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Diffstat (limited to 'library/alloc/src/raw_vec.rs')
| -rw-r--r-- | library/alloc/src/raw_vec.rs | 536 |
1 files changed, 536 insertions, 0 deletions
diff --git a/library/alloc/src/raw_vec.rs b/library/alloc/src/raw_vec.rs new file mode 100644 index 00000000000..ed81ce71ddf --- /dev/null +++ b/library/alloc/src/raw_vec.rs @@ -0,0 +1,536 @@ +#![unstable(feature = "raw_vec_internals", reason = "implementation detail", issue = "none")] +#![doc(hidden)] + +use core::alloc::{LayoutErr, MemoryBlock}; +use core::cmp; +use core::mem::{self, ManuallyDrop, MaybeUninit}; +use core::ops::Drop; +use core::ptr::{NonNull, Unique}; +use core::slice; + +use crate::alloc::{ + handle_alloc_error, + AllocInit::{self, *}, + AllocRef, Global, Layout, + ReallocPlacement::{self, *}, +}; +use crate::boxed::Box; +use crate::collections::TryReserveError::{self, *}; + +#[cfg(test)] +mod tests; + +/// A low-level utility for more ergonomically allocating, reallocating, and deallocating +/// a buffer of memory on the heap without having to worry about all the corner cases +/// involved. This type is excellent for building your own data structures like Vec and VecDeque. +/// In particular: +/// +/// * Produces `Unique::dangling()` on zero-sized types. +/// * Produces `Unique::dangling()` on zero-length allocations. +/// * Avoids freeing `Unique::dangling()`. +/// * Catches all overflows in capacity computations (promotes them to "capacity overflow" panics). +/// * Guards against 32-bit systems allocating more than isize::MAX bytes. +/// * Guards against overflowing your length. +/// * Calls `handle_alloc_error` for fallible allocations. +/// * Contains a `ptr::Unique` and thus endows the user with all related benefits. +/// * Uses the excess returned from the allocator to use the largest available capacity. +/// +/// This type does not in anyway inspect the memory that it manages. When dropped it *will* +/// free its memory, but it *won't* try to drop its contents. It is up to the user of `RawVec` +/// to handle the actual things *stored* inside of a `RawVec`. +/// +/// Note that the excess of a zero-sized types is always infinite, so `capacity()` always returns +/// `usize::MAX`. This means that you need to be careful when round-tripping this type with a +/// `Box<[T]>`, since `capacity()` won't yield the length. +#[allow(missing_debug_implementations)] +pub struct RawVec<T, A: AllocRef = Global> { + ptr: Unique<T>, + cap: usize, + alloc: A, +} + +impl<T> RawVec<T, Global> { + /// HACK(Centril): This exists because `#[unstable]` `const fn`s needn't conform + /// to `min_const_fn` and so they cannot be called in `min_const_fn`s either. + /// + /// If you change `RawVec<T>::new` or dependencies, please take care to not + /// introduce anything that would truly violate `min_const_fn`. + /// + /// NOTE: We could avoid this hack and check conformance with some + /// `#[rustc_force_min_const_fn]` attribute which requires conformance + /// with `min_const_fn` but does not necessarily allow calling it in + /// `stable(...) const fn` / user code not enabling `foo` when + /// `#[rustc_const_unstable(feature = "foo", issue = "01234")]` is present. + pub const NEW: Self = Self::new(); + + /// Creates the biggest possible `RawVec` (on the system heap) + /// without allocating. If `T` has positive size, then this makes a + /// `RawVec` with capacity `0`. If `T` is zero-sized, then it makes a + /// `RawVec` with capacity `usize::MAX`. Useful for implementing + /// delayed allocation. + pub const fn new() -> Self { + Self::new_in(Global) + } + + /// Creates a `RawVec` (on the system heap) with exactly the + /// capacity and alignment requirements for a `[T; capacity]`. This is + /// equivalent to calling `RawVec::new` when `capacity` is `0` or `T` is + /// zero-sized. Note that if `T` is zero-sized this