Data-oriented design or why you might shoot yourself in the foot with OOP (2009)

Re:Data-oriented design or why you might shoot yourself in t... by tsimionescu on Monday June 28, 2021 @01:20PM (#27664651)

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.

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Re:Data-oriented design or why you might shoot yourself in t... by jhgb on Monday June 28, 2021 @01:25PM (#27664713)

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".

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