[PATCH] match.pd: Optimize __builtin_mul_overflow_p (x, cst, (utype)0) to x > ~(utype)0 / cst [PR30314]

[Jakub Jelinek <jakub@redhat.com>]

Hi!

A comparison with a constant is most likely always faster than
.MUL_OVERFLOW from which we only check whether it overflowed and not the
multiplication result, and even if not, it is simpler operation on GIMPLE
and even if a target exists where such multiplications with overflow checking
are cheaper than comparisons, because comparisons are so much more common
than overflow checking multiplications, it would be nice if it simply
arranged for comparisons to be emitted like those multiplications on its
own...

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2022-06-01  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/30314
	* match.pd (__builtin_mul_overflow_p (x, cst, (utype) 0) ->
	x > ~(utype)0 / cst): New simplification.

	* gcc.dg/tree-ssa/pr30314.c: New test.

--- gcc/match.pd.jj	2022-06-01 13:54:32.000654151 +0200
+++ gcc/match.pd	2022-06-01 15:13:35.473084402 +0200
@@ -5969,6 +5969,17 @@ (define_operator_list SYNC_FETCH_AND_AND
        && (!TYPE_UNSIGNED (TREE_TYPE (@2)) || TYPE_UNSIGNED (TREE_TYPE (@0))))
    (ovf @1 @0))))
 
+/* Optimize __builtin_mul_overflow_p (x, cst, (utype) 0) if all 3 types
+   are unsigned to x > (umax / cst).  */
+(simplify
+ (imagpart (IFN_MUL_OVERFLOW:cs@2 @0 integer_nonzerop@1))
+  (if (INTEGRAL_TYPE_P (TREE_TYPE (@0))
+       && TYPE_UNSIGNED (TREE_TYPE (@0))
+       && TYPE_MAX_VALUE (TREE_TYPE (@0))
+       && types_match (TREE_TYPE (@0), TREE_TYPE (TREE_TYPE (@2)))
+       && int_fits_type_p (@1, TREE_TYPE (@0)))
+   (convert (gt @0 (trunc_div! { TYPE_MAX_VALUE (TREE_TYPE (@0)); } @1)))))
+
 /* Simplification of math builtins.  These rules must all be optimizations
    as well as IL simplifications.  If there is a possibility that the new
    form could be a pessimization, the rule should go in the canonicalization
--- gcc/testsuite/gcc.dg/tree-ssa/pr30314.c.jj	2022-06-01 15:22:53.201271365 +0200
+++ gcc/testsuite/gcc.dg/tree-ssa/pr30314.c	2022-06-01 15:13:24.725196482 +0200
@@ -0,0 +1,18 @@
+/* PR middle-end/30314 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+/* { dg-final { scan-tree-dump-not "\.MUL_OVERFLOW " "optimized" } } */
+/* { dg-final { scan-tree-dump " > 122713351" "optimized" { target int32 } } } */
+/* { dg-final { scan-tree-dump " > 527049830677415760" "optimized" { target lp64 } } } */
+
+int
+foo (unsigned int x)
+{
+  return __builtin_mul_overflow_p (x, 35U, 0U);
+}
+
+int
+bar (unsigned long int x)
+{
+  return __builtin_mul_overflow_p (x, 35UL, 0UL);
+}

	Jakub