vect-generic: Fix up expand_vector_mult [PR123656]

The alg_sub_factor handling in expand_vector_mult had the arguments
reversed.
As documented in expmed.h, the algorithms should be
   These are the operations:
   alg_zero             total := 0;
   alg_m                total := multiplicand;
   alg_shift            total := total * coeff
   alg_add_t_m2         total := total + multiplicand * coeff;
   alg_sub_t_m2         total := total - multiplicand * coeff;
   alg_add_factor       total := total * coeff + total;
   alg_sub_factor       total := total * coeff - total;
   alg_add_t2_m         total := total * coeff + multiplicand;
   alg_sub_t2_m         total := total * coeff - multiplicand;

   The first operand must be either alg_zero or alg_m.  */
So, alg_sub_factor should be identical to alg_sub_t2_m with the
difference that one subtracts accumulator and the other subtracts
op0.  I went through all the other ones and they seem to match
the description except for alg_sub_factor and tree-vect-patterns.cc
seems to be fully correct.  expand_vector_mult at times has
pretty random order of PLUS_EXPR arguments, but that is a commutative
operation, so makes no difference.

Furthermore, I saw weird formatting in the alg_add_t_m2 case, so fixed
that too.

2026-01-19  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/123656
	* tree-vect-generic.cc (expand_vector_mult): Fix up alg_sub_factor
	handling.  Fix up formatting in alg_add_t_m2 handling.

	* gcc.dg/pr123656.c: New test.

diff --git a/gcc/testsuite/gcc.dg/pr123656.c b/gcc/testsuite/gcc.dg/pr123656.c
new file mode 100644 (file)
index 0000000..9843985
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr123656.c
@@ -0,0 +1,21 @@
+/* PR tree-optimization/123656 */
+/* { dg-do run } */
+/* { dg-options "-Og -Wno-psabi" } */
+
+#define C 210
+
+typedef __attribute__((__vector_size__ (2))) unsigned char V;
+
+V
+foo (V v)
+{
+  return v * C;
+}
+
+int
+main ()
+{
+  V x = foo ((V) {1, 1});
+  if (x[0] != C || x[1] != C)
+    __builtin_abort ();
+}

diff --git a/gcc/tree-vect-generic.cc b/gcc/tree-vect-generic.cc
index d34d2c5fa48d04ea6f9e11f31a77ba039ba3685f..77066bbb2e1b307983262a7d055c8052ee897fcb 100644 (file)
--- a/gcc/tree-vect-generic.cc
+++ b/gcc/tree-vect-generic.cc
@@ -643,9 +643,9 @@ expand_vector_mult (gimple_stmt_iterator *gsi, tree type, tree op0,
                                       LSHIFT_EXPR);
              break;
            case alg_add_t_m2:
-              tmp_var = add_shift (gsi, vectype, op0, shifts, LSHIFT_EXPR);
+             tmp_var = add_shift (gsi, vectype, op0, shifts, LSHIFT_EXPR);
              accumulator = gimplify_build2 (gsi, PLUS_EXPR, vectype, tmp_var,
-                                           accumulator);
+                                            accumulator);
              break;
            case alg_sub_t_m2:
              tmp_var = add_shift (gsi, vectype, op0, shifts, LSHIFT_EXPR);
@@ -674,7 +674,7 @@ expand_vector_mult (gimple_stmt_iterator *gsi, tree type, tree op0,
              tmp_var = add_shift (gsi, vectype, accumulator, shifts,
                                   LSHIFT_EXPR);
              accumulator = gimplify_build2 (gsi, MINUS_EXPR, vectype,
-                                            accumulator, tmp_var);
+                                            tmp_var, accumulator);
              break;
            default:
              gcc_unreachable ();