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//! A minimal arena allocator inspired by `rustc_arena::DroplessArena`.
//!
//! This is unfortunately a minimal re-implementation rather than a dependency
//! as it is difficult to depend on crates from within `proc_macro`, due to it
//! being built at the same time as `std`.
use std::cell::{Cell, RefCell};
use std::cmp;
use std::mem::MaybeUninit;
use std::ops::Range;
use std::ptr;
use std::slice;
use std::str;
// 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;
/// A minimal arena allocator inspired by `rustc_arena::DroplessArena`.
///
/// This is unfortunately a complete re-implementation rather than a dependency
/// as it is difficult to depend on crates from within `proc_macro`, due to it
/// being built at the same time as `std`.
///
/// This arena doesn't have support for allocating anything other than byte
/// slices, as that is all that is necessary.
pub(crate) struct Arena {
start: Cell<*mut MaybeUninit<u8>>,
end: Cell<*mut MaybeUninit<u8>>,
chunks: RefCell<Vec<Box<[MaybeUninit<u8>]>>>,
}
impl Arena {
pub(crate) fn new() -> Self {
Arena {
start: Cell::new(ptr::null_mut()),
end: Cell::new(ptr::null_mut()),
chunks: RefCell::new(Vec::new()),
}
}
/// Add a new chunk with at least `additional` free bytes.
#[inline(never)]
#[cold]
fn grow(&self, additional: usize) {
let mut chunks = self.chunks.borrow_mut();
let mut new_cap;
if let Some(last_chunk) = chunks.last_mut() {
// 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.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 = Box::new_uninit_slice(new_cap);
let Range { start, end } = chunk.as_mut_ptr_range();
self.start.set(start);
self.end.set(end);
chunks.push(chunk);
}
/// Allocates a byte slice with specified size from the current memory
/// chunk. Returns `None` if there is no free space left to satisfy the
/// request.
fn alloc_raw_without_grow(&self, bytes: usize) -> Option<&mut [MaybeUninit<u8>]> {
let start = self.start.get().addr();
let old_end = self.end.get();
let end = old_end.addr();
let new_end = end.checked_sub(bytes)?;
if start <= new_end {
let new_end = old_end.with_addr(new_end);
self.end.set(new_end);
// SAFETY: `bytes` bytes starting at `new_end` were just reserved.
Some(unsafe { slice::from_raw_parts_mut(new_end, bytes) })
} else {
None
}
}
fn alloc_raw(&self, bytes: usize) -> &mut [MaybeUninit<u8>] {
if bytes == 0 {
return &mut [];
}
loop {
if let Some(a) = self.alloc_raw_without_grow(bytes) {
break a;
}
// No free space left. Allocate a new chunk to satisfy the request.
// On failure the grow will panic or abort.
self.grow(bytes);
}
}
pub(crate) fn alloc_str<'a>(&'a self, string: &str) -> &'a mut str {
let alloc = self.alloc_raw(string.len());
let bytes = MaybeUninit::write_slice(alloc, string.as_bytes());
// SAFETY: we convert from `&str` to `&[u8]`, clone it into the arena,
// and immediately convert the clone back to `&str`.
unsafe { str::from_utf8_unchecked_mut(bytes) }
}
}
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