library/core/src/alloc/layout.rs

   1 use crate::cmp;
   2 use crate::fmt;
   3 use crate::mem;
   4 use crate::num::NonZeroUsize;
   5 use crate::ptr::NonNull;
   6
   7 // While this function is used in one place and its implementation
   8 // could be inlined, the previous attempts to do so made rustc
   9 // slower:
  10 //
  11 // * https://github.com/rust-lang/rust/pull/72189
  12 // * https://github.com/rust-lang/rust/pull/79827
  13 const fn size_align<T>() -> (usize, usize) {
  14     (mem::size_of::<T>(), mem::align_of::<T>())
  15 }
  16
  17 /// Layout of a block of memory.
  18 ///
  19 /// An instance of `Layout` describes a particular layout of memory.
  20 /// You build a `Layout` up as an input to give to an allocator.
  21 ///
  22 /// All layouts have an associated size and a power-of-two alignment.
  23 ///
  24 /// (Note that layouts are *not* required to have non-zero size,
  25 /// even though `GlobalAlloc` requires that all memory requests
  26 /// be non-zero in size. A caller must either ensure that conditions
  27 /// like this are met, use specific allocators with looser
  28 /// requirements, or use the more lenient `Allocator` interface.)
  29 #[stable(feature = "alloc_layout", since = "1.28.0")]
  30 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
  31 #[lang = "alloc_layout"]
  32 pub struct Layout {
  33     // size of the requested block of memory, measured in bytes.
  34     size_: usize,
  35
  36     // alignment of the requested block of memory, measured in bytes.
  37     // we ensure that this is always a power-of-two, because API's
  38     // like `posix_memalign` require it and it is a reasonable
  39     // constraint to impose on Layout constructors.
  40     //
  41     // (However, we do not analogously require `align >= sizeof(void*)`,
  42     //  even though that is *also* a requirement of `posix_memalign`.)
  43     align_: NonZeroUsize,
  44 }
  45
  46 impl Layout {
  47     /// Constructs a `Layout` from a given `size` and `align`,
  48     /// or returns `LayoutError` if any of the following conditions
  49     /// are not met:
  50     ///
  51     /// * `align` must not be zero,
  52     ///
  53     /// * `align` must be a power of two,
  54     ///
  55     /// * `size`, when rounded up to the nearest multiple of `align`,
  56     ///    must not overflow (i.e., the rounded value must be less than
  57     ///    or equal to `usize::MAX`).
  58     #[stable(feature = "alloc_layout", since = "1.28.0")]
  59     #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
  60     #[inline]
  61     pub const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutError> {
  62         if !align.is_power_of_two() {
  63             return Err(LayoutError { private: () });
  64         }
  65
  66         // (power-of-two implies align != 0.)
  67
  68         // Rounded up size is:
  69         //   size_rounded_up = (size + align - 1) & !(align - 1);
  70         //
  71         // We know from above that align != 0. If adding (align - 1)
  72         // does not overflow, then rounding up will be fine.
  73         //
  74         // Conversely, &-masking with !(align - 1) will subtract off
  75         // only low-order-bits. Thus if overflow occurs with the sum,
  76         // the &-mask cannot subtract enough to undo that overflow.
  77         //
  78         // Above implies that checking for summation overflow is both
  79         // necessary and sufficient.
  80         if size > usize::MAX - (align - 1) {
  81             return Err(LayoutError { private: () });
  82         }
  83
  84         // SAFETY: the conditions for `from_size_align_unchecked` have been
  85         // checked above.
  86         unsafe { Ok(Layout::from_size_align_unchecked(size, align)) }
  87     }
  88
  89     /// Creates a layout, bypassing all checks.
  90     ///
  91     /// # Safety
  92     ///
  93     /// This function is unsafe as it does not verify the preconditions from
  94     /// [`Layout::from_size_align`].
