Wasm: Opt: Fuse casts into custom JS helpers. #5229
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When the `expr` of a cast (including unchecked `asInstanceOf`s) is a node implemented with a custom JS helper, we now fuse the unboxing/cast operation into the JS-to-Wasm boundary, when possible. This removes one round-trip to JS. For example, a call to `js.Math.cos(x)` now results in a single Wasm-to-JS call, where the JS helper has type `[f64] -> [f64]` in Wasm.
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tanishiking
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Sep 12, 2025
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| * Exception: two JSBinaryOp's have `tpe == BooleanType`, but this is also | ||
| * fine as all the supertypes of `BooleanType` can be used as well. |
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What do you mean by:
this is also fine as all the supertypes of BooleanType can be used as well.
?
If I understand correctly, it's fine to use castTo = AnyType when tpe = BooleanType (=== and !==) because of val resultType = if (castTo == AnyType) tree.tpe else castTo in genJSBinary/UnaryOp. The helper function returns boolean, not anyref.
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| if (sourceTpe == AnyType && targetTpeCastableThroughJSWasmBoundary) { | ||
| // Try and push the cast inside nodes that use a custom JS helper |
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It might be worth adding some more explanation, similar to what's in the PR description here ?
This removes one round-trip to JS. For example, a call to js.Math.cos(x) now results in a single Wasm-to-JS call, where the JS helper has type [f64] -> [f64] in Wasm.
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When the
exprof a cast (including uncheckedasInstanceOfs) is a node implemented with a custom JS helper, we now fuse the unboxing/cast operation into the JS-to-Wasm boundary, when possible.This removes one round-trip to JS. For example, a call to
js.Math.cos(x)now results in a single Wasm-to-JS call, where the JS helper has type[f64] -> [f64]in Wasm.