https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105513
--- Comment #5 from Hongtao.liu <crazylht at gmail dot com> --- And for constraint like 'vm', it's different from 'v,m' in calculating mem_cost which will impact RA when op is REG_P. For 'v,m' mem_cost is just 1 * frequency, but for 'vm' mem_cost is much bigger(memory_move_cost - 1) * frequency. It means the impact of writing 'v,m' instead of 'vm' makes RA more likely to choose mem instead of register. 928 if (op_class == NO_REGS) 929 /* Although we don't need insn to reload from 930 memory, still accessing memory is usually more 931 expensive than a register. */ 932 pp->mem_cost = frequency; 933 else 934 /* If the alternative actually allows memory, make 935 things a bit cheaper since we won't need an 936 extra insn to load it. */ 937 pp->mem_cost 938 = ((out_p ? ira_memory_move_cost[mode][op_class][0] : 0) 939 + (in_p ? ira_memory_move_cost[mode][op_class][1] : 0) 940=> - allows_mem[i]) * frequency;
