https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103932
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
And the reason why the == 0 does not combine (at the RTL level) with the
builtin is because the shift clobbers CC:
(insn 13 12 14 2 (parallel [
(set (reg:DI 90)
(ashift:DI (reg:DI 89 [ n ])
(const_int 4 [0x4])))
(clobber (reg:CC 17 flags))
]) "/app/example.cpp":58:13 710 {*ashldi3_1}
(expr_list:REG_DEAD (reg:DI 89 [ n ])
(expr_list:REG_UNUSED (reg:CC 17 flags)
(nil))))
