xref: /openbmc/linux/rust/alloc/raw_vec.rs (revision a6978d1b7bb8f3a25305e8ff7d367f7289614c5d)
1  // SPDX-License-Identifier: Apache-2.0 OR MIT
2  
3  #![unstable(feature = "raw_vec_internals", reason = "unstable const warnings", issue = "none")]
4  
5  use core::alloc::LayoutError;
6  use core::cmp;
7  use core::intrinsics;
8  use core::mem::{self, ManuallyDrop, MaybeUninit, SizedTypeProperties};
9  use core::ptr::{self, NonNull, Unique};
10  use core::slice;
11  
12  #[cfg(not(no_global_oom_handling))]
13  use crate::alloc::handle_alloc_error;
14  use crate::alloc::{Allocator, Global, Layout};
15  use crate::boxed::Box;
16  use crate::collections::TryReserveError;
17  use crate::collections::TryReserveErrorKind::*;
18  
19  #[cfg(test)]
20  mod tests;
21  
22  enum AllocInit {
23      /// The contents of the new memory are uninitialized.
24      Uninitialized,
25      /// The new memory is guaranteed to be zeroed.
26      #[allow(dead_code)]
27      Zeroed,
28  }
29  
30  /// A low-level utility for more ergonomically allocating, reallocating, and deallocating
31  /// a buffer of memory on the heap without having to worry about all the corner cases
32  /// involved. This type is excellent for building your own data structures like Vec and VecDeque.
33  /// In particular:
34  ///
35  /// * Produces `Unique::dangling()` on zero-sized types.
36  /// * Produces `Unique::dangling()` on zero-length allocations.
37  /// * Avoids freeing `Unique::dangling()`.
38  /// * Catches all overflows in capacity computations (promotes them to "capacity overflow" panics).
39  /// * Guards against 32-bit systems allocating more than isize::MAX bytes.
40  /// * Guards against overflowing your length.
41  /// * Calls `handle_alloc_error` for fallible allocations.
42  /// * Contains a `ptr::Unique` and thus endows the user with all related benefits.
43  /// * Uses the excess returned from the allocator to use the largest available capacity.
44  ///
45  /// This type does not in anyway inspect the memory that it manages. When dropped it *will*
46  /// free its memory, but it *won't* try to drop its contents. It is up to the user of `RawVec`
47  /// to handle the actual things *stored* inside of a `RawVec`.
48  ///
49  /// Note that the excess of a zero-sized types is always infinite, so `capacity()` always returns
50  /// `usize::MAX`. This means that you need to be careful when round-tripping this type with a
51  /// `Box<[T]>`, since `capacity()` won't yield the length.
52  #[allow(missing_debug_implementations)]
53  pub(crate) struct RawVec<T, A: Allocator = Global> {
54      ptr: Unique<T>,
55      cap: usize,
56      alloc: A,
57  }
58  
59  impl<T> RawVec<T, Global> {
60      /// HACK(Centril): This exists because stable `const fn` can only call stable `const fn`, so
61      /// they cannot call `Self::new()`.
62      ///
63      /// If you change `RawVec<T>::new` or dependencies, please take care to not introduce anything
64      /// that would truly const-call something unstable.
65      pub const NEW: Self = Self::new();
66  
67      /// Creates the biggest possible `RawVec` (on the system heap)
68      /// without allocating. If `T` has positive size, then this makes a
69      /// `RawVec` with capacity `0`. If `T` is zero-sized, then it makes a
70      /// `RawVec` with capacity `usize::MAX`. Useful for implementing
71      /// delayed allocation.
72      #[must_use]
73      pub const fn new() -> Self {
74          Self::new_in(Global)
75      }
76  
77      /// Creates a `RawVec` (on the system heap) with exactly the
78      /// capacity and alignment requirements for a `[T; capacity]`. This is
79      /// equivalent to calling `RawVec::new` when `capacity` is `0` or `T` is
80      /// zero-sized. Note that if `T` is zero-sized this means you will
81      /// *not* get a `RawVec` with the requested capacity.
