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 { new() -> Self38 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] grow(&self, additional: usize)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. alloc_raw_without_grow(&self, bytes: usize) -> Option<&mut [MaybeUninit<u8>]>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 alloc_raw(&self, bytes: usize) -> &mut [MaybeUninit<u8>]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 alloc_str<'a>(&'a self, string: &str) -> &'a mut str105 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 } 114