[gcc r12-9255] tree-optimization/107451 - SLP load vectorization issue

[Richard Biener]

https://gcc.gnu.org/g:c722c6b061a5e909267eae53ffe5910fbe0a7d5e

commit r12-9255-gc722c6b061a5e909267eae53ffe5910fbe0a7d5e
Author: Richard Biener <rguenther@suse.de>
Date:   Thu Dec 22 09:36:17 2022 +0100

    tree-optimization/107451 - SLP load vectorization issue
    
    When vectorizing SLP loads with permutations we can access excess
    elements when the load vector type is bigger than the group size
    and the vectorization factor covers less groups than necessary
    to fill it.  Since we know the code will only access up to
    group_size * VF elements in the unpermuted vector we can simply
    fill the rest of the vector with whatever we want.  For simplicity
    this patch chooses to repeat the last group.
    
            PR tree-optimization/107451
            * tree-vect-stmts.cc (vectorizable_load): Avoid loading
            SLP group members from group numbers in excess of the
            vectorization factor.
    
            * gcc.dg/torture/pr107451.c: New testcase.
    
    (cherry picked from commit 7b2cf5041460859ca4f58e5da1308b7ef9129d8b)

Diff:
---
 gcc/testsuite/gcc.dg/torture/pr107451.c | 27 +++++++++++++++++++++++++++
 gcc/tree-vect-stmts.cc                  | 20 ++++++++++++++------
 2 files changed, 41 insertions(+), 6 deletions(-)

diff --git a/gcc/testsuite/gcc.dg/torture/pr107451.c b/gcc/testsuite/gcc.dg/torture/pr107451.c
new file mode 100644
index 00000000000..a17574c6896
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr107451.c
@@ -0,0 +1,27 @@
+/* { dg-do run } */
+/* { dg-additional-options "-ftree-vectorize -fno-vect-cost-model" } */
+/* { dg-additional-options "-mavx2" { target avx2_runtime } } */
+
+double getdot(int n, const double *x, int inc_x, const double *y)
+{
+  int i, ix = 0;
+  double dot[4] = { 0.0, 0.0, 0.0, 0.0 } ;
+
+  for(i = 0; i < n; i++) {
+      dot[0] += x[ix]   * y[ix]   ;
+      dot[1] += x[ix+1] * y[ix+1] ;
+      dot[2] += x[ix]   * y[ix+1] ;
+      dot[3] += x[ix+1] * y[ix]   ;
+      ix += inc_x ;
+  }
+
+  return dot[0] + dot[1] + dot[2] + dot[3];
+}
+
+int main()
+{
+  double x[2] = {0, 0}, y[2] = {0, 0};
+  if (getdot(1, x, 4096*4096, y) != 0.)
+    __builtin_abort ();
+  return 0;
+}
diff --git a/gcc/tree-vect-stmts.cc b/gcc/tree-vect-stmts.cc
index 4c5d20a0e2c..2498948fad2 100644
--- a/gcc/tree-vect-stmts.cc
+++ b/gcc/tree-vect-stmts.cc
@@ -9202,6 +9202,7 @@ vectorizable_load (vec_info *vinfo,
       unsigned int group_el = 0;
       unsigned HOST_WIDE_INT
 	elsz = tree_to_uhwi (TYPE_SIZE_UNIT (TREE_TYPE (vectype)));
+      unsigned int n_groups = 0;
       for (j = 0; j < ncopies; j++)
 	{
 	  if (nloads > 1)
@@ -9223,12 +9224,19 @@ vectorizable_load (vec_info *vinfo,
 	      if (! slp
 		  || group_el == group_size)
 		{
-		  tree newoff = copy_ssa_name (running_off);
-		  gimple *incr = gimple_build_assign (newoff, POINTER_PLUS_EXPR,
-						      running_off, stride_step);
-		  vect_finish_stmt_generation (vinfo, stmt_info, incr, gsi);
-
-		  running_off = newoff;
+		  n_groups++;
+		  /* When doing SLP make sure to not load elements from
+		     the next vector iteration, those will not be accessed
+		     so just use the last element again.  See PR107451.  */
+		  if (!slp || known_lt (n_groups, vf))
+		    {
+		      tree newoff = copy_ssa_name (running_off);
+		      gimple *incr
+			= gimple_build_assign (newoff, POINTER_PLUS_EXPR,
+					       running_off, stride_step);
+		      vect_finish_stmt_generation (vinfo, stmt_info, incr, gsi);
+		      running_off = newoff;
+		    }
 		  group_el = 0;
 		}
 	    }