82      ///
83      /// # Panics
84      ///
85      /// Panics if the requested capacity exceeds `isize::MAX` bytes.
86      ///
87      /// # Aborts
88      ///
89      /// Aborts on OOM.
90      #[cfg(not(any(no_global_oom_handling, test)))]
91      #[must_use]
92      #[inline]
93      pub fn with_capacity(capacity: usize) -> Self {
94          Self::with_capacity_in(capacity, Global)
95      }
96  
97      /// Like `with_capacity`, but guarantees the buffer is zeroed.
98      #[cfg(not(any(no_global_oom_handling, test)))]
99      #[must_use]
100      #[inline]
101      pub fn with_capacity_zeroed(capacity: usize) -> Self {
102          Self::with_capacity_zeroed_in(capacity, Global)
103      }
104  }
105  
106  impl<T, A: Allocator> RawVec<T, A> {
107      // Tiny Vecs are dumb. Skip to:
108      // - 8 if the element size is 1, because any heap allocators is likely
109      //   to round up a request of less than 8 bytes to at least 8 bytes.
110      // - 4 if elements are moderate-sized (<= 1 KiB).
111      // - 1 otherwise, to avoid wasting too much space for very short Vecs.
112      pub(crate) const MIN_NON_ZERO_CAP: usize = if mem::size_of::<T>() == 1 {
113          8
114      } else if mem::size_of::<T>() <= 1024 {
115          4
116      } else {
117          1
118      };
119  
120      /// Like `new`, but parameterized over the choice of allocator for
121      /// the returned `RawVec`.
122      pub const fn new_in(alloc: A) -> Self {
123          // `cap: 0` means "unallocated". zero-sized types are ignored.
124          Self { ptr: Unique::dangling(), cap: 0, alloc }
125      }
126  
127      /// Like `with_capacity`, but parameterized over the choice of
128      /// allocator for the returned `RawVec`.
129      #[cfg(not(no_global_oom_handling))]
130      #[inline]
131      pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
132          Self::allocate_in(capacity, AllocInit::Uninitialized, alloc)
133      }
134  
135      /// Like `try_with_capacity`, but parameterized over the choice of
136      /// allocator for the returned `RawVec`.
137      #[inline]
138      pub fn try_with_capacity_in(capacity: usize, alloc: A) -> Result<Self, TryReserveError> {
139          Self::try_allocate_in(capacity, AllocInit::Uninitialized, alloc)
140      }
141  
142      /// Like `with_capacity_zeroed`, but parameterized over the choice
143      /// of allocator for the returned `RawVec`.
144      #[cfg(not(no_global_oom_handling))]
145      #[inline]
146      pub fn with_capacity_zeroed_in(capacity: usize, alloc: A) -> Self {
147          Self::allocate_in(capacity, AllocInit::Zeroed, alloc)
148      }
149  
150      /// Converts the entire buffer into `Box<[MaybeUninit<T>]>` with the specified `len`.
151      ///
152      /// Note that this will correctly reconstitute any `cap` changes
153      /// that may have been performed. (See description of type for details.)
154      ///
155      /// # Safety
156      ///
157      /// * `len` must be greater than or equal to the most recently requested capacity, and
158      /// * `len` must be less than or equal to `self.capacity()`.
159      ///
160      /// Note, that the requested capacity and `self.capacity()` could differ, as
161      /// an allocator could overallocate and return a greater memory block than requested.
162      pub unsafe fn into_box(self, len: usize) -> Box<[MaybeUninit<T>], A> {
163          // Sanity-check one half of the safety requirement (we cannot check the other half).
164          debug_assert!(
165              len <= self.capacity(),
166              "`len` must be smaller than or equal to `self.capacity()`"
167          );
168  
169          let me = ManuallyDrop::new(self);
170          unsafe {
171              let slice = slice::from_raw_parts_mut(me.ptr() as *mut MaybeUninit<T>, len);
172              Box::from_raw_in(slice, ptr::read(&me.alloc))
173          }
174      }
175  
176      #[cfg(not(no_global_oom_handling))]
177      fn allocate_in(capacity: usize, init: AllocInit, alloc: A) -> Self {
178          // Don't allocate here because `Drop` will not deallocate when `capacity` is 0.
