[Bug tree-optimization/88315] New: SAD and DOT_PROD SLP reductions with initial value != 0 create wrong code

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88315

            Bug ID: 88315
           Summary: SAD and DOT_PROD SLP reductions with initial value !=
                    0 create wrong code
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

Index: gcc/testsuite/gcc.dg/vect/slp-reduc-sad.c
===================================================================
--- gcc/testsuite/gcc.dg/vect/slp-reduc-sad.c   (revision 266739)
+++ gcc/testsuite/gcc.dg/vect/slp-reduc-sad.c   (working copy)
@@ -12,7 +12,7 @@ extern void abort (void);
 int __attribute__((noinline,noclone))
 foo (uint8_t *pix1, uint8_t *pix2, int i_stride_pix2)
 {
-  int i_sum = 0;
+  int i_sum = 5;
   for( int y = 0; y < 16; y++ )
     {
       i_sum += abs ( pix1[0] - pix2[0] );
@@ -52,7 +52,7 @@ main ()
       __asm__ volatile ("");
     }

-  if (foo (X, Y, 16) != 32512)
+  if (foo (X, Y, 16) != 32512 + 5)
     abort ();

   return 0;


FAILs at runtime.  This is because

  number_of_copies = nunits * number_of_vectors / group_size;

is zero as both SAD and DOT_PROD reduce to half the number of lanes
and thus for example nunits == 4, number_of_vectors == 1 but group_size == 8.

Looks like GCC 7, 8 and trunk are affected.