  95     #[stable(feature = "alloc_layout", since = "1.28.0")]
  96     #[rustc_const_stable(feature = "alloc_layout", since = "1.28.0")]
  97     #[inline]
  98     pub const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self {
  99         // SAFETY: the caller must ensure that `align` is greater than zero.
 100         Layout { size_: size, align_: unsafe { NonZeroUsize::new_unchecked(align) } }
 101     }
 102
 103     /// The minimum size in bytes for a memory block of this layout.
 104     #[stable(feature = "alloc_layout", since = "1.28.0")]
 105     #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
 106     #[inline]
 107     pub const fn size(&self) -> usize {
 108         self.size_
 109     }
 110
 111     /// The minimum byte alignment for a memory block of this layout.
 112     #[stable(feature = "alloc_layout", since = "1.28.0")]
 113     #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
 114     #[inline]
 115     pub const fn align(&self) -> usize {
 116         self.align_.get()
 117     }
 118
 119     /// Constructs a `Layout` suitable for holding a value of type `T`.
 120     #[stable(feature = "alloc_layout", since = "1.28.0")]
 121     #[rustc_const_stable(feature = "alloc_layout_const_new", since = "1.42.0")]
 122     #[inline]
 123     pub const fn new<T>() -> Self {
 124         let (size, align) = size_align::<T>();
 125         // SAFETY: the align is guaranteed by Rust to be a power of two and
 126         // the size+align combo is guaranteed to fit in our address space. As a
 127         // result use the unchecked constructor here to avoid inserting code
 128         // that panics if it isn't optimized well enough.
 129         unsafe { Layout::from_size_align_unchecked(size, align) }
 130     }
 131
 132     /// Produces layout describing a record that could be used to
 133     /// allocate backing structure for `T` (which could be a trait
 134     /// or other unsized type like a slice).
 135     #[stable(feature = "alloc_layout", since = "1.28.0")]
 136     #[inline]
 137     pub fn for_value<T: ?Sized>(t: &T) -> Self {
 138         let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
 139         debug_assert!(Layout::from_size_align(size, align).is_ok());
 140         // SAFETY: see rationale in `new` for why this is using the unsafe variant
 141         unsafe { Layout::from_size_align_unchecked(size, align) }
 142     }
 143
 144     /// Produces layout describing a record that could be used to
 145     /// allocate backing structure for `T` (which could be a trait
 146     /// or other unsized type like a slice).
 147     ///
 148     /// # Safety
 149     ///
 150     /// This function is only safe to call if the following conditions hold:
 151     ///
 152     /// - If `T` is `Sized`, this function is always safe to call.
 153     /// - If the unsized tail of `T` is:
 154     ///     - a [slice], then the length of the slice tail must be an intialized
 155     ///       integer, and the size of the *entire value*
 156     ///       (dynamic tail length + statically sized prefix) must fit in `isize`.
 157     ///     - a [trait object], then the vtable part of the pointer must point
 158     ///       to a valid vtable for the type `T` acquired by an unsizing coersion,
 159     ///       and the size of the *entire value*
 160     ///       (dynamic tail length + statically sized prefix) must fit in `isize`.
 161     ///     - an (unstable) [extern type], then this function is always safe to
 162     ///       call, but may panic or otherwise return the wrong value, as the
 163     ///       extern type's layout is not known. This is the same behavior as
 164     ///       [`Layout::for_value`] on a reference to an extern type tail.
 165     ///     - otherwise, it is conservatively not allowed to call this function.
