| //! The arena, a fast but limited type of allocator. |
| //! |
| //! Arenas are a type of allocator that destroy the objects within, all at |
| //! once, once the arena itself is destroyed. They do not support deallocation |
| //! of individual objects while the arena itself is still alive. The benefit |
| //! of an arena is very fast allocation; just a pointer bump. |
| //! |
| //! This crate implements `TypedArena`, a simple arena that can only hold |
| //! objects of a single type. |
| |
| #![doc(html_root_url = "https://doc.rust-lang.org/nightly/", |
| test(no_crate_inject, attr(deny(warnings))))] |
| |
| #![feature(core_intrinsics)] |
| #![feature(dropck_eyepatch)] |
| #![feature(raw_vec_internals)] |
| #![cfg_attr(test, feature(test))] |
| |
| #![allow(deprecated)] |
| |
| extern crate alloc; |
| |
| use rustc_data_structures::cold_path; |
| use rustc_data_structures::sync::MTLock; |
| use smallvec::SmallVec; |
| |
| use std::cell::{Cell, RefCell}; |
| use std::cmp; |
| use std::intrinsics; |
| use std::marker::{PhantomData, Send}; |
| use std::mem; |
| use std::ptr; |
| use std::slice; |
| |
| use alloc::raw_vec::RawVec; |
| |
| /// An arena that can hold objects of only one type. |
| pub struct TypedArena<T> { |
| /// A pointer to the next object to be allocated. |
| ptr: Cell<*mut T>, |
| |
| /// A pointer to the end of the allocated area. When this pointer is |
| /// reached, a new chunk is allocated. |
| end: Cell<*mut T>, |
| |
| /// A vector of arena chunks. |
| chunks: RefCell<Vec<TypedArenaChunk<T>>>, |
| |
| /// Marker indicating that dropping the arena causes its owned |
| /// instances of `T` to be dropped. |
| _own: PhantomData<T>, |
| } |
| |
| struct TypedArenaChunk<T> { |
| /// The raw storage for the arena chunk. |
| storage: RawVec<T>, |
| /// The number of valid entries in the chunk. |
| entries: usize, |
| } |
| |
| impl<T> TypedArenaChunk<T> { |
| #[inline] |
| unsafe fn new(capacity: usize) -> TypedArenaChunk<T> { |
| TypedArenaChunk { |
| storage: RawVec::with_capacity(capacity), |
| entries: 0, |
| } |
| } |
| |
| /// Destroys this arena chunk. |
| #[inline] |
| unsafe fn destroy(&mut self, len: usize) { |
| // The branch on needs_drop() is an -O1 performance optimization. |
| // Without the branch, dropping TypedArena<u8> takes linear time. |
| if mem::needs_drop::<T>() { |
| let mut start = self.start(); |
| // Destroy all allocated objects. |
| for _ in 0..len { |
| ptr::drop_in_place(start); |
| start = start.offset(1); |
| } |
| } |
| } |
| |
| // Returns a pointer to the first allocated object. |
| #[inline] |
| fn start(&self) -> *mut T { |
| self.storage.ptr() |
| } |
| |
| // Returns a pointer to the end of the allocated space. |
| #[inline] |
| fn end(&self) -> *mut T { |
| unsafe { |
| if mem::size_of::<T>() == 0 { |
| // A pointer as large as possible for zero-sized elements. |
| !0 as *mut T |
| } else { |
| self.start().add(self.storage.capacity()) |
| } |
| } |
| } |
| } |
| |
| const PAGE: usize = 4096; |
| |
| impl<T> Default for TypedArena<T> { |
| /// Creates a new `TypedArena`. |
| fn default() -> TypedArena<T> { |
| TypedArena { |
| // We set both `ptr` and `end` to 0 so that the first call to |
| // alloc() will trigger a grow(). |
| ptr: Cell::new(ptr::null_mut()), |
| end: Cell::new(ptr::null_mut()), |
| chunks: RefCell::new(vec![]), |
| _own: PhantomData, |
| } |
| } |
| } |
| |
| impl<T> TypedArena<T> { |
| pub fn in_arena(&self, ptr: *const T) -> bool { |
| let ptr = ptr as *const T as *mut T; |
