Unreachable propagation: preserve information about unreachable values #77800
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Unreachable propagation pass removes all unreachable terminators. As a consequence, for switch int terminators, where two or more targets remain after the optimization, where otherwise branch is reachable, switching on unreachable values will use otherwise branch after the optimization. Consequently, the information about unreachable values is irrevocably lost. To preserve the information about unreachable values, keep the existing terminator if otherwise branch is reachable. Additionally, since now unreachable blocks might remain in use, remove their statements.
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@bors try @rust-timer queue |
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Awaiting bors try build completion |
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⌛ Trying commit 1ad305e with merge 48e255ae10a076b047a4ee7b01befa7cdde97d38... |
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☀️ Try build successful - checks-actions, checks-azure |
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Queued 48e255ae10a076b047a4ee7b01befa7cdde97d38 with parent 87b71ed, future comparison URL. |
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Finished benchmarking try commit (48e255ae10a076b047a4ee7b01befa7cdde97d38): comparison url. Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up. @bors rollup=never |
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No impact on compilation time. We should teach unreachable propagator about unreachable intrinsic or change how MIR is built. |
UnreachableProp: Preserve unreachable branches for multiple targets
Before, UnreachablePropagation removed all unreachable branches. This was a pessimization, as it removed information about reachability that was used later in the optimization pipeline.
For example, this code
```rust
pub enum Two { A, B }
pub fn identity(x: Two) -> Two {
match x {
Two::A => Two::A,
Two::B => Two::B,
}
}
```
basically has `switchInt() -> [0: 0, 1: 1, otherwise: unreachable]` for the match. This allows it to be transformed into a simple `x`. If we remove the unreachable branch, the transformation becomes illegal.
This was the problem keeping `UnreachablePropagation` from being enabled, so we can enable it now.
Something similar already happened in rust-lang#77800, but it did not show a perf improvement there. Let's try it again anyways!
Fixes rust-lang#68105, although that issue has been fixed for a long time (see rust-lang#77680).
Unreachable propagation pass removes all unreachable terminators.
As a consequence, for switch int terminators, where two or more targets
remain after the optimization, where otherwise branch is reachable,
switching on unreachable values will use otherwise branch after the
optimization. Consequently, the information about unreachable values is
irrevocably lost.
To preserve the information about unreachable values, keep the existing
terminator if otherwise branch is reachable. Additionally, since now
unreachable blocks might remain in use, remove their statements.
Based on analysis from #77680, also enables unreachable propagation by default.