library/alloc/src/raw_vec.rs

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