https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99830
--- Comment #10 from Segher Boessenkool <segher at gcc dot gnu.org> --- (In reply to Jakub Jelinek from comment #8) > In particular, it is combine_simplify_rtx that is called on: > (zero_extend:SI (subreg:QI (ior:TI (and:TI (reg/v:TI 103 [ f ]) > (const_int -16711681 [0xffffffffff00ffff])) > (ashift:TI (and:TI (clobber:TI (const_int 0 [0])) > (const_int 255 [0xff])) > (const_int 16 [0x10]))) 0)) > which simplifies it into > (and:SI (subreg:SI (reg/v:TI 103 [ f ]) 0) > (const_int 255 [0xff])) That is very wrong. A clobber of 0 should *never* be removed. Various parts of generic code know about that already, btw. A clobber of 0 means "Abort! Abort!" It does not mean "well, here is something you can optimise away more easily". Do you want to investigate further, or shall I?