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// Copyright (c) Microsoft Corporation.
// Licensed under the MIT License.
use core::alloc::Layout;
use core::ptr::NonNull;
use allocator_api2::alloc::{AllocError, Allocator};
use crate::Arena;
use crate::internal::in_chunk::InLocalChunk;
use crate::internal::local_chunk::LocalChunk;
/// `&Arena<A>` is the allocator handle: cheap to copy and backed by
/// local chunks. `allocate` bumps the chunk refcount; `deallocate`
/// releases it.
///
/// # Safety
///
/// `deallocate` must get a pointer returned by `allocate` on the same
/// arena. The chunk-header mask would misidentify a foreign pointer.
// SAFETY: the chunk refcount keeps each allocation alive until the
// matching `deallocate`.
unsafe impl<A: Allocator + Clone> Allocator for &Arena<A> {
fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
let ptr = self.allocate_layout(layout)?;
let fat: *mut [u8] = core::ptr::slice_from_raw_parts_mut(ptr.as_ptr(), layout.size());
// SAFETY: `ptr.as_ptr()` is non-null.
Ok(unsafe { NonNull::new_unchecked(fat) })
}
unsafe fn deallocate(&self, ptr: NonNull<u8>, _layout: Layout) {
// SAFETY: chunk-header invariant — `ptr` was returned by
// `allocate` above, which only hands out pointers into chunks
// produced by this arena.
let in_chunk = unsafe { InLocalChunk::<_, A>::new(ptr) };
let chunk = in_chunk.chunk_ptr();
// SAFETY: refcount-positive — `allocate` left a +1 for this
// pointer.
unsafe { LocalChunk::dec_ref(chunk) };
}
unsafe fn grow(&self, ptr: NonNull<u8>, old_layout: Layout, new_layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
// Grow in place when the buffer still reaches the bump cursor;
// otherwise fall back to allocate-copy-deallocate.
if old_layout.align() == new_layout.align() && new_layout.size() >= old_layout.size() {
// SAFETY: `ptr` was previously returned by `self.allocate`
// and `old_layout` is the layout it was allocated with.
if let Some(grown) = unsafe { self.try_grow_in_place(ptr, old_layout, new_layout) } {
let fat: *mut [u8] = core::ptr::slice_from_raw_parts_mut(grown.as_ptr(), new_layout.size());
// SAFETY: `grown.as_ptr()` is non-null.
return Ok(unsafe { NonNull::new_unchecked(fat) });
}
}
let new_ptr = self.allocate(new_layout)?;
// SAFETY: `ptr` is valid for old_layout.size() bytes; new_ptr
// is fresh and has at least new_layout.size() bytes.
unsafe {
core::ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_ptr().cast::<u8>(), old_layout.size());
self.deallocate(ptr, old_layout);
}
self.bump_relocation();
Ok(new_ptr)
}
}