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()
}
}
```
rustbot
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dianqk
commented
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.
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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