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Correct outdated object size limit

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The comment here about 48 bit addresses being enough was written in 2016
but was made incorrect in 2019 by 5-level paging, and then persisted for
another 5 years before being noticed and corrected.
This commit is contained in:
Jubilee Young 2024-07-09 18:09:56 -07:00
parent 506f22b466
commit 42dbf29b48
1 changed files with 4 additions and 6 deletions
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10
compiler/rustc_abi/src/lib.rs
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@ -337,23 +337,21 @@ impl TargetDataLayout {
Ok(dl) Ok(dl)
} }
/// Returns exclusive upper bound on object size. /// Returns **exclusive** upper bound on object size.
/// ///
/// The theoretical maximum object size is defined as the maximum positive `isize` value. /// The theoretical maximum object size is defined as the maximum positive `isize` value.
/// This ensures that the `offset` semantics remain well-defined by allowing it to correctly /// This ensures that the `offset` semantics remain well-defined by allowing it to correctly
/// index every address within an object along with one byte past the end, along with allowing /// index every address within an object along with one byte past the end, along with allowing
/// `isize` to store the difference between any two pointers into an object. /// `isize` to store the difference between any two pointers into an object.
/// ///
/// The upper bound on 64-bit currently needs to be lower because LLVM uses a 64-bit integer /// LLVM uses a 64-bit integer to represent object size in bits, but we care only for bytes,
/// to represent object size in bits. It would need to be 1 << 61 to account for this, but is /// so we adopt such a more-constrained address space due to its technical limitations.
/// currently conservatively bounded to 1 << 47 as that is enough to cover the current usable
/// address space on 64-bit ARMv8 and x86_64.
#[inline] #[inline]
pub fn obj_size_bound(&self) -> u64 { pub fn obj_size_bound(&self) -> u64 {
match self.pointer_size.bits() { match self.pointer_size.bits() {
16 => 1 << 15, 16 => 1 << 15,
32 => 1 << 31, 32 => 1 << 31,
64 => 1 << 47, 64 => 1 << 61,
bits => panic!("obj_size_bound: unknown pointer bit size {bits}"), bits => panic!("obj_size_bound: unknown pointer bit size {bits}"),
} }
} }