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| 1 | +//! A minimal arena allocator inspired by `rustc_arena::DroplessArena`. |
| 2 | +//! |
| 3 | +//! This is unfortunately a minimal re-implementation rather than a dependency |
| 4 | +//! as it is difficult to depend on crates from within `proc_macro`, due to it |
| 5 | +//! being built at the same time as `std`. |
| 6 | +
|
| 7 | +use std::cell::{Cell, RefCell}; |
| 8 | +use std::cmp; |
| 9 | +use std::mem::MaybeUninit; |
| 10 | +use std::ops::Range; |
| 11 | +use std::ptr; |
| 12 | +use std::slice; |
| 13 | +use std::str; |
| 14 | + |
| 15 | +// The arenas start with PAGE-sized chunks, and then each new chunk is twice as |
| 16 | +// big as its predecessor, up until we reach HUGE_PAGE-sized chunks, whereupon |
| 17 | +// we stop growing. This scales well, from arenas that are barely used up to |
| 18 | +// arenas that are used for 100s of MiBs. Note also that the chosen sizes match |
| 19 | +// the usual sizes of pages and huge pages on Linux. |
| 20 | +const PAGE: usize = 4096; |
| 21 | +const HUGE_PAGE: usize = 2 * 1024 * 1024; |
| 22 | + |
| 23 | +/// A minimal arena allocator inspired by `rustc_arena::DroplessArena`. |
| 24 | +/// |
| 25 | +/// This is unfortunately a complete re-implementation rather than a dependency |
| 26 | +/// as it is difficult to depend on crates from within `proc_macro`, due to it |
| 27 | +/// being built at the same time as `std`. |
| 28 | +/// |
| 29 | +/// This arena doesn't have support for allocating anything other than byte |
| 30 | +/// slices, as that is all that is necessary. |
| 31 | +pub(crate) struct Arena { |
| 32 | + start: Cell<*mut MaybeUninit<u8>>, |
| 33 | + end: Cell<*mut MaybeUninit<u8>>, |
| 34 | + chunks: RefCell<Vec<Box<[MaybeUninit<u8>]>>>, |
| 35 | +} |
| 36 | + |
| 37 | +impl Arena { |
| 38 | + pub(crate) fn new() -> Self { |
| 39 | + Arena { |
| 40 | + start: Cell::new(ptr::null_mut()), |
| 41 | + end: Cell::new(ptr::null_mut()), |
| 42 | + chunks: RefCell::new(Vec::new()), |
| 43 | + } |
| 44 | + } |
| 45 | + |
| 46 | + /// Add a new chunk with at least `additional` free bytes. |
| 47 | + #[inline(never)] |
| 48 | + #[cold] |
| 49 | + fn grow(&self, additional: usize) { |
| 50 | + let mut chunks = self.chunks.borrow_mut(); |
| 51 | + let mut new_cap; |
| 52 | + if let Some(last_chunk) = chunks.last_mut() { |
| 53 | + // If the previous chunk's len is less than HUGE_PAGE |
| 54 | + // bytes, then this chunk will be least double the previous |
| 55 | + // chunk's size. |
| 56 | + new_cap = last_chunk.len().min(HUGE_PAGE / 2); |
| 57 | + new_cap *= 2; |
| 58 | + } else { |
| 59 | + new_cap = PAGE; |
| 60 | + } |
| 61 | + // Also ensure that this chunk can fit `additional`. |
| 62 | + new_cap = cmp::max(additional, new_cap); |
| 63 | + |
| 64 | + let mut chunk = Box::new_uninit_slice(new_cap); |
| 65 | + let Range { start, end } = chunk.as_mut_ptr_range(); |
| 66 | + self.start.set(start); |
| 67 | + self.end.set(end); |
| 68 | + chunks.push(chunk); |
| 69 | + } |
| 70 | + |
| 71 | + /// Allocates a byte slice with specified size from the current memory |
| 72 | + /// chunk. Returns `None` if there is no free space left to satisfy the |
| 73 | + /// request. |
| 74 | + fn alloc_raw_without_grow(&self, bytes: usize) -> Option<&mut [MaybeUninit<u8>]> { |
| 75 | + let start = self.start.get().addr(); |
| 76 | + let old_end = self.end.get(); |
| 77 | + let end = old_end.addr(); |
| 78 | + |
| 79 | + let new_end = end.checked_sub(bytes)?; |
| 80 | + if start <= new_end { |
| 81 | + let new_end = old_end.with_addr(new_end); |
| 82 | + self.end.set(new_end); |
| 83 | + // SAFETY: `bytes` bytes starting at `new_end` were just reserved. |
| 84 | + Some(unsafe { slice::from_raw_parts_mut(new_end, bytes) }) |
| 85 | + } else { |
| 86 | + None |
| 87 | + } |
| 88 | + } |
| 89 | + |
| 90 | + fn alloc_raw(&self, bytes: usize) -> &mut [MaybeUninit<u8>] { |
| 91 | + if bytes == 0 { |
| 92 | + return &mut []; |
| 93 | + } |
| 94 | + |
| 95 | + loop { |
| 96 | + if let Some(a) = self.alloc_raw_without_grow(bytes) { |
| 97 | + break a; |
| 98 | + } |
| 99 | + // No free space left. Allocate a new chunk to satisfy the request. |
| 100 | + // On failure the grow will panic or abort. |
| 101 | + self.grow(bytes); |
| 102 | + } |
| 103 | + } |
| 104 | + |
| 105 | + pub(crate) fn alloc_str<'a>(&'a self, string: &str) -> &'a mut str { |
| 106 | + let alloc = self.alloc_raw(string.len()); |
| 107 | + let bytes = MaybeUninit::write_slice(alloc, string.as_bytes()); |
| 108 | + |
| 109 | + // SAFETY: we convert from `&str` to `&[u8]`, clone it into the arena, |
| 110 | + // and immediately convert the clone back to `&str`. |
| 111 | + unsafe { str::from_utf8_unchecked_mut(bytes) } |
| 112 | + } |
| 113 | +} |
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