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Auto merge of #113892 - RalfJung:uninit-undef-poison, r=wesleywiser

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clarify MIR uninit vs LLVM undef/poison

In [this LLVM discussion](https://discourse.llvm.org/t/rfc-load-instruction-uninitialized-memory-semantics/67481) 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.
This commit is contained in:
bors 2023-07-21 19:32:17 +00:00
commit d908a5b08e
2 changed files with 7 additions and 1 deletions
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6
compiler/rustc_codegen_ssa/src/traits/consts.rs
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@ -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.
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;

2
compiler/rustc_const_eval/src/interpret/operand.rs
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@ -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,
}