https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71289
Bug ID: 71289
Summary: Fails to optimize unsigned mul overflow check 'A > -1
/ B'
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: amonakov at gcc dot gnu.org
Target Milestone: ---
If A and B are both unsigned, then 'A > -1 / B' is a nice predicate for
checking whether A*B would overflow. The compiler should be able to optimize
the otherwise-unused division to a multiplication followed by branch on
overflow.
I've tried to add the corresponding transform to match.pd, but it seems
something else needs to be wired up as well, because it doesn't trigger. What
am I missing? TIA.
diff --git a/gcc/match.pd b/gcc/match.pd
index e511e9a..83c02a8 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -2582,6 +2582,16 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
&& types_match (TREE_TYPE (@0), TREE_TYPE (@1)))
(out (imagpart @2) { build_zero_cst (TREE_TYPE (@0)); }))))
+/* Simplify unsigned multiplication overflow check A > -1 / B to a builtin.
*/
+(for cmp (gt le)
+ out (ne eq)
+ (simplify
+ (cmp @0 (trunc_div:s integer_all_onesp @1))
+ (if (TYPE_UNSIGNED (TREE_TYPE (@0))
+ && types_match (TREE_TYPE (@0), TREE_TYPE (@1)))
+ (out
+ (imagpart (IFN_MUL_OVERFLOW @0 @1))
+ { build_zero_cst (TREE_TYPE (@0)); }))))
/* Simplification of math builtins. These rules must all be optimizations
as well as IL simplifications. If there is a possibility that the new
