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Diffstat (limited to 'library/core/src/alloc/layout.rs')
| -rw-r--r-- | library/core/src/alloc/layout.rs | 443 |
1 files changed, 443 insertions, 0 deletions
diff --git a/library/core/src/alloc/layout.rs b/library/core/src/alloc/layout.rs new file mode 100644 index 000000000..2f378836c --- /dev/null +++ b/library/core/src/alloc/layout.rs @@ -0,0 +1,443 @@ +use crate::cmp; +use crate::fmt; +use crate::mem::{self, ValidAlign}; +use crate::ptr::NonNull; + +// While this function is used in one place and its implementation +// could be inlined, the previous attempts to do so made rustc +// slower: +// +// * https://github.com/rust-lang/rust/pull/72189 +// * https://github.com/rust-lang/rust/pull/79827 +const fn size_align<T>() -> (usize, usize) { + (mem::size_of::<T>(), mem::align_of::<T>()) +} + +/// Layout of a block of memory. +/// +/// An instance of `Layout` describes a particular layout of memory. +/// You build a `Layout` up as an input to give to an allocator. +/// +/// All layouts have an associated size and a power-of-two alignment. +/// +/// (Note that layouts are *not* required to have non-zero size, +/// even though `GlobalAlloc` requires that all memory requests +/// be non-zero in size. A caller must either ensure that conditions +/// like this are met, use specific allocators with looser +/// requirements, or use the more lenient `Allocator` interface.) +#[stable(feature = "alloc_layout", since = "1.28.0")] +#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)] +#[lang = "alloc_layout"] +pub struct Layout { + // size of the requested block of memory, measured in bytes. + size: usize, + + // alignment of the requested block of memory, measured in bytes. + // we ensure that this is always a power-of-two, because API's + // like `posix_memalign` require it and it is a reasonable + // constraint to impose on Layout constructors. + // + // (However, we do not analogously require `align >= sizeof(void*)`, + // even though that is *also* a requirement of `posix_memalign`.) + align: ValidAlign, +} + +impl Layout { + /// Constructs a `Layout` from a given `size` and `align`, + /// or returns `LayoutError` if any of the following conditions + /// are not met: + /// + /// * `align` must not be zero, + /// + /// * `align` must be a power of two, + /// + /// * `size`, when rounded up to the nearest multiple of `align`, + /// must not overflow (i.e., the rounded value must be less than + /// or equal to `usize::MAX`). + #[stable(feature = "alloc_layout", since = "1.28.0")] + #[rustc_const_stable(feature = "const_alloc_layout_size_align", since = "1.50.0")] + #[inline] + pub const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutError> { + if !align.is_power_of_two() { + return Err(LayoutError); + } + + // (power-of-two implies align != 0.) + + // Rounded up size is: + // size_rounded_up = (size + align - 1) & !(align - 1); + // + // We know from above that align != 0. If adding (align - 1) + // does not overflow, then rounding up will be fine. + // + // Conversely, &-masking with !(align - 1) will subtract off + // only low-order-bits. Thus if overflow occurs with the sum, + // the &-mask cannot subtract enough to undo that overflow. + // + // Above implies that checking for summation overflow is both + // necessary and sufficient. + if size > usize::MAX - (align - 1) { + return Err(LayoutError); + } + + // SAFETY: the conditions for `from_size_align_unchecked` have been + // checked above. + unsafe { Ok(Layout::from_size_align_unchecked(size, align)) } + } + + /// Creates a layout, bypassing all checks. + /// + /// # Safety + /// + /// This function is unsafe as it does not verify the preconditions from + /// [`Layout::from_size_align`]. + #[stable(feature = "alloc_layout", since = "1.28.0")] + #[rustc_const_stable(feature = "const_alloc_layout_unchecked", since = "1.36.0")] + #[must_use] + #[inline] + pub const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self { + // SAFETY: the caller must ensure that `align` is a power of two. + Layout { size, align: unsafe { ValidAlign::new_unchecked(align) } } + } + + /// The minimum size in bytes for a memory block of this layout. + #[stable(feature = "alloc_layout", since = "1.28.0")] + #[rustc_const_stable(feature = "const_alloc_layout_size_align", since = "1.50.0")] + #[must_use] + #[inline] + pub const fn size(&self) -> usize { + self.size + } + + /// The minimum byte alignment for a memory block of this layout. + #[stable(feature = "alloc_layout", since = "1.28.0")] + #[rustc_const_stable(feature = "const_alloc_layout_size_align", since = "1.50.0")] + #[must_use = "this returns the minimum alignment, \ + without modifying the layout"] + #[inline] + pub