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.
