tree-optimization/96075 - fix bogus misalignment calculation

This fixes bogus misalignment calculation for negative steps
since an assertion a previous comment indicated no longer holds:

      /* DR_STEP(dr) is the same as -TYPE_SIZE of the scalar type,
         otherwise we wouldn't be here.  */

Thus the following replaces DR_STEP by -TYPE_SIZE.

2020-07-06  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/96075
	* tree-vect-data-refs.c (vect_compute_data_ref_alignment): Use
	TYPE_SIZE_UNIT of the vector component type instead of DR_STEP
	for the misalignment calculation for negative step.

	* gcc.dg/vect/slp-46.c: New testcase.

(cherry picked from commit dccbf1e2a6e544f71b4a5795f0c79015db019fc3)

gcc/testsuite/gcc.dg/vect/slp-46.c

+/* { dg-require-effective-target vect_double } */
+
+#include "tree-vect.h"
+
+double x[1024], y[1024];
+
+void __attribute__((noipa)) foo()
+{
+  for (int i = 0; i < 512; ++i)
+    {
+      x[2*i] = y[i];
+      x[2*i+1] = y[i];
+    }
+}
+
+void __attribute__((noipa)) bar()
+{
+  for (int i = 0; i < 512; ++i)
+    {
+      x[2*i] = y[2*i];
+      x[2*i+1] = y[2*i];
+    }
+}
+
+void __attribute__((noipa)) baz()
+{
+  for (int i = 0; i < 512; ++i)
+    {
+      x[2*i] = y[511-i];
+      x[2*i+1] = y[511-i];
+    }
+}
+
+void __attribute__((noipa)) boo()
+{
+  for (int i = 0; i < 512; ++i)
+    {
+      x[2*i] = y[2*(511-i)];
+      x[2*i+1] = y[2*(511-i)];
+    }
+}
+
+int 
+main ()
+{
+  check_vect ();
+
+  for (int i = 0; i < 1024; ++i)
+    {
+      x[i] = 0;
+      y[i] = i;
+      __asm__ volatile ("");
+    }
+
+  foo ();
+  for (int i = 0; i < 1024; ++i)
+    if (x[i] != y[i/2])
+      abort ();
+
+  for (int i = 0; i < 1024; ++i)
+    {
+      x[i] = 0;
+      __asm__ volatile ("");
+    }
+
+  bar ();
+  for (int i = 0; i < 1024; ++i)
+    if (x[i] != y[2*(i/2)])
+      abort ();
+
+  for (int i = 0; i < 1024; ++i)
+    {
+      x[i] = 0;
+      __asm__ volatile ("");
+    }
+
+  baz ();
+  for (int i = 0; i < 1024; ++i)
+    if (x[i] != y[511 - i/2])
+      abort ();
+
+  for (int i = 0; i < 1024; ++i)
+    {
+      x[i] = 0;
+      __asm__ volatile ("");
+    }
+
+  boo ();
+  for (int i = 0; i < 1024; ++i)
+    if (x[i] != y[2*(511 - i/2)])
+      abort ();
+
+  return 0;
+}
+
+/* { dg-final { scan-tree-dump-times "vectorizing stmts using SLP" 2 "vect" } } */

gcc/tree-vect-data-refs.c

@@ -1056,7 +1056,7 @@ vect_compute_data_ref_alignment (dr_vec_info *dr_info)
   if (tree_int_cst_sgn (drb->step) < 0)
     /* PLUS because STEP is negative.  */
     misalignment += ((TYPE_VECTOR_SUBPARTS (vectype) - 1)
-		     * TREE_INT_CST_LOW (drb->step));
+		     * -TREE_INT_CST_LOW (TYPE_SIZE_UNIT (TREE_TYPE (vectype))));
 
   unsigned int const_misalignment;
   if (!known_misalignment (misalignment, vect_align_c, &const_misalignment))