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#![no_std]
#![cfg_attr(
feature = "allocator",
feature(allocator_api, nonnull_slice_from_raw_parts, slice_ptr_get)
)]
//! This is a linked list allocator, inspired by the dlmalloc algorithm, to be
//! used in `no_std` environments such as operating system kernels. The overhead
//! for each allocation is a single `usize`. The implementation prioritizes
//! runtime efficiency over memory efficiency, but also provides very good
//! memory utilization. The allocator is heavily tested with test cases covering
//! almost all code paths; fuzzing is used to cover the rest.
//!
//! ## Usage
//!
//! Create a static allocator:
//!
//! ```ignore
//! use good_memory_allocator::SpinLockedAllocator;
//!
//! #[global_allocator]
//! static ALLOCATOR: SpinLockedAllocator = SpinLockedAllocator::empty();
//! ```
//!
//! Before using this allocator, you need to initialize it:
//!
//! ```ignore
//! pub fn init_heap() {
//! unsafe {
//! ALLOCATOR.init(heap_start, heap_size);
//! }
//! }
//! ```
//!
//! ## `SMALLBINS_AMOUNT` and `ALIGNMENT_SUB_BINS_AMOUNT`.
//!
//! The allocator allows configuring the amount of smallbins and alignment
//! sub-bins that it uses. For those of you not familiar with smallbins, they
//! are data structures used by the allocator to keep track of free chunks. Each
//! smallbin is made up of multiple alignment sub-bins. The default amounts of
//! smallbins and alignment sub-bins used by the allocator are stored in their
//! respective constant [`DEFAULT_SMALLBINS_AMOUNT`] and
//! [`DEFAULT_ALIGNMENT_SUB_BINS_AMOUNT`].
//!
//! Increasing the amount of smallbins will improve runtime performance and
//! memory utilization, especially when you are making a lot of relatively small
//! allocation, but it will also increase the size of the [`Allocator`] struct.
//!
//! Increasing the amount of alignment sub-bins will will also improve runtime
//! performance and memory utilization, especially when you are making a lot of
//! allocations with relatively large alignments, but it will also increase the
//! size of the [`Allocator`] struct.
//! It is recommended to choose a value that is a power of 2 for the alignemnt
//! sub-bins amount, since that will improve the memory utilization of the
//! smallbins' memory.
//! The amount of alignment sub bins must be at least 2, otherwise you will get
//! a compilation error.
//!
//! If you are in a memory constrained environment, you might want to use lower
//! values for these constants, since the size of the [`Allocator`] struct using
//! the default values is relatively large.
//!
//! ## Features
//!
//! - **`spin`** (default): Provide a `SpinLockedAllocator` type that implements
//! the `GlobalAlloc` trait by using a spinlock.
//! - **`allocator`**: Provides an implementation of the unstable `Allocator`
//! trait for the `SpinLockedAllocator` type.
#[cfg(test)]
#[macro_use]
extern crate std;
mod alignment;
mod bins;
mod chunks;
mod divisible_by_4_usize;
mod smallest_type_which_has_at_least_n_bits;
#[cfg(test)]
mod tests;
use core::{alloc::Layout, ptr::NonNull};
use alignment::*;
use bins::SmallBins;
pub use bins::{DEFAULT_ALIGNMENT_SUB_BINS_AMOUNT, DEFAULT_SMALLBINS_AMOUNT};
use chunks::*;
use smallest_type_which_has_at_least_n_bits::{
SmallestTypeWhichHasAtLeastNBitsStruct, SmallestTypeWhichHasAtLeastNBitsTrait,
};
const USIZE_ALIGNMENT: usize = core::mem::align_of::<usize>();
const USIZE_SIZE: usize = core::mem::size_of::<usize>();
// IMPORTANT:
// `MIN_ALIGNMENT` must be larger than 4, so that storing the size as a
// `DivisibleBy4Usize` is safe.
const MIN_ALIGNMENT: usize = if USIZE_ALIGNMENT < 8 {
8
} else {
USIZE_ALIGNMENT
};
const MIN_FREE_CHUNK_SIZE_INCLUDING_HEADER: usize = core::mem::size_of::<FreeChunk>() + USIZE_SIZE;
const HEADER_SIZE: usize = core::mem::size_of::<Chunk>();
/// A linked list memory allocator.
///
/// This allocator allows configuring the amount of smallbins and alignment
/// sub-bins that it uses. For more information about this value, read the
/// crate's top level documentation.
#[derive(Debug)]
pub struct Allocator<
const SMALLBINS_AMOUNT: usize = DEFAULT_SMALLBINS_AMOUNT,
const ALIGNMENT_SUB_BINS_AMOUNT: usize = DEFAULT_ALIGNMENT_SUB_BINS_AMOUNT,
> where
SmallestTypeWhichHasAtLeastNBitsStruct<ALIGNMENT_SUB_BINS_AMOUNT>:
SmallestTypeWhichHasAtLeastNBitsTrait,
{
smallbins: SmallBins<SMALLBINS_AMOUNT, ALIGNMENT_SUB_BINS_AMOUNT>,
heap_end_addr: usize,
fake_chunk_of_other_bin: FakeFreeChunk,
}
impl<const SMALLBINS_AMOUNT: usize, const ALIGNMENT_SUB_BINS_AMOUNT: usize>
Allocator<SMALLBINS_AMOUNT, ALIGNMENT_SUB_BINS_AMOUNT>
where
SmallestTypeWhichHasAtLeastNBitsStruct<ALIGNMENT_SUB_BINS_AMOUNT>:
SmallestTypeWhichHasAtLeastNBitsTrait,
{
/// Creates an empty heap allocator without any heap memory region, which
/// will always return null on allocation requests.
