mirror of https://github.com/rust-lang/rust
115 lines
4.1 KiB
Rust
115 lines
4.1 KiB
Rust
//! A minimal arena allocator inspired by `rustc_arena::DroplessArena`.
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//!
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//! This is unfortunately a minimal re-implementation rather than a dependency
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//! as it is difficult to depend on crates from within `proc_macro`, due to it
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//! being built at the same time as `std`.
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use std::cell::{Cell, RefCell};
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use std::cmp;
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use std::mem::MaybeUninit;
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use std::ops::Range;
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use std::ptr;
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use std::slice;
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use std::str;
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// The arenas start with PAGE-sized chunks, and then each new chunk is twice as
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// big as its predecessor, up until we reach HUGE_PAGE-sized chunks, whereupon
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// we stop growing. This scales well, from arenas that are barely used up to
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// arenas that are used for 100s of MiBs. Note also that the chosen sizes match
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// the usual sizes of pages and huge pages on Linux.
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const PAGE: usize = 4096;
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const HUGE_PAGE: usize = 2 * 1024 * 1024;
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/// A minimal arena allocator inspired by `rustc_arena::DroplessArena`.
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///
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/// This is unfortunately a complete re-implementation rather than a dependency
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/// as it is difficult to depend on crates from within `proc_macro`, due to it
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/// being built at the same time as `std`.
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///
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/// This arena doesn't have support for allocating anything other than byte
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/// slices, as that is all that is necessary.
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pub(crate) struct Arena {
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start: Cell<*mut MaybeUninit<u8>>,
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end: Cell<*mut MaybeUninit<u8>>,
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chunks: RefCell<Vec<Box<[MaybeUninit<u8>]>>>,
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}
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impl Arena {
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pub(crate) fn new() -> Self {
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Arena {
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start: Cell::new(ptr::null_mut()),
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end: Cell::new(ptr::null_mut()),
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chunks: RefCell::new(Vec::new()),
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}
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}
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/// Add a new chunk with at least `additional` free bytes.
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#[inline(never)]
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#[cold]
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fn grow(&self, additional: usize) {
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let mut chunks = self.chunks.borrow_mut();
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let mut new_cap;
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if let Some(last_chunk) = chunks.last_mut() {
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// If the previous chunk's len is less than HUGE_PAGE
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// bytes, then this chunk will be least double the previous
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// chunk's size.
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new_cap = last_chunk.len().min(HUGE_PAGE / 2);
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new_cap *= 2;
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} else {
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new_cap = PAGE;
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}
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// Also ensure that this chunk can fit `additional`.
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new_cap = cmp::max(additional, new_cap);
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let mut chunk = Box::new_uninit_slice(new_cap);
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let Range { start, end } = chunk.as_mut_ptr_range();
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self.start.set(start);
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self.end.set(end);
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chunks.push(chunk);
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}
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/// Allocates a byte slice with specified size from the current memory
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/// chunk. Returns `None` if there is no free space left to satisfy the
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/// request.
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fn alloc_raw_without_grow(&self, bytes: usize) -> Option<&mut [MaybeUninit<u8>]> {
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let start = self.start.get().addr();
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let old_end = self.end.get();
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let end = old_end.addr();
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let new_end = end.checked_sub(bytes)?;
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if start <= new_end {
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let new_end = old_end.with_addr(new_end);
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self.end.set(new_end);
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// SAFETY: `bytes` bytes starting at `new_end` were just reserved.
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Some(unsafe { slice::from_raw_parts_mut(new_end, bytes) })
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} else {
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None
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}
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}
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fn alloc_raw(&self, bytes: usize) -> &mut [MaybeUninit<u8>] {
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if bytes == 0 {
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return &mut [];
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}
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loop {
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if let Some(a) = self.alloc_raw_without_grow(bytes) {
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break a;
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}
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// No free space left. Allocate a new chunk to satisfy the request.
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// On failure the grow will panic or abort.
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self.grow(bytes);
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}
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}
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#[allow(clippy::mut_from_ref)] // arena allocator
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pub(crate) fn alloc_str<'a>(&'a self, string: &str) -> &'a mut str {
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let alloc = self.alloc_raw(string.len());
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let bytes = MaybeUninit::copy_from_slice(alloc, string.as_bytes());
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// SAFETY: we convert from `&str` to `&[u8]`, clone it into the arena,
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// and immediately convert the clone back to `&str`.
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unsafe { str::from_utf8_unchecked_mut(bytes) }
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}
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}
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