src/librustc_metadata/rmeta/table.rs

   1 use crate::rmeta::*;
   2
   3 use rustc_index::vec::Idx;
   4 use rustc_serialize::{Encodable, opaque::Encoder};
   5 use std::convert::TryInto;
   6 use std::marker::PhantomData;
   7 use std::num::NonZeroUsize;
   8 use log::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(
  46                     b.as_ptr() as *const [u8; BYTE_LEN],
  47                     b.len() / BYTE_LEN,
  48                 )
  49             };
  50             b.get(i).map(|b| FixedSizeEncoding::from_bytes(b))
  51         }
  52         fn write_to_bytes_at(self, b: &mut [u8], i: usize) {
  53             const BYTE_LEN: usize = $byte_len;
  54             // HACK(eddyb) ideally this would be done with fully safe code,
  55             // see similar comment in `read_from_bytes_at` for why it can't yet.
  56             let b = unsafe {
  57                 std::slice::from_raw_parts_mut(
  58                     b.as_mut_ptr() as *mut [u8; BYTE_LEN],
  59                     b.len() / BYTE_LEN,
  60                 )
  61             };
  62             self.write_to_bytes(&mut b[i]);
  63         }
  64     }
  65 }
  66
  67 impl FixedSizeEncoding for u32 {
  68     fixed_size_encoding_byte_len_and_defaults!(4);
  69
  70     fn from_bytes(b: &[u8]) -> Self {
  71         let mut bytes = [0; Self::BYTE_LEN];
  72         bytes.copy_from_slice(&b[..Self::BYTE_LEN]);
  73         Self::from_le_bytes(bytes)
  74     }
  75
  76     fn write_to_bytes(self, b: &mut [u8]) {
  77         b[..Self::BYTE_LEN].copy_from_slice(&self.to_le_bytes());
  78     }
  79 }
  80
  81 // NOTE(eddyb) there could be an impl for `usize`, which would enable a more
  82 // generic `Lazy<T>` impl, but in the general case we might not need / want to
  83 // fit every `usize` in `u32`.
  84 impl<T: Encodable> FixedSizeEncoding for Option<Lazy<T>> {
  85     fixed_size_encoding_byte_len_and_defaults!(u32::BYTE_LEN);
  86
  87     fn from_bytes(b: &[u8]) -> Self {
  88         Some(Lazy::from_position(NonZeroUsize::new(u32::from_bytes(b) as usize)?))
  89     }
  90
  91     fn write_to_bytes(self, b: &mut [u8]) {
  92         let position = self.map_or(0, |lazy| lazy.position.get());
  93         let position: u32 = position.try_into().unwrap();
  94
  95         position.write_to_bytes(b)
  96     }
  97 }
  98
  99 impl<T: Encodable> FixedSizeEncoding for Option<Lazy<[T]>> {
 100     fixed_size_encoding_byte_len_and_defaults!(u32::BYTE_LEN * 2);
 101
 102     fn from_bytes(b: &[u8]) -> Self {
 103         Some(Lazy::from_position_and_meta(
 104             <Option<Lazy<T>>>::from_bytes(b)?.position,
 105             u32::from_bytes(&b[u32::BYTE_LEN..]) as usize,
 106         ))
 107     }
 108
 109     fn write_to_bytes(self, b: &mut [u8]) {
 110         self.map(|lazy| Lazy::<T>::from_position(lazy.position))
 111             .write_to_bytes(b);
 112
 113         let len = self.map_or(0, |lazy| lazy.meta);
 114         let len: u32 = len.try_into().unwrap();
 115
 116         len.write_to_bytes(&mut b[u32::BYTE_LEN..]);
 117     }
 118 }
 119
 120 /// Random-access table (i.e. offering constant-time `get`/`set`), similar to
 121 /// `Vec<Option<T>>`, but without requiring encoding or decoding all the values
 122 /// eagerly and in-order.
 123 /// A total of `(max_idx + 1) * <Option<T> as FixedSizeEncoding>::BYTE_LEN` bytes
 124 /// are used for a table, where `max_idx` is the largest index passed to
 125 /// `TableBuilder::set`.
 126 pub(super) struct Table<I: Idx, T> where Option<T>: FixedSizeEncoding {
 127     _marker: PhantomData<(fn(&I), T)>,
 128     // NOTE(eddyb) this makes `Table` not implement `Sized`, but no
 129     // value of `Table` is ever created (it's always behind `Lazy`).
 130     _bytes: [u8],
 131 }
 132
 133 /// Helper for constructing a table's serialization (also see `Table`).
 134 pub(super) struct TableBuilder<I: Idx, T> where Option<T>: FixedSizeEncoding {
 135     // FIXME(eddyb) use `IndexVec<I, [u8; <Option<T>>::BYTE_LEN]>` instead of
 136     // `Vec<u8>`, once that starts working (i.e. lazy normalization).
 137     // Then again, that has the downside of not allowing `TableBuilder::encode` to
 138     // obtain a `&[u8]` entirely in safe code, for writing the bytes out.
 139     bytes: Vec<u8>,
 140     _marker: PhantomData<(fn(&I), T)>,
 141 }
 142
 143 impl<I: Idx, T> Default for TableBuilder<I, T> where Option<T>: FixedSizeEncoding {
 144     fn default() -> Self {
 145         TableBuilder {
 146             bytes: vec![],
 147             _marker: PhantomData,
 148         }
 149     }
 150 }
 151
 152 impl<I: Idx, T> TableBuilder<I, T> where Option<T>: FixedSizeEncoding {
 153     pub(super) fn set(&mut self, i: I, value: T) {
 154         // FIXME(eddyb) investigate more compact encodings for sparse tables.
 155         // On the PR @michaelwoerister mentioned:
 156         // > Space requirements could perhaps be optimized by using the HAMT `popcnt`
 157         // > trick (i.e. divide things into buckets of 32 or 64 items and then
 158         // > store bit-masks of which item in each bucket is actually serialized).
 159         let i = i.index();
 160         let needed = (i + 1) * <Option<T>>::BYTE_LEN;
 161         if self.bytes.len() < needed {
 162             self.bytes.resize(needed, 0);
 163         }
 164
 165         Some(value).write_to_bytes_at(&mut self.bytes, i);
 166     }
 167
 168     pub(super) fn encode(&self, buf: &mut Encoder) -> Lazy<Table<I, T>> {
 169         let pos = buf.position();
 170         buf.emit_raw_bytes(&self.bytes);
 171         Lazy::from_position_and_meta(
 172             NonZeroUsize::new(pos as usize).unwrap(),
 173             self.bytes.len(),
 174         )
 175     }
 176 }
 177
 178 impl<I: Idx, T> LazyMeta for Table<I, T> where Option<T>: FixedSizeEncoding {
 179     type Meta = usize;
 180
 181     fn min_size(len: usize) -> usize {
 182         len
 183     }
 184 }
 185
 186 impl<I: Idx, T> Lazy<Table<I, T>> where Option<T>: FixedSizeEncoding {
 187     /// Given the metadata, extract out the value at a particular index (if any).
 188     #[inline(never)]
 189     pub(super) fn get<'a, 'tcx, M: Metadata<'a, 'tcx>>(
 190         &self,
 191         metadata: M,
 192         i: I,
 193     ) -> Option<T> {
 194         debug!("Table::lookup: index={:?} len={:?}", i, self.meta);
 195
 196         let start = self.position.get();
 197         let bytes = &metadata.raw_bytes()[start..start + self.meta];
 198         <Option<T>>::maybe_read_from_bytes_at(bytes, i.index())?
 199     }
 200 }