///
/// To intiialize this allocator, use the `init` method.
pub const fn empty() -> Self {
Self {
heap_end_addr: 0,
fake_chunk_of_other_bin: FakeFreeChunk {
fd: None,
ptr_to_fd_of_bk: core::ptr::null_mut(),
},
smallbins: SmallBins::new(),
}
}
/// Checks if the heap memory region was already initialized by calling
/// `init`.
pub fn was_initialized(&self) -> bool {
self.heap_end_addr != 0
}
/// Initializes the heap allocator with the given memory region.
///
/// # Safety
///
/// If the allocator was already initialized, this function will panic.
///
/// The `Allocator` on which this was called must not be moved, and its
/// address in memory must not change, otherwise undefined behaviour
/// will occur. This is because the heap region now contains pointers to
/// fields of this struct, and if this struct will move, the address of
/// its fields will change, and those pointers will now be invalid.
///
/// The provided memory region must be valid and non-null, and must not be
/// used by anything else.
///
/// If after aligning the start and end addresses, the size of the heap is
/// 0, the function panics.
pub unsafe fn init(&mut self, heap_start_addr: usize, heap_size: usize) {
if self.was_initialized() {
panic!("the heap was already initialized");
}
let aligned_heap_start_addr = align_up(heap_start_addr, MIN_ALIGNMENT);
let heap_end_addr = heap_start_addr + heap_size;
let aligned_heap_end_addr = align_down(heap_end_addr, MIN_ALIGNMENT);
let aligned_size = aligned_heap_end_addr.saturating_sub(aligned_heap_start_addr);
// if after aligning the start and end addresses, the heap size is 0
if aligned_size == 0 {
panic!("heap size is 0 after aligning heap start and end addresses");
}
// make the fake chunk point to itself.
self.fake_chunk_of_other_bin.fd = Some(self.fake_chunk_of_other_bin_ptr());
self.fake_chunk_of_other_bin.ptr_to_fd_of_bk = self.ptr_to_fd_of_fake_chunk_of_other_bin();
// create a free chunk for the entire heap.
// no need to update the next chunk because there is no next chunk, this chunk
// covers the entire heap.
let chunk_size = aligned_size - HEADER_SIZE;
let _ = FreeChunk::create_new_without_updating_next_chunk(
aligned_heap_start_addr,
chunk_size,
self,
);
// update the heap end address.
self.heap_end_addr = aligned_heap_end_addr;
}
/// Returns a pointer to the fake chunk of the other bin.
///
/// # Safety
///
/// This chunk is missing the chunk header, so when using it as a free
/// chunk, you must make sure that you never access its header.
unsafe fn fake_chunk_of_other_bin_ptr(&self) -> FreeChunkPtr {
self.fake_chunk_of_other_bin.free_chunk_ptr()
}
/// Returns a pointer to the first free chunk in the other bin, if any.
fn first_free_chunk_in_other_bin(&self) -> Option<FreeChunkPtr> {
let fd_of_fake_chunk = self.fake_chunk_of_other_bin.fd?;
if fd_of_fake_chunk == unsafe { self.fake_chunk_of_other_bin_ptr() } {
return None;
}
Some(fd_of_fake_chunk)
}
/// Returns a pointer to the fd of the fake chunk of the other bin.
fn ptr_to_fd_of_fake_chunk_of_other_bin(&mut self) -> *mut Option<FreeChunkPtr> {
&mut self.fake_chunk_of_other_bin.fd
}
/// Prepares an allocation size according to the requirements of
/// the allocator. Returns the prepared size.
fn prepare_size(size: usize) -> usize {
unsafe {
align_up(
core::cmp::max(size, MIN_FREE_CHUNK_SIZE_INCLUDING_HEADER - HEADER_SIZE),
MIN_ALIGNMENT,
)
}
}
/// Returns fd and bk pointers for inserting a new free chunk to its
/// matching bin.
///
/// In case the chunk should be inserted into a smallbin, updates the
/// smallbin's bitmap to indicate that it now contains that chunk.
///
/// # Safety
///
/// - `chunk_size` must be the size of an actual chunk.
/// - After calling this function, the free chunk must be inserted using
/// the fd and bk pointers, otherwise the smallbins' bitmaps might be in
/// an invalid state.
unsafe fn get_fd_and_bk_pointers_for_inserting_new_free_chunk(
&mut self,
chunk_size: usize,
alignment_index_of_chunk_content_addr: usize,
) -> (Option<FreeChunkPtr>, *mut Option<FreeChunkPtr>) {
// SAFETY: this is safe because `chunk_size` is the size of an actual chunk, so
// it must have already been prepared.
if let Some(smallbin_index) =
SmallBins::<SMALLBINS_AMOUNT, ALIGNMENT_SUB_BINS_AMOUNT>::smallbin_index(chunk_size)
{
self.smallbins
.get_fd_and_bk_pointers_for_inserting_to_smallbin(
smallbin_index,
alignment_index_of_chunk_content_addr,
)
} else {
// if this chunk does not fit in the smallbins, put it in the other bin
self.get_fd_and_bk_pointers_for_inserting_new_free_chunk_in_other_bin(chunk_size)
}
}
/// Returns fd and bk pointers for inserting a new free chunk to the
/// other bin.
fn get_fd_and_bk_pointers_for_inserting_new_free_chunk_in_other_bin(
&mut self,
chunk_size: usize,
) -> (Option<FreeChunkPtr>, *mut Option<FreeChunkPtr>) {
match self.first_free_chunk_in_other_bin() {
// if the other bin is not empty
Some(mut first_free_chunk) => {
// check if the given chunk size is bigger than the current first chunk,
// and if so, put the given chunk at the start of the bin.
//
// otherwise if the given chunk is smaller than the current first chunk,
// then put the given chunk at the end of the bin.
if chunk_size > unsafe { first_free_chunk.as_mut() }.size() {
(
Some(first_free_chunk),
self.ptr_to_fd_of_fake_chunk_of_other_bin(),
)
} else {
(
// SAFETY: the fake chunk will be used as the fd of some other chunk, but
// chunks never access the header of their fd, only their fd and bk, so
// this is safe.
