qemu转换中间码的规则

Question

case 4: /* mul /
switch(ot) {
case MO_8:
gen_op_mov_v_reg(MO_8, cpu_T[1], R_EAX);
tcg_gen_ext8u_tl(cpu_T[0], cpu_T[0]);
tcg_gen_ext8u_tl(cpu_T[1], cpu_T[1]);
/ XXX: use 32 bit mul which could be faster */
tcg_gen_mul_tl(cpu_T[0], cpu_T[0], cpu_T[1]);
gen_op_mov_reg_v(MO_16, R_EAX, cpu_T[0]);
tcg_gen_mov_tl(cpu_cc_dst, cpu_T[0]);
tcg_gen_andi_tl(cpu_cc_src, cpu_T[0], 0xff00);
set_cc_op(s, CC_OP_MULB);
break;

MUL为什么能够转换成-
tcg_gen_mul_tl(cpu_T[0], cpu_T[0], cpu_T[1]);
gen_op_mov_reg_v(MO_16, R_EAX, cpu_T[0]);
tcg_gen_mov_tl(cpu_cc_dst, cpu_T[0]);
tcg_gen_andi_tl(cpu_cc_src, cpu_T[0], 0xff00);