means you will + /// *not* get a `RawVec` with the requested capacity. + /// + /// # Panics + /// + /// Panics if the requested capacity exceeds `isize::MAX` bytes. + /// + /// # Aborts + /// + /// Aborts on OOM. + #[inline] + pub fn with_capacity(capacity: usize) -> Self { + Self::with_capacity_in(capacity, Global) + } + + /// Like `with_capacity`, but guarantees the buffer is zeroed. + #[inline] + pub fn with_capacity_zeroed(capacity: usize) -> Self { + Self::with_capacity_zeroed_in(capacity, Global) + } + + /// Reconstitutes a `RawVec` from a pointer and capacity. + /// + /// # Safety + /// + /// The `ptr` must be allocated (on the system heap), and with the given `capacity`. + /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit + /// systems). ZST vectors may have a capacity up to `usize::MAX`. + /// If the `ptr` and `capacity` come from a `RawVec`, then this is guaranteed. + #[inline] + pub unsafe fn from_raw_parts(ptr: *mut T, capacity: usize) -> Self { + unsafe { Self::from_raw_parts_in(ptr, capacity, Global) } + } + + /// Converts a `Box<[T]>` into a `RawVec<T>`. + pub fn from_box(slice: Box<[T]>) -> Self { + unsafe { + let mut slice = ManuallyDrop::new(slice); + RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len()) + } + } + + /// Converts the entire buffer into `Box<[MaybeUninit<T>]>` with the specified `len`. + /// + /// Note that this will correctly reconstitute any `cap` changes + /// that may have been performed. (See description of type for details.) + /// + /// # Safety + /// + /// * `len` must be greater than or equal to the most recently requested capacity, and + /// * `len` must be less than or equal to `self.capacity()`. + /// + /// Note, that the requested capacity and `self.capacity()` could differ, as + /// an allocator could overallocate and return a greater memory block than requested. + pub unsafe fn into_box(self, len: usize) -> Box<[MaybeUninit<T>]> { + // Sanity-check one half of the safety requirement (we cannot check the other half). + debug_assert!( + len <= self.capacity(), + "`len` must be smaller than or equal to `self.capacity()`" + ); + + let me = ManuallyDrop::new(self); + unsafe { + let slice = slice::from_raw_parts_mut(me.ptr() as *mut MaybeUninit<T>, len); + Box::from_raw(slice) + } + } +} + +impl<T, A: AllocRef> RawVec<T, A> { + /// Like `new`, but parameterized over the choice of allocator for + /// the returned `RawVec`. + pub const fn new_in(alloc: A) -> Self { + // `cap: 0` means "unallocated". zero-sized types are ignored. + Self { ptr: Unique::dangling(), cap: 0, alloc } + } + + /// Like `with_capacity`, but parameterized over the choice of + /// allocator for the returned `RawVec`. + #[inline] + pub fn with_capacity_in(capacity: usize, alloc: A) -> Self { + Self::allocate_in(capacity, Uninitialized, alloc) + } + + /// Like `with_capacity_zeroed`, but parameterized over the choice + /// of allocator for the returned `RawVec`. + #[inline] + pub fn with_capacity_zeroed_in(capacity: usize, alloc: A) -> Self { + Self::allocate_in(capacity, Zeroed, alloc) + } + + fn allocate_in(capacity: usize, init: AllocInit, mut alloc: A) -> Self { + if mem::size_of::<T>() == 0 { + Self::new_in(alloc) + } else { + // We avoid `unwrap_or_else` here because it bloats the amount of + // LLVM IR generated. + let layout = match Layout::array::<T>(capacity) { + Ok(layout) => layout, + Err(_) => capacity_overflow(), + }; + match alloc_guard(layout.size()) { + Ok(_) => {} + Err(_) => capacity_overflow(), + } + let memory = match alloc.alloc(layout, init) { + Ok(memory) => memory, + Err(_) => handle_alloc_error(layout), + }; + + Self { + ptr: unsafe { Unique::new_unchecked(memory.ptr.cast().as_ptr()) }, + cap: Self::capacity_from_bytes(memory.size), + alloc, + } + } + } + + /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator. + /// + /// # Safety + /// + /// The `ptr` must