https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98865
Jakub Jelinek <jakub at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |jakub at gcc dot gnu.org --- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> --- PR middle-end/98865 * match.pd (a * (b >> (prec-1)) to ((signed)b >> (prec-1)) & a): New simplification. --- gcc/match.pd.jj 2021-01-22 11:50:09.882909120 +0100 +++ gcc/match.pd 2021-01-28 15:20:20.536238614 +0100 @@ -793,6 +793,16 @@ (define_operator_list COND_TERNARY && tree_nop_conversion_p (type, TREE_TYPE (@1))) (lshift @0 @2))) +/* Fold (a * (b >> (prec-1))) with logical shift into + ((signed)b >> (prec-1)) & a. */ +(simplify + (mult:c @0 (nop_convert? (rshift @1 INTEGER_CST@2))) + (if (INTEGRAL_TYPE_P (TREE_TYPE (@1)) + && TYPE_UNSIGNED (TREE_TYPE (@1)) + && wi::to_widest (@2) + 1 == TYPE_PRECISION (TREE_TYPE (@1))) + (with { tree stype = signed_type_for (TREE_TYPE (@1)); } + (bit_and (convert:type (rshift (convert:stype @1) @2)) @0)))) + /* Fold (1 << (C - x)) where C = precision(type) - 1 into ((1 << C) >> x). */ (simplify (completely untested) does that. It doesn't handle vector types, whether that is a good idea or not depends on how do we deal with the match.pd simplifications after last veclower pass issue. And, given: unsigned long long foo (unsigned long long a, unsigned long long b) { return (a >> 63) * b; } long long bar (long long a, long long b) { return -(a >> 63) * b; } long long baz (long long a, long long b) { long long c = a >> 63; long long d = -c; return d * b; } we optimize with it for and bar but not baz, apparently the -(a >> 63) arithmetic to (a >> 63) logical shift is done only in GENERIC folding and not later.