Some(unsafe { self.fake_chunk_of_other_bin_ptr() }),
self.fake_chunk_of_other_bin.ptr_to_fd_of_bk,
)
}
}
// if the other bin is empty, put this chunk as the first chunk in the bin.
None => (
// SAFETY: the fake chunk will be used as the fd of some other chunk, but chunks
// never access the header of their fd, only their fd and bk, so this is safe.
Some(unsafe { self.fake_chunk_of_other_bin_ptr() }),
self.ptr_to_fd_of_fake_chunk_of_other_bin(),
),
}
}
/// Prepares a layout's size and alignment according to the requirements of
/// the allocator. Returns the prepared size and alignment.
fn prepare_layout(layout: Layout) -> (usize, usize) {
let layout_size = Self::prepare_size(layout.size());
let layout_align = core::cmp::max(layout.align(), MIN_ALIGNMENT);
(layout_size, layout_align)
}
/// Allocates memory.
pub unsafe fn alloc(&mut self, layout: core::alloc::Layout) -> *mut u8 {
if !self.was_initialized() {
return core::ptr::null_mut();
}
let (layout_size, layout_align) = Self::prepare_layout(layout);
let alignment_index_if_size_is_smallbin_size =
if SmallBins::<SMALLBINS_AMOUNT, ALIGNMENT_SUB_BINS_AMOUNT>::is_smallbin_size(
layout_size,
) {
Some(
SmallBins::<SMALLBINS_AMOUNT, ALIGNMENT_SUB_BINS_AMOUNT>::alignment_index(
layout_align,
),
)
} else {
None
};
// if the allocation size is the size of a smallbin, allocate from optimal
// chunks from the smallbins.
if let Some(alignment_index) = alignment_index_if_size_is_smallbin_size {
if let Some(ptr) =
self.alloc_from_optimal_chunk(layout_size, layout_align, alignment_index)
{
return ptr.as_ptr();
}
}
if let Some(ptr) = self.alloc_from_other_bin(layout_size, layout_align) {
return ptr.as_ptr();
}
// if the allocation size is the size of a smallbin, allocate from suboptimal
// chunks from the smallbins.
if let Some(alignment_index) = alignment_index_if_size_is_smallbin_size {
if let Some(ptr) = self.alloc_from_aligned_suboptimal_chunks(
layout_size,
layout_align,
alignment_index,
) {
return ptr.as_ptr();
}
if let Some(ptr) = self.alloc_from_unaligned_suboptimal_chunks(
layout_size,
layout_align,
alignment_index,
) {
return ptr.as_ptr();
}
}
core::ptr::null_mut()
}
/// Tries to allocate an optimal chunks for the given allocation
/// requirements.
///
/// # Safety
///
/// - The size and align must have been prepared.
/// - The size must be the size of a small bin.
unsafe fn alloc_from_optimal_chunk(
&mut self,
layout_size: usize,
layout_align: usize,
alignment_index: usize,
) -> Option<NonNull<u8>> {
let mut optimal_chunk =
self.smallbins
.optimal_chunk(layout_size, layout_align, alignment_index)?;
let chunk = optimal_chunk.as_mut();
Some(self.alloc_aligned_no_end_padding(chunk))
}
/// Tries to allocate a chunk from the other bin for the given allocation
/// requirements.
///
/// # Safety
///
/// The size and align must have been prepared.
unsafe fn alloc_from_other_bin(
&mut self,
layout_size: usize,
layout_align: usize,
) -> Option<NonNull<u8>> {
// try to allocate from the other bin.
let fake_chunk_of_other_bin_ptr = self.fake_chunk_of_other_bin_ptr();
// the other bin is circular so the fd will never be `None`.
let mut cur_chunk_ptr = self.fake_chunk_of_other_bin.fd.unwrap();
loop {
// if the current chunk is the fake chunk of the other bin, we reached the end
// of the list.
if cur_chunk_ptr == fake_chunk_of_other_bin_ptr {
break;
}
let cur_chunk = cur_chunk_ptr.as_mut();
// check if the chunk is large enough
if cur_chunk.size() >= layout_size {
if is_aligned(cur_chunk.content_addr(), layout_align) {
// the chunk is already aligned
//
// SAFETY: we know that the chunk is large enough
return Some(self.alloc_aligned(layout_size, cur_chunk_ptr.as_mut()));
} else {
// the chunk is not aligned
// check if we can use the current chunk for an unaligned allocation.
let cur_chunk_ref = cur_chunk_ptr.as_mut();
if let Some(aligned_start_addr) =
self.can_alloc_unaligned(layout_size, layout_align, cur_chunk_ref)
{
return Some(self.alloc_unaligned(
layout_size,
cur_chunk_ptr.as_mut(),
aligned_start_addr,
));
}
}
}
// the other bin is circular so the fd will never be `None`.
cur_chunk_ptr = cur_chunk.fd().unwrap();
}
None
}
/// Tries to allocate a chunk from the aligned sub-optimal chunks for the
/// given allocation requirements.
///
/// This strategy should be called after trying the other bin.
///
/// # Safety
///
/// - The size and align must have been prepared.
/// - The size must be the size of a small bin.
unsafe fn alloc_from_aligned_suboptimal_chunks(
&mut self,
layout_size: usize,
layout_align: usize,
alignment_index: usize,
) -> Option<NonNull<u8>> {
let mut aligned_suboptimal_chunk =
self.smallbins
.aligned_suboptimal_chunk(layout_size, layout_align, alignment_index)?;
// the aligned suboptimal chunks are aligned, but are also large enough so that
// an end padding chunk must be created.
Some(self.alloc_aligned_split_end_padding_chunk(
layout_size,
aligned_suboptimal_chunk.end_padding,
aligned_suboptimal_chunk.chunk_ptr.as_mut(),
))
}
/// Tries to allocate a chunk from the unaligned sub-optimal chunks for the
/// given allocation requirements.
