.org>
+
 	* selftest.h (selftest::vec_perm_indices_c_tests): Declare.
 	* selftest-run-tests.c (selftest::run_tests): Call it.
 	* vector-builder.h (vector_builder::operator ==): New function.

gcc/optabs.c

@@ -5395,20 +5395,20 @@ vector_compare_rtx (machine_mode cmp_mode, enum tree_code tcode,
 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_shift_amount (mode, first * bitsize);
 }

gcc/tree-vect-generic.c

@@ -1320,15 +1320,20 @@ lower_vec_perm (gimple_stmt_iterator *gsi)
 	  && indices[0]
 	  && indices[0] < elements)
 	{
-	  for (i = 1; i < elements; ++i)
+	  bool ok_p = indices.series_p (0, 1, indices[0], 1);
+	  if (!ok_p)
 	    {
-	      unsigned int expected = i + indices[0];
-	      /* Indices into the second vector are all equivalent.  */
-	      if (MIN (elements, (unsigned) indices[i])
-		  != MIN (elements, expected))
- 		break;
+	      for (i = 1; i < elements; ++i)
+		{
+		  unsigned int expected = i + indices[0];
+		  /* Indices into the second vector are all equivalent.  */
+		  if (MIN (elements, (unsigned) indices[i])
+		      != MIN (elements, expected))
+		    break;
+		}
+	      ok_p = i == elements;
 	    }
-	  if (i == elements)
+	  if (ok_p)
 	    {
 	      gimple_assign_set_rhs3 (stmt, mask);
 	      update_stmt (stmt);