src/libcore/alloc/layout.rs

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