Fix DR_GROUP_GAP for strided accesses (PR 92677)

When dissolving an SLP-only group of accesses, we should only set
the gap to group_size - 1 for normal non-strided groups.

2019-11-29  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/92677
	* tree-vect-loop.c (vect_dissolve_slp_only_groups): Set the gap
	to zero when dissolving a group of strided accesses.

gcc/testsuite/
	PR tree-optimization/92677
	* gcc.dg/vect/pr92677.c: New test.

From-SVN: r278852

gcc/ChangeLog

 2019-11-29  Richard Sandiford  <richard.sandiford@arm.com>
 
+	PR tree-optimization/92677
+	* tree-vect-loop.c (vect_dissolve_slp_only_groups): Set the gap
+	to zero when dissolving a group of strided accesses.
+
+2019-11-29  Richard Sandiford  <richard.sandiford@arm.com>
+
 	PR tree-optimization/92596
 	* tree-vect-stmts.c (vectorizable_call): Punt on hybrid mask/nonmask
 	operations.

gcc/testsuite/ChangeLog

 2019-11-29  Richard Sandiford  <richard.sandiford@arm.com>
 
+	PR tree-optimization/92677
+	* gcc.dg/vect/pr92677.c: New test.
+
+2019-11-29  Richard Sandiford  <richard.sandiford@arm.com>
+
 	PR tree-optimization/92596
 	* gcc.dg/vect/bb-slp-pr92596.c: New test.
 	* gcc.dg/vect/bb-slp-43.c: Likewise.

gcc/testsuite/gcc.dg/vect/pr92677.c

+/* { dg-do compile } */
+/* { dg-additional-options "-O3" } */
+
+int a, c;
+int *b;
+long d;
+double *e;
+
+void fn1() {
+  long f;
+  double g, h;
+  while (c) {
+    if (d) {
+      g = *e;
+      *(b + 4) = g;
+    }
+    if (f) {
+      h = *(e + 2);
+      *(b + 6) = h;
+    }
+    e += a;
+    b += 8;
+    c--;
+    d += 2;
+  }
+}

gcc/tree-vect-loop.c

@@ -1829,7 +1829,10 @@ vect_dissolve_slp_only_groups (loop_vec_info loop_vinfo)
 		  DR_GROUP_FIRST_ELEMENT (vinfo) = vinfo;
 		  DR_GROUP_NEXT_ELEMENT (vinfo) = NULL;
 		  DR_GROUP_SIZE (vinfo) = 1;
-		  DR_GROUP_GAP (vinfo) = group_size - 1;
+		  if (STMT_VINFO_STRIDED_P (first_element))
+		    DR_GROUP_GAP (vinfo) = 0;
+		  else
+		    DR_GROUP_GAP (vinfo) = group_size - 1;
 		  vinfo = next;
 		}
 	    }