179          if T::IS_ZST || capacity == 0 {
180              Self::new_in(alloc)
181          } else {
182              // We avoid `unwrap_or_else` here because it bloats the amount of
183              // LLVM IR generated.
184              let layout = match Layout::array::<T>(capacity) {
185                  Ok(layout) => layout,
186                  Err(_) => capacity_overflow(),
187              };
188              match alloc_guard(layout.size()) {
189                  Ok(_) => {}
190                  Err(_) => capacity_overflow(),
191              }
192              let result = match init {
193                  AllocInit::Uninitialized => alloc.allocate(layout),
194                  AllocInit::Zeroed => alloc.allocate_zeroed(layout),
195              };
196              let ptr = match result {
197                  Ok(ptr) => ptr,
198                  Err(_) => handle_alloc_error(layout),
199              };
200  
201              // Allocators currently return a `NonNull<[u8]>` whose length
202              // matches the size requested. If that ever changes, the capacity
203              // here should change to `ptr.len() / mem::size_of::<T>()`.
204              Self {
205                  ptr: unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) },
206                  cap: capacity,
207                  alloc,
208              }
209          }
210      }
211  
212      fn try_allocate_in(capacity: usize, init: AllocInit, alloc: A) -> Result<Self, TryReserveError> {
213          // Don't allocate here because `Drop` will not deallocate when `capacity` is 0.
214          if T::IS_ZST || capacity == 0 {
215              return Ok(Self::new_in(alloc));
216          }
217  
218          let layout = Layout::array::<T>(capacity).map_err(|_| CapacityOverflow)?;
219          alloc_guard(layout.size())?;
220          let result = match init {
221              AllocInit::Uninitialized => alloc.allocate(layout),
222              AllocInit::Zeroed => alloc.allocate_zeroed(layout),
223          };
224          let ptr = result.map_err(|_| AllocError { layout, non_exhaustive: () })?;
225  
226          // Allocators currently return a `NonNull<[u8]>` whose length
227          // matches the size requested. If that ever changes, the capacity
228          // here should change to `ptr.len() / mem::size_of::<T>()`.
229          Ok(Self {
230              ptr: unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) },
231              cap: capacity,
232              alloc,
233          })
234      }
235  
236      /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
237      ///
238      /// # Safety
239      ///
240      /// The `ptr` must be allocated (via the given allocator `alloc`), and with the given
241      /// `capacity`.
242      /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
243      /// systems). ZST vectors may have a capacity up to `usize::MAX`.
244      /// If the `ptr` and `capacity` come from a `RawVec` created via `alloc`, then this is
245      /// guaranteed.
246      #[inline]
247      pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, alloc: A) -> Self {
248          Self { ptr: unsafe { Unique::new_unchecked(ptr) }, cap: capacity, alloc }
249      }
250  
251      /// Gets a raw pointer to the start of the allocation. Note that this is
252      /// `Unique::dangling()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
253      /// be careful.
254      #[inline]
255      pub fn ptr(&self) -> *mut T {
256          self.ptr.as_ptr()
257      }
258  
259      /// Gets the capacity of the allocation.
260      ///
261      /// This will always be `usize::MAX` if `T` is zero-sized.
262      #[inline(always)]
263      pub fn capacity(&self) -> usize {
264          if T::IS_ZST { usize::MAX } else { self.cap }
265      }
266  
267      /// Returns a shared reference to the allocator backing this `RawVec`.
268      pub fn allocator(&self) -> &A {
269          &self.alloc
270      }
271  
272      fn current_memory(&self) -> Option<(NonNull<u8>, Layout)> {
273          if T::IS_ZST || self.cap == 0 {
274              None
275          } else {
276              // We could use Layout::array here which ensures the absence of isize and usize overflows
277              // and could hypothetically handle differences between stride and size, but this memory
278              // has already been allocated so we know it can't overflow and currently rust does not
279              // support such types. So we can do better by skipping some checks and avoid an unwrap.