| |
| self.chunks.borrow().iter().any(|chunk| chunk.start() <= ptr && ptr < chunk.end()) |
| } |
| /// Allocates an object in the `TypedArena`, returning a reference to it. |
| #[inline] |
| pub fn alloc(&self, object: T) -> &mut T { |
| if self.ptr == self.end { |
| self.grow(1) |
| } |
| |
| unsafe { |
| if mem::size_of::<T>() == 0 { |
| self.ptr |
| .set(intrinsics::arith_offset(self.ptr.get() as *mut u8, 1) |
| as *mut T); |
| let ptr = mem::align_of::<T>() as *mut T; |
| // Don't drop the object. This `write` is equivalent to `forget`. |
| ptr::write(ptr, object); |
| &mut *ptr |
| } else { |
| let ptr = self.ptr.get(); |
| // Advance the pointer. |
| self.ptr.set(self.ptr.get().offset(1)); |
| // Write into uninitialized memory. |
| ptr::write(ptr, object); |
| &mut *ptr |
| } |
| } |
| } |
| |
| #[inline] |
| fn can_allocate(&self, len: usize) -> bool { |
| let available_capacity_bytes = self.end.get() as usize - self.ptr.get() as usize; |
| let at_least_bytes = len.checked_mul(mem::size_of::<T>()).unwrap(); |
| available_capacity_bytes >= at_least_bytes |
| } |
| |
| /// Ensures there's enough space in the current chunk to fit `len` objects. |
| #[inline] |
| fn ensure_capacity(&self, len: usize) { |
| if !self.can_allocate(len) { |
| self.grow(len); |
| debug_assert!(self.can_allocate(len)); |
| } |
| } |
| |
| #[inline] |
| unsafe fn alloc_raw_slice(&self, len: usize) -> *mut T { |
| assert!(mem::size_of::<T>() != 0); |
| assert!(len != 0); |
| |
| self.ensure_capacity(len); |
| |
| let start_ptr = self.ptr.get(); |
| self.ptr.set(start_ptr.add(len)); |
| start_ptr |
| } |
| |
| /// Allocates a slice of objects that are copied into the `TypedArena`, returning a mutable |
| /// reference to it. Will panic if passed a zero-sized types. |
| /// |
| /// Panics: |
| /// |
| /// - Zero-sized types |
| /// - Zero-length slices |
| #[inline] |
| pub fn alloc_slice(&self, slice: &[T]) -> &mut [T] |
| where |
| T: Copy, |
| { |
| unsafe { |
| let len = slice.len(); |
| let start_ptr = self.alloc_raw_slice(len); |
| slice.as_ptr().copy_to_nonoverlapping(start_ptr, len); |
| slice::from_raw_parts_mut(start_ptr, len) |
| } |
| } |
| |
| #[inline] |
| pub fn alloc_from_iter<I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] { |
| assert!(mem::size_of::<T>() != 0); |
| let mut iter = iter.into_iter(); |
| let size_hint = iter.size_hint(); |
| |
| match size_hint { |
| (min, Some(max)) if min == max => { |
| // We know the exact number of elements the iterator will produce here |
| let len = min; |
| |
| if len == 0 { |
| return &mut []; |
| } |
| |
| self.ensure_capacity(len); |
| |
| let slice = self.ptr.get(); |
| |
| unsafe { |
| let mut ptr = self.ptr.get(); |
| for _ in 0..len { |
| // Write into uninitialized memory. |
| ptr::write(ptr, iter.next().unwrap()); |
| // Advance the pointer. |
| ptr = ptr.offset(1); |
| // Update the pointer per iteration so if `iter.next()` panics |
| // we destroy the correct amount |
| self.ptr.set(ptr); |
| } |
| slice::from_raw_parts_mut(slice, len) |
| } |
| } |
| _ => { |
| cold_path(move || -> &mut [T] { |
| let mut vec: SmallVec<[_; 8]> = iter.collect(); |
| if vec.is_empty() { |
| return &mut []; |
| } |
| // Move the content to the arena by copying it and then forgetting |
| // the content of the SmallVec |
| unsafe { |
| let len = vec.len(); |
| let start_ptr = self.alloc_raw_slice(len); |
| vec.as_ptr().copy_to_nonoverlapping(start_ptr, len); |
| vec.set_len(0); |
