Heejin Ahn 473ef10b0f
[WebAssembly] Demote PHIs in catchswitch BB only (#81570)
`DemoteCatchSwitchPHIOnly` option in `WinEHPrepare` pass was added in
99d60e0dab,
because Wasm EH uses `WinEHPrepare`, but it doesn't need to demote all
PHIs. PHIs in `catchswitch` BBs have to be removed (= demoted) because
`catchswitch`s are removed in ISel and `catchswitch` BBs are removed as
well, so they can't have other instructions.

But because Wasm EH doesn't use funclets, so PHIs in `catchpad` or
`cleanuppad` BBs don't need to be demoted. That was the reason
`DemoteCatchSwitchPHIOnly` option was added, in order not to demote more
instructions unnecessarily.

The problem is it should have been set to `true` for Wasm EH. (Its
default value is `false` for WinEH) And I mistakenly set it to `false`
and wasn't aware about this for more than 5 years. This was not the end
of the world; it just means we've been demoting more instructions than
we should, possibly huting code size. In practice I think it would've
had hardly any effect in real performance given that the occurrence of
PHIs in `catchpad` or `cleanuppad` BBs are not very frequent and many
people run other optimizers like Binaryen anyway.
2024-02-13 13:43:21 -08:00
..
2023-09-26 14:25:11 +01:00
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2023-01-26 10:26:24 +00:00
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