Avoid unwinding the stack on hot path in method call lookups #15002
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The method lookup code uses exceptions to retry lookups using auto-casting. This is effectively using an exception for execution control, which is not what they are intended for. On `raise`, Crystal will try to unwind the call stack and save it to be able to report the original place where the exception was thrown, and this is a very expensive operation. To avoid that, we initialize the callstack of the special `RetryLookupWithLiterals` exception class always with the same fixed value.
ysbaddaden
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Sep 15, 2024
straight-shoota
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Sep 15, 2024
straight-shoota
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This is a follow-up on #15002 which explicitly assigns a dummy callstack to `RetryLookupWithLiterals` for performance reasons. `CallStack.empty` is intended to make this use case a bit more ergonomical. It doesn't require allocating a dummy instance with fake data. Instead, it's an explicitly empty callstack. This makes this mechanism easier to re-use in other places (ref #11658 (comment)).
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The method lookup code uses an exception to retry lookups using auto-casting. This is effectively using an exception for execution control, which is not what they are intended for (in Crystal). On
raise, Crystal will try to unwind the call stack and save it to be able to report the original place where the exception was thrown, and this is a very expensive operation. To avoid that, initialize the callstack of the specialRetryLookupWithLiteralsexception class always with the same fixed value.This cuts the compilation times for the compiler itself in about 1 second (both using a development and a release optimized compiler). More importantly, since the affected code path happens during semantic analysis, it also cuts that time but approximately 1 second, which should benefit development tools such as crystalline.
Some numbers from non-rigorous measurement, using a release build of the compiler building the compiler itself:
GC_DONT_GC=1 make -B): from 15.0s (baseline) to 13.9s (with this patch)GC_DONT_GC=1 make -B FLAGS="--no-codegen"): from 7.3s (baseline) to 6s (with this patch)Btw, I think a better solution would be to refactor the code so that we don't use exceptions for flow control. But that's a lot more work.