3 ms·

How does it determine it was wrong without checking the condition? Or does it?

• vardump 11y ago
  It catches up with the branch condition with some delay, because CPUs have a lot of pipelined stages. CPU is not executing one or two instructions at a time, but a window of 10-50 (guesstimate, maybe more) instructions over roughly 15 clock cycles -- pipeline depth. So after it knows the condition result that determines the taken branch say 15 cycles later, it compares the guess to the actual path that needs to be taken. If they agree, speculative results are marked valid. If not, they're thrown away, CPU pipeline is flushed and execution starts again from the other path. CPUs also have hundreds (current crop is about 200 uops (read: instructions) reorder buffer (ROB), Intel Haswell has 192), where CPU tries to sort the cross instruction dependencies in an order that's faster to execute. Deep pipeline means if it can't reorder the instructions, it'll have to stall waiting for the earlier results -- it just doesn't know the value of certain register (or cached memory location), until the earlier computation dependency chain is finished. They're complicated and weird machines. They don't really execute the code sequentially at all, just make it look like as if they did. Everything I said above is oversimplified. I left out register renaming, cross core / CPU socket cache coherency -- and so much more. I can't really say I completely understand the beast myself.