1 // Seemingly inconsequential code changes to this file can lead to measurable
2 // performance impact on compilation times, due at least in part to the fact
3 // that the layout code gets called from many instantiations of the various
4 // collections, resulting in having to optimize down excess IR multiple times.
5 // Your performance intuition is useless. Run perf.
8 use crate::error::Error;
10 use crate::mem::{self, ValidAlign};
11 use crate::ptr::NonNull;
13 // While this function is used in one place and its implementation
14 // could be inlined, the previous attempts to do so made rustc
17 // * https://github.com/rust-lang/rust/pull/72189
18 // * https://github.com/rust-lang/rust/pull/79827
19 const fn size_align<T>() -> (usize, usize) {
20 (mem::size_of::<T>(), mem::align_of::<T>())
23 /// Layout of a block of memory.
25 /// An instance of `Layout` describes a particular layout of memory.
26 /// You build a `Layout` up as an input to give to an allocator.
28 /// All layouts have an associated size and a power-of-two alignment.
30 /// (Note that layouts are *not* required to have non-zero size,
31 /// even though `GlobalAlloc` requires that all memory requests
32 /// be non-zero in size. A caller must either ensure that conditions
33 /// like this are met, use specific allocators with looser
34 /// requirements, or use the more lenient `Allocator` interface.)
35 #[stable(feature = "alloc_layout", since = "1.28.0")]
36 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
37 #[lang = "alloc_layout"]
39 // size of the requested block of memory, measured in bytes.
42 // alignment of the requested block of memory, measured in bytes.
43 // we ensure that this is always a power-of-two, because API's
44 // like `posix_memalign` require it and it is a reasonable
45 // constraint to impose on Layout constructors.
47 // (However, we do not analogously require `align >= sizeof(void*)`,
48 // even though that is *also* a requirement of `posix_memalign`.)
53 /// Constructs a `Layout` from a given `size` and `align`,
54 /// or returns `LayoutError` if any of the following conditions
57 /// * `align` must not be zero,
59 /// * `align` must be a power of two,
61 /// * `size`, when rounded up to the nearest multiple of `align`,
62 /// must not overflow isize (i.e., the rounded value must be
63 /// less than or equal to `isize::MAX`).
64 #[stable(feature = "alloc_layout", since = "1.28.0")]
65 #[rustc_const_stable(feature = "const_alloc_layout_size_align", since = "1.50.0")]
67 pub const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutError> {
68 if !align.is_power_of_two() {
69 return Err(LayoutError);
72 // SAFETY: just checked that align is a power of two.
73 Layout::from_size_valid_align(size, unsafe { ValidAlign::new_unchecked(align) })
77 const fn max_size_for_align(align: ValidAlign) -> usize {
78 // (power-of-two implies align != 0.)
80 // Rounded up size is:
81 // size_rounded_up = (size + align - 1) & !(align - 1);
83 // We know from above that align != 0. If adding (align - 1)
84 // does not overflow, then rounding up will be fine.
86 // Conversely, &-masking with !(align - 1) will subtract off
87 // only low-order-bits. Thus if overflow occurs with the sum,
88 // the &-mask cannot subtract enough to undo that overflow.
90 // Above implies that checking for summation overflow is both
91 // necessary and sufficient.
92 isize::MAX as usize - (align.as_usize() - 1)
95 /// Internal helper constructor to skip revalidating alignment validity.
97 const fn from_size_valid_align(size: usize, align: ValidAlign) -> Result<Self, LayoutError> {
98 if size > Self::max_size_for_align(align) {
99 return Err(LayoutError);
102 // SAFETY: Layout::size invariants checked above.
103 Ok(Layout { size, align })
106 /// Creates a layout, bypassing all checks.
110 /// This function is unsafe as it does not verify the preconditions from
111 /// [`Layout::from_size_align`].
112 #[stable(feature = "alloc_layout", since = "1.28.0")]
113 #[rustc_const_stable(feature = "const_alloc_layout_unchecked", since = "1.36.0")]
116 pub const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self {
117 // SAFETY: the caller is required to uphold the preconditions.
118 unsafe { Layout { size, align: ValidAlign::new_unchecked(align) } }
121 /// The minimum size in bytes for a memory block of this layout.
122 #[stable(feature = "alloc_layout", since = "1.28.0")]
123 #[rustc_const_stable(feature = "const_alloc_layout_size_align", since = "1.50.0")]
126 pub const fn size(&self) -> usize {
130 /// The minimum byte alignment for a memory block of this layout.
