4 use rustc_index::vec::Idx;
5 use rustc_serialize::{opaque::Encoder, Encodable};
6 use std::convert::TryInto;
7 use std::marker::PhantomData;
8 use std::num::NonZeroUsize;
10 /// Helper trait, for encoding to, and decoding from, a fixed number of bytes.
11 /// Used mainly for Lazy positions and lengths.
12 /// Unchecked invariant: `Self::default()` should encode as `[0; BYTE_LEN]`,
13 /// but this has no impact on safety.
14 pub(super) trait FixedSizeEncoding: Default {
15 const BYTE_LEN: usize;
17 // FIXME(eddyb) convert to and from `[u8; Self::BYTE_LEN]` instead,
18 // once that starts being allowed by the compiler (i.e. lazy normalization).
19 fn from_bytes(b: &[u8]) -> Self;
20 fn write_to_bytes(self, b: &mut [u8]);
22 // FIXME(eddyb) make these generic functions, or at least defaults here.
23 // (same problem as above, needs `[u8; Self::BYTE_LEN]`)
24 // For now, a macro (`fixed_size_encoding_byte_len_and_defaults`) is used.
26 /// Read a `Self` value (encoded as `Self::BYTE_LEN` bytes),
27 /// from `&b[i * Self::BYTE_LEN..]`, returning `None` if `i`
28 /// is not in bounds, or `Some(Self::from_bytes(...))` otherwise.
29 fn maybe_read_from_bytes_at(b: &[u8], i: usize) -> Option<Self>;
30 /// Write a `Self` value (encoded as `Self::BYTE_LEN` bytes),
31 /// at `&mut b[i * Self::BYTE_LEN..]`, using `Self::write_to_bytes`.
32 fn write_to_bytes_at(self, b: &mut [u8], i: usize);
35 // HACK(eddyb) this shouldn't be needed (see comments on the methods above).
36 macro_rules! fixed_size_encoding_byte_len_and_defaults {
38 const BYTE_LEN: usize = $byte_len;
39 fn maybe_read_from_bytes_at(b: &[u8], i: usize) -> Option<Self> {
40 const BYTE_LEN: usize = $byte_len;
41 // HACK(eddyb) ideally this would be done with fully safe code,
42 // but slicing `[u8]` with `i * N..` is optimized worse, due to the
43 // possibility of `i * N` overflowing, than indexing `[[u8; N]]`.
45 std::slice::from_raw_parts(b.as_ptr() as *const [u8; BYTE_LEN], b.len() / BYTE_LEN)
47 b.get(i).map(|b| FixedSizeEncoding::from_bytes(b))
49 fn write_to_bytes_at(self, b: &mut [u8], i: usize) {
50 const BYTE_LEN: usize = $byte_len;
51 // HACK(eddyb) ideally this would be done with fully safe code,
52 // see similar comment in `read_from_bytes_at` for why it can't yet.
54 std::slice::from_raw_parts_mut(
55 b.as_mut_ptr() as *mut [u8; BYTE_LEN],
59 self.write_to_bytes(&mut b[i]);
64 impl FixedSizeEncoding for u32 {
65 fixed_size_encoding_byte_len_and_defaults!(4);
67 fn from_bytes(b: &[u8]) -> Self {
68 let mut bytes = [0; Self::BYTE_LEN];
69 bytes.copy_from_slice(&b[..Self::BYTE_LEN]);
70 Self::from_le_bytes(bytes)
73 fn write_to_bytes(self, b: &mut [u8]) {
74 b[..Self::BYTE_LEN].copy_from_slice(&self.to_le_bytes());
78 // NOTE(eddyb) there could be an impl for `usize`, which would enable a more
79 // generic `Lazy<T>` impl, but in the general case we might not need / want to
80 // fit every `usize` in `u32`.
