[experiment] Cache mapper instantiations #61505
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Andarist
commented
Mar 30, 2025
| @@ -19960,7 +19961,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker { | |||
| } | |||
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| function makeFunctionTypeMapper(func: (t: Type) => Type, debugInfo: () => string): TypeMapper { | |||
| return Debug.attachDebugPrototypeIfDebug({ kind: TypeMapKind.Function, func, debugInfo: Debug.isDebugging ? debugInfo : undefined }); | |||
| const mapper = Debug.attachDebugPrototypeIfDebug({ kind: TypeMapKind.Function, func, debugInfo: Debug.isDebugging ? debugInfo : undefined }); | |||
| mapper.instantiations = voidMap; | |||
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those mappers are "global" and this is just a quick hack to prevent this caching mechanism to kick in on them
Andarist
commented
Mar 30, 2025
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| if (context) { | ||
| context.nonFixingMapper.instantiations = undefined; | ||
| } |
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nonFixingMapper isn't idempotent so whenever inferences are cleared it's instantiation cache has to be cleared too
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@typescript-bot test it |
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@typescript-bot test it |
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Hey @jakebailey, the results of running the DT tests are ready. Everything looks the same! |
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@jakebailey Here are the results of running the user tests with tsc comparing Something interesting changed - please have a look. Details
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@jakebailey Here they are:
tscComparison Report - baseline..pr
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@ssalbdivad found out that a chain of somewhat trivial operations can easily end up with dreaded "Type instantiation is excessively deep and possibly infinite".
Two different reproductions were created:
We were able to bisect both to two different past PRs - one of which is quite old:
I dug into this and what I found out is, those types refer to the same "base" types - they are chained from them and
instantiateTyperepeats the work on them a lot. At least part of the problem is thatgetObjectTypeInstantiationcallsinstantiateTypes(type.aliasTypeArguments, mapper)to create part of its cache key. So it can't even check if the result is cached before it instantiates those. This problem compounds heavily in longer chains like this.So I toyed with possible solutions and I realized that caching results on mappers could help here... to some extent at least. I don't think it's the best solution but I also don't yet understand this problem to its core to propose anything better. The fact that property lookup still leads to the problem is especially worrying here.
Tha being said... it looks like this has very positive impact on instantiation counts across the test suite. It surely trades some memory for it though. It would be interesting to see perf results for this and the extended test suite run (even if only to get new data points for further investigation).