diff options
| -rw-r--r-- | compiler/rustc_data_structures/src/tagged_ptr.rs | 55 | ||||
| -rw-r--r-- | compiler/rustc_middle/src/ty/list.rs | 3 |
2 files changed, 42 insertions, 16 deletions
diff --git a/compiler/rustc_data_structures/src/tagged_ptr.rs b/compiler/rustc_data_structures/src/tagged_ptr.rs index 651bc556c98..c96d7835b2b 100644 --- a/compiler/rustc_data_structures/src/tagged_ptr.rs +++ b/compiler/rustc_data_structures/src/tagged_ptr.rs @@ -24,32 +24,45 @@ mod drop; pub use copy::CopyTaggedPtr; pub use drop::TaggedPtr; -/// This describes the pointer type encapsulated by TaggedPtr. +/// This describes the pointer type encapsulated by [`TaggedPtr`] and +/// [`CopyTaggedPtr`]. /// /// # Safety /// /// The usize returned from `into_usize` must be a valid, dereferenceable, -/// pointer to `<Self as Deref>::Target`. Note that pointers to `Pointee` must -/// be thin, even though `Pointee` may not be sized. +/// pointer to [`<Self as Deref>::Target`]. Note that pointers to +/// [`Self::Target`] must be thin, even though [`Self::Target`] may not be +/// `Sized`. /// /// Note that the returned pointer from `into_usize` should be castable to `&mut -/// <Self as Deref>::Target` if `Pointer: DerefMut`. +/// <Self as Deref>::Target` if `Self: DerefMut`. /// /// The BITS constant must be correct. At least `BITS` bits, least-significant, /// must be zero on all returned pointers from `into_usize`. /// -/// For example, if the alignment of `Pointee` is 2, then `BITS` should be 1. +/// For example, if the alignment of [`Self::Target`] is 2, then `BITS` should be 1. +/// +/// [`<Self as Deref>::Target`]: Deref::Target +/// [`Self::Target`]: Deref::Target pub unsafe trait Pointer: Deref { + /// Number of unused (always zero) **least significant bits** in this + /// pointer, usually related to the pointees alignment. + /// /// Most likely the value you want to use here is the following, unless - /// your Pointee type is unsized (e.g., `ty::List<T>` in rustc) in which - /// case you'll need to manually figure out what the right type to pass to - /// align_of is. + /// your [`Self::Target`] type is unsized (e.g., `ty::List<T>` in rustc) + /// or your pointer is over/under aligned, in which case you'll need to + /// manually figure out what the right type to pass to [`bits_for`] is, or + /// what the value to set here. /// - /// ```ignore UNSOLVED (what to do about the Self) + /// ```rust /// # use std::ops::Deref; - /// std::mem::align_of::<<Self as Deref>::Target>().trailing_zeros() as usize; + /// # type Self = &'static u64; + /// bits_for::<Self::Target>() /// ``` + /// + /// [`Self::Target`]: Deref::Target const BITS: usize; + fn into_usize(self) -> usize; /// # Safety @@ -90,7 +103,7 @@ pub unsafe trait Tag: Copy { } unsafe impl<T> Pointer for Box<T> { - const BITS: usize = std::mem::align_of::<T>().trailing_zeros() as usize; + const BITS: usize = bits_for::<Self::Target>(); #[inline] fn into_usize(self) -> usize { Box::into_raw(self) as usize @@ -106,7 +119,7 @@ unsafe impl<T> Pointer for Box<T> { } unsafe impl<T> Pointer for Rc<T> { - const BITS: usize = std::mem::align_of::<T>().trailing_zeros() as usize; + const BITS: usize = bits_for::<Self::Target>(); #[inline] fn into_usize(self) -> usize { Rc::into_raw(self) as usize @@ -122,7 +135,7 @@ unsafe impl<T> Pointer for Rc<T> { } unsafe impl<T> Pointer for Arc<T> { - const BITS: usize = std::mem::align_of::<T>().trailing_zeros() as usize; + const BITS: usize = bits_for::<Self::Target>(); #[inline] fn into_usize(self) -> usize { Arc::into_raw(self) as usize @@ -138,7 +151,7 @@ unsafe impl<T> Pointer for Arc<T> { } unsafe impl<'a, T: 'a> Pointer for &'a T { - const BITS: usize = std::mem::align_of::<T>().trailing_zeros() as usize; + const BITS: usize = bits_for::<Self::Target>(); #[inline] fn into_usize(self) -> usize { self as *const T as usize @@ -153,7 +166,7 @@ unsafe impl<'a, T: 'a> Pointer for &'a T { } unsafe impl<'a, T: 'a> Pointer for &'a mut T { - const BITS: usize = std::mem::align_of::<T>().trailing_zeros() as usize; + const BITS: usize = bits_for::<Self::Target>(); #[inline] fn into_usize(self) -> usize { self as *mut T as usize @@ -166,3 +179,15 @@ unsafe impl<'a, T: 'a> Pointer for &'a mut T { f(&*(&ptr as *const usize as *const Self)) } } + +/// Returns the number of bits available for use for tags in a pointer to `T` +/// (this is based on `T`'s alignment). +pub const fn bits_for<T>() -> usize { + let bits = std::mem::align_of::<T>().trailing_zeros(); + + // This is a replacement for `.try_into().unwrap()` unavailable in `const` + // (it's fine to make an assert here, since this is only called in compile time) + assert!((bits as u128) < usize::MAX as u128); + + bits as usize +} diff --git a/compiler/rustc_middle/src/ty/list.rs b/compiler/rustc_middle/src/ty/list.rs index 79365ef281b..4526487cf1d 100644 --- a/compiler/rustc_middle/src/ty/list.rs +++ b/compiler/rustc_middle/src/ty/list.rs @@ -1,4 +1,5 @@ use crate::arena::Arena; +use rustc_data_structures::tagged_ptr::bits_for; use rustc_serialize::{Encodable, Encoder}; use std::alloc::Layout; use std::cmp::Ordering; @@ -199,7 +200,7 @@ impl<'a, T: Copy> IntoIterator for &'a List<T> { unsafe impl<T: Sync> Sync for List<T> {} unsafe impl<'a, T: 'a> rustc_data_structures::tagged_ptr::Pointer for &'a List<T> { - const BITS: usize = std::mem::align_of::<usize>().trailing_zeros() as usize; + const BITS: usize = bits_for::<usize>(); #[inline] fn into_usize(self) -> usize { |