https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124808
--- Comment #4 from Drea Pinski <pinskia at gcc dot gnu.org> ---
>From combine.cc:
/* If we have (compare (xshiftrt FOO N) (const_int C)) and
the low order N bits of FOO are known to be zero, we can do this
by comparing FOO with C shifted left N bits so long as no
overflow occurs. Even if the low order N bits of FOO aren't known
to be zero, if the comparison is >= or < we can use the same
optimization and for > or <= by setting all the low
order N bits in the comparison constant. */
