src/librustc_metadata/rmeta/table.rs

   1 use crate::rmeta::*;
   2
   3 use rustc_index::vec::Idx;
   4 use rustc_serialize::opaque::Encoder;
   5 use std::convert::TryInto;
   6 use std::marker::PhantomData;
   7 use std::num::NonZeroUsize;
   8 use tracing::debug;
   9
  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;
  16
  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]);
  21
  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.
  25
  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);
  33 }
  34
  35 // HACK(eddyb) this shouldn't be needed (see comments on the methods above).
  36 macro_rules! fixed_size_encoding_byte_len_and_defaults {
  37     ($byte_len:expr) => {
  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]]`.
  44             let b = unsafe {
  45                 std::slice::from_raw_parts(b.as_ptr() as *const [u8; BYTE_LEN], b.len() / BYTE_LEN)
  46             };
  47             b.get(i).map(|b| FixedSizeEncoding::from_bytes(b))
  48         }
  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.
  53             let b = unsafe {
  54                 std::slice::from_raw_parts_mut(
  55                     b.as_mut_ptr() as *mut [u8; BYTE_LEN],
  56                     b.len() / BYTE_LEN,
  57                 )
  58             };
  59             self.write_to_bytes(&mut b[i]);
  60         }
  61     };
  62 }
  63
  64 impl FixedSizeEncoding for u32 {
  65     fixed_size_encoding_byte_len_and_defaults!(4);
  66
  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)
  71     }
  72
  73     fn write_to_bytes(self, b: &mut [u8]) {
  74         b[..Self::BYTE_LEN].copy_from_slice(&self.to_le_bytes());
  75     }
  76 }
  77
  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> FixedSizeEncoding for Option<Lazy<T>> {
  82     fixed_size_encoding_byte_len_and_defaults!(u32::BYTE_LEN);
  83
  84     fn from_bytes(b: &[u8]) -> Self {
  85         Some(Lazy::from_position(NonZeroUsize::new(u32::from_bytes(b) as usize)?))
  86     }
  87
  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();
  91
  92         position.write_to_bytes(b)
  93     }
  94 }
  95
  96 impl<T> FixedSizeEncoding for Option<Lazy<[T]>> {
  97     fixed_size_encoding_byte_len_and_defaults!(u32::BYTE_LEN * 2);
  98
  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,
 103         ))
 104     }
 105
 106     fn write_to_bytes(self, b: &mut [u8]) {
 107         self.map(|lazy| Lazy::<T>::from_position(lazy.position)).write_to_bytes(b);
 108
 109         let len = self.map_or(0, |lazy| lazy.meta);
 110         let len: u32 = len.try_into().unwrap();
 111
 112         len.write_to_bytes(&mut b[u32::BYTE_LEN..]);
 113     }
 114 }
 115
 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>
 123 where
 124     Option<T>: FixedSizeEncoding,
 125 {
 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`).
 129     _bytes: [u8],
 130 }
 131
 132 /// Helper for constructing a table's serialization (also see `Table`).
 133 pub(super) struct TableBuilder<I: Idx, T>
 134 where
 135     Option<T>: FixedSizeEncoding,
 136 {
 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.
 141     bytes: Vec<u8>,
 142     _marker: PhantomData<(fn(&I), T)>,
 143 }
 144
 145 impl<I: Idx, T> Default for TableBuilder<I, T>
 146 where
 147     Option<T>: FixedSizeEncoding,
 148 {
 149     fn default() -> Self {
 150         TableBuilder { bytes: vec![], _marker: PhantomData }
 151     }
 152 }
 153
 154 impl<I: Idx, T> TableBuilder<I, T>
 155 where
 156     Option<T>: FixedSizeEncoding,
 157 {
 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).
 164         let i = i.index();
 165         let needed = (i + 1) * <Option<T>>::BYTE_LEN;
 166         if self.bytes.len() < needed {
 167             self.bytes.resize(needed, 0);
 168         }
 169
 170         Some(value).write_to_bytes_at(&mut self.bytes, i);
 171     }
 172
 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())
 177     }
 178 }
 179
 180 impl<I: Idx, T> LazyMeta for Table<I, T>
 181 where
 182     Option<T>: FixedSizeEncoding,
 183 {
 184     type Meta = usize;
 185
 186     fn min_size(len: usize) -> usize {
 187         len
 188     }
 189 }
 190
 191 impl<I: Idx, T> Lazy<Table<I, T>>
 192 where
 193     Option<T>: FixedSizeEncoding,
 194 {
 195     /// Given the metadata, extract out the value at a particular index (if any).
 196     #[inline(never)]
 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);
 199
 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())?
 203     }
 204
 205     /// Size of the table in entries, including possible gaps.
 206     pub(super) fn size(&self) -> usize {
 207         self.meta / <Option<T>>::BYTE_LEN
 208     }
 209 }