///
/// This strategy should be called after trying to find an aligned
/// suboptimal chunk.
///
/// # Safety
///
/// - The size and align must have been prepared.
/// - The size must be the size of a small bin.
unsafe fn alloc_from_unaligned_suboptimal_chunks(
&mut self,
layout_size: usize,
layout_align: usize,
alignment_index: usize,
) -> Option<NonNull<u8>> {
let (allocated_chunk, aligned_start_addr) = {
let mut chunks_that_can_allocate_unaligned = self
.smallbins
.unaligned_suboptimal_chunks(layout_size, layout_align, alignment_index)?
.filter_map(|mut chunk_ptr| {
// for each unaligned suboptimal chunk, check if it can fit the allocation
// requirements.
let chunk = chunk_ptr.as_mut();
if let Some(aligned_start_addr) =
self.can_alloc_unaligned(layout_size, layout_align, chunk_ptr.as_mut())
{
Some((chunk, aligned_start_addr))
} else {
None
}
});
// get the first chunk which can fit the allocation requirements.
chunks_that_can_allocate_unaligned.next()?
};
Some(self.alloc_unaligned(layout_size, allocated_chunk, aligned_start_addr))
}
/// Deallocates memory.
pub unsafe fn dealloc(&mut self, ptr: *mut u8) {
let chunk = UsedChunk::from_addr(ptr as usize - HEADER_SIZE);
match (
chunk.prev_chunk_if_free(),
chunk.next_chunk_if_free(self.heap_end_addr),
) {
(None, None) => {
// mark the chunk as free, and adds it to the start of the linked list of free
// chunks.
//
// SAFETY: the provided chunk size is the size of an actual chunk, and the
// alignment returned from `Chunk::alignment` is always a power of 2.
let (fd, bk) = self.get_fd_and_bk_pointers_for_inserting_new_free_chunk(
chunk.0.size(),
SmallBins::<SMALLBINS_AMOUNT, ALIGNMENT_SUB_BINS_AMOUNT>::alignment_index_of_chunk_content_addr(
chunk.content_addr(),
),
);
let _ = chunk.mark_as_free(fd, bk, self.heap_end_addr);
}
(None, Some(next_chunk_free)) => {
// for this case, we create a free chunk where the deallocated chunk is,
// which will consolidate itself and the next chunk into one big free chunk.
// first resize the chunk (before marking it as free), so that we won't write
// the postfix size twice.
//
// the chunk should include itself, and the entire next free chunk,
// which is made up of its header, and its content.
chunk.set_size(chunk.0.size() + HEADER_SIZE + next_chunk_free.size());
// unlink the next free chunk, because we include its memory region in `chunk`.
next_chunk_free.unlink(&mut self.smallbins);
// mark the chunk as free.
//
// no need to update the next chunk, because it already knows that its prev is
// free, because before marking `chunk` as free, the next chunk's previous chunk
// was `next_chunk_free`.
let (fd, bk) = self.get_fd_and_bk_pointers_for_inserting_new_free_chunk(
chunk.0.size(),
SmallBins::<SMALLBINS_AMOUNT, ALIGNMENT_SUB_BINS_AMOUNT>::alignment_index_of_chunk_content_addr(
chunk.content_addr(),
),
);
let _ = chunk.mark_as_free_without_updating_next_chunk(fd, bk);
}
(Some(prev_chunk_free), None) => {
// for this case, just resize the prev chunk to consolidate it with the current
// chunk. in other words, make it large enough so that it includes the entire
// current chunk.
//
// the resized prev chunk should include itself, the current chunk's header, and
// the current chunk's content.
prev_chunk_free.set_size_and_update_bin(
prev_chunk_free.size() + HEADER_SIZE + chunk.0.size(),
self,
);
// we must also update the prev in use bit of the next chunk, if any, because
// its prev is now free.
if let Some(next_chunk_addr) = chunk.0.next_chunk_addr(self.heap_end_addr) {
Chunk::set_prev_in_use_for_chunk_with_addr(next_chunk_addr, false);
}
}
(Some(prev_chunk_free), Some(next_chunk_free)) => {
// for this case, we want to make the prev chunk large enough to include both
// this and the next chunk.
//
// we must also make sure to unlink the next chunk since it's now part of
// another chunk.
// SAFETY: this is safe because this chunk will now be incorporated in the prev
// chunk, so no memory is lost.
next_chunk_free.unlink(&mut self.smallbins);
// the prev chunk will now include the following:
// - itself
// - the current chunk's header
// - the current chunk's content
// - the next chunk's header
// - the next chunk's content
prev_chunk_free.set_size_and_update_bin(
prev_chunk_free.size()
+ HEADER_SIZE
+ chunk.0.size()
+ HEADER_SIZE
+ next_chunk_free.size(),
self,
);
// also, there's no need to update the next chunk of
// `next_free_chunk`, because that chunk already knows that its
// prev is free.
}
}
}
/// Resizes an allocation previously returned from `alloc` or `realloc`.
/// The alignment of the content will stay the same.
pub unsafe fn realloc(&mut self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
let previously_requested_size = Self::prepare_size(layout.size());
let new_size = Self::prepare_size(new_size);
let chunk = UsedChunk::from_addr(ptr as usize - HEADER_SIZE);
// first try to realloc in place, to avoid copying memory.
if self.try_realloc_in_place(chunk, previously_requested_size, new_size) {
return ptr;
}
// if reallocation in place fails, realloc a new region for and copy the data
// there.
let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
self.realloc_new_region(ptr, layout, new_layout)
}
/// Resizes an allocation previously returned from `alloc` or `realloc`, by
/// allocating a new memory region for it, copying the memory, and
/// dealloacting the provided pointer.
unsafe fn realloc_new_region(
&mut self,
ptr: *mut u8,
old_layout: Layout,
new_layout: Layout,
) -> *mut u8 {
// SAFETY: the caller must ensure that `new_layout` is greater than zero.
let new_ptr = self.alloc(new_layout);
if !new_ptr.is_null() {
// SAFETY: the previously allocated block cannot overlap the newly allocated
// block. The safety contract for `dealloc` must be upheld by the
// caller.
core::ptr::copy_nonoverlapping(
ptr,
new_ptr,
core::cmp::min(old_layout.size(), new_layout.size()),
);
self.dealloc(ptr);
}
new_ptr
}
/// Tries to reallocate the given chunk to the given new size, in place.
