[Bug middle-end/90693] Missing popcount simplifications

[wilco at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90693

--- Comment #2 from Wilco <wilco at gcc dot gnu.org> ---
(In reply to Dávid Bolvanský from comment #1)
> >> __builtin_popcount (x) == 1 into x == (x & -x)
> 
> 
> This will not work for x = 0.
> 
> Should work:
> x && x == (x & -x)
> x && (x & x-1) == 0

Good point, though that's not needed for (x & (x-1)) != 0 given you can only
have 2 or more bits set if x was non-zero to start with. It's worth finding a
branchless sequence, otherwise it may not be faster.

Maybe (x << clz (x)) == INT_MIN or (x-1) <u (x & -x) work better.