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clarify MIR uninit vs LLVM undef/poison #113892

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Jul 21, 2023
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clarify MIR uninit vs LLVM undef/poison #113892

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6 changes: 6 additions & 0 deletions compiler/rustc_codegen_ssa/src/traits/consts.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -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.

fn const_undef(&self, t: Self::Type) -> Self::Value;
/// Generate a fake value. Poison always affects the entire value, even if just a single byte is
/// poison. This can only be used in codepaths that are already UB, i.e., UB-free Rust code
/// (including code that e.g. copies uninit memory with `MaybeUninit`) can never encounter a
/// poison value.
fn const_poison(&self, t: Self::Type) -> Self::Value;
fn const_int(&self, t: Self::Type, i: i64) -> Self::Value;
fn const_uint(&self, t: Self::Type, i: u64) -> Self::Value;
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2 changes: 1 addition & 1 deletion compiler/rustc_const_eval/src/interpret/operand.rs
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Expand Up @@ -31,7 +31,7 @@ pub enum Immediate<Prov: Provenance = AllocId> {
/// A pair of two scalar value (must have `ScalarPair` ABI where both fields are
/// `Scalar::Initialized`).
ScalarPair(Scalar<Prov>, Scalar<Prov>),
/// A value of fully uninitialized memory. Can have and size and layout.
/// A value of fully uninitialized memory. Can have arbitrary size and layout.
Uninit,
}

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