///
/// # Safety
///
/// `previously_requested_size` and `new_size` must have been prepared.
unsafe fn try_realloc_in_place(
&mut self,
chunk: UsedChunkRef,
previously_requested_size: usize,
new_size: usize,
) -> bool {
if new_size > previously_requested_size {
// the user requested to grow his allocation.
self.try_grow_in_place(chunk, new_size)
} else {
// the user requested to shrink his allocation, shrink in place.
self.shrink_in_place(chunk, new_size);
true
}
}
/// Tries to grow the given chunk in place, to the given new size. Please
/// note that this means that the alignment of the content of this chunk
/// will stay exactly the same.
///
/// Returns whether or not the operation was successful.
///
/// # Safety
///
/// `new_size` must have been prepared.
unsafe fn try_grow_in_place(&mut self, chunk: UsedChunkRef, new_size: usize) -> bool {
// sometimes chunks are bigger than their content due to end padding that's not
// big enough to fit a chunk, check if the chunk is already large enough for the
// new size.
if chunk.0.size() >= new_size {
// if the chunk is already large enough, no need to do anything.
return true;
}
// to grow this chunk in place we need its next chunk to be free, make sure that
// the next chunk is free.
let next_chunk_free = match chunk.next_chunk_if_free(self.heap_end_addr) {
Some(next_chunk_free) => next_chunk_free,
// if the next chunk is not free, we can't grow this chunk in place.
None => {
return false;
}
};
// calculate the new end addresss of the chunk.
let new_end_addr = chunk.content_addr() + new_size;
// make sure that if we grow this chunk in place using the next chunk,
// the chunk's end address stays within the bounds of the next chunk and
// doesn't go beyound it.
//
// if the new end address is beyond the next free chunk, we can't grow in place,
// because the chunk after the next chunk must be used because there are no 2
// adjacent free chunks, and the next chunk alone is not enough.
let next_chunk_end_addr = next_chunk_free.end_addr();
if new_end_addr > next_chunk_end_addr {
return false;
}
// we can grow in place using the next free chunk.
//
// we now need to check if the space left in the next chunk after
// growing (if there even is any space left) is large enough to store a
// free chunk there.
let space_left_at_end_of_next_free_chunk = next_chunk_end_addr - new_end_addr;
if space_left_at_end_of_next_free_chunk > MIN_FREE_CHUNK_SIZE_INCLUDING_HEADER {
// the space left at the end of the next free chunk is enough to
// store a free chunk, so we must move the next free chunk forward to the new
// end address and resize it accordingly, and then we can resize `chunk`
// to include the space between it and the new free chunk.
//
// also note that there is no need to update the prev in use of the
// next chunk of the next chunk, because its prev will be the new
// free chunk that we're creating, and its prev in use bit is
// already set to false.
next_chunk_free.move_and_resize_chunk_without_updating_next_chunk(
new_end_addr,
space_left_at_end_of_next_free_chunk - HEADER_SIZE,
self,
);
// resize `chunk` to the desired size. this will make `chunk` include all the
// space up to where the new free chunk was just created.
chunk.set_size(new_size);
} else {
// the space left at the end of the next free chunk is not enough to
// store a free chunk, so just include that space when growing this
// chunk.
// now we should just unlink the next free chunk and increase the
// size of the current chunk to include all of it.
//
// we must also update the prev in use bit of the next chunk of the
// next chunk, because its prev is now `chunk`, which is in use,
// while before calling this function its prev was `next_chunk_free`, which is
// free.
if let Some(next_chunk_of_next_chunk_addr) =
next_chunk_free.header.next_chunk_addr(self.heap_end_addr)
{
// its prev is now `chunk`, which is used.
Chunk::set_prev_in_use_for_chunk_with_addr(next_chunk_of_next_chunk_addr, true);
}
next_chunk_free.unlink(&mut self.smallbins);
// make sure that it includes all of the next chunk
chunk.set_size(next_chunk_end_addr - chunk.content_addr());
}
// we have successfully grew the chunk in place.
true
}
/// Shrinks the given chunk in place, to the given new size. Please note
/// that this means that the alignment of the content of this chunk will
/// stay exactly the same.
///
/// # Safety
///
/// `new_size` must be lower than the chunk's size, and must have been
/// prepared.
unsafe fn shrink_in_place(&mut self, chunk: UsedChunkRef, new_size: usize) {
// calculate the new end addresss of the chunk.
let new_end_addr = chunk.content_addr() + new_size;
match chunk.next_chunk_if_free(self.heap_end_addr) {
Some(next_chunk_free) => {
// we can just move the next chunk backwards to include the extra space that's
// left due to shrinking in place.
next_chunk_free.move_and_resize_chunk_without_updating_next_chunk(
new_end_addr,
// the size is calculated by subtracting the new start address of the free
// chunk from its current end address, and subtracting the header size because
// we only want the size of the content, excluding the header.
next_chunk_free.end_addr() - new_end_addr - HEADER_SIZE,
self,
);
// resize `chunk` to the desired size.
chunk.set_size(new_size);
}
None => {
// calculate how much space we have left at the end of this chunk after
// shrinking.
let space_left_at_end = chunk.0.size() - new_size;
// check if we now have enough space at the end to create an entire free chunk.
if space_left_at_end >= MIN_FREE_CHUNK_SIZE_INCLUDING_HEADER {
// if we have enough space at the end, create a free chunk there.
