Backport "Opt: Get rid of the LiftTry phase; instead handle things in the back-end." to LTS #20638
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…end. When we enter a `try-catch` at the JVM level, we have to make sure that the stack is empty. That's because, upon exception, the JVM wipes the stack, and we must not lose operands that are already on the stack that we will still use. Previously, this was achieved with a transformation phase, `LiftTry`, which lifted problematic `try-catch`es in local `def`s, called `liftedTree$x`. It analyzed the tree to predict which `try-catch`es would execute on a non-empty stack when eventually compiled to the JVM. This approach has several shortcomings. It exhibits performance cliffs, as the generated def can then cause more variables to be boxed in to `XRef`s. These were the only extra defs created for implementation reasons rather than for language reasons. As a user of the language, it is hard to predict when such a lifted def will be needed. The additional `liftedTree` methods also show up on stack traces and obfuscate them. Debugging can be severely hampered as well. Phases executing after `LiftTry`, notably `CapturedVars`, also had to take care not to create more problematic situations as a result of their transformations, which is hard to predict and to remember. Finally, Scala.js and Scala Native do not have the same restriction, so they received suboptimal code for no reason. In this commit, we entirely remove the `LiftTry` phase. Instead, we enhance the JVM back-end to deal with the situation. When starting a `try-catch` on a non-empty stack, we stash the entire contents of the stack into local variables. After the `try-catch`, we pop all those local variables back onto the stack. We also null out the leftover vars not to prevent garbage collection. This new approach solves all of the problems mentioned above. [Cherry-picked 52e8e74]
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No regressions detected in the community build up to lts-18731. |
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Backports #18619 to the LTS branch.
PR submitted by the release tooling.
[skip ci]