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Commit 5f1d5cf

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Scala.js: Remove spurious js.AsInstanceOf around js.LinkTimeIf.
When B <: A and C <: A, `linkTimeIf[A](cond) { B }{ C }` would generate an `.asInstanceOf[A]` that casts the resulting value to a supertype. However, this cast is redundant, as the linker will prune one branch at link time, and the remaining expression is already known to be a compatible type. This commit eliminates the surrounding `.asInstanceOf` around the `linkTimeIf[T]` when both `thenp` and `elsep` are known to be subtypes of `T`. The optimizer already routinely performs this optimization. However, that comes too late for the module instance field alias analysis performed by `IncOptimizer`. In that case, while the desugarer removes the `LinkTimeIf`, the extra `AsInstanceOf` prevents aliasing the field. Removing the cast ahead of time in the compiler allows field aliases to be recognized in the presence of `LinkTimeIf`s. This commit is a forward port of the Scala.js commit scala-js/scala-js@9bb267c 
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‎compiler/src/dotty/tools/backend/sjs/JSCodeGen.scala‎

Lines changed: 23 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -3321,7 +3321,29 @@ class JSCodeGen()(using genCtx: Context) {
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val genReceiver = genExpr(receiver)
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if (sym == defn.Any_asInstanceOf) {
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genAsInstanceOf(genReceiver, to)
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receiver match {
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/* This is an optimization for `linkTimeIf(cond)(thenp)(elsep).asInstanceOf[T]`.
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* If both `thenp` and `elsep` are subtypes of `T`, the `asInstanceOf`
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* is redundant and can be removed. The optimizer already routinely performs
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* this optimization. However, that comes too late for the module instance
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* field alias analysis performed by `IncOptimizer`. In that case, while the
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* desugarer removes the `LinkTimeIf`, the extra `AsInstanceOf` prevents
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* aliasing the field. Removing the cast ahead of time in the compiler allows
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* field aliases to be recognized in the presence of `LinkTimeIf`s.
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*/
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case Apply(innerFun, List(cond, thenp, elsep))
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if innerFun.symbol == jsdefn.LinkingInfo_linkTimeIf &&
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thenp.tpe <:< to && elsep.tpe <:< to =>
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val genReceiver1 = genReceiver match {
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case genReceiver: js.LinkTimeIf =>
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genReceiver
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case _ =>
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throw FatalError(s"Unexpected tree $genReceiver is generated for $innerFun at: ${tree.sourcePos}")
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}
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js.LinkTimeIf(genReceiver1.cond, genReceiver1.thenp, genReceiver1.elsep)(toIRType(to))(using genReceiver1.pos)
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case _ =>
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genAsInstanceOf(genReceiver, to)
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}
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} else if (sym == defn.Any_isInstanceOf) {
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genIsInstanceOf(genReceiver, to)
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} else {

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