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by fwsgonzo 1262 days ago
Don't take this as anything other than speculation: I wonder if wasm3 is using musttail with opaque function calls in the instruction handlers. It will demolish performance, which is why I am only using computed gotos in mine (when available). Even switch-case is faster than musttail when you have to leave the tco-jumps. Which is (as an example) why one should not measure performance by fibonacci number generation. :)
1 comments
haberman 1262 days ago
> I wonder if wasm3 is using musttail with opaque function calls in the instruction handlers. It will demolish performance, which is why I am only using computed gotos in mine (when available). Even switch-case is faster than musttail when you have to leave the tco-jumps.

This doesn't match with my experience. After working on this problem a lot, I came to the conclusion that musttail with opaque function calls is one of the best ways of getting good code out of the compiler: https://blog.reverberate.org/2021/04/21/musttail-efficient-i...

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fwsgonzo 1262 days ago
I meant having an opaque function inside your instruction handler. My assembly looks like crap if something doesn't get inlined. Because I have no way of achieving this I simply cannot use TCO. It runs fibonacci faster, but anything that uses memory is way worse because it pushes and pops a ton of registers on the instruction handler itself, and not the slow-path opaque function.

An instruction handler here being a dispatch function. It handles a single instruction.

Reading your post it says so under Limitations. Opaque calls trashes performance. I guess we agree, but then again I was just reading my assembly, so I had no reason to doubt myself.

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haberman 1262 days ago
Yes our solution was to make all fallback functions into tail calls. It solves the problem, but requires a lot of discipline and can be a bit awkward.

I recently saw this, which is a very interesting approach for using non-tail-call fallback functions without trashing the code: https://chromium-review.googlesource.com/c/v8/v8/+/4116584

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fwsgonzo 1262 days ago
That's very interesting! Thanks!
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