be allocated (via the given allocator `a`), and with the given `capacity`. + /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit + /// systems). ZST vectors may have a capacity up to `usize::MAX`. + /// If the `ptr` and `capacity` come from a `RawVec` created via `a`, then this is guaranteed. + #[inline] + pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, a: A) -> Self { + Self { ptr: unsafe { Unique::new_unchecked(ptr) }, cap: capacity, alloc: a } + } + + /// Gets a raw pointer to the start of the allocation. Note that this is + /// `Unique::dangling()` if `capacity == 0` or `T` is zero-sized. In the former case, you must + /// be careful. + pub fn ptr(&self) -> *mut T { + self.ptr.as_ptr() + } + + /// Gets the capacity of the allocation. + /// + /// This will always be `usize::MAX` if `T` is zero-sized. + #[inline(always)] + pub fn capacity(&self) -> usize { + if mem::size_of::<T>() == 0 { usize::MAX } else { self.cap } + } + + /// Returns a shared reference to the allocator backing this `RawVec`. + pub fn alloc(&self) -> &A { + &self.alloc + } + + /// Returns a mutable reference to the allocator backing this `RawVec`. + pub fn alloc_mut(&mut self) -> &mut A { + &mut self.alloc + } + + fn current_memory(&self) -> Option<(NonNull<u8>, Layout)> { + if mem::size_of::<T>() == 0 || self.cap == 0 { + None + } else { + // We have an allocated chunk of memory, so we can bypass runtime + // checks to get our current layout. + unsafe { + let align = mem::align_of::<T>(); + let size = mem::size_of::<T>() * self.cap; + let layout = Layout::from_size_align_unchecked(size, align); + Some((self.ptr.cast().into(), layout)) + } + } + } + + /// Ensures that the buffer contains at least enough space to hold `len + + /// additional` elements. If it doesn't already have enough capacity, will + /// reallocate enough space plus comfortable slack space to get amortized + /// `O(1)` behavior. Will limit this behavior if it would needlessly cause + /// itself to panic. + /// + /// If `len` exceeds `self.capacity()`, this may fail to actually allocate + /// the requested space. This is not really unsafe, but the unsafe + /// code *you* write that relies on the behavior of this function may break. + /// + /// This is ideal for implementing a bulk-push operation like `extend`. + /// + /// # Panics + /// + /// Panics if the new capacity exceeds `isize::MAX` bytes. + /// + /// # Aborts + /// + /// Aborts on OOM. + /// + /// # Examples + /// + /// ``` + /// # #![feature(raw_vec_internals)] + /// # extern crate alloc; + /// # use std::ptr; + /// # use alloc::raw_vec::RawVec; + /// struct MyVec<T> { + /// buf: RawVec<T>, + /// len: usize, + /// } + /// + /// impl<T: Clone> MyVec<T> { + /// pub fn push_all(&mut self, elems: &[T]) { + /// self.buf.reserve(self.len, elems.len()); + /// // reserve would have aborted or panicked if the len exceeded + /// // `isize::MAX` so this is safe to do unchecked now. + /// for x in elems { + /// unsafe { + /// ptr::write(self.buf.ptr().add(self.len), x.clone()); + /// } + /// self.len += 1; + /// } + /// } + /// } + /// # fn main() { + /// # let mut vector = MyVec { buf: RawVec::new(), len: 0 }; + /// # vector.push_all(&[1, 3, 5, 7, 9]); + /// # } + /// ``` + pub fn reserve(&mut self, len: usize, additional: usize) { + match self.try_reserve(len, additional) { + Err(CapacityOverflow) => capacity_overflow(), + Err(AllocError { layout, .. }) => handle_alloc_error(layout), + Ok(()) => { /* yay */ } + } + } + + /// The same as `reserve`, but returns on errors instead of panicking or aborting. + pub fn try_reserve(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> { + if self.needs_to_grow(len, additional) { + self.grow_amortized(len, additional) + } else { + Ok(()) + } + } + + /// Ensures that the buffer contains at least enough space to hold `len + + /// additional` elements. If it doesn't