predcom: Punt for steps which aren't multiples of access size [PR111683]

On the following testcases, there is no overlap between data references
within a single iteration, but the data references have size which is twice
as large as the step, which means the data references overlap with the next
iteration which predcom doesn't take into account.
As discussed in the PR, even if the reference size is smaller than step,
if step isn't a multiple of the reference size, there could be overlaps with
some other iteration later on.
The initial version of the patch regressed (test still passed, but predcom
didn't optimize anymore) pr71083.c which has a packed char, short structure
and was reading/writing the short 2 bytes in there with step 3.
The following patch deals with that by retrying for COMPONENT_REFs also the
aggregate sizes etc., so that it then compares 3 bytes against step 3.
In make check-gcc/check-g++ this patch I believe affects code generation
for only the 2 new testcases according to statistics I've gathered.

2024-03-23  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/111683
	* tree-predcom.cc (pcom_worker::suitable_component_p): If has_write
	and comp_step is RS_NONZERO, return false if any reference in the
	component doesn't have DR_STEP a multiple of access size.

	* gcc.dg/pr111683-1.c: New test.
	* gcc.dg/pr111683-2.c: New test.

gcc/testsuite/gcc.dg/pr111683-1.c

@@ -0,0 +1,22 @@
+/* PR middle-end/111683 */
+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+long long b[6] = { 3, 4, 5, 6, 7, 8 }, c[16];
+long long d[9] = { 3, 7, 12, 18, 22, 26, 21, 15, 8 };
+typedef long long U __attribute__ ((vector_size(16), may_alias, aligned(1)));
+typedef long long V __attribute__ ((vector_size(16), may_alias));
+
+int
+main ()
+{
+  for (int f = 0; f < 6; f++)
+    {
+      *(U *) &c[f] = *(U *) &c[f] + (V) { b[f], b[f] };
+      *(U *) &c[f + 2] = *(U *) &c[f + 2] + (V) { b[f], b[f] };
+    }
+  for (int f = 0; f < 9; f++)
+    if (c[f] != d[f])
+      __builtin_abort ();
+  return 0;
+}

gcc/testsuite/gcc.dg/pr111683-2.c

@@ -0,0 +1,27 @@
+/* PR middle-end/111683 */
+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+int b[6] = { 3, 4, 5, 6, 7, 8 }, c[12];
+int d[16] = { 0, 1, 3, 6, 10, 14, 12, 9, 5, 0, 0, 0 };
+
+int
+main ()
+{
+  int i;
+  if (sizeof (int) * 2 != sizeof (long long))
+    return 0;
+  for (i = 0; i < 6; i++)
+    {
+      long long a;
+      __builtin_memcpy (&a, &c[i], sizeof (a));
+      a += (((long long) i) << (sizeof (int) * __CHAR_BIT__)) + i;
+      __builtin_memcpy (&c[i], &a, sizeof (a));
+      __builtin_memcpy (&a, &c[i + 2], sizeof (a));
+      a += (((long long) i) << (sizeof (int) * __CHAR_BIT__)) + i;
+      __builtin_memcpy (&c[i + 2], &a, sizeof (a));
+    }
+  if (__builtin_memcmp (&c[0], &d[0], sizeof (c)))
+    __builtin_abort ();
+  return 0;
+}

gcc/tree-predcom.cc

@@ -1102,8 +1102,39 @@ pcom_worker::suitable_component_p (struct component *comp)
   gcc_assert (ok);
   first->offset = 0;
 
-  for (i = 1; comp->refs.iterate (i, &a); i++)
+  FOR_EACH_VEC_ELT (comp->refs, i, a)
     {
+      if (has_write && comp->comp_step == RS_NONZERO)
+	{
+	  /* Punt for non-invariant references where step isn't a multiple
+	     of reference size.  If step is smaller than reference size,
+	     it overlaps the access in next iteration, if step is larger,
+	     but not multiple of the access size, there could be overlap
+	     in some later iteration.  There might be more latent issues
+	     about this in predcom or data reference analysis.  If the
+	     reference is a COMPONENT_REF, also check if step isn't a
+	     multiple of the containg aggregate size.  See PR111683.  */
+	  tree ref = DR_REF (a->ref);
+	  tree step = DR_STEP (a->ref);
+	  if (TREE_CODE (ref) == COMPONENT_REF
+	      && DECL_BIT_FIELD (TREE_OPERAND (ref, 1)))
+	    ref = TREE_OPERAND (ref, 0);
+	  do
+	    {
+	      tree sz = TYPE_SIZE_UNIT (TREE_TYPE (ref));
+	      if (TREE_CODE (sz) != INTEGER_CST)
+		return false;
+	      if (wi::multiple_of_p (wi::to_offset (step),
+				     wi::to_offset (sz), SIGNED))
+		break;
+	      if (TREE_CODE (ref) != COMPONENT_REF)
+		return false;
+	      ref = TREE_OPERAND (ref, 0);
+	    }
+	  while (1);
+	}
+      if (i == 0)
+	continue;
       /* Polynomial offsets are no use, since we need to know the
 	 gap between iteration numbers at compile time.  */
       poly_widest_int offset;