3 ms·

Not if it's renamed. Instruction 2 does need to run after instruction 1, but instruction 3 doesn't need to run after them. xor eax, eax doesn't depend on the

• brucedawson 5y ago
  Yep, exactly. The rule of thumb is that whenever a register is written to a new register mapping is created. For instruction #1 eax might be assigned to physical register 103. It maintains that identity for instruction #2. For instruction #3 eax can be assigned to, let's say, physical register 67. Then for instruction #4 it could be assigned physical register 92. The only tricky thing is that instruction #3 also reads from eax, which normally would cause a dependency, but because XOR doesn't depend on the register contents this clever optimization applies. Here's another variant: > 1: add eax, 1 > 2: mov ebx, eax > 3: mov eax, 42 > 4: add eax, ecx Here instruction 3 writes the constant 42 to the register. This might make it clearer why instruction #3 can run in parallel with or before instruction #1. Note that the results must be _retired_ in order, but execution can happen out of order.