3 ms·

If you want to encode a series of indirect jumps between concatenated functions, you can do that already and the return address stack won’t get involved; you’ll

If you want to encode a series of indirect jumps between concatenated functions, you can do that already and the return address stack won’t get involved; you’ll simply get the normal branch predictors. But generalized branch prediction is expensive (multiple entries in the BTB hashed by the previous program flow, and mispredicts if the next function in the chain changes); the point of an RAS is that it’s a very cheap way to keep some 100% predictable branches from using those general resources. Concatenation pretty much requires branches without unique characteristics, so no fast path shortcuts and everything is a bit slower.

mncharity 4y ago
> Concatenation pretty much requires branches without unique characteristics, so no fast path Discussion the other day, of optimizing dispatch overhead in no-jit-allowed lightweight-word vm's, raised a brainstormy idea of having multiple copies of some words, and the compiler juggling them with awareness of their tailcall-next-word varied branch predictor states. Sort of a dynamic version of reducing dispatch by identifying hot phrases and creating multi-word-composite new words for them.
- tmzt 4y ago
  Would that be feasible on something like iOS that doesn't allow third-party JIT? Could this allow for translating WASM or emulator workloads to something that runs fast with those restrictions?
  - mncharity 4y ago
    Re iOS, yes. But runs fast? The words have to be small enough that the branch misprediction penalty (order-10 cycles) is significant. So for example, if a word is a single 3 cycle op then the dispatch matters, but with a dependency chain just a few ops long, the potential win drops under 50%. And having multiple copies of words will increase pressure on the instruction cache, so there will be a tradeoff, which may or may not be a win. So... perhaps this could be a way to dynamically optimize loop interiors? Or... perhaps to permit low-cost register shuffling words to glue together larger words with varied hard-wired register expectations? Or... insert creative idea here? There are blog posts around where someone writes a simple vm and incrementally optimizes it. But I don't recall seeing one yet with an iOS-ish focus of no-JIT, lots of registers, modern branch prediction... and so throws creative stuff against the wall and measures if anything sticks. It might be interesting to take such a blog post/series and do a followup?

mncharity 4y ago

> Concatenation pretty much requires branches without unique characteristics, so no fast path Discussion the other day, of optimizing dispatch overhead in no-jit-allowed lightweight-word vm's, raised a brainstormy idea of having multiple copies of some words, and the compiler juggling them with awareness of their tailcall-next-word varied branch predictor states. Sort of a dynamic version of reducing dispatch by identifying hot phrases and creating multi-word-composite new words for them.

tmzt 4y ago
Would that be feasible on something like iOS that doesn't allow third-party JIT? Could this allow for translating WASM or emulator workloads to something that runs fast with those restrictions?
- mncharity 4y ago
  Re iOS, yes. But runs fast? The words have to be small enough that the branch misprediction penalty (order-10 cycles) is significant. So for example, if a word is a single 3 cycle op then the dispatch matters, but with a dependency chain just a few ops long, the potential win drops under 50%. And having multiple copies of words will increase pressure on the instruction cache, so there will be a tradeoff, which may or may not be a win. So... perhaps this could be a way to dynamically optimize loop interiors? Or... perhaps to permit low-cost register shuffling words to glue together larger words with varied hard-wired register expectations? Or... insert creative idea here? There are blog posts around where someone writes a simple vm and incrementally optimizes it. But I don't recall seeing one yet with an iOS-ish focus of no-JIT, lots of registers, modern branch prediction... and so throws creative stuff against the wall and measures if anything sticks. It might be interesting to take such a blog post/series and do a followup?