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I’m unconvinced because the instruction set sizes are already becoming fairly bloated (increasing microcode translation cost and i-cache pressure) but also beca

I’m unconvinced because the instruction set sizes are already becoming fairly bloated (increasing microcode translation cost and i-cache pressure) but also because CPUs already do speculative execution which has a nearly equivalent effect to the idiom you’re referring to. *edit: wording

MauranKilom 6y ago
To be more precise, some CPUs do the "both sides of the conditional" execution (EDIT This link is incorrect! Refer to "Eager execution" in the last link below. /EDIT https://en.wikipedia.org/wiki/Eager_evaluation https://en.wikipedia.org/wiki/Eager_evaluation) - years ago I heard that mobile processors do so for power reasons. No idea if that's still the case. The problem in question is due to predictive execution in which the CPU has to flush the pipeline on misprediction. This is dominant in desktop PC CPUs. I have no knowledge on what is prevalent in e.g. server or mobile CPUs. Both are (according to https://en.wikipedia.org/wiki/Speculative_execution https://en.wikipedia.org/wiki/Speculative_execution) forms of speculative execution.
- banachtarski 6y ago
  I'm speaking broadly about desktop/console CPUs which I have low-level familiarity with. I don't know of any CPUs in this class that don't perform some form of speculative execution. I'm not sure if eager evaluation is related to the topic at hand and was speaking specifically about speculative execution. Eager evaluation is just what pretty much every language except Haskell (or Haskell-like) does.
  - MauranKilom 6y ago
    Sorry, that first link was nonsense. I just copied the wiki link under "Eager execution" in the Speculative Execution page, but that is of course far from the same as "eager evaluation" (which is what the link goes to). I apologized for the confusion. The point is that there are at least two ways CPUs can speculatively execute a conditional: Either predict which branch is taken and flush the pipeline if the prediction was wrong, or execute both arms of the conditional and discard the one that was not actually taken. The top-level SIMD comment is about the latter, but most optimization around speculative execution is for the former. Yes, CPUs do speculative execution. The "flush pipeline on mispredict" kind ("predictive execution"). That is not the same as the "execute both and discard the untaken one" kind ("eager execution"). Your first reply suggests you consider them equivalent when they are not. I just wanted to clear that up.
    - banachtarski 6y ago
      Ah OK I understand now thanks for clearing that up. I meant that the end effect was similar but not so much that the mechanism was identical.

MauranKilom 6y ago

To be more precise, some CPUs do the "both sides of the conditional" execution (EDIT This link is incorrect! Refer to "Eager execution" in the last link below. /EDIT https://en.wikipedia.org/wiki/Eager_evaluation https://en.wikipedia.org/wiki/Eager_evaluation) - years ago I heard that mobile processors do so for power reasons. No idea if that's still the case. The problem in question is due to predictive execution in which the CPU has to flush the pipeline on misprediction. This is dominant in desktop PC CPUs. I have no knowledge on what is prevalent in e.g. server or mobile CPUs. Both are (according to https://en.wikipedia.org/wiki/Speculative_execution https://en.wikipedia.org/wiki/Speculative_execution) forms of speculative execution.

banachtarski 6y ago
I'm speaking broadly about desktop/console CPUs which I have low-level familiarity with. I don't know of any CPUs in this class that don't perform some form of speculative execution. I'm not sure if eager evaluation is related to the topic at hand and was speaking specifically about speculative execution. Eager evaluation is just what pretty much every language except Haskell (or Haskell-like) does.
- MauranKilom 6y ago
  Sorry, that first link was nonsense. I just copied the wiki link under "Eager execution" in the Speculative Execution page, but that is of course far from the same as "eager evaluation" (which is what the link goes to). I apologized for the confusion. The point is that there are at least two ways CPUs can speculatively execute a conditional: Either predict which branch is taken and flush the pipeline if the prediction was wrong, or execute both arms of the conditional and discard the one that was not actually taken. The top-level SIMD comment is about the latter, but most optimization around speculative execution is for the former. Yes, CPUs do speculative execution. The "flush pipeline on mispredict" kind ("predictive execution"). That is not the same as the "execute both and discard the untaken one" kind ("eager execution"). Your first reply suggests you consider them equivalent when they are not. I just wanted to clear that up.
  - banachtarski 6y ago
    Ah OK I understand now thanks for clearing that up. I meant that the end effect was similar but not so much that the mechanism was identical.