131 #[stable(feature = "alloc_layout", since = "1.28.0")]
132 #[rustc_const_stable(feature = "const_alloc_layout_size_align", since = "1.50.0")]
133 #[must_use = "this returns the minimum alignment, \
134 without modifying the layout"]
136 pub const fn align(&self) -> usize {
137 self.align.as_usize()
140 /// Constructs a `Layout` suitable for holding a value of type `T`.
141 #[stable(feature = "alloc_layout", since = "1.28.0")]
142 #[rustc_const_stable(feature = "alloc_layout_const_new", since = "1.42.0")]
145 pub const fn new<T>() -> Self {
146 let (size, align) = size_align::<T>();
147 // SAFETY: if the type is instantiated, rustc already ensures that its
148 // layout is valid. Use the unchecked constructor to avoid inserting a
149 // panicking codepath that needs to be optimized out.
150 unsafe { Layout::from_size_align_unchecked(size, align) }
153 /// Produces layout describing a record that could be used to
154 /// allocate backing structure for `T` (which could be a trait
155 /// or other unsized type like a slice).
156 #[stable(feature = "alloc_layout", since = "1.28.0")]
159 pub fn for_value<T: ?Sized>(t: &T) -> Self {
160 let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
161 // SAFETY: see rationale in `new` for why this is using the unsafe variant
162 unsafe { Layout::from_size_align_unchecked(size, align) }
165 /// Produces layout describing a record that could be used to
166 /// allocate backing structure for `T` (which could be a trait
167 /// or other unsized type like a slice).
171 /// This function is only safe to call if the following conditions hold:
173 /// - If `T` is `Sized`, this function is always safe to call.
174 /// - If the unsized tail of `T` is:
175 /// - a [slice], then the length of the slice tail must be an initialized
176 /// integer, and the size of the *entire value*
177 /// (dynamic tail length + statically sized prefix) must fit in `isize`.
178 /// - a [trait object], then the vtable part of the pointer must point
179 /// to a valid vtable for the type `T` acquired by an unsizing coercion,
180 /// and the size of the *entire value*
181 /// (dynamic tail length + statically sized prefix) must fit in `isize`.
182 /// - an (unstable) [extern type], then this function is always safe to
183 /// call, but may panic or otherwise return the wrong value, as the
184 /// extern type's layout is not known. This is the same behavior as
185 /// [`Layout::for_value`] on a reference to an extern type tail.
186 /// - otherwise, it is conservatively not allowed to call this function.
188 /// [trait object]: ../../book/ch17-02-trait-objects.html
189 /// [extern type]: ../../unstable-book/language-features/extern-types.html
190 #[unstable(feature = "layout_for_ptr", issue = "69835")]
192 pub unsafe fn for_value_raw<T: ?Sized>(t: *const T) -> Self {
193 // SAFETY: we pass along the prerequisites of these functions to the caller
194 let (size, align) = unsafe { (mem::size_of_val_raw(t), mem::align_of_val_raw(t)) };
195 // SAFETY: see rationale in `new` for why this is using the unsafe variant
196 unsafe { Layout::from_size_align_unchecked(size, align) }
199 /// Creates a `NonNull` that is dangling, but well-aligned for this Layout.
201 /// Note that the pointer value may potentially represent a valid pointer,
202 /// which means this must not be used as a "not yet initialized"
203 /// sentinel value. Types that lazily allocate must track initialization by
204 /// some other means.
205 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
206 #[rustc_const_unstable(feature = "alloc_layout_extra", issue = "55724")]
209 pub const fn dangling(&self) -> NonNull<u8> {
210 // SAFETY: align is guaranteed to be non-zero
211 unsafe { NonNull::new_unchecked(crate::ptr::invalid_mut::<u8>(self.align())) }
214 /// Creates a layout describing the record that can hold a value
215 /// of the same layout as `self`, but that also is aligned to
216 /// alignment `align` (measured in bytes).
218 /// If `self` already meets the prescribed alignment, then returns
221 /// Note that this method does not add any padding to the overall
222 /// size, regardless of whether the returned layout has a different
223 /// alignment. In other words, if `K` has size 16, `K.align_to(32)`
224 /// will *still* have size 16.
226 /// Returns an error if the combination of `self.size()` and the given
227 /// `align` violates the conditions listed in [`Layout::from_size_align`].
228 #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
230 pub fn align_to(&self, align: usize) -> Result<Self, LayoutError> {
231 Layout::from_size_align(self.size(), cmp::max(self.align(), align))
234 /// Returns the amount of padding we must insert after `self`
235 /// to ensure that the following address will satisfy `align`
236 /// (measured in bytes).
238 /// e.g., if `self.size()` is 9, then `self.padding_needed_for(4)`
239 /// returns 3, because that is the minimum number of bytes of
240 /// padding required to get a 4-aligned address (assuming that the
241 /// corresponding memory block starts at a 4-aligned address).
