Improve Invoke*: explicit catch types
#1565
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kind: enhancement ✨
A new feature or use case
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The structure of
Invoke, with its separate catch and resume labels, mirrors LLVM. In order to translate Java's catch list to a catch label, we employ an algorithm which generatesif+instanceoftrees for each potential throw site. This is done at the front end as the method is parsed.However one downside to this approach is that we may end up generating multiple identical
iftrees if many call sites exist within range of the same catch handler list. It also makes the IR somewhat more complex to read since one must trace the catch execution through several blocks.It might be worth trying to modify
Invoke*to instead accept a list of (type, label) pairs, somewhat like a type switch.On the front end, the method parser (or a BBB like today) would be able to determine the catch list fairly easily for a given call site and carry out the automatic promotion of
Call*toInvoke*. The interpreter can perform the requisite type checks and dispatches fairly easily as well.For LLVM, during lowering we would transform
Invoke*so that it only has a single catch of typeThrowableby generating aniftree much like today. However, theiftree which is generated can be reused by every call site with the same list; furthermore, theiftree for a given list ofnitems (wheren > 1) can delegate to theiftree for the corresponding tail list ofn - 1items much like the original catch generator did. This would be simpler to do on the back end than on the front end.Once this transformation has occurred, from the perspective of LLVM,
Invoke*is identical to what it was before.It might be necessary to add a catch list to
Throwas well. This would prevent the necessity of translatingThrowtoGotoon the front end (this can be done during lower instead).By using an ordered catch list, lowering to WASM might become simpler as well (see WASM exception handling for more info).
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