280              let _: () = const { assert!(mem::size_of::<T>() % mem::align_of::<T>() == 0) };
281              unsafe {
282                  let align = mem::align_of::<T>();
283                  let size = mem::size_of::<T>().unchecked_mul(self.cap);
284                  let layout = Layout::from_size_align_unchecked(size, align);
285                  Some((self.ptr.cast().into(), layout))
286              }
287          }
288      }
289  
290      /// Ensures that the buffer contains at least enough space to hold `len +
291      /// additional` elements. If it doesn't already have enough capacity, will
292      /// reallocate enough space plus comfortable slack space to get amortized
293      /// *O*(1) behavior. Will limit this behavior if it would needlessly cause
294      /// itself to panic.
295      ///
296      /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
297      /// the requested space. This is not really unsafe, but the unsafe
298      /// code *you* write that relies on the behavior of this function may break.
299      ///
300      /// This is ideal for implementing a bulk-push operation like `extend`.
301      ///
302      /// # Panics
303      ///
304      /// Panics if the new capacity exceeds `isize::MAX` bytes.
305      ///
306      /// # Aborts
307      ///
308      /// Aborts on OOM.
309      #[cfg(not(no_global_oom_handling))]
310      #[inline]
311      pub fn reserve(&mut self, len: usize, additional: usize) {
312          // Callers expect this function to be very cheap when there is already sufficient capacity.
313          // Therefore, we move all the resizing and error-handling logic from grow_amortized and
314          // handle_reserve behind a call, while making sure that this function is likely to be
315          // inlined as just a comparison and a call if the comparison fails.
316          #[cold]
317          fn do_reserve_and_handle<T, A: Allocator>(
318              slf: &mut RawVec<T, A>,
319              len: usize,
320              additional: usize,
321          ) {
322              handle_reserve(slf.grow_amortized(len, additional));
323          }
324  
325          if self.needs_to_grow(len, additional) {
326              do_reserve_and_handle(self, len, additional);
327          }
328      }
329  
330      /// A specialized version of `reserve()` used only by the hot and
331      /// oft-instantiated `Vec::push()`, which does its own capacity check.
332      #[cfg(not(no_global_oom_handling))]
333      #[inline(never)]
334      pub fn reserve_for_push(&mut self, len: usize) {
335          handle_reserve(self.grow_amortized(len, 1));
336      }
337  
338      /// The same as `reserve`, but returns on errors instead of panicking or aborting.
339      pub fn try_reserve(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
340          if self.needs_to_grow(len, additional) {
341              self.grow_amortized(len, additional)
342          } else {
343              Ok(())
344          }
345      }
346  
347      /// The same as `reserve_for_push`, but returns on errors instead of panicking or aborting.
348      #[inline(never)]
349      pub fn try_reserve_for_push(&mut self, len: usize) -> Result<(), TryReserveError> {
350          self.grow_amortized(len, 1)
351      }
352  
353      /// Ensures that the buffer contains at least enough space to hold `len +
354      /// additional` elements. If it doesn't already, will reallocate the
355      /// minimum possible amount of memory necessary. Generally this will be
356      /// exactly the amount of memory necessary, but in principle the allocator
357      /// is free to give back more than we asked for.
358      ///
359      /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
360      /// the requested space. This is not really unsafe, but the unsafe code
361      /// *you* write that relies on the behavior of this function may break.
362      ///
363      /// # Panics
364      ///
365      /// Panics if the new capacity exceeds `isize::MAX` bytes.
366      ///
367      /// # Aborts
368      ///
369      /// Aborts on OOM.
