4 ms·

Are you saying that branches can be used as a way of dividing code into several parts, so that several instruction decoders can be fed at the same time? Is it

• phire 2y ago
  Yep. It uses the branch predictor to slice up the instruction stream and feed it to multiple instruction decoders. Because the variable length nature of x86 instructions makes it hard to implement extremely wide instruction decoders. To reach a throughput of 4 instructions per cycle of the typical 4-wide x86 decoder you actually need 16 length decoders running in parallel, one for every byte position. The next pipeline stage then picks the 4 outputs that don't overlap before feeding it into proper decoders. Intel have actually implemented a 6-wide decoder for Golden Cove, but they must really be pushing the limits of propagation delays. So it's attractive to just use two or three smaller decoders in parallel. > If branches can be used in this way, unconditional jumps (e.g. gotos) and function calls can probably be used for similar purposes. It's a common misconception that the branch predictor only predicts conditional branches. They need to predict anything that causes control flow to diverge. So unconditional branches, calls, returns, and maybe even syscalls, all count as "branches" and get handled by the branch predictor. (On most RISC instruction sets, these instruction are often labeled branch. Call is just "Branch And Link" which saves the return address in a register. To return, you just use the "Branch to Register" instruction) This is necessary because the frontend is pipelined. It takes a minimum of four cycles to fetch the instruction data from instruction cache and decode it before the frontend can even know there is a call, return or unconditional branch instruction. If we are decoding four instruction per cycle, we are already 16 instructions past the branch before we even know it's there, and it's common to have a branch every 3-4 instructions. Basically, the frontend is blind and can't see any branches. Predicting the direction of conditional branches is almost a secondary task for modern branch predictors. It's much more important to predict where the branches are and their destination, so that instruction fetch stage can request the correct data from the instruction cache and keep the pipeline fed.
  • tigrou 2y ago
    Thanks for your reply. I learned a lot. For people wondering why you need a lot of decoders with x86, this answer explain it well (see "Decode problem" part) : https://qr.ae/p2IEt9 https://qr.ae/p2IEt9