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| -rw-r--r-- | compiler/rustc_arena/src/lib.rs | 645 |
1 files changed, 645 insertions, 0 deletions
diff --git a/compiler/rustc_arena/src/lib.rs b/compiler/rustc_arena/src/lib.rs new file mode 100644 index 000000000..a5f1cbc96 --- /dev/null +++ b/compiler/rustc_arena/src/lib.rs @@ -0,0 +1,645 @@ +//! 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 several kinds of arena. + +#![doc( + html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/", + test(no_crate_inject, attr(deny(warnings))) +)] +#![feature(dropck_eyepatch)] +#![feature(new_uninit)] +#![feature(maybe_uninit_slice)] +#![feature(min_specialization)] +#![feature(decl_macro)] +#![feature(rustc_attrs)] +#![cfg_attr(test, feature(test))] +#![feature(strict_provenance)] +#![feature(ptr_const_cast)] + +use smallvec::SmallVec; + +use std::alloc::Layout; +use std::cell::{Cell, RefCell}; +use std::cmp; +use std::marker::{PhantomData, Send}; +use std::mem::{self, MaybeUninit}; +use std::ptr::{self, NonNull}; +use std::slice; + +#[inline(never)] +#[cold] +fn cold_path<F: FnOnce() -> R, R>(f: F) -> R { + f() +} + +/// 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<ArenaChunk<T>>>, + + /// Marker indicating that dropping the arena causes its owned + /// instances of `T` to be dropped. + _own: PhantomData<T>, +} + +struct ArenaChunk<T = u8> { + /// The raw storage for the arena chunk. + storage: NonNull<[MaybeUninit<T>]>, + /// The number of valid entries in the chunk. + entries: usize, +} + +unsafe impl<#[may_dangle] T> Drop for ArenaChunk<T> { + fn drop(&mut self) { + unsafe { Box::from_raw(self.storage.as_mut()) }; + } +} + +impl<T> ArenaChunk<T> { + #[inline] + unsafe fn new(capacity: usize) -> ArenaChunk<T> { + ArenaChunk { + storage: NonNull::new(Box::into_raw(Box::new_uninit_slice(capacity))).unwrap(), + 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 slice = &mut *(self.storage.as_mut()); + ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(&mut slice[..len])); + } + } + + // Returns a pointer to the first allocated object. + #[inline] + fn start(&mut self) -> *mut T { + self.storage.as_ptr() as *mut T + } + + // Returns a pointer to the end of the allocated space. + #[inline] + fn end(&mut self) -> *mut T { + unsafe { + if mem::size_of::<T>() == 0 { + // A pointer as large as possible for zero-sized elements. + ptr::invalid_mut(!0) + } else { + self.start().add((*self.storage.as_ptr()).len()) + } + } + } +} + +// The arenas start with PAGE-sized chunks, and then each new chunk is twice as +// big as its predecessor, up until we reach HUGE_PAGE-sized chunks, whereupon +// we stop growing. This scales well, from arenas that are barely used up to +// arenas that are used for 100s of MiBs. Note also that the chosen sizes match +// the usual sizes of pages and huge pages on Linux. +const PAGE: usize = 4096; +const HUGE_PAGE: usize = 2 * 1024 * 1024; + +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: Default::default(), + _own: PhantomData, + } + } +} + +trait IterExt<T> { + fn alloc_from_iter(self, arena: &TypedArena<T>) -> &mut [T]; +} + +impl<I, T> IterExt<T> for I +where + I: IntoIterator<Item = T>, +{ + // This default collects into a `SmallVec` and then allocates by copying + // from it. The specializations below for types like `Vec` are more + // efficient, copying directly without the intermediate collecting step. + // This default could be made more efficient, like + // `DroplessArena::alloc_from_iter`, but it's not hot enough to bother. + #[inline] + default fn alloc_from_iter(self, arena: &TypedArena<T>) -> &mut [T] { + let vec: SmallVec<[_; 