//
// we must also update the next chunk of the free chunk that we create, because
// before creating this free chunk, its prev chunk was `chunk`, which is used,
// but now its prev is a free chunk.
let end_padding_chunk_size = space_left_at_end - HEADER_SIZE;
let _ = FreeChunk::create_new_and_update_next_chunk(
// the end padding chunk starts right where the new end address is, and the
// content comes right after the header.
new_end_addr,
end_padding_chunk_size,
self,
);
// resize `chunk` to the desired size.
chunk.set_size(new_size);
} else {
// if we don't have enough space to create a free chunk
// there, just leave it there. this prioritizes runtime
// efficiency over memory efficiency, because it prevents
// copying the entire memory, but it wastes a little bit of
// memory (up to 32 bytes).
}
}
}
}
/// Checks if the given chunk can be used to allocate the given allocation
/// requirements, knowing that the chunk's content start address is not well
/// aligned to the required alignment.
///
/// If the chunk can be used for the allocation, returns the aligned start
/// address of the allocated chunk, which can then be used to allocate the
/// chunk by calling `alloc_unaligned`. Otherwise, if the chunk can't be
/// used, returns `None`.
///
/// # Safety
///
/// The size and alignment must have been prepared.
unsafe fn can_alloc_unaligned(
&self,
layout_size: usize,
layout_align: usize,
chunk: FreeChunkRef,
) -> Option<usize> {
// find an aligned start address starting from the end of the chunk which leaves
// enough space for the content to fit.
let aligned_content_start_addr = align_down(chunk.end_addr() - layout_size, layout_align);
let aligned_start_addr = aligned_content_start_addr - HEADER_SIZE;
// make sure that after aligning the start address, there is enough space left
// at the start of `cur_chunk` to store a free chunk there.
//
// note that we must store a chunk there because there's no way that the
// `aligned_start_addr` fits exactly into `cur_chunk`, otherwise alloc aligned
// would have been called, so there must be space left at the start.
let space_left_at_start = aligned_start_addr.saturating_sub(chunk.addr());
if space_left_at_start < MIN_FREE_CHUNK_SIZE_INCLUDING_HEADER {
return None;
}
Some(aligned_start_addr)
}
/// Allocates an unaligned chunk by splitting a start padding chunk from it,
/// and then proceeding as usual.
///
/// # Safety
///
/// - The provided size and align must have been prepared.
/// - The provided `aligned_start_addr` must have been returned from a call
/// to `can_alloc_unaligned` with the same parameters given to this
/// function call.
unsafe fn alloc_unaligned(
&mut self,
layout_size: usize,
chunk: FreeChunkRef,
aligned_start_addr: usize,
) -> NonNull<u8> {
let aligned_content_start_addr = aligned_start_addr + HEADER_SIZE;
// calculate the size left from the aligned start addr to the end of the chunk.
let left_size = chunk.end_addr().saturating_sub(aligned_content_start_addr);
// shrink the current chunk to leave some space for the new aligned allocated
// chunk.
let cur_chunk_new_size = aligned_start_addr - chunk.content_addr();
chunk.set_size_and_update_bin(cur_chunk_new_size, self);
self.alloc_unaligned_after_splitting_start_padding(
layout_size,
aligned_start_addr,
left_size,
)
}
/// Allocates an unaligned chunk after splitting its start padding to a
/// different chunk, given the address and size of the allocated chunk that
/// is next to the start padding chunk.
///
/// # Safety
///
/// The provided size and align must have been prepared.
unsafe fn alloc_unaligned_after_splitting_start_padding(
&mut self,
layout_size: usize,
allocated_chunk_addr: usize,
allocated_chunk_size: usize,
) -> NonNull<u8> {
// check if we need any end padding, and if that padding is large enough to
// store an entire free chunk there.
let end_padding = allocated_chunk_size - layout_size;
if end_padding >= MIN_FREE_CHUNK_SIZE_INCLUDING_HEADER {
// if the end padding is large enough to hold a free chunk, create a chunk
// there and allocate the rest.
//
// this is safe because we checked that `end_padding` is big enough
self.alloc_unaligned_after_splitting_start_padding_split_end_padding_chunk(
layout_size,
end_padding,
allocated_chunk_addr,
)
} else {
// if there is no end padding, or the end padding is not large enough to store
// an entire free chunk there, just include it in the allocated chunk instead of
// splitting an end padding chunk for it.
self.alloc_unaligned_after_splitting_start_padding_no_end_padding(
allocated_chunk_addr,
allocated_chunk_size,
)
}
}
/// Allocates an unaligned chunk after splitting its start padding, without
/// splitting the end padding.
///
/// # Safety
///
/// - `allocated_chunk_addr` must be a valid non-null memory address.
/// - `allocated_chunk_size` must be aligned.
/// - the range of memory
/// `allocated_chunk_addr..allocated_chunk_addr+allocated_chunk_size`
/// must be valid and must not be used by any other chunk.
unsafe fn alloc_unaligned_after_splitting_start_padding_no_end_padding(
&mut self,
allocated_chunk_addr: usize,
allocated_chunk_size: usize,
) -> NonNull<u8> {
// we already split of the start padding and we don't need no end padding, so
// just create a used chunk in the available space.
//
// third argument is the `prev_in_use` bit of the created chunk. the chunk
// before the allocated chunk is the start padding chunk, which is not in
// use, thus the third argument is `false`.
let chunk = UsedChunk::create_new(
allocated_chunk_addr,
allocated_chunk_size,
false,
self.heap_end_addr,
);
NonNull::new_unchecked(chunk.content_addr() as *mut _)
}
/// Allocates an unaligned chunk after splitting its start padding, and
/// splits an end padding chunk from it.
///
/// # Safety
///
/// - `end_padding` must be greater than or equal to
/// [`MIN_FREE_CHUNK_SIZE_INCLUDING_HEADER`].
/// - `allocated_chunk_addr` must be a valid non-null memory address.
