https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84071
--- Comment #6 from Eric Botcazou <ebotcazou at gcc dot gnu.org> --- > They are always written but have an undefined value. Adding 2 8-bit values > results in a 9-bit value with WORD_REGISTER_OPERATIONS. If they have an undefined value, then WORD_REGISTER_OPERATIONS must not be defined for ARM. Here's the definition: -- Macro: WORD_REGISTER_OPERATIONS Define this macro to 1 if operations between registers with integral mode smaller than a word are always performed on the entire register. Most RISC machines have this property and most CISC machines do not. If the 8-bit addition is not performed on the entire 32-bit register, then this is not a WORD_REGISTER_OPERATIONS target.