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Backport "Opt: Get rid of the LiftTry phase; instead handle things in the back-end." to LTS #20691

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merged 2 commits into from
Jun 20, 2024

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WojciechMazur
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Backports #18619 to the LTS branch.

PR submitted by the release tooling.
[skip ci]

sjrd added 2 commits June 20, 2024 15:13
…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]
Base automatically changed from lts-18617 to lts-3.3 June 20, 2024 14:56
@WojciechMazur
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No regressions detected in the community build up to lts-18731.

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@WojciechMazur WojciechMazur merged commit d54491c into lts-3.3 Jun 20, 2024
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@WojciechMazur WojciechMazur deleted the lts-18619 branch June 20, 2024 14:56
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2 participants