already, will reallocate the + /// minimum possible amount of memory necessary. Generally this will be + /// exactly the amount of memory necessary, but in principle the allocator + /// is free to give back more than we asked for. + /// + /// If `len` exceeds `self.capacity()`, this may fail to actually allocate + /// the requested space. This is not really unsafe, but the unsafe code + /// *you* write that relies on the behavior of this function may break. + /// + /// # Panics + /// + /// Panics if the new capacity exceeds `isize::MAX` bytes. + /// + /// # Aborts + /// + /// Aborts on OOM. + pub fn reserve_exact(&mut self, len: usize, additional: usize) { + match self.try_reserve_exact(len, additional) { + Err(CapacityOverflow) => capacity_overflow(), + Err(AllocError { layout, .. }) => handle_alloc_error(layout), + Ok(()) => { /* yay */ } + } + } + + /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting. + pub fn try_reserve_exact( + &mut self, + len: usize, + additional: usize, + ) -> Result<(), TryReserveError> { + if self.needs_to_grow(len, additional) { self.grow_exact(len, additional) } else { Ok(()) } + } + + /// Shrinks the allocation down to the specified amount. If the given amount + /// is 0, actually completely deallocates. + /// + /// # Panics + /// + /// Panics if the given amount is *larger* than the current capacity. + /// + /// # Aborts + /// + /// Aborts on OOM. + pub fn shrink_to_fit(&mut self, amount: usize) { + match self.shrink(amount, MayMove) { + Err(CapacityOverflow) => capacity_overflow(), + Err(AllocError { layout, .. }) => handle_alloc_error(layout), + Ok(()) => { /* yay */ } + } + } +} + +impl<T, A: AllocRef> RawVec<T, A> { + /// Returns if the buffer needs to grow to fulfill the needed extra capacity. + /// Mainly used to make inlining reserve-calls possible without inlining `grow`. + fn needs_to_grow(&self, len: usize, additional: usize) -> bool { + additional > self.capacity().wrapping_sub(len) + } + + fn capacity_from_bytes(excess: usize) -> usize { + debug_assert_ne!(mem::size_of::<T>(), 0); + excess / mem::size_of::<T>() + } + + fn set_memory(&mut self, memory: MemoryBlock) { + self.ptr = unsafe { Unique::new_unchecked(memory.ptr.cast().as_ptr()) }; + self.cap = Self::capacity_from_bytes(memory.size); + } + + // This method is usually instantiated many times. So we want it to be as + // small as possible, to improve compile times. But we also want as much of + // its contents to be statically computable as possible, to make the + // generated code run faster. Therefore, this method is carefully written + // so that all of the code that depends on `T` is within it, while as much + // of the code that doesn't depend on `T` as possible is in functions that + // are non-generic over `T`. + fn grow_amortized(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> { + // This is ensured by the calling contexts. + debug_assert!(additional > 0); + + if mem::size_of::<T>() == 0 { + // Since we return a capacity of `usize::MAX` when `elem_size` is + // 0, getting to here necessarily means the `RawVec` is overfull. + return Err(CapacityOverflow); + } + + // Nothing we can really do about these checks, sadly. + let required_cap = len.checked_add(additional).ok_or(CapacityOverflow)?; + + // This guarantees exponential growth. The doubling cannot overflow + // because `cap <= isize::MAX` and the type of `cap` is `usize`. + let cap = cmp::max(self.cap * 2, required_cap); + + // Tiny Vecs are dumb. Skip to: + // - 8 if the element size is 1, because any heap allocators is likely + // to round up a request of less than 8 bytes to at least 8 bytes. + // - 4 if elements are moderate-sized (<= 1 KiB). + // - 1 otherwise, to avoid wasting too much space for very short Vecs. + // Note that `min_non_zero_cap` is computed statically. + let elem_size = mem::size_of::<T>(); + let min_non_zero_cap = if elem_size == 1 { + 8 + } else if elem_size <= 1024 { + 4 + } else { + 1 + }; + let cap = cmp::max(min_non_zero_cap, cap); + + let new_layout = Layout::array::<T>(cap); + + // `finish_grow` is non-generic over `T`. + let memory = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?; + self.set_memory(memory); + Ok(()) + } + + // The constraints on this method are much the same as those on + // `grow_amortized`, but this method is usually instantiated less often so + // it's less critical. + fn grow_exact(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> { + if mem::size_of::<T>() == 0 { + // Since we return a capacity of `usize::MAX` when the type size is + // 0, getting to here necessarily means the `RawVec` is overfull. + return Err(CapacityOverflow); + } + + let cap = len.checked_add(additional).ok_or(CapacityOverflow)?; + let new_layout = Layout::array::<T>(cap); + + // `finish_grow` is non-generic over `T`. + let memory = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?; + self.set_memory(memory); + Ok(()) + } + + fn shrink( + &mut self, + amount: usize, + placement: ReallocPlacement, + ) -> Result<(), TryReserveError> { + assert!(amount <= self.capacity(), "Tried to shrink to a larger capacity"); + + let (ptr, layout) = if let Some(mem) = self.current_memory() { mem } else { return Ok(()) }; + let new_size = amount * mem::size_of::<T>(); + + let memory = unsafe { + self.alloc.shrink(ptr, layout, new_size, placement).map_err(|_| { + TryReserveError::AllocError { + layout: Layout::from_size_align_unchecked(new_size, layout.align()), + non_exhaustive: (), + } + })? + }; + self.set_memory(memory); + Ok(()) + } +} + +// This function is outside `RawVec` to minimize compile times. See the comment +// above `RawVec::grow_amortized` for details. (The `A` parameter isn't +// significant, because the number of different `A` types seen in practice is +// much smaller than the number of `T` types.) +fn finish_grow<A>( + new_layout: Result<Layout, LayoutErr>, + current_memory: Option<(NonNull<u8>, Layout)>, + alloc: &mut A, +) -> Result<MemoryBlock, TryReserveError> +where + A: AllocRef, +{ + // Check for the error here to minimize the size of `RawVec::grow_*`. + let new_layout = new_layout.map_err(|_| CapacityOverflow)?; + + alloc_guard(new_layout.size())?; + + let memory = if let Some((ptr, old_layout)) = current_memory { + debug_assert_eq!(old_layout.align(), new_layout.align()); + unsafe { alloc.grow(ptr, old_layout, new_layout.size(), MayMove, Uninitialized) } + } else { + alloc.alloc(new_layout, Uninitialized) + } + .map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })?; + + Ok(memory) +} + +unsafe impl<#[may_dangle] T, A: AllocRef> Drop for RawVec<T, A> { + /// Frees the memory owned by the `RawVec` *without* trying to drop its contents. + fn drop(&mut self) { + if let Some((ptr, layout)) = self.current_memory() { + unsafe { self.alloc.dealloc(ptr, layout) } + } + } +} + +// We need to guarantee the following: +// * We don't ever allocate `> isize::MAX` byte-size objects. +// * We don't overflow `usize::MAX` and actually allocate too little. +// +// On 64-bit we just need to check for overflow since trying to allocate +// `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add +// an extra guard for this in case we're running on a platform which can use +// all 4GB in user-space, e.g., PAE or x32. + +#[inline] +fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> { + if mem::size_of::<usize>() < 8 && alloc_size > isize::MAX as usize { + Err(CapacityOverflow) + } else { + Ok(()) + } +} + +// One central function responsible for reporting capacity overflows. This'll +// ensure that the code generation related to these panics is minimal as there's +// only one location which panics rather than a bunch throughout the module. +fn capacity_overflow() -> ! { + panic!("capacity overflow"); +} |