Allow Return arg to be a void if the target Labeled is a void.
#5074
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gzm0
reviewed
Nov 16, 2024
linker/shared/src/main/scala/org/scalajs/linker/frontend/optimizer/OptimizerCore.scala
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| case Lhs.ReturnFromFunction => | ||
| js.Block( | ||
| transformStat(rhs, tailPosLabels = Set.empty), | ||
| js.Return(js.Undefined())) |
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It feels odd that we do not have tail label optimization here, but IIUC that's on par with the status quo.
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We can't have tail label optimization here. Indeed, there is control-flow-disrupting code after the transformed rhs, namely this js.Return(js.Undefined()). The labels that are in tail position after the js.Return(js.Undefined()) are definitely not in tail position of the rhs.
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Ah, sorry, I wasn't clear. I understand why tailPosLabels = Set.empty is necessary.
What I didn't get is why there is no attempt to eliminate the return we are emitting itself here.
But I think now I understand: We only get here if an actual label is transformed to an Lhs.ReturnFromFunction. If the function we are translating has void return type (and no top-level label), we invoke transformStat immediately.
Maybe that warrants a comment :P
linker/shared/src/main/scala/org/scalajs/linker/backend/emitter/FunctionEmitter.scala
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Updated with the requested comment + commit message explanation. |
gzm0
approved these changes
Nov 17, 2024
| /* If we get here, it is because desugarToFunctionInternal() | ||
| * found a top-level Labeled and eliminated it. Therefore, unless | ||
| * we're mistaken, by construction we cannot be in tail position | ||
| * of the whole function. That means there is no point trying to |
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| * of the whole function. That means there is no point trying to | |
| * of the whole function (otherwise doReturnToLabel would have | |
| * eliminated the lhs). That means there is no point trying to |
Previously, even if the target label of a `Return` declared a `void` result type, the argument to the `Return` had to be an expression, with a non-`void` type. Because of that rule, we generated fake `Undefined()` arguments to the `Return`s. In this commit, we remove that restriction. This simplifies the IR specification and removes some ad hoc code paths to generate the fake `Undefined()`. We do have to add some handling in the JS function emitter. The Wasm function emitter requires no change. The changes in `doReturnToLabel()` are necessary to maintain the status quo in terms of the emitted JS. My understanding is that before, we would get a lot of `Block(stats, Return(Undefined))` which are now `Return(Block(stats))`. If the `stats` contain `If`s and other branches, the new `Return` can be pushed down a lot more. When it wasn't pushed down, we would have regular fall-through behavior without `js.Break`s. Now with the `Return`s pushed down, we get a lot of `js.Break`s that are not actually necessary. The new optimization in `doReturnToLabel()` removes them.
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sjrd
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Jan 17, 2025
And port the changes that were made to the compiler backend. Most nobably, the changes from the following upstream PRs: * Introduce non-nullable reference types in the IR. scala-js/scala-js#5018 * Opt: Remove useless *Ident indirections in the IR model. scala-js/scala-js#5092 * Merge the IR node This into VarRef, with a magic LocalName. scala-js/scala-js#5090 * Merge several IR nodes into UnaryOp. scala-js/scala-js#5088 * Restrict usages of StoreModule even further. scala-js/scala-js#5059 * Allow Return arg to be a void if the target Labeled is a void. scala-js/scala-js#5074 * Rename NoType to VoidType and print it as "void". scala-js/scala-js#5061
sjrd
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Jan 17, 2025
And port the changes that were made to the compiler backend. Most notably, the changes from the following upstream PRs: * Introduce non-nullable reference types in the IR. scala-js/scala-js#5018 * Opt: Remove useless *Ident indirections in the IR model. scala-js/scala-js#5092 * Merge the IR node This into VarRef, with a magic LocalName. scala-js/scala-js#5090 * Merge several IR nodes into UnaryOp. scala-js/scala-js#5088 * Restrict usages of StoreModule even further. scala-js/scala-js#5059 * Allow Return arg to be a void if the target Labeled is a void. scala-js/scala-js#5074 * Rename NoType to VoidType and print it as "void". scala-js/scala-js#5061
sjrd
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Jan 22, 2025
And port the changes that were made to the compiler backend. Most notably, the changes from the following upstream PRs: * scala-js/scala-js#5018 * scala-js/scala-js#5092 * scala-js/scala-js#5090 * scala-js/scala-js#5088 * scala-js/scala-js#5059 * scala-js/scala-js#5074 * scala-js/scala-js#5061
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Previously, even if the target label of a
Returndeclared avoidresult type, the argument to theReturnhad to be an expression, with a non-voidtype. Because of that rule, we generated fakeUndefined()arguments to theReturns.In this commit, we remove that restriction. This simplifies the IR specification and removes some ad hoc code paths to generate the fake
Undefined(). We do have to add some handling in the JS function emitter. The Wasm function emitter requires no change.The changes in
doReturnToLabel()are necessary to maintain the status quo in terms of the emitted JS. My understanding is that before, we would get a lot ofBlock(stats, Return(Undefined))which are nowReturn(Block(stats)). If thestatscontainIfs and other branches, the newReturncan be pushed down a lot more. When it wasn't pushed down, we would have regular fall-through behavior withoutjs.Breaks. Now with theReturns pushed down, we get a lot ofjs.Breaks that are not actually necessary. The new optimization indoReturnToLabel()removes them.