lib.rs

//! 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;