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#[cfg(test)]
mod tests;
use