const fn align(&self) -> usize { + self.align.as_nonzero().get() + } + + /// Constructs a `Layout` suitable for holding a value of type `T`. + #[stable(feature = "alloc_layout", since = "1.28.0")] + #[rustc_const_stable(feature = "alloc_layout_const_new", since = "1.42.0")] + #[must_use] + #[inline] + pub const fn new<T>() -> Self { + let (size, align) = size_align::<T>(); + // SAFETY: the align is guaranteed by Rust to be a power of two and + // the size+align combo is guaranteed to fit in our address space. As a + // result use the unchecked constructor here to avoid inserting code + // that panics if it isn't optimized well enough. + unsafe { Layout::from_size_align_unchecked(size, align) } + } + + /// Produces layout describing a record that could be used to + /// allocate backing structure for `T` (which could be a trait + /// or other unsized type like a slice). + #[stable(feature = "alloc_layout", since = "1.28.0")] + #[must_use] + #[inline] + pub fn for_value<T: ?Sized>(t: &T) -> Self { + let (size, align) = (mem::size_of_val(t), mem::align_of_val(t)); + debug_assert!(Layout::from_size_align(size, align).is_ok()); + // SAFETY: see rationale in `new` for why this is using the unsafe variant + unsafe { Layout::from_size_align_unchecked(size, align) } + } + + /// Produces layout describing a record that could be used to + /// allocate backing structure for `T` (which could be a trait + /// or other unsized type like a slice). + /// + /// # Safety + /// + /// This function is only safe to call if the following conditions hold: + /// + /// - If `T` is `Sized`, this function is always safe to call. + /// - If the unsized tail of `T` is: + /// - a [slice], then the length of the slice tail must be an initialized + /// integer, and the size of the *entire value* + /// (dynamic tail length + statically sized prefix) must fit in `isize`. + /// - a [trait object], then the vtable part of the pointer must point + /// to a valid vtable for the type `T` acquired by an unsizing coercion, + /// and the size of the *entire value* + /// (dynamic tail length + statically sized prefix) must fit in `isize`. + /// - an (unstable) [extern type], then this function is always safe to + /// call, but may panic or otherwise return the wrong value, as the + /// extern type's layout is not known. This is the same behavior as + /// [`Layout::for_value`] on a reference to an extern type tail. + /// - otherwise, it is conservatively not allowed to call this function. + /// + /// [trait object]: ../../book/ch17-02-trait-objects.html + /// [extern type]: ../../unstable-book/language-features/extern-types.html + #[unstable(feature = "layout_for_ptr", issue = "69835")] + #[must_use] + pub unsafe fn for_value_raw<T: ?Sized>(t: *const T) -> Self { + // SAFETY: we pass along the prerequisites of these functions to the caller + let (size, align) = unsafe { (mem::size_of_val_raw(t), mem::align_of_val_raw(t)) }; + debug_assert!(Layout::from_size_align(size, align).is_ok()); + // SAFETY: see rationale in `new` for why this is using the unsafe variant + unsafe { Layout::from_size_align_unchecked(size, align) } + } + + /// Creates a `NonNull` that is dangling, but well-aligned for this Layout. + /// + /// Note that the pointer value may potentially represent a valid pointer, + /// which means this must not be used as a "not yet initialized" + /// sentinel value. Types that lazily allocate must track initialization by + /// some other means. + #[unstable(feature = "alloc_layout_extra", issue = "55724")] + #[rustc_const_unstable(feature = "alloc_layout_extra", issue = "55724")] + #[must_use] + #[inline] + pub const fn dangling(&self) -> NonNull<u8> { + // SAFETY: align is guaranteed to be non-zero + unsafe { NonNull::new_unchecked(crate::ptr::invalid_mut::<u8>(self.align())) } + } + + /// Creates a layout describing the record that can hold a value + /// of the same layout as `self`, but that also is aligned to + /// alignment `align` (measured in bytes). + /// + /// If `self` already meets the prescribed alignment, then returns + /// `self`. + /// + /// Note that this method does not add any padding to the overall + /// size, regardless of whether the returned layout has a different + /// alignment. In other words, if `K` has size 16, `K.align_to(32)` + /// will *still* have size 16. + /// + /// Returns an error if the combination of `self.size()` and the given + /// `align` violates the conditions listed in [`Layout::from_size_align`]. + #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")] + #[inline] + pub fn align_to(&self, align: usize) -> Result<Self, LayoutError> { + Layout::from_size_align(self.size(), cmp::max(self.align(), align)) + } + + /// Returns the amount of