| slice::from_raw_parts_mut(start_ptr, len) |
| } |
| }) |
| } |
| } |
| } |
| |
| /// Grows the arena. |
| #[inline(never)] |
| #[cold] |
| fn grow(&self, n: usize) { |
| unsafe { |
| let mut chunks = self.chunks.borrow_mut(); |
| let (chunk, mut new_capacity); |
| if let Some(last_chunk) = chunks.last_mut() { |
| let used_bytes = self.ptr.get() as usize - last_chunk.start() as usize; |
| let currently_used_cap = used_bytes / mem::size_of::<T>(); |
| last_chunk.entries = currently_used_cap; |
| if last_chunk.storage.reserve_in_place(currently_used_cap, n) { |
| self.end.set(last_chunk.end()); |
| return; |
| } else { |
| new_capacity = last_chunk.storage.capacity(); |
| loop { |
| new_capacity = new_capacity.checked_mul(2).unwrap(); |
| if new_capacity >= currently_used_cap + n { |
| break; |
| } |
| } |
| } |
| } else { |
| let elem_size = cmp::max(1, mem::size_of::<T>()); |
| new_capacity = cmp::max(n, PAGE / elem_size); |
| } |
| chunk = TypedArenaChunk::<T>::new(new_capacity); |
| self.ptr.set(chunk.start()); |
| self.end.set(chunk.end()); |
| chunks.push(chunk); |
| } |
| } |
| |
| /// Clears the arena. Deallocates all but the longest chunk which may be reused. |
| pub fn clear(&mut self) { |
| unsafe { |
| // Clear the last chunk, which is partially filled. |
| let mut chunks_borrow = self.chunks.borrow_mut(); |
| if let Some(mut last_chunk) = chunks_borrow.last_mut() { |
| self.clear_last_chunk(&mut last_chunk); |
| let len = chunks_borrow.len(); |
| // If `T` is ZST, code below has no effect. |
| for mut chunk in chunks_borrow.drain(..len-1) { |
| chunk.destroy(chunk.entries); |
| } |
| } |
| } |
| } |
| |
| // Drops the contents of the last chunk. The last chunk is partially empty, unlike all other |
| // chunks. |
| fn clear_last_chunk(&self, last_chunk: &mut TypedArenaChunk<T>) { |
| // Determine how much was filled. |
| let start = last_chunk.start() as usize; |
| // We obtain the value of the pointer to the first uninitialized element. |
| let end = self.ptr.get() as usize; |
| // We then calculate the number of elements to be dropped in the last chunk, |
| // which is the filled area's length. |
| let diff = if mem::size_of::<T>() == 0 { |
| // `T` is ZST. It can't have a drop flag, so the value here doesn't matter. We get |
| // the number of zero-sized values in the last and only chunk, just out of caution. |
| // Recall that `end` was incremented for each allocated value. |
| end - start |
| } else { |
| (end - start) / mem::size_of::<T>() |
| }; |
| // Pass that to the `destroy` method. |
| unsafe { |
| last_chunk.destroy(diff); |
| } |
| // Reset the chunk. |
| self.ptr.set(last_chunk.start()); |
| } |
| } |
| |
| unsafe impl<#[may_dangle] T> Drop for TypedArena<T> { |
| fn drop(&mut self) { |
| unsafe { |
| // Determine how much was filled. |
| let mut chunks_borrow = self.chunks.borrow_mut(); |
| if let Some(mut last_chunk) = chunks_borrow.pop() { |
| // Drop the contents of the last chunk. |
| self.clear_last_chunk(&mut last_chunk); |
| // The last chunk will be dropped. Destroy all other chunks. |
| for chunk in chunks_borrow.iter_mut() { |
| chunk.destroy(chunk.entries); |
| } |
| } |
| // RawVec handles deallocation of `last_chunk` and `self.chunks`. |
| } |
| } |
| } |
| |
| unsafe impl<T: Send> Send for TypedArena<T> {} |
| |
| pub struct DroplessArena { |
| /// A pointer to the next object to be allocated. |
| ptr: Cell<*mut u8>, |
| |
| /// A pointer to the end of the allocated area. When this pointer is |