370      #[cfg(not(no_global_oom_handling))]
371      pub fn reserve_exact(&mut self, len: usize, additional: usize) {
372          handle_reserve(self.try_reserve_exact(len, additional));
373      }
374  
375      /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
376      pub fn try_reserve_exact(
377          &mut self,
378          len: usize,
379          additional: usize,
380      ) -> Result<(), TryReserveError> {
381          if self.needs_to_grow(len, additional) { self.grow_exact(len, additional) } else { Ok(()) }
382      }
383  
384      /// Shrinks the buffer down to the specified capacity. If the given amount
385      /// is 0, actually completely deallocates.
386      ///
387      /// # Panics
388      ///
389      /// Panics if the given amount is *larger* than the current capacity.
390      ///
391      /// # Aborts
392      ///
393      /// Aborts on OOM.
394      #[cfg(not(no_global_oom_handling))]
395      pub fn shrink_to_fit(&mut self, cap: usize) {
396          handle_reserve(self.shrink(cap));
397      }
398  }
399  
400  impl<T, A: Allocator> RawVec<T, A> {
401      /// Returns if the buffer needs to grow to fulfill the needed extra capacity.
402      /// Mainly used to make inlining reserve-calls possible without inlining `grow`.
403      fn needs_to_grow(&self, len: usize, additional: usize) -> bool {
404          additional > self.capacity().wrapping_sub(len)
405      }
406  
407      fn set_ptr_and_cap(&mut self, ptr: NonNull<[u8]>, cap: usize) {
408          // Allocators currently return a `NonNull<[u8]>` whose length matches
409          // the size requested. If that ever changes, the capacity here should
410          // change to `ptr.len() / mem::size_of::<T>()`.
411          self.ptr = unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) };
412          self.cap = cap;
413      }
414  
415      // This method is usually instantiated many times. So we want it to be as
416      // small as possible, to improve compile times. But we also want as much of
417      // its contents to be statically computable as possible, to make the
418      // generated code run faster. Therefore, this method is carefully written
419      // so that all of the code that depends on `T` is within it, while as much
420      // of the code that doesn't depend on `T` as possible is in functions that
421      // are non-generic over `T`.
422      fn grow_amortized(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
423          // This is ensured by the calling contexts.
424          debug_assert!(additional > 0);
425  
426          if T::IS_ZST {
427              // Since we return a capacity of `usize::MAX` when `elem_size` is
428              // 0, getting to here necessarily means the `RawVec` is overfull.
429              return Err(CapacityOverflow.into());
430          }
431  
432          // Nothing we can really do about these checks, sadly.
433          let required_cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
434  
435          // This guarantees exponential growth. The doubling cannot overflow
436          // because `cap <= isize::MAX` and the type of `cap` is `usize`.
437          let cap = cmp::max(self.cap * 2, required_cap);
438          let cap = cmp::max(Self::MIN_NON_ZERO_CAP, cap);
439  
440          let new_layout = Layout::array::<T>(cap);
441  
442          // `finish_grow` is non-generic over `T`.
443          let ptr = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
444          self.set_ptr_and_cap(ptr, cap);
445          Ok(())
446      }
447  
448      // The constraints on this method are much the same as those on
449      // `grow_amortized`, but this method is usually instantiated less often so
450      // it's less critical.
451      fn grow_exact(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
452          if T::IS_ZST {
453              // Since we return a capacity of `usize::MAX` when the type size is
454              // 0, getting to here necessarily means the `RawVec` is overfull.
455              return Err(CapacityOverflow.into());
456          }
457  
458          let cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
459          let new_layout = Layout::array::<T>(cap);
460  
461          // `finish_grow` is non-generic over `T`.
462          let ptr = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
463          self.set_ptr_and_cap(ptr, cap);
464          Ok(())
465      }
466  
467      #[cfg(not(no_global_oom_handling))]
468      fn shrink(&mut self, cap: usize) -> Result<(), TryReserveError> {
469          assert!(cap <= self.capacity(), "Tried to shrink to a larger capacity");
470  
471          let (ptr, layout) = if let Some(mem) = self.current_memory() { mem } else { return Ok(()) };
472          // See current_memory() why this assert is here
473          let _: () = const { assert!(mem::size_of::<T>() % mem::align_of::<T>() == 0) };
474  
475          // If shrinking to 0, deallocate the buffer. We don't reach this point
476          // for the T::IS_ZST case since current_memory() will have returned
477          // None.
