tree-optimization/119057 - bogus double reduction detection

We are detecting a cycle as double reduction where the inner loop
cycle has extra out-of-loop uses.  This clashes at least with
assumptions from the SLP discovery code which says the cycle
isn't reachable from another SLP instance.  It also was not intended
to support this case, in fact with GCC 14 we seem to generate wrong
code here.

	PR tree-optimization/119057
	* tree-vect-loop.cc (check_reduction_path): Add argument
	specifying whether we're analyzing the inner loop of a
	double reduction.  Do not allow extra uses outside of the
	double reduction cycle in this case.
	(vect_is_simple_reduction): Adjust.

	* gcc.dg/vect/pr119057.c: New testcase.

diff --git a/gcc/testsuite/gcc.dg/vect/pr119057.c b/gcc/testsuite/gcc.dg/vect/pr119057.c
new file mode 100644 (file)
index 0000000..582bb8f
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/pr119057.c
@@ -0,0 +1,19 @@
+/* { dg-do compile } */
+/* { dg-additional-options "-fno-tree-vrp -fno-tree-forwprop" } */
+
+int a, b, c, d;
+unsigned e;
+static void f(void)
+{
+  unsigned h;
+  for (d = 0; d < 2; d++)
+    b |= e;
+  h = b;
+  c |= h;
+}
+int main()
+{
+  for (; a; a++)
+    f();
+  return 0;
+}

diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
index b279ebe2793bb409c6d1d5348e81226f58c3feef..dc15b955aadf58c0a1e36f8d8d294647d91f1b54 100644 (file)
--- a/gcc/tree-vect-loop.cc
+++ b/gcc/tree-vect-loop.cc
@@ -4044,7 +4044,8 @@ needs_fold_left_reduction_p (tree type, code_helper code)
 static bool
 check_reduction_path (dump_user_location_t loc, loop_p loop, gphi *phi,
                      tree loop_arg, code_helper *code,
-                     vec<std::pair<ssa_op_iter, use_operand_p> > &path)
+                     vec<std::pair<ssa_op_iter, use_operand_p> > &path,
+                     bool inner_loop_of_double_reduc)
 {
   auto_bitmap visited;
   tree lookfor = PHI_RESULT (phi);
@@ -4181,7 +4182,8 @@ pop:
          break;
        }
       /* Check there's only a single stmt the op is used on.  For the
-        not value-changing tail and the last stmt allow out-of-loop uses.
+        not value-changing tail and the last stmt allow out-of-loop uses,
+        but not when this is the inner loop of a double reduction.
         ???  We could relax this and handle arbitrary live stmts by
         forcing a scalar epilogue for example.  */
       imm_use_iterator imm_iter;
@@ -4216,7 +4218,7 @@ pop:
                }
            }
          else if (!is_gimple_debug (op_use_stmt)
-                  && (*code != ERROR_MARK
+                  && ((*code != ERROR_MARK || inner_loop_of_double_reduc)
                       || flow_bb_inside_loop_p (loop,
                                                 gimple_bb (op_use_stmt))))
            FOR_EACH_IMM_USE_ON_STMT (use_p, imm_iter)
@@ -4238,7 +4240,7 @@ check_reduction_path (dump_user_location_t loc, loop_p loop, gphi *phi,
 {
   auto_vec<std::pair<ssa_op_iter, use_operand_p> > path;
   code_helper code_;
-  return (check_reduction_path (loc, loop, phi, loop_arg, &code_, path)
+  return (check_reduction_path (loc, loop, phi, loop_arg, &code_, path, false)
          && code_ == code);
 }
 
@@ -4449,7 +4451,7 @@ vect_is_simple_reduction (loop_vec_info loop_info, stmt_vec_info phi_info,
   auto_vec<std::pair<ssa_op_iter, use_operand_p> > path;
   code_helper code;
   if (check_reduction_path (vect_location, loop, phi, latch_def, &code,
-                           path))
+                           path, inner_loop_of_double_reduc))
     {
       STMT_VINFO_REDUC_CODE (phi_info) = code;
       if (code == COND_EXPR && !nested_in_vect_loop)