padding we must insert after `self` + /// to ensure that the following address will satisfy `align` + /// (measured in bytes). + /// + /// e.g., if `self.size()` is 9, then `self.padding_needed_for(4)` + /// returns 3, because that is the minimum number of bytes of + /// padding required to get a 4-aligned address (assuming that the + /// corresponding memory block starts at a 4-aligned address). + /// + /// The return value of this function has no meaning if `align` is + /// not a power-of-two. + /// + /// Note that the utility of the returned value requires `align` + /// to be less than or equal to the alignment of the starting + /// address for the whole allocated block of memory. One way to + /// satisfy this constraint is to ensure `align <= self.align()`. + #[unstable(feature = "alloc_layout_extra", issue = "55724")] + #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")] + #[must_use = "this returns the padding needed, \ + without modifying the `Layout`"] + #[inline] + pub const fn padding_needed_for(&self, align: usize) -> usize { + let len = self.size(); + + // Rounded up value is: + // len_rounded_up = (len + align - 1) & !(align - 1); + // and then we return the padding difference: `len_rounded_up - len`. + // + // We use modular arithmetic throughout: + // + // 1. align is guaranteed to be > 0, so align - 1 is always + // valid. + // + // 2. `len + align - 1` can overflow by at most `align - 1`, + // so the &-mask with `!(align - 1)` will ensure that in the + // case of overflow, `len_rounded_up` will itself be 0. + // Thus the returned padding, when added to `len`, yields 0, + // which trivially satisfies the alignment `align`. + // + // (Of course, attempts to allocate blocks of memory whose + // size and padding overflow in the above manner should cause + // the allocator to yield an error anyway.) + + let len_rounded_up = len.wrapping_add(align).wrapping_sub(1) & !align.wrapping_sub(1); + len_rounded_up.wrapping_sub(len) + } + + /// Creates a layout by rounding the size of this layout up to a multiple + /// of the layout's alignment. + /// + /// This is equivalent to adding the result of `padding_needed_for` + /// to the layout's current size. + #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")] + #[must_use = "this returns a new `Layout`, \ + without modifying the original"] + #[inline] + pub fn pad_to_align(&self) -> Layout { + let pad = self.padding_needed_for(self.align()); + // This cannot overflow. Quoting from the invariant of Layout: + // > `size`, when rounded up to the nearest multiple of `align`, + // > must not overflow (i.e., the rounded value must be less than + // > `usize::MAX`) + let new_size = self.size() + pad; + + // SAFETY: self.align is already known to be valid and new_size has been + // padded already. + unsafe { Layout::from_size_align_unchecked(new_size, self.align()) } + } + + /// Creates a layout describing the record for `n` instances of + /// `self`, with a suitable amount of padding between each to + /// ensure that each instance is given its requested size and + /// alignment. On success, returns `(k, offs)` where `k` is the + /// layout of the array and `offs` is the distance between the start + /// of each element in the array. + /// + /// On arithmetic overflow, returns `LayoutError`. + #[unstable(feature = "alloc_layout_extra", issue = "55724")] + #[inline] + pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutError> { + // This cannot overflow. Quoting from the invariant of Layout: + // > `size`, when rounded up to the nearest multiple of `align`, + // > must not overflow (i.e., the rounded value must be less than + // > `usize::MAX`) + let padded_size = self.size() + self.padding_needed_for(self.align()); + let alloc_size = padded_size.checked_mul(n).ok_or(LayoutError)?; + + // SAFETY: self.align is already known to be valid and alloc_size has been + // padded already. + unsafe { Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), padded_size)) } + } + + /// Creates a layout describing the record for `self` followed by + /// `next`, including any necessary padding to ensure that `next` + /// will be properly aligned, but *no trailing padding*. + /// + /// In order to match C representation layout `repr(C)`, you should + /// call `pad_to_align` after extending the layout with all fields. + /// (There is no way to match the default Rust representation + /// layout `repr(Rust)`, as it is unspecified.) + /// + /// Note that the alignment of the resulting layout will be the maximum of + /// those of `self` and `next`, in order to ensure alignment of both parts. + /// + /// Returns `Ok((k, offset))`, where `k` is layout of the concatenated + /// record and `offset` is the relative location, in