81 impl<T: Encodable> FixedSizeEncoding for Option<Lazy<T>> {
82 fixed_size_encoding_byte_len_and_defaults!(u32::BYTE_LEN);
84 fn from_bytes(b: &[u8]) -> Self {
85 Some(Lazy::from_position(NonZeroUsize::new(u32::from_bytes(b) as usize)?))
88 fn write_to_bytes(self, b: &mut [u8]) {
89 let position = self.map_or(0, |lazy| lazy.position.get());
90 let position: u32 = position.try_into().unwrap();
92 position.write_to_bytes(b)
96 impl<T: Encodable> FixedSizeEncoding for Option<Lazy<[T]>> {
97 fixed_size_encoding_byte_len_and_defaults!(u32::BYTE_LEN * 2);
99 fn from_bytes(b: &[u8]) -> Self {
100 Some(Lazy::from_position_and_meta(
101 <Option<Lazy<T>>>::from_bytes(b)?.position,
102 u32::from_bytes(&b[u32::BYTE_LEN..]) as usize,
106 fn write_to_bytes(self, b: &mut [u8]) {
107 self.map(|lazy| Lazy::<T>::from_position(lazy.position)).write_to_bytes(b);
109 let len = self.map_or(0, |lazy| lazy.meta);
110 let len: u32 = len.try_into().unwrap();
112 len.write_to_bytes(&mut b[u32::BYTE_LEN..]);
116 /// Random-access table (i.e. offering constant-time `get`/`set`), similar to
117 /// `Vec<Option<T>>`, but without requiring encoding or decoding all the values
118 /// eagerly and in-order.
119 /// A total of `(max_idx + 1) * <Option<T> as FixedSizeEncoding>::BYTE_LEN` bytes
120 /// are used for a table, where `max_idx` is the largest index passed to
121 /// `TableBuilder::set`.
122 pub(super) struct Table<I: Idx, T>
124 Option<T>: FixedSizeEncoding,
126 _marker: PhantomData<(fn(&I), T)>,
127 // NOTE(eddyb) this makes `Table` not implement `Sized`, but no
128 // value of `Table` is ever created (it's always behind `Lazy`).
132 /// Helper for constructing a table's serialization (also see `Table`).
133 pub(super) struct TableBuilder<I: Idx, T>
135 Option<T>: FixedSizeEncoding,
137 // FIXME(eddyb) use `IndexVec<I, [u8; <Option<T>>::BYTE_LEN]>` instead of
138 // `Vec<u8>`, once that starts working (i.e. lazy normalization).
139 // Then again, that has the downside of not allowing `TableBuilder::encode` to
140 // obtain a `&[u8]` entirely in safe code, for writing the bytes out.
142 _marker: PhantomData<(fn(&I), T)>,
145 impl<I: Idx, T> Default for TableBuilder<I, T>
147 Option<T>: FixedSizeEncoding,
149 fn default() -> Self {
150 TableBuilder { bytes: vec![], _marker: PhantomData }
154 impl<I: Idx, T> TableBuilder<I, T>
156 Option<T>: FixedSizeEncoding,
158 pub(crate) fn set(&mut self, i: I, value: T) {
159 // FIXME(eddyb) investigate more compact encodings for sparse tables.
160 // On the PR @michaelwoerister mentioned:
161 // > Space requirements could perhaps be optimized by using the HAMT `popcnt`
162 // > trick (i.e. divide things into buckets of 32 or 64 items and then
163 // > store bit-masks of which item in each bucket is actually serialized).
165 let needed = (i + 1) * <Option<T>>::BYTE_LEN;
166 if self.bytes.len() < needed {
167 self.bytes.resize(needed, 0);
170 Some(value).write_to_bytes_at(&mut self.bytes, i);
173 pub(crate) fn encode(&self, buf: &mut Encoder) -> Lazy<Table<I, T>> {
174 let pos = buf.position();
175 buf.emit_raw_bytes(&self.bytes);
176 Lazy::from_position_and_meta(NonZeroUsize::new(pos as usize).unwrap(), self.bytes.len())
180 impl<I: Idx, T> LazyMeta for Table<I, T>
182 Option<T>: FixedSizeEncoding,
186 fn min_size(len: usize) -> usize {
191 impl<I: Idx, T> Lazy<Table<I, T>>
193 Option<T>: FixedSizeEncoding,
195 /// Given the metadata, extract out the value at a particular index (if any).
197 pub(super) fn get<'a, 'tcx, M: Metadata<'a, 'tcx>>(&self, metadata: M, i: I) -> Option<T> {
198 debug!("Table::lookup: index={:?} len={:?}", i, self.meta);
200 let start = self.position.get();
201 let bytes = &metadata.raw_bytes()[start..start + self.meta];
202 <Option<T>>::maybe_read_from_bytes_at(bytes, i.index())?