 166     ///
 167     /// [slice]: ../../std/primitive.slice.html
 168     /// [trait object]: ../../book/ch17-02-trait-objects.html
 169     /// [extern type]: ../../unstable-book/language-features/extern-types.html
 170     #[unstable(feature = "layout_for_ptr", issue = "69835")]
 171     pub unsafe fn for_value_raw<T: ?Sized>(t: *const T) -> Self {
 172         // SAFETY: we pass along the prerequisites of these functions to the caller
 173         let (size, align) = unsafe { (mem::size_of_val_raw(t), mem::align_of_val_raw(t)) };
 174         debug_assert!(Layout::from_size_align(size, align).is_ok());
 175         // SAFETY: see rationale in `new` for why this is using the unsafe variant
 176         unsafe { Layout::from_size_align_unchecked(size, align) }
 177     }
 178
 179     /// Creates a `NonNull` that is dangling, but well-aligned for this Layout.
 180     ///
 181     /// Note that the pointer value may potentially represent a valid pointer,
 182     /// which means this must not be used as a "not yet initialized"
 183     /// sentinel value. Types that lazily allocate must track initialization by
 184     /// some other means.
 185     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
 186     #[rustc_const_unstable(feature = "alloc_layout_extra", issue = "55724")]
 187     #[inline]
 188     pub const fn dangling(&self) -> NonNull<u8> {
 189         // SAFETY: align is guaranteed to be non-zero
 190         unsafe { NonNull::new_unchecked(self.align() as *mut u8) }
 191     }
 192
 193     /// Creates a layout describing the record that can hold a value
 194     /// of the same layout as `self`, but that also is aligned to
 195     /// alignment `align` (measured in bytes).
 196     ///
 197     /// If `self` already meets the prescribed alignment, then returns
 198     /// `self`.
 199     ///
 200     /// Note that this method does not add any padding to the overall
 201     /// size, regardless of whether the returned layout has a different
 202     /// alignment. In other words, if `K` has size 16, `K.align_to(32)`
 203     /// will *still* have size 16.
 204     ///
 205     /// Returns an error if the combination of `self.size()` and the given
 206     /// `align` violates the conditions listed in [`Layout::from_size_align`].
 207     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
 208     #[inline]
 209     pub fn align_to(&self, align: usize) -> Result<Self, LayoutError> {
 210         Layout::from_size_align(self.size(), cmp::max(self.align(), align))
 211     }
 212
 213     /// Returns the amount of padding we must insert after `self`
 214     /// to ensure that the following address will satisfy `align`
 215     /// (measured in bytes).
 216     ///
 217     /// e.g., if `self.size()` is 9, then `self.padding_needed_for(4)`
 218     /// returns 3, because that is the minimum number of bytes of
 219     /// padding required to get a 4-aligned address (assuming that the
 220     /// corresponding memory block starts at a 4-aligned address).
 221     ///
 222     /// The return value of this function has no meaning if `align` is
 223     /// not a power-of-two.
 224     ///
 225     /// Note that the utility of the returned value requires `align`
 226     /// to be less than or equal to the alignment of the starting
 227     /// address for the whole allocated block of memory. One way to
 228     /// satisfy this constraint is to ensure `align <= self.align()`.
 229     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
 230     #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
 231     #[inline]
 232     pub const fn padding_needed_for(&self, align: usize) -> usize {
 233         let len = self.size();
 234
 235         // Rounded up value is:
 236         //   len_rounded_up = (len + align - 1) & !(align - 1);
 237         // and then we return the padding difference: `len_rounded_up - len`.
 238         //
 239         // We use modular arithmetic throughout:
 240         //
 241         // 1. align is guaranteed to be > 0, so align - 1 is always
 242         //    valid.
 243         //
 244         // 2. `len + align - 1` can overflow by at most `align - 1`,
 245         //    so the &-mask with `!(align - 1)` will ensure that in the
 246         //    case of overflow, `len_rounded_up` will itself be 0.
 247         //    Thus the returned padding, when added to `len`, yields 0,
 248         //    which trivially satisfies the alignment `align`.
 249         //
 250         // (Of course, attempts to allocate blocks of memory whose
 251         // size and padding overflow in the above manner should cause
 252         // the allocator to yield an error anyway.)
