clarify MIR uninit vs LLVM undef/poison #113892
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r? @wesleywiser (rustbot has picked a reviewer for you, use r? to override) |
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Some changes occurred to the CTFE / Miri engine cc @rust-lang/miri |
| @@ -5,7 +5,13 @@ use rustc_target::abi; | |||
| pub trait ConstMethods<'tcx>: BackendTypes { | |||
| // Constant constructors | |||
| fn const_null(&self, t: Self::Type) -> Self::Value; | |||
| /// Generate an uninitialized value (matching uninitialized memory in MIR). | |||
| /// Whether memory is initialized or not is tracked byte-for-byte. | |||
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I don't know whether it matters, but I think it might be bit-for-bit in LLVM? The way I understand langref, (undef << 1) & 1 in LLVM is definitely 0, though in Rust I think we'd still treat it as uninit.
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Not sure if that's true for all backends though. Ultimately what matters is MIR semantics; it is then the backend's responsibility to find an appropriate translation.
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@bors r+ rollup |
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☀️ Test successful - checks-actions |
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Finished benchmarking commit (d908a5b): comparison URL. Overall result: ✅ improvements - no action needed@rustbot label: -perf-regression Instruction countThis is a highly reliable metric that was used to determine the overall result at the top of this comment.
Max RSS (memory usage)ResultsThis is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.
CyclesResultsThis is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.
Binary sizeThis benchmark run did not return any relevant results for this metric. Bootstrap: 658.525s -> 658.594s (0.01%) |
In this LLVM discussion I learned that mapping our uninitialized memory in MIR to poison in LLVM would be quite problematic due to the lack of a byte type. I am not sure where to write down this insight but this seems like a reasonable start.