gvn: bail out unavoidable non-ssa locals in repeat #141252
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We cannot transform `*elem` to `array[idx1]` in the following code,
as `idx1` has already been modified.
```rust
mir! {
let array;
let elem;
{
array = [*val; 5];
elem = &array[idx1];
idx1 = idx2;
RET = *elem;
Return()
}
}
```
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dianqk
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May 19, 2025
| _4 = [copy (*_3); 5]; | ||
| _5 = &_4[_1]; | ||
| _1 = copy _2; | ||
| _0 = copy (*_5); |
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|
Your change does work, but I feel like we have a systematic problem that this patch doesn't fix. Isn't GVN supposed to notice that |
dianqk
commented
May 19, 2025
| @@ -682,6 +683,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> { | |||
| } | |||
| ProjectionElem::Index(idx) => { | |||
| if let Value::Repeat(inner, _) = self.get(value) { | |||
| *from_non_ssa_index |= self.locals[idx].is_none(); | |||
| return Some(*inner); | |||
| } | |||
| let idx = self.locals[idx]?; | |||
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@saethlin Yes, right here. This is an exception for the repeat array, because whichever value we access, it's all the same. However, we have overlooked the out-of-bounds case here.
|
Sorry it took me a while to convince myself this is a good strategy for fixing this. I now agree. @bors r+ |
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Rollup of 8 pull requests Successful merges: - #140367 (add `asm_cfg`: `#[cfg(...)]` within `asm!`) - #140894 (Make check-cfg diagnostics work in `#[doc(cfg(..))]`) - #141252 (gvn: bail out unavoidable non-ssa locals in repeat) - #141517 (rustdoc: use descriptive tooltip if doctest is conditionally ignored) - #141551 (Make two transmute-related MIR lints into HIR lint) - #141591 (ci: fix llvm test coverage) - #141647 (Bump master `stage0` compiler) - #141659 (Add `Result::map_or_default` and `Option::map_or_default`) r? `@ghost` `@rustbot` modify labels: rollup
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Rollup merge of #141252 - dianqk:gvn-repeat-index, r=saethlin gvn: bail out unavoidable non-ssa locals in repeat Fixes #141251. We cannot transform `*elem` to `array[idx1]` in the following code, as `idx1` has already been modified. ```rust mir! { let array; let elem; { array = [*val; 5]; elem = &array[idx1]; idx1 = idx2; RET = *elem; Return() } } ``` Perhaps I could transform it to `array[0]`, but I prefer the conservative approach. r? mir-opt
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Rollup of 8 pull requests Successful merges: - rust-lang/rust#140367 (add `asm_cfg`: `#[cfg(...)]` within `asm!`) - rust-lang/rust#140894 (Make check-cfg diagnostics work in `#[doc(cfg(..))]`) - rust-lang/rust#141252 (gvn: bail out unavoidable non-ssa locals in repeat) - rust-lang/rust#141517 (rustdoc: use descriptive tooltip if doctest is conditionally ignored) - rust-lang/rust#141551 (Make two transmute-related MIR lints into HIR lint) - rust-lang/rust#141591 (ci: fix llvm test coverage) - rust-lang/rust#141647 (Bump master `stage0` compiler) - rust-lang/rust#141659 (Add `Result::map_or_default` and `Option::map_or_default`) r? `@ghost` `@rustbot` modify labels: rollup
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Rollup of 8 pull requests Successful merges: - rust-lang#140367 (add `asm_cfg`: `#[cfg(...)]` within `asm!`) - rust-lang#140894 (Make check-cfg diagnostics work in `#[doc(cfg(..))]`) - rust-lang#141252 (gvn: bail out unavoidable non-ssa locals in repeat) - rust-lang#141517 (rustdoc: use descriptive tooltip if doctest is conditionally ignored) - rust-lang#141551 (Make two transmute-related MIR lints into HIR lint) - rust-lang#141591 (ci: fix llvm test coverage) - rust-lang#141647 (Bump master `stage0` compiler) - rust-lang#141659 (Add `Result::map_or_default` and `Option::map_or_default`) r? `@ghost` `@rustbot` modify labels: rollup
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Fixes #141251.
We cannot transform
*elemtoarray[idx1]in the following code, asidx1has already been modified.Perhaps I could transform it to
array[0], but I prefer the conservative approach.r? mir-opt