Store PCall alternate return addresses in frame info #188
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As suggested by @JohnReppy at POPL'24, one can exploit the static relationship among the return addresses at a PCall to avoid storing the alternate return addresses on the (dynamic) call stack; rather, the `cont`/`seq` return address on the stack should be sufficient to determine the corresponding `parl`/`sync` and `parr`/`spwn` return addresses. In MLton, we can use the `GC_frameInfo` that is associated with each return address to store the `parl`/`sync` and `parr`/`spwn` return addresses in a `PCALL_CONT_FRAME`. One advantage is that it avoids two writes (of the `parl`/`sync` and `parr`/`spwn` return addresses) at each `PCall`. Although it is expected that this overhead is small (the top of the call stack ought to be in cache), it does nicely reduce the overhead of `PCall` to exactly the same as `Call`.
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Nice! I was able to measure a few percent improvement in a couple examples (fully parallel |
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As suggested by @JohnReppy, rather than storing the alternate return addresses of a
PCallon the (dynamic) call stack, one can store them in a (static) table. In MLton, we can store the alternate return addresses in thestruct GC_frameInfothat corresponds to thePCALL_CONT_FRAME.