478          if cap == 0 {
479              unsafe { self.alloc.deallocate(ptr, layout) };
480              self.ptr = Unique::dangling();
481              self.cap = 0;
482          } else {
483              let ptr = unsafe {
484                  // `Layout::array` cannot overflow here because it would have
485                  // overflowed earlier when capacity was larger.
486                  let new_size = mem::size_of::<T>().unchecked_mul(cap);
487                  let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
488                  self.alloc
489                      .shrink(ptr, layout, new_layout)
490                      .map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })?
491              };
492              self.set_ptr_and_cap(ptr, cap);
493          }
494          Ok(())
495      }
496  }
497  
498  // This function is outside `RawVec` to minimize compile times. See the comment
499  // above `RawVec::grow_amortized` for details. (The `A` parameter isn't
500  // significant, because the number of different `A` types seen in practice is
501  // much smaller than the number of `T` types.)
502  #[inline(never)]
503  fn finish_grow<A>(
504      new_layout: Result<Layout, LayoutError>,
505      current_memory: Option<(NonNull<u8>, Layout)>,
506      alloc: &mut A,
507  ) -> Result<NonNull<[u8]>, TryReserveError>
508  where
509      A: Allocator,
510  {
511      // Check for the error here to minimize the size of `RawVec::grow_*`.
512      let new_layout = new_layout.map_err(|_| CapacityOverflow)?;
513  
514      alloc_guard(new_layout.size())?;
515  
516      let memory = if let Some((ptr, old_layout)) = current_memory {
517          debug_assert_eq!(old_layout.align(), new_layout.align());
518          unsafe {
519              // The allocator checks for alignment equality
520              intrinsics::assume(old_layout.align() == new_layout.align());
521              alloc.grow(ptr, old_layout, new_layout)
522          }
523      } else {
524          alloc.allocate(new_layout)
525      };
526  
527      memory.map_err(|_| AllocError { layout: new_layout, non_exhaustive: () }.into())
528  }
529  
530  unsafe impl<#[may_dangle] T, A: Allocator> Drop for RawVec<T, A> {
531      /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
532      fn drop(&mut self) {
533          if let Some((ptr, layout)) = self.current_memory() {
534              unsafe { self.alloc.deallocate(ptr, layout) }
535          }
536      }
537  }
538  
539  // Central function for reserve error handling.
540  #[cfg(not(no_global_oom_handling))]
541  #[inline]
542  fn handle_reserve(result: Result<(), TryReserveError>) {
543      match result.map_err(|e| e.kind()) {
544          Err(CapacityOverflow) => capacity_overflow(),
545          Err(AllocError { layout, .. }) => handle_alloc_error(layout),
546          Ok(()) => { /* yay */ }
547      }
548  }
549  
550  // We need to guarantee the following:
551  // * We don't ever allocate `> isize::MAX` byte-size objects.
552  // * We don't overflow `usize::MAX` and actually allocate too little.
553  //
554  // On 64-bit we just need to check for overflow since trying to allocate
555  // `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
556  // an extra guard for this in case we're running on a platform which can use
557  // all 4GB in user-space, e.g., PAE or x32.
558  
559  #[inline]
560  fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
561      if usize::BITS < 64 && alloc_size > isize::MAX as usize {
562          Err(CapacityOverflow.into())
563      } else {
564          Ok(())
565      }
566  }
567  
568  // One central function responsible for reporting capacity overflows. This'll
569  // ensure that the code generation related to these panics is minimal as there's
570  // only one location which panics rather than a bunch throughout the module.
571  #[cfg(not(no_global_oom_handling))]
572  fn capacity_overflow() -> ! {
573      panic!("capacity overflow");
574  }
575