 253
 254         let len_rounded_up = len.wrapping_add(align).wrapping_sub(1) & !align.wrapping_sub(1);
 255         len_rounded_up.wrapping_sub(len)
 256     }
 257
 258     /// Creates a layout by rounding the size of this layout up to a multiple
 259     /// of the layout's alignment.
 260     ///
 261     /// This is equivalent to adding the result of `padding_needed_for`
 262     /// to the layout's current size.
 263     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
 264     #[inline]
 265     pub fn pad_to_align(&self) -> Layout {
 266         let pad = self.padding_needed_for(self.align());
 267         // This cannot overflow. Quoting from the invariant of Layout:
 268         // > `size`, when rounded up to the nearest multiple of `align`,
 269         // > must not overflow (i.e., the rounded value must be less than
 270         // > `usize::MAX`)
 271         let new_size = self.size() + pad;
 272
 273         Layout::from_size_align(new_size, self.align()).unwrap()
 274     }
 275
 276     /// Creates a layout describing the record for `n` instances of
 277     /// `self`, with a suitable amount of padding between each to
 278     /// ensure that each instance is given its requested size and
 279     /// alignment. On success, returns `(k, offs)` where `k` is the
 280     /// layout of the array and `offs` is the distance between the start
 281     /// of each element in the array.
 282     ///
 283     /// On arithmetic overflow, returns `LayoutError`.
 284     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
 285     #[inline]
 286     pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutError> {
 287         // This cannot overflow. Quoting from the invariant of Layout:
 288         // > `size`, when rounded up to the nearest multiple of `align`,
 289         // > must not overflow (i.e., the rounded value must be less than
 290         // > `usize::MAX`)
 291         let padded_size = self.size() + self.padding_needed_for(self.align());
 292         let alloc_size = padded_size.checked_mul(n).ok_or(LayoutError { private: () })?;
 293
 294         // SAFETY: self.align is already known to be valid and alloc_size has been
 295         // padded already.
 296         unsafe { Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), padded_size)) }
 297     }
 298
 299     /// Creates a layout describing the record for `self` followed by
 300     /// `next`, including any necessary padding to ensure that `next`
 301     /// will be properly aligned, but *no trailing padding*.
 302     ///
 303     /// In order to match C representation layout `repr(C)`, you should
 304     /// call `pad_to_align` after extending the layout with all fields.
 305     /// (There is no way to match the default Rust representation
 306     /// layout `repr(Rust)`, as it is unspecified.)
 307     ///
 308     /// Note that the alignment of the resulting layout will be the maximum of
 309     /// those of `self` and `next`, in order to ensure alignment of both parts.
 310     ///
 311     /// Returns `Ok((k, offset))`, where `k` is layout of the concatenated
 312     /// record and `offset` is the relative location, in bytes, of the
 313     /// start of the `next` embedded within the concatenated record
 314     /// (assuming that the record itself starts at offset 0).
 315     ///
 316     /// On arithmetic overflow, returns `LayoutError`.
 317     ///
 318     /// # Examples
 319     ///
 320     /// To calculate the layout of a `#[repr(C)]` structure and the offsets of
 321     /// the fields from its fields' layouts:
 322     ///
 323     /// ```rust
 324     /// # use std::alloc::{Layout, LayoutError};
 325     /// pub fn repr_c(fields: &[Layout]) -> Result<(Layout, Vec<usize>), LayoutError> {
 326     ///     let mut offsets = Vec::new();
 327     ///     let mut layout = Layout::from_size_align(0, 1)?;
 328     ///     for &field in fields {
 329     ///         let (new_layout, offset) = layout.extend(field)?;
 330     ///         layout = new_layout;
 331     ///         offsets.push(offset);
 332     ///     }
 333     ///     // Remember to finalize with `pad_to_align`!