| /// reached, a new chunk is allocated. |
| end: Cell<*mut u8>, |
| |
| /// A vector of arena chunks. |
| chunks: RefCell<Vec<TypedArenaChunk<u8>>>, |
| } |
| |
| unsafe impl Send for DroplessArena {} |
| |
| impl Default for DroplessArena { |
| #[inline] |
| fn default() -> DroplessArena { |
| DroplessArena { |
| ptr: Cell::new(ptr::null_mut()), |
| end: Cell::new(ptr::null_mut()), |
| chunks: Default::default(), |
| } |
| } |
| } |
| |
| impl DroplessArena { |
| pub fn in_arena<T: ?Sized>(&self, ptr: *const T) -> bool { |
| let ptr = ptr as *const u8 as *mut u8; |
| |
| self.chunks.borrow().iter().any(|chunk| chunk.start() <= ptr && ptr < chunk.end()) |
| } |
| |
| #[inline] |
| fn align(&self, align: usize) { |
| let final_address = ((self.ptr.get() as usize) + align - 1) & !(align - 1); |
| self.ptr.set(final_address as *mut u8); |
| assert!(self.ptr <= self.end); |
| } |
| |
| #[inline(never)] |
| #[cold] |
| fn grow(&self, needed_bytes: usize) { |
| unsafe { |
| let mut chunks = self.chunks.borrow_mut(); |
| let (chunk, mut new_capacity); |
| if let Some(last_chunk) = chunks.last_mut() { |
| let used_bytes = self.ptr.get() as usize - last_chunk.start() as usize; |
| if last_chunk |
| .storage |
| .reserve_in_place(used_bytes, needed_bytes) |
| { |
| self.end.set(last_chunk.end()); |
| return; |
| } else { |
| new_capacity = last_chunk.storage.capacity(); |
| loop { |
| new_capacity = new_capacity.checked_mul(2).unwrap(); |
| if new_capacity >= used_bytes + needed_bytes { |
| break; |
| } |
| } |
| } |
| } else { |
| new_capacity = cmp::max(needed_bytes, PAGE); |
| } |
| chunk = TypedArenaChunk::<u8>::new(new_capacity); |
| self.ptr.set(chunk.start()); |
| self.end.set(chunk.end()); |
| chunks.push(chunk); |
| } |
| } |
| |
| #[inline] |
| pub fn alloc_raw(&self, bytes: usize, align: usize) -> &mut [u8] { |
| unsafe { |
| assert!(bytes != 0); |
| |
| self.align(align); |
| |
| let future_end = intrinsics::arith_offset(self.ptr.get(), bytes as isize); |
| if (future_end as *mut u8) >= self.end.get() { |
| self.grow(bytes); |
| } |
| |
| let ptr = self.ptr.get(); |
| // Set the pointer past ourselves |
| self.ptr.set( |
| intrinsics::arith_offset(self.ptr.get(), bytes as isize) as *mut u8, |
| ); |
| slice::from_raw_parts_mut(ptr, bytes) |
| } |
| } |
| |
| #[inline] |
| pub fn alloc<T>(&self, object: T) -> &mut T { |
| assert!(!mem::needs_drop::<T>()); |
| |
| let mem = self.alloc_raw( |
| mem::size_of::<T>(), |
| mem::align_of::<T>()) as *mut _ as *mut T; |
| |
| unsafe { |
| // Write into uninitialized memory. |
| ptr::write(mem, object); |
| &mut *mem |
| } |
| } |
| |
| /// Allocates a slice of objects that are copied into the `DroplessArena`, returning a mutable |
| /// reference to it. Will panic if passed a zero-sized type. |
| /// |
| /// Panics: |
| /// |
| /// - Zero-sized types |
| /// - Zero-length slices |
| #[inline] |
| pub fn alloc_slice<T>(&self, slice: &[T]) -> &mut [T] |
| where |
| T: Copy, |
| { |
| assert!(!mem::needs_drop::<T>()); |
| assert!(mem::size_of::<T>() != 0); |
| assert!(!slice.is_empty()); |
| |
| let mem = self.alloc_raw( |
| slice.len() * mem::size_of::<T>(), |
| mem::align_of::<T>()) as *mut _ as *mut T; |
| |
| unsafe { |
| let arena_slice = slice::from_raw_parts_mut(mem, slice.len()); |
| arena_slice.copy_from_slice(slice); |
| arena_slice |
| } |
| } |
| |
| #[inline] |
| unsafe fn write_from_iter<T, I: Iterator<Item = T>>( |
| &self, |
| mut iter: I, |
| len: usize, |
| mem: *mut T, |
| ) -> &mut [T] { |
