On 1/4/2026 2:54 PM, Daniel Henrique Barboza wrote:
On 1/4/26 4:56 PM, Andrew Pinski wrote:
On Sun, Jan 4, 2026 at 11:32 AM Jeffrey Law
<[email protected]> wrote:
On 12/30/2025 2:34 PM, Daniel Barboza wrote:
PR 102486
gcc/ChangeLog:
* match.pd (`popcount (X & -X) -> (X & -X) != 0`): New
pattern.
gcc/testsuite/ChangeLog:
* gcc.dg/tree-ssa/pr102486.c: New test.
So isn't it transformed into "X != 0" rather than "(X & -X) != 0)"?
Which means the subject line and ChangeLog entry need minor updates.
I'm not sure why Andrew suggested optimizing to (X & -X) != 0 since the
X & -X is zero when X == 0 and nonzero when X != 0.
I don't remember why I suggested optimizing it to `(X & -X) != 0`
instead of just `X != 0`. The bug report was filed 4 years ago and I
can't find my notes for what I was looking at that time or why 2 years
later mentioned `(X&-X) != 0` either. But yes it should be to `X !=
0`.
Do we still care about the extra checks we introduced in v2 w.r.t the
result
of the optional conversion? We have this:
+ (POPCOUNT (convert?@1 (bit_and:c @0 (negate @0))))
+ (with { tree type0 = TREE_TYPE (@0);
+ tree type1 = TREE_TYPE (@1); }
+ (if (INTEGRAL_TYPE_P (type0)
+ && (TYPE_UNSIGNED (type0)
+ || TYPE_PRECISION (type1) <= TYPE_PRECISION (type0)))
+ /* Use an explicit bool conversion to avoid errors when the
+ POPCOUNT result is assigned to a type (e.g. int) that
+ will generate a "bogus comparison result type" error. */
+ (convert:type
+ (ne:boolean_type_node @1 { build_zero_cst (type1); })))))
I assume that these checks
+ && (TYPE_UNSIGNED (type0)
+ || TYPE_PRECISION (type1) <= TYPE_PRECISION (type0)))
Are still relevant because the conversion can still add additional
ones aside from
the signal bit if we happen to go from a signed type that is larger.
I can't offhand think of a case were the extension would be
problematical. The only case that's close in my mind would be
0x80000000 (assuming a 32bit type), where its negation would also be
0x80000000. But I think the property still holds in that case.
Other thoughts Andrew?
jeff
