https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120296
Roger Sayle <roger at nextmovesoftware dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |roger at nextmovesoftware dot
com
Last reconfirmed| |2025-05-19
Status|UNCONFIRMED |NEW
Ever confirmed|0 |1
--- Comment #1 from Roger Sayle <roger at nextmovesoftware dot com> ---
A step to implementing this would be to add folding optimizations to match.pd:
__builtin_popcount(x<<__builtin_clz(x)) => __builtin_popcount(x)
__builtin_popcount(x>>__builtin_ctz(x)) => __builtin_popcount(x)
when the right shift is unsigned or x is known to be non-negative
I don't think it's necessary to also check that x can't be zero (which
introduces undefined behavior (UB) in the c[tl]z, and from there UB in the
shift) as the result has strictly less UB than the input.
