Rustup #2668
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Rustup #2668
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Add new MIR constant propagation based on dataflow analysis
The current constant propagation in `rustc_mir_transform/src/const_prop.rs` fails to handle many cases that would be expected from a constant propagation optimization. For example:
```rust
let x = if true { 0 } else { 0 };
```
This pull request adds a new constant propagation MIR optimization pass based on the existing dataflow analysis framework. Since most of the analysis is not unique to constant propagation, a generic framework has been extracted. It works on top of the existing framework and could be reused for other optimzations.
Closes #80038. Closes #81605.
## Todo
### Essential
- [x] [Writes to inactive enum variants](rust-lang/rust#101168 (review)). Resolved by rejecting the registration of places with downcast projections for now. Could be improved by flooding other variants if mutable access to a variant is observed.
- [X] Handle [`StatementKind::CopyNonOverlapping`](rust-lang/rust#101168 (comment)). Resolved by flooding the destination.
- [x] Handle `UnsafeCell` / `!Freeze` correctly.
- [X] Overflow propagation of `CheckedBinaryOp`: Decided to not propagate if overflow flag is `true` (`false` will still be propagated)
- [x] More documentation in general.
- [x] Arguments for correctness, documentation of necessary assumptions.
- [x] Better performance, or alternatively, require `-Zmir-opt-level=3` for now.
### Extra
- [x] Add explicit unreachability, i.e. upgrading the lattice from $\mathbb{P} \to \mathbb{V}$ to $\set{\bot} \cup (\mathbb{P} \to \mathbb{V})$.
- [x] Use storage statements to improve precision.
- [ ] Consider opening issue for duplicate diagnostics: rust-lang/rust#101168 (comment)
- [ ] Flood moved-from places with $\bot$ (requires some changes for places with tracked projections).
- [ ] Add downcast projections back in.
- [ ] [Algebraic simplifications](rust-lang/rust#101168 (comment)) (possibly with a shared API; done by old const prop).
- [ ] Propagation through slices / arrays.
- [ ] Find other optimizations that are done by old `const_prop.rs`, but not by this one.
interpret: support for per-byte provenance Also factors the provenance map into its own module. The third commit does the same for the init mask. I can move it in a separate PR if you prefer. Fixes rust-lang#2181 r? `@oli-obk`
Perform simple scalar replacement of aggregates (SROA) MIR opt This is a re-open of rust-lang/rust#85796 I copied the debuginfo implementation (first commit) from `@eddyb's` own SROA PR. This pass replaces plain field accesses by simple locals when possible. To be eligible, the replaced locals: - must not be enums or unions; - must not be used whole; - must not have their address taken. The storage and deinit statements are duplicated on each created local. cc `@tmiasko` who reviewed the former version of this PR.
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@bors r+ |
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☀️ Test successful - checks-actions |
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