/// - the range of memory
/// `allocated_chunk_addr..allocated_chunk_addr+allocated_chunk_size`
/// must be valid and must not be used by any other chunk.
/// - the provided size must have been prepared.
unsafe fn alloc_unaligned_after_splitting_start_padding_split_end_padding_chunk(
&mut self,
layout_size: usize,
end_padding: usize,
allocated_chunk_addr: usize,
) -> NonNull<u8> {
// the end address of the allocated chunk that will be returned to the user.
// this is also the start address of the end padding chunk.
let end_padding_chunk_start_addr = allocated_chunk_addr + HEADER_SIZE + layout_size;
// create an end padding chunk.
//
// there is no need to update the next chunk because it's prev in use bit is
// already set to false, as it should be.
let end_padding_chunk_size = end_padding - HEADER_SIZE;
let _ = FreeChunk::create_new_without_updating_next_chunk(
end_padding_chunk_start_addr,
end_padding_chunk_size,
self,
);
// now create a used chunk for the allocated chunk.
//
// note that there is no need to update the next chunk, which will be the end
// padding chunk, because when creating a free chunk the prev in use flag is
// already set to false.
//
// also note that the chunk before the allocated chunk is the start padding
// chunk which is free and not in use, thus the third argument is `false`.
let allocated_chunk = UsedChunk::create_new_without_updating_next_chunk(
allocated_chunk_addr,
layout_size,
false,
);
NonNull::new_unchecked(allocated_chunk.content_addr() as *mut _)
}
/// Allocates a chunk that is already aligned to the desired alignment of
/// the content.
///
/// # Safety
///
/// The provided size must have been prepared, and the provided chunk must
/// be large enough to fit the allocation.
unsafe fn alloc_aligned(&mut self, layout_size: usize, chunk: FreeChunkRef) -> NonNull<u8> {
let cur_chunk_size = chunk.size();
// check if we need any end padding, and if that end padding is large enough to
// store an entire free chunk there.
let end_padding = cur_chunk_size - layout_size;
if end_padding >= MIN_FREE_CHUNK_SIZE_INCLUDING_HEADER {
// if the end padding is large enough to hold a free chunk, create a chunk
// there.
//
// this is safe because we checked that `end_padding` is big enough
self.alloc_aligned_split_end_padding_chunk(layout_size, end_padding, chunk)
} else {
// if there is no end padding, or the end padding is not large enough to store
// an entire free chunk there, consider the end padding a part of the allocated
// chunk.
self.alloc_aligned_no_end_padding(chunk)
}
}
/// Allocates an aligned chunk without any end padding.
fn alloc_aligned_no_end_padding(&mut self, chunk: FreeChunkRef) -> NonNull<u8> {
// mark the chunk as used and make the necessary updates
chunk.mark_as_used_unlink(self.heap_end_addr, &mut self.smallbins);
// retrun the chunk to the user
unsafe { NonNull::new_unchecked(chunk.content_addr() as *mut u8) }
}
/// Allocates the given aligned chunk and splits an end padding chunk from
/// it.
///
/// # Safety
///
/// - `end_padding` must be greater than or equal to
/// [`MIN_FREE_CHUNK_SIZE_INCLUDING_HEADER`].
/// - the provided size must have been prepared.
unsafe fn alloc_aligned_split_end_padding_chunk(
&mut self,
layout_size: usize,
end_padding: usize,
chunk: FreeChunkRef,
) -> NonNull<u8> {
// the end address of the allocated chunk that will be returned to the user.
// this is also the start address of the end padding chunk.
let end_padding_chunk_start_addr = chunk.content_addr() + layout_size;
// unlink the current chunk and mark it as used.
//
// SAFETY: the next chunk is the end padding chunk, and will soon be created as
// a free chunk, and when creating a free chunk its prev in use bit is
// automatically set to true, so no need to update it.
//
// also please note that if we tried to update the next chunk here it would
// update the chunk *after* the end padding chunk and not the end padding chunk
// itself, because we haven't yet updated the size of `cur_chunk`.
chunk.unlink(&mut self.smallbins);
let cur_chunk_as_used = chunk.mark_as_used_without_updating_freelist_and_next_chunk();
// create a free chunk for the end padding.
//
// there is no need to update the next chunk because it's prev in use bit is
// already set to false, as it should be.
let end_padding_chunk_size = end_padding - HEADER_SIZE;
let _ = FreeChunk::create_new_without_updating_next_chunk(
end_padding_chunk_start_addr,
end_padding_chunk_size,
self,
);
// shrink cur chunk to only include the content and not the end padding.
cur_chunk_as_used.set_size(layout_size);
// return a pointer to the allocated chunk
NonNull::new_unchecked(chunk.content_addr() as *mut u8)
}
}
unsafe impl<const SMALLBINS_AMOUNT: usize, const ALIGNMENT_SUB_BINS_AMOUNT: usize> Send
for Allocator<SMALLBINS_AMOUNT, ALIGNMENT_SUB_BINS_AMOUNT>
where
SmallestTypeWhichHasAtLeastNBitsStruct<ALIGNMENT_SUB_BINS_AMOUNT>:
SmallestTypeWhichHasAtLeastNBitsTrait,
{
}
/// A spin locked memory allocator that can be used as the global allocator.
#[cfg(feature = "spin")]
pub struct SpinLockedAllocator<
const SMALLBINS_AMOUNT: usize = DEFAULT_SMALLBINS_AMOUNT,
const ALIGNMENT_SUB_BINS_AMOUNT: usize = DEFAULT_ALIGNMENT_SUB_BINS_AMOUNT,
>(spin::Mutex<Allocator<SMALLBINS_AMOUNT, ALIGNMENT_SUB_BINS_AMOUNT>>)
where
SmallestTypeWhichHasAtLeastNBitsStruct<ALIGNMENT_SUB_BINS_AMOUNT>:
SmallestTypeWhichHasAtLeastNBitsTrait;
#[cfg(feature = "spin")]
impl<const SMALLBINS_AMOUNT: usize, const ALIGNMENT_SUB_BINS_AMOUNT: usize>
SpinLockedAllocator<SMALLBINS_AMOUNT, ALIGNMENT_SUB_BINS_AMOUNT>
where
SmallestTypeWhichHasAtLeastNBitsStruct<ALIGNMENT_SUB_BINS_AMOUNT>:
SmallestTypeWhichHasAtLeastNBitsTrait,
{
/// Creates an empty locked heap allocator without any heap memory region,
/// which will always return null on allocation requests.