243 /// The return value of this function has no meaning if `align` is
244 /// not a power-of-two.
246 /// Note that the utility of the returned value requires `align`
247 /// to be less than or equal to the alignment of the starting
248 /// address for the whole allocated block of memory. One way to
249 /// satisfy this constraint is to ensure `align <= self.align()`.
250 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
251 #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
252 #[must_use = "this returns the padding needed, \
253 without modifying the `Layout`"]
255 pub const fn padding_needed_for(&self, align: usize) -> usize {
256 let len = self.size();
258 // Rounded up value is:
259 // len_rounded_up = (len + align - 1) & !(align - 1);
260 // and then we return the padding difference: `len_rounded_up - len`.
262 // We use modular arithmetic throughout:
264 // 1. align is guaranteed to be > 0, so align - 1 is always
267 // 2. `len + align - 1` can overflow by at most `align - 1`,
268 // so the &-mask with `!(align - 1)` will ensure that in the
269 // case of overflow, `len_rounded_up` will itself be 0.
270 // Thus the returned padding, when added to `len`, yields 0,
271 // which trivially satisfies the alignment `align`.
273 // (Of course, attempts to allocate blocks of memory whose
274 // size and padding overflow in the above manner should cause
275 // the allocator to yield an error anyway.)
277 let len_rounded_up = len.wrapping_add(align).wrapping_sub(1) & !align.wrapping_sub(1);
278 len_rounded_up.wrapping_sub(len)
281 /// Creates a layout by rounding the size of this layout up to a multiple
282 /// of the layout's alignment.
284 /// This is equivalent to adding the result of `padding_needed_for`
285 /// to the layout's current size.
286 #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
287 #[must_use = "this returns a new `Layout`, \
288 without modifying the original"]
290 pub fn pad_to_align(&self) -> Layout {
291 let pad = self.padding_needed_for(self.align());
292 // This cannot overflow. Quoting from the invariant of Layout:
293 // > `size`, when rounded up to the nearest multiple of `align`,
294 // > must not overflow isize (i.e., the rounded value must be
295 // > less than or equal to `isize::MAX`)
296 let new_size = self.size() + pad;
298 // SAFETY: padded size is guaranteed to not exceed `isize::MAX`.
299 unsafe { Layout::from_size_align_unchecked(new_size, self.align()) }
302 /// Creates a layout describing the record for `n` instances of
303 /// `self`, with a suitable amount of padding between each to
304 /// ensure that each instance is given its requested size and
305 /// alignment. On success, returns `(k, offs)` where `k` is the
306 /// layout of the array and `offs` is the distance between the start
307 /// of each element in the array.
309 /// On arithmetic overflow, returns `LayoutError`.
310 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
312 pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutError> {
313 // This cannot overflow. Quoting from the invariant of Layout:
314 // > `size`, when rounded up to the nearest multiple of `align`,
315 // > must not overflow isize (i.e., the rounded value must be
316 // > less than or equal to `isize::MAX`)
317 let padded_size = self.size() + self.padding_needed_for(self.align());
318 let alloc_size = padded_size.checked_mul(n).ok_or(LayoutError)?;
320 // The safe constructor is called here to enforce the isize size limit.
321 Layout::from_size_valid_align(alloc_size, self.align).map(|layout| (layout, padded_size))
324 /// Creates a layout describing the record for `self` followed by
325 /// `next`, including any necessary padding to ensure that `next`
326 /// will be properly aligned, but *no trailing padding*.
328 /// In order to match C representation layout `repr(C)`, you should
329 /// call `pad_to_align` after extending the layout with all fields.
330 /// (There is no way to match the default Rust representation
331 /// layout `repr(Rust)`, as it is unspecified.)
333 /// Note that the alignment of the resulting layout will be the maximum of
334 /// those of `self` and `next`, in order to ensure alignment of both parts.
336 /// Returns `Ok((k, offset))`, where `k` is layout of the concatenated
337 /// record and `offset` is the relative location, in bytes, of the
338 /// start of the `next` embedded within the concatenated record
339 /// (assuming that the record itself starts at offset 0).
341 /// On arithmetic overflow, returns `LayoutError`.
345 /// To calculate the layout of a `#[repr(C)]` structure and the offsets of
346 /// the fields from its fields' layouts:
349 /// # use std::alloc::{Layout, LayoutError};
350 /// pub fn repr_c(fields: &[Layout]) -> Result<(Layout, Vec<usize>), LayoutError> {
351 /// let mut offsets = Vec::new();
352 /// let mut layout = Layout::from_size_align(0, 1)?;
353 /// for &field in fields {
354 /// let (new_layout, offset) = layout.extend(field)?;
355 /// layout = new_layout;
356 /// offsets.push(offset);
358 /// // Remember to finalize with `pad_to_align`!