| let mut i = 0; |
| // Use a manual loop since LLVM manages to optimize it better for |
| // slice iterators |
| loop { |
| let value = iter.next(); |
| if i >= len || value.is_none() { |
| // We only return as many items as the iterator gave us, even |
| // though it was supposed to give us `len` |
| return slice::from_raw_parts_mut(mem, i); |
| } |
| ptr::write(mem.offset(i as isize), value.unwrap()); |
| i += 1; |
| } |
| } |
| |
| #[inline] |
| pub fn alloc_from_iter<T, I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] { |
| let iter = iter.into_iter(); |
| assert!(mem::size_of::<T>() != 0); |
| assert!(!mem::needs_drop::<T>()); |
| |
| let size_hint = iter.size_hint(); |
| |
| match size_hint { |
| (min, Some(max)) if min == max => { |
| // We know the exact number of elements the iterator will produce here |
| let len = min; |
| |
| if len == 0 { |
| return &mut [] |
| } |
| let size = len.checked_mul(mem::size_of::<T>()).unwrap(); |
| let mem = self.alloc_raw(size, mem::align_of::<T>()) as *mut _ as *mut T; |
| unsafe { |
| self.write_from_iter(iter, len, mem) |
| } |
| } |
| (_, _) => { |
| cold_path(move || -> &mut [T] { |
| let mut vec: SmallVec<[_; 8]> = iter.collect(); |
| if vec.is_empty() { |
| return &mut []; |
| } |
| // Move the content to the arena by copying it and then forgetting |
| // the content of the SmallVec |
| unsafe { |
| let len = vec.len(); |
| let start_ptr = self.alloc_raw( |
| len * mem::size_of::<T>(), |
| mem::align_of::<T>() |
| ) as *mut _ as *mut T; |
| vec.as_ptr().copy_to_nonoverlapping(start_ptr, len); |
| vec.set_len(0); |
| slice::from_raw_parts_mut(start_ptr, len) |
| } |
| }) |
| } |
| } |
| } |
| } |
| |
| #[derive(Default)] |
| // FIXME(@Zoxc): this type is entirely unused in rustc |
| pub struct SyncTypedArena<T> { |
| lock: MTLock<TypedArena<T>>, |
| } |
| |
| impl<T> SyncTypedArena<T> { |
| #[inline(always)] |
| pub fn alloc(&self, object: T) -> &mut T { |
| // Extend the lifetime of the result since it's limited to the lock guard |
| unsafe { &mut *(self.lock.lock().alloc(object) as *mut T) } |
| } |
| |
| #[inline(always)] |
| pub fn alloc_slice(&self, slice: &[T]) -> &mut [T] |
| where |
| T: Copy, |
| { |
| // Extend the lifetime of the result since it's limited to the lock guard |
| unsafe { &mut *(self.lock.lock().alloc_slice(slice) as *mut [T]) } |
| } |
| |
| #[inline(always)] |
| pub fn clear(&mut self) { |
| self.lock.get_mut().clear(); |
| } |
| } |
| |
| #[derive(Default)] |
| pub struct SyncDroplessArena { |
| lock: MTLock<DroplessArena>, |
| } |
| |
| impl SyncDroplessArena { |
| #[inline(always)] |
| pub fn in_arena<T: ?Sized>(&self, ptr: *const T) -> bool { |
| self.lock.lock().in_arena(ptr) |
| } |
| |
| #[inline(always)] |
| pub fn alloc_raw(&self, bytes: usize, align: usize) -> &mut [u8] { |
| // Extend the lifetime of the result since it's limited to the lock guard |
| unsafe { &mut *(self.lock.lock().alloc_raw(bytes, align) as *mut [u8]) } |
| } |
| |
| #[inline(always)] |
| pub fn alloc<T>(&self, object: T) -> &mut T { |
| // Extend the lifetime of the result since it's limited to the lock guard |
| unsafe { &mut *(self.lock.lock().alloc(object) as *mut T) } |
| } |
| |
| #[inline(always)] |
| pub fn alloc_slice<T>(&self, slice: &[T]) -> &mut [T] |
| where |
| T: Copy, |
| { |
| // Extend the lifetime of the result since it's limited to the lock guard |
| unsafe { &mut *(self.lock.lock().alloc_slice(slice) as *mut [T]) } |
| } |
| } |
| |
| #[cfg(test)] |
| mod tests; |