///
/// To intiialize this allocator, use the `init` method.
pub const fn empty() -> Self {
Self(spin::Mutex::new(Allocator::empty()))
}
/// Initializes the heap allocator with the given memory region.
///
/// # Safety
///
/// If the allocator was already initialized, this function will panic.
///
/// The `SpinLockedAllocator` on which this was called must not be moved,
/// and its address in memory must not change, otherwise undefined
/// behaviour will occur. This is because the heap region now contains
/// pointers to fields of this struct, and if this struct will move, the
/// address of its fields will change, and those pointers will now be
/// invalid.
///
/// The provided memory region must be valid and non-null, and must not be
/// used by anything else.
///
/// If after aligning the start and end addresses, the size of the heap is
/// 0, the function panics.
pub unsafe fn init(&self, heap_start_addr: usize, heap_size: usize) {
let mut allocator = self.0.lock();
allocator.init(heap_start_addr, heap_size);
}
/// Checks if the heap memory region was already initialized by calling init.
pub fn was_initialized(&self) -> bool {
let allocator = self.0.lock();
allocator.was_initialized()
}
}
#[cfg(feature = "spin")]
unsafe impl<const SMALLBINS_AMOUNT: usize, const ALIGNMENT_SUB_BINS_AMOUNT: usize>
core::alloc::GlobalAlloc for SpinLockedAllocator<SMALLBINS_AMOUNT, ALIGNMENT_SUB_BINS_AMOUNT>
where
SmallestTypeWhichHasAtLeastNBitsStruct<ALIGNMENT_SUB_BINS_AMOUNT>:
SmallestTypeWhichHasAtLeastNBitsTrait,
{
unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
let mut allocator = self.0.lock();
allocator.alloc(layout)
}
unsafe fn dealloc(&self, ptr: *mut u8, _layout: Layout) {
let mut allocator = self.0.lock();
allocator.dealloc(ptr)
}
unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
let mut allocator = self.0.lock();
allocator.realloc(ptr, layout, new_size)
}
}
#[cfg(feature = "allocator")]
unsafe impl<const SMALLBINS_AMOUNT: usize, const ALIGNMENT_SUB_BINS_AMOUNT: usize>
core::alloc::Allocator for SpinLockedAllocator<SMALLBINS_AMOUNT, ALIGNMENT_SUB_BINS_AMOUNT>
where
SmallestTypeWhichHasAtLeastNBitsStruct<ALIGNMENT_SUB_BINS_AMOUNT>:
SmallestTypeWhichHasAtLeastNBitsTrait,
{
fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, core::alloc::AllocError> {
let mut allocator = self.0.lock();
let ptr =
NonNull::new(unsafe { allocator.alloc(layout) }).ok_or(core::alloc::AllocError)?;
Ok(NonNull::slice_from_raw_parts(ptr, layout.size()))
}
unsafe fn deallocate(&self, ptr: NonNull<u8>, _layout: Layout) {
let mut allocator = self.0.lock();
allocator.dealloc(ptr.as_ptr());
}
unsafe fn shrink(
&self,
ptr: NonNull<u8>,
old_layout: Layout,
new_layout: Layout,
) -> Result<NonNull<[u8]>, core::alloc::AllocError> {
let mut allocator = self.0.lock();
let chunk_content_addr = ptr.as_ptr() as usize;
if is_aligned(chunk_content_addr, new_layout.align()) {
let chunk = UsedChunk::from_addr(chunk_content_addr - HEADER_SIZE);
allocator.shrink_in_place(chunk, new_layout.size());
return Ok(NonNull::slice_from_raw_parts(ptr, new_layout.size()));
}
// we can't shrink in place, so realloc a new region and copy the data there.
let ptr = NonNull::new(allocator.realloc_new_region(ptr.as_ptr(), old_layout, new_layout))
.ok_or(core::alloc::AllocError)?;
Ok(NonNull::slice_from_raw_parts(ptr, new_layout.size()))
}
unsafe fn grow(
&self,
ptr: NonNull<u8>,
old_layout: Layout,
new_layout: Layout,
) -> Result<NonNull<[u8]>, core::alloc::AllocError> {
let mut allocator = self.0.lock();
let chunk_content_addr = ptr.as_ptr() as usize;
if is_aligned(chunk_content_addr, new_layout.align()) {
let chunk = UsedChunk::from_addr(chunk_content_addr - HEADER_SIZE);
if allocator.try_grow_in_place(chunk, new_layout.size()) {
return Ok(NonNull::slice_from_raw_parts(ptr, new_layout.size()));
}
}
// we can't grow in place, so realloc a new region and copy the data there.
let ptr = NonNull::new(allocator.realloc_new_region(ptr.as_ptr(), old_layout, new_layout))
.ok_or(core::alloc::AllocError)?;
Ok(NonNull::slice_from_raw_parts(ptr, new_layout.size()))
}
unsafe fn grow_zeroed(
&self,
ptr: NonNull<u8>,
old_layout: Layout,
new_layout: Layout,
) -> Result<NonNull<[u8]>, core::alloc::AllocError> {
let result = self.grow(ptr, old_layout, new_layout)?;
let ptr = result.as_mut_ptr() as *mut u8;
ptr.add(old_layout.size())
.write_bytes(0, new_layout.size() - old_layout.size());
Ok(result)
}
}