Fix DR_GROUP_GAP for strided accesses (PR 92677)

[Richard Sandiford]

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

Tested on aarch64-linux-gnu and x86_64-linux-gnu.  OK to install?

Richard


2019-11-29  Richard Sandiford  <richard.sandif...@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.

Index: gcc/tree-vect-loop.c
===================================================================
--- gcc/tree-vect-loop.c        2019-11-29 09:13:43.764143091 +0000
+++ gcc/tree-vect-loop.c        2019-11-29 10:52:30.475476141 +0000
@@ -1829,7 +1829,10 @@ vect_dissolve_slp_only_groups (loop_vec_
                  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;
                }
            }
Index: gcc/testsuite/gcc.dg/vect/pr92677.c
===================================================================
--- /dev/null   2019-09-17 11:41:18.176664108 +0100
+++ gcc/testsuite/gcc.dg/vect/pr92677.c 2019-11-29 10:52:30.475476141 +0000
@@ -0,0 +1,26 @@
+/* { 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;
+  }
+}