4 ms·

When you say vectorize, are you referring to loop unrolling? Or SIMD or something?

simiones 5y ago
I have never heard vectorization to refer to anything other than SIMD. Loop unrolling is usually only a useful technique to enable SIMD, as far as I know (at least on modern processors, where branch prediction has greatly decreased the cost of jump instructions).
- jhgb 5y ago
  > as far as I know (at least on modern processors, where branch prediction has greatly decreased the cost of jump instructions). What about ILP? Can't that benefit from an unrolled loop in some cases? For example if there's a fairly long dependency chain but you might still be able to go through two loop bodies at once instead.
  - tsimionescu 5y ago
    I don't know for sure at all, but I don't think it's impossible that speculative execution could also achieve the same at the processor level.
    - jhgb 5y ago
      I don't see how this has anything to do with speculation? In most cases where you care about this you don't have to speculate if all the loop iterations are needed. For example in matrix multiplication all of those iterations will be needed.
      - tsimionescu 5y ago
        What I'm thinking is that the processor has an instruction stream that looks like this: loop: instr_1 instr_2 ... instr_n jcond loop Now, assuming the loop is not unrolled, it would need to speculate that `jcond loop` will jump to be able to execute 2 copies of instr_1 in parallel - I'm saying that it may be able to do that, though I am by no means sure.
        jhgb 5y ago
        Oh, I see what you mean -- I was talking (and thinking) about the unrolled version so it didn't make sense how speculation could help there. But I imagine that typically the kind of long chains that you might want to do in parallel in a single basic block are perhaps something that wouldn't get executed that far after a branch, if the only purpose is to not waste time after a branch misprediction. Plus from what I understand you'd still be wasting execution units here, just not by idling them but rather by speculating the "I'm done" branch repeatedly. EDIT: I just found that the idea that I had in my head actually exists and is called "modulo scheduling".

I have never heard vectorization to refer to anything other than SIMD. Loop unrolling is usually only a useful technique to enable SIMD, as far as I know (at least on modern processors, where branch prediction has greatly decreased the cost of jump instructions).

jhgb 5y ago
> as far as I know (at least on modern processors, where branch prediction has greatly decreased the cost of jump instructions). What about ILP? Can't that benefit from an unrolled loop in some cases? For example if there's a fairly long dependency chain but you might still be able to go through two loop bodies at once instead.
- tsimionescu 5y ago
  I don't know for sure at all, but I don't think it's impossible that speculative execution could also achieve the same at the processor level.
  - jhgb 5y ago
    I don't see how this has anything to do with speculation? In most cases where you care about this you don't have to speculate if all the loop iterations are needed. For example in matrix multiplication all of those iterations will be needed.
    - tsimionescu 5y ago
      What I'm thinking is that the processor has an instruction stream that looks like this: loop: instr_1 instr_2 ... instr_n jcond loop Now, assuming the loop is not unrolled, it would need to speculate that `jcond loop` will jump to be able to execute 2 copies of instr_1 in parallel - I'm saying that it may be able to do that, though I am by no means sure.
      - jhgb 5y ago
        Oh, I see what you mean -- I was talking (and thinking) about the unrolled version so it didn't make sense how speculation could help there. But I imagine that typically the kind of long chains that you might want to do in parallel in a single basic block are perhaps something that wouldn't get executed that far after a branch, if the only purpose is to not waste time after a branch misprediction. Plus from what I understand you'd still be wasting execution units here, just not by idling them but rather by speculating the "I'm done" branch repeatedly. EDIT: I just found that the idea that I had in my head actually exists and is called "modulo scheduling".