 334     ///     Ok((layout.pad_to_align(), offsets))
 335     /// }
 336     /// # // test that it works
 337     /// # #[repr(C)] struct S { a: u64, b: u32, c: u16, d: u32 }
 338     /// # let s = Layout::new::<S>();
 339     /// # let u16 = Layout::new::<u16>();
 340     /// # let u32 = Layout::new::<u32>();
 341     /// # let u64 = Layout::new::<u64>();
 342     /// # assert_eq!(repr_c(&[u64, u32, u16, u32]), Ok((s, vec![0, 8, 12, 16])));
 343     /// ```
 344     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
 345     #[inline]
 346     pub fn extend(&self, next: Self) -> Result<(Self, usize), LayoutError> {
 347         let new_align = cmp::max(self.align(), next.align());
 348         let pad = self.padding_needed_for(next.align());
 349
 350         let offset = self.size().checked_add(pad).ok_or(LayoutError { private: () })?;
 351         let new_size = offset.checked_add(next.size()).ok_or(LayoutError { private: () })?;
 352
 353         let layout = Layout::from_size_align(new_size, new_align)?;
 354         Ok((layout, offset))
 355     }
 356
 357     /// Creates a layout describing the record for `n` instances of
 358     /// `self`, with no padding between each instance.
 359     ///
 360     /// Note that, unlike `repeat`, `repeat_packed` does not guarantee
 361     /// that the repeated instances of `self` will be properly
 362     /// aligned, even if a given instance of `self` is properly
 363     /// aligned. In other words, if the layout returned by
 364     /// `repeat_packed` is used to allocate an array, it is not
 365     /// guaranteed that all elements in the array will be properly
 366     /// aligned.
 367     ///
 368     /// On arithmetic overflow, returns `LayoutError`.
 369     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
 370     #[inline]
 371     pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutError> {
 372         let size = self.size().checked_mul(n).ok_or(LayoutError { private: () })?;
 373         Layout::from_size_align(size, self.align())
 374     }
 375
 376     /// Creates a layout describing the record for `self` followed by
 377     /// `next` with no additional padding between the two. Since no
 378     /// padding is inserted, the alignment of `next` is irrelevant,
 379     /// and is not incorporated *at all* into the resulting layout.
 380     ///
 381     /// On arithmetic overflow, returns `LayoutError`.
 382     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
 383     #[inline]
 384     pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutError> {
 385         let new_size = self.size().checked_add(next.size()).ok_or(LayoutError { private: () })?;
 386         Layout::from_size_align(new_size, self.align())
 387     }
 388
 389     /// Creates a layout describing the record for a `[T; n]`.
 390     ///
 391     /// On arithmetic overflow, returns `LayoutError`.
 392     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
 393     #[inline]
 394     pub fn array<T>(n: usize) -> Result<Self, LayoutError> {
 395         let (layout, offset) = Layout::new::<T>().repeat(n)?;
 396         debug_assert_eq!(offset, mem::size_of::<T>());
 397         Ok(layout.pad_to_align())
 398     }
 399 }
 400
 401 #[stable(feature = "alloc_layout", since = "1.28.0")]
 402 #[rustc_deprecated(
 403     since = "1.52.0",
 404     reason = "Name does not follow std convention, use LayoutError",
 405     suggestion = "LayoutError"
 406 )]
 407 pub type LayoutErr = LayoutError;
 408
 409 /// The parameters given to `Layout::from_size_align`
 410 /// or some other `Layout` constructor
 411 /// do not satisfy its documented constraints.
 412 #[stable(feature = "alloc_layout_error", since = "1.50.0")]
 413 #[derive(Clone, PartialEq, Eq, Debug)]
 414 pub struct LayoutError {
 415     private: (),
 416 }
 417
 418 // (we need this for downstream impl of trait Error)
 419 #[stable(feature = "alloc_layout", since = "1.28.0")]
 420 impl fmt::Display for LayoutError {
 421     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 422         f.write_str("invalid parameters to Layout::from_size_align")
 423     }
 424 }