bytes, of the + /// start of the `next` embedded within the concatenated record + /// (assuming that the record itself starts at offset 0). + /// + /// On arithmetic overflow, returns `LayoutError`. + /// + /// # Examples + /// + /// To calculate the layout of a `#[repr(C)]` structure and the offsets of + /// the fields from its fields' layouts: + /// + /// ```rust + /// # use std::alloc::{Layout, LayoutError}; + /// pub fn repr_c(fields: &[Layout]) -> Result<(Layout, Vec<usize>), LayoutError> { + /// let mut offsets = Vec::new(); + /// let mut layout = Layout::from_size_align(0, 1)?; + /// for &field in fields { + /// let (new_layout, offset) = layout.extend(field)?; + /// layout = new_layout; + /// offsets.push(offset); + /// } + /// // Remember to finalize with `pad_to_align`! + /// Ok((layout.pad_to_align(), offsets)) + /// } + /// # // test that it works + /// # #[repr(C)] struct S { a: u64, b: u32, c: u16, d: u32 } + /// # let s = Layout::new::<S>(); + /// # let u16 = Layout::new::<u16>(); + /// # let u32 = Layout::new::<u32>(); + /// # let u64 = Layout::new::<u64>(); + /// # assert_eq!(repr_c(&[u64, u32, u16, u32]), Ok((s, vec![0, 8, 12, 16]))); + /// ``` + #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")] + #[inline] + pub fn extend(&self, next: Self) -> Result<(Self, usize), LayoutError> { + let new_align = cmp::max(self.align(), next.align()); + let pad = self.padding_needed_for(next.align()); + + let offset = self.size().checked_add(pad).ok_or(LayoutError)?; + let new_size = offset.checked_add(next.size()).ok_or(LayoutError)?; + + let layout = Layout::from_size_align(new_size, new_align)?; + Ok((layout, offset)) + } + + /// Creates a layout describing the record for `n` instances of + /// `self`, with no padding between each instance. + /// + /// Note that, unlike `repeat`, `repeat_packed` does not guarantee + /// that the repeated instances of `self` will be properly + /// aligned, even if a given instance of `self` is properly + /// aligned. In other words, if the layout returned by + /// `repeat_packed` is used to allocate an array, it is not + /// guaranteed that all elements in the array will be properly + /// aligned. + /// + /// On arithmetic overflow, returns `LayoutError`. + #[unstable(feature = "alloc_layout_extra", issue = "55724")] + #[inline] + pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutError> { + let size = self.size().checked_mul(n).ok_or(LayoutError)?; + Layout::from_size_align(size, self.align()) + } + + /// Creates a layout describing the record for `self` followed by + /// `next` with no additional padding between the two. Since no + /// padding is inserted, the alignment of `next` is irrelevant, + /// and is not incorporated *at all* into the resulting layout. + /// + /// On arithmetic overflow, returns `LayoutError`. + #[unstable(feature = "alloc_layout_extra", issue = "55724")] + #[inline] + pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutError> { + let new_size = self.size().checked_add(next.size()).ok_or(LayoutError)?; + Layout::from_size_align(new_size, self.align()) + } + + /// Creates a layout describing the record for a `[T; n]`. + /// + /// On arithmetic overflow, returns `LayoutError`. + #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")] + #[inline] + pub fn array<T>(n: usize) -> Result<Self, LayoutError> { + let array_size = mem::size_of::<T>().checked_mul(n).ok_or(LayoutError)?; + + // SAFETY: + // - Size: `array_size` cannot be too big because `size_of::<T>()` must + // be a multiple of `align_of::<T>()`. Therefore, `array_size` + // rounded up to the nearest multiple of `align_of::<T>()` is just + // `array_size`. And `array_size` cannot be too big because it was + // just checked by the `checked_mul()`. + // - Alignment: `align_of::<T>()` will always give an acceptable + // (non-zero, power of two) alignment. + Ok(unsafe { Layout::from_size_align_unchecked(array_size, mem::align_of::<T>()) }) + } +} + +#[stable(feature = "alloc_layout", since = "1.28.0")] +#[deprecated( + since = "1.52.0", + note = "Name does not follow std convention, use LayoutError", + suggestion = "LayoutError" +)] +pub type LayoutErr = LayoutError; + +/// The parameters given to `Layout::from_size_align` +/// or some other `Layout` constructor +/// do not satisfy its documented constraints. +#[stable(feature = "alloc_layout_error", since = "1.50.0")] +#[non_exhaustive] +#[derive(Clone, PartialEq, Eq, Debug)] +pub struct LayoutError; + +// (we need this for downstream impl of trait Error) +#[stable(feature = "alloc_layout", since = "1.28.0")] +impl fmt::Display for LayoutError { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + f.write_str("invalid parameters to Layout::from_size_align") + } +} |