https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115092
Jakub Jelinek <jakub at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |segher at gcc dot gnu.org --- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> --- Indeed, combine_simplify_rtx on (set (reg:CCGC 17 flags) (compare:CCGC (sign_extract:SI (reg/v:SI 101 [ g ]) (const_int 1 [0x1]) (const_int 0 [0])) (const_int -1 [0xffffffffffffffff]))) with VOIDmode, false, false remaining arguments is optimizing it to (set (reg:CCZ 17 flags) (compare:CCZ (zero_extract:SI (reg/v:SI 101 [ g ]) (const_int 1 [0x1]) (const_int 0 [0])) (const_int 0 [0]))) which is ok if it would be used solely in equality/non-equality comparisons, but is not ok when it is used in other comparisons. 1-bit sign_extract has range [-1,0] and [-1,0] < -1 is always false.