359 /// Ok((layout.pad_to_align(), offsets))
361 /// # // test that it works
362 /// # #[repr(C)] struct S { a: u64, b: u32, c: u16, d: u32 }
363 /// # let s = Layout::new::<S>();
364 /// # let u16 = Layout::new::<u16>();
365 /// # let u32 = Layout::new::<u32>();
366 /// # let u64 = Layout::new::<u64>();
367 /// # assert_eq!(repr_c(&[u64, u32, u16, u32]), Ok((s, vec![0, 8, 12, 16])));
369 #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
371 pub fn extend(&self, next: Self) -> Result<(Self, usize), LayoutError> {
372 let new_align = cmp::max(self.align, next.align);
373 let pad = self.padding_needed_for(next.align());
375 let offset = self.size().checked_add(pad).ok_or(LayoutError)?;
376 let new_size = offset.checked_add(next.size()).ok_or(LayoutError)?;
378 // The safe constructor is called here to enforce the isize size limit.
379 let layout = Layout::from_size_valid_align(new_size, new_align)?;
383 /// Creates a layout describing the record for `n` instances of
384 /// `self`, with no padding between each instance.
386 /// Note that, unlike `repeat`, `repeat_packed` does not guarantee
387 /// that the repeated instances of `self` will be properly
388 /// aligned, even if a given instance of `self` is properly
389 /// aligned. In other words, if the layout returned by
390 /// `repeat_packed` is used to allocate an array, it is not
391 /// guaranteed that all elements in the array will be properly
394 /// On arithmetic overflow, returns `LayoutError`.
395 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
397 pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutError> {
398 let size = self.size().checked_mul(n).ok_or(LayoutError)?;
399 // The safe constructor is called here to enforce the isize size limit.
400 Layout::from_size_valid_align(size, self.align)
403 /// Creates a layout describing the record for `self` followed by
404 /// `next` with no additional padding between the two. Since no
405 /// padding is inserted, the alignment of `next` is irrelevant,
406 /// and is not incorporated *at all* into the resulting layout.
408 /// On arithmetic overflow, returns `LayoutError`.
409 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
411 pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutError> {
412 let new_size = self.size().checked_add(next.size()).ok_or(LayoutError)?;
413 // The safe constructor is called here to enforce the isize size limit.
414 Layout::from_size_valid_align(new_size, self.align)
417 /// Creates a layout describing the record for a `[T; n]`.
419 /// On arithmetic overflow or when the total size would exceed
420 /// `isize::MAX`, returns `LayoutError`.
421 #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
423 pub fn array<T>(n: usize) -> Result<Self, LayoutError> {
424 // Reduce the amount of code we need to monomorphize per `T`.
425 return inner(mem::size_of::<T>(), ValidAlign::of::<T>(), n);
428 fn inner(element_size: usize, align: ValidAlign, n: usize) -> Result<Layout, LayoutError> {
429 // We need to check two things about the size:
430 // - That the total size won't overflow a `usize`, and
431 // - That the total size still fits in an `isize`.
432 // By using division we can check them both with a single threshold.
433 // That'd usually be a bad idea, but thankfully here the element size
434 // and alignment are constants, so the compiler will fold all of it.
435 if element_size != 0 && n > Layout::max_size_for_align(align) / element_size {
436 return Err(LayoutError);
439 let array_size = element_size * n;
441 // SAFETY: We just checked above that the `array_size` will not
442 // exceed `isize::MAX` even when rounded up to the alignment.
443 // And `ValidAlign` guarantees it's a power of two.
444 unsafe { Ok(Layout::from_size_align_unchecked(array_size, align.as_usize())) }
449 #[stable(feature = "alloc_layout", since = "1.28.0")]
452 note = "Name does not follow std convention, use LayoutError",
453 suggestion = "LayoutError"
455 pub type LayoutErr = LayoutError;
457 /// The parameters given to `Layout::from_size_align`
458 /// or some other `Layout` constructor
459 /// do not satisfy its documented constraints.
460 #[stable(feature = "alloc_layout_error", since = "1.50.0")]
462 #[derive(Clone, PartialEq, Eq, Debug)]
463 pub struct LayoutError;
465 #[stable(feature = "alloc_layout", since = "1.28.0")]
466 impl Error for LayoutError {}
468 // (we need this for downstream impl of trait Error)
469 #[stable(feature = "alloc_layout", since = "1.28.0")]
470 impl fmt::Display for LayoutError {
471 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
472 f.write_str("invalid parameters to Layout::from_size_align")