https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114591
--- Comment #12 from Hongtao Liu <liuhongt at gcc dot gnu.org> --- short a; short c; short d; void foo (short b, short f) { c = b + a; d = f + a; } foo(short, short): addw a(%rip), %di addw a(%rip), %si movw %di, c(%rip) movw %si, d(%rip) ret this one is bad since gcc10.1 and there's no subreg, The problem is if the operand is used by more than 1 insn, and they all support separate m constraint, mem_cost is quite small(just 1, reg move cost is 2), and this makes RA more inclined to propagate memory across insns. I guess RA assumes the separate m means the insn only support memory_operand? 961 if (op_class == NO_REGS) 962 /* Although we don't need insn to reload from 963 memory, still accessing memory is usually more 964 expensive than a register. */ 965 pp->mem_cost = frequency; 966 else