8]> = self.into_iter().collect(); + vec.alloc_from_iter(arena) + } +} + +impl<T, const N: usize> IterExt<T> for std::array::IntoIter<T, N> { + #[inline] + fn alloc_from_iter(self, arena: &TypedArena<T>) -> &mut [T] { + let len = self.len(); + if len == 0 { + return &mut []; + } + // Move the content to the arena by copying and then forgetting it. + unsafe { + let start_ptr = arena.alloc_raw_slice(len); + self.as_slice().as_ptr().copy_to_nonoverlapping(start_ptr, len); + mem::forget(self); + slice::from_raw_parts_mut(start_ptr, len) + } + } +} + +impl<T> IterExt<T> for Vec<T> { + #[inline] + fn alloc_from_iter(mut self, arena: &TypedArena<T>) -> &mut [T] { + let len = self.len(); + if len == 0 { + return &mut []; + } + // Move the content to the arena by copying and then forgetting it. + unsafe { + let start_ptr = arena.alloc_raw_slice(len); + self.as_ptr().copy_to_nonoverlapping(start_ptr, len); + self.set_len(0); + slice::from_raw_parts_mut(start_ptr, len) + } + } +} + +impl<A: smallvec::Array> IterExt<A::Item> for SmallVec<A> { + #[inline] + fn alloc_from_iter(mut self, arena: &TypedArena<A::Item>) -> &mut [A::Item] { + let len = self.len(); + if len == 0 { + return &mut []; + } + // Move the content to the arena by copying and then forgetting it. + unsafe { + let start_ptr = arena.alloc_raw_slice(len); + self.as_ptr().copy_to_nonoverlapping(start_ptr, len); + self.set_len(0); + slice::from_raw_parts_mut(start_ptr, len) + } + } +} + +impl<T> TypedArena<T> { + /// 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((self.ptr.get() as *mut u8).wrapping_offset(1) as *mut T); + let ptr = ptr::NonNull::<T>::dangling().as_ptr(); + // 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, additional: usize) -> bool { + // FIXME: this should *likely* use `offset_from`, but more + // investigation is needed (including running tests in miri). + let available_bytes = self.end.get().addr() - self.ptr.get().addr(); + let additional_bytes = additional.checked_mul(mem::size_of::<T>()).unwrap(); + available_bytes >= additional_bytes + } + + /// Ensures there's enough space in the current chunk to fit `len` objects. + #[inline] + fn ensure_capacity(&self, additional: usize) { + if !self.can_allocate(additional) { + self.grow(additional); + debug_assert!(self.can_allocate(additional)); + } + } + + #[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 + } + + #[inline] + pub fn alloc_from_iter<I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] { + assert!(mem::size_of::<T>() != 0); + iter.alloc_from_iter(self) + } + + /// Grows the arena. + #[inline(never)] + #[cold] + fn grow(&self, additional: usize) { + unsafe { + // We need the element size to convert chunk sizes (ranging from + // PAGE to HUGE_PAGE bytes) to element counts. + let elem_size = cmp::max(1, mem::size_of::<T>()); + let mut chunks = self.chunks.borrow_mut(); + let mut new_cap; + if let Some(last_chunk) = chunks.last_mut() { + // If a type is `!needs_drop`, we don't need to keep track of how many elements + // the chunk stores - the field will be ignored anyway. + if mem::needs_drop::<T>() { + // FIXME: this should *likely* use `offset_from`, but more + // investigation is needed (including running tests in miri). + let used_bytes = self.ptr.get().addr() - last_chunk.start().addr(); + last_chunk.entries = used_bytes / mem::size_of::<T>(); + } + + // If the previous chunk's len is less than HUGE_PAGE + // bytes, then this chunk will be least double the previous + // chunk's size. + new_cap = (*last_chunk.storage.as_ptr()).len().min(HUGE_PAGE / elem_size / 2); + new_cap *= 2; + } else { + new_cap = PAGE / elem_size; + } + // Also ensure that this chunk can fit `additional`. + new_cap = cmp::max(additional, new_cap); + + let mut chunk = ArenaChunk::<T>::new(new_cap); + self.ptr.set(chunk.start()); + self.end.set(chunk.end()); + chunks.push(chunk); + } + } + + // 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 ArenaChunk<T>) { + // Determine how much was filled. + let start = last_chunk.start().addr(); + // We obtain the value of the pointer to the first uninitialized element. + let end = self.ptr.get().addr(); + // 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 { + // FIXME: this should *likely* use `offset_from`, but more + // investigation is needed (including running tests in miri). + (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); + } + } + // Box handles deallocation of `last_chunk` and `self.chunks`. + } + } +} + +unsafe impl<T: Send> Send for TypedArena<T> {} + +/// An arena that can hold objects of multiple different types that impl `Copy` +/// and/or satisfy `!mem::needs_drop`. +pub struct DroplessArena { + /// A pointer to the start of the free space. + start: Cell<*mut u8>, + + /// A pointer to the end of free space. + /// + /// The allocation proceeds downwards from the end of the chunk towards the + /// start. (This is slightly simpler and faster than allocating upwards, + /// see <https://fitzgeraldnick.com/2019/11/01/always-bump-downwards.html>.) + /// When this pointer crosses the start pointer, a new chunk is allocated. + end: Cell<*mut u8>, + + /// A vector of arena chunks. + chunks: RefCell<Vec<ArenaChunk>>, +} + +unsafe impl Send for DroplessArena {} + +impl Default for DroplessArena { + #[inline] + fn default() -> DroplessArena { + DroplessArena { + start: Cell::new(ptr::null_mut()), + end: Cell::new(ptr::null_mut()), + chunks: Default::default(), + } + } +} + +impl DroplessArena { + #[inline(never)] + #[cold] + fn grow(&self, additional: usize) { + unsafe { + let mut chunks = self.chunks.borrow_mut(); + let mut new_cap; + if let Some(last_chunk) = chunks.last_mut() { + // There is no need to update `last_chunk.entries` because that + // field isn't used by `DroplessArena`. + + // If the previous chunk's len is less than HUGE_PAGE + // bytes, then this chunk will be least double the previous + // chunk's size. + new_cap = (*last_chunk.storage.as_ptr()).len().min(HUGE_PAGE / 2); + new_cap *= 2; + } else { + new_cap = PAGE; + } + // Also ensure that this chunk can fit `additional`. + new_cap = cmp::max(additional, new_cap); + + let mut chunk = ArenaChunk::new(new_cap); + self.start.set(chunk.start()); + self.end.set(chunk.end()); + chunks.push(chunk); + } + } + + /// Allocates a byte slice with specified layout from the current memory + /// chunk. Returns `None` if there is no free space left to satisfy the + /// request. + #[inline] + fn alloc_raw_without_grow(&self, layout: Layout) -> Option<*mut u8> { + let start = self.start.get().addr(); + let old_end = self.end.get(); + let end = old_end.addr(); + + let align = layout.align(); + let bytes = layout.size(); + + let new_end = end.checked_sub(bytes)? & !(align - 1); + if start <= new_end { + let new_end = old_end.with_addr(new_end); + self.end.set(new_end); + Some(new_end) + } else { + None + } + } + + #[inline] + pub fn alloc_raw(&self, layout: Layout) -> *mut u8 { + assert!(layout.size() != 0); + loop { + if let Some(a) = self.alloc_raw_without_grow(layout) { + break a; + } + // No free space left. Allocate a new chunk to satisfy the request. + // On failure the grow will panic or abort. + self.grow(layout.size()); + } + } + + #[inline] + pub fn alloc<T>(&self, object: T) -> &mut T { + assert!(!mem::needs_drop::<T>()); + + let mem = self.alloc_raw(Layout::for_value::<T>(&object)) 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(Layout::for_value::<[T]>(slice)) as *mut T; + + unsafe { + mem.copy_from_nonoverlapping(slice.as_ptr(), slice.len()); + slice::from_raw_parts_mut(mem, slice.len()) + } + } + + #[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.add(i), 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 mem = self.alloc_raw(Layout::array::<T>(len).unwrap()) 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(Layout::for_value::<[T]>(vec.as_slice())) as *mut T; + vec.as_ptr().copy_to_nonoverlapping(start_ptr, len); + vec.set_len(0); + slice::from_raw_parts_mut(start_ptr, len) + } + }) + } + } + } +} + +/// Declare an `Arena` containing one dropless arena and many typed arenas (the +/// types of the typed arenas are specified by the arguments). +/// +/// There are three cases of interest. +/// - Types that are `Copy`: these need not be specified in the arguments. They +/// will use the `DroplessArena`. +/// - Types that are `!Copy` and `!Drop`: these must be specified in the +/// arguments. An empty `TypedArena` will be created for each one, but the +/// `DroplessArena` will always be used and the `TypedArena` will stay empty. +/// This is odd but harmless, because an empty arena allocates no memory. +/// - Types that are `!Copy` and `Drop`: these must be specified in the +/// arguments. The `TypedArena` will be used for them. +/// +#[rustc_macro_transparency = "semitransparent"] +pub macro declare_arena([$($a:tt $name:ident: $ty:ty,)*]) { + #[derive(Default)] + pub struct Arena<'tcx> { + pub dropless: $crate::DroplessArena, + $($name: $crate::TypedArena<$ty>,)* + } + + pub trait ArenaAllocatable<'tcx, C = rustc_arena::IsNotCopy>: Sized { + fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut Self; + fn allocate_from_iter<'a>( + arena: &'a Arena<'tcx>, + iter: impl ::std::iter::IntoIterator<Item = Self>, + ) -> &'a mut [Self]; + } + + // Any type that impls `Copy` can be arena-allocated in the `DroplessArena`. + impl<'tcx, T: Copy> ArenaAllocatable<'tcx, rustc_arena::IsCopy> for T { + #[inline] + fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut Self { + arena.dropless.alloc(self) + } + #[inline] + fn allocate_from_iter<'a>( + arena: &'a Arena<'tcx>, + iter: impl ::std::iter::IntoIterator<Item = Self>, + ) -> &'a mut [Self] { + arena.dropless.alloc_from_iter(iter) + } + } + $( + impl<'tcx> ArenaAllocatable<'tcx, rustc_arena::IsNotCopy> for $ty { + #[inline] + fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut Self { + if !::std::mem::needs_drop::<Self>() { + arena.dropless.alloc(self) + } else { + arena.$name.alloc(self) + } + } + + #[inline] + fn allocate_from_iter<'a>( + arena: &'a Arena<'tcx>, + iter: impl ::std::iter::IntoIterator<Item = Self>, + ) -> &'a mut [Self] { + if !::std::mem::needs_drop::<Self>() { + arena.dropless.alloc_from_iter(iter) + } else { + arena.$name.alloc_from_iter(iter) + } + } + } + )* + + impl<'tcx> Arena<'tcx> { + #[inline] + pub fn alloc<T: ArenaAllocatable<'tcx, C>, C>(&self, value: T) -> &mut T { + value.allocate_on(self) + } + + // Any type that impls `Copy` can have slices be arena-allocated in the `DroplessArena`. + #[inline] + pub fn alloc_slice<T: ::std::marker::Copy>(&self, value: &[T]) -> &mut [T] { + if value.is_empty() { + return &mut []; + } + self.dropless.alloc_slice(value) + } + + pub fn alloc_from_iter<'a, T: ArenaAllocatable<'tcx, C>, C>( + &'a self, + iter: impl ::std::iter::IntoIterator<Item = T>, + ) -> &'a mut [T] { + T::allocate_from_iter(self, iter) + } + } +} + +// Marker types that let us give different behaviour for arenas allocating +// `Copy` types vs `!Copy` types. +pub struct IsCopy; +pub struct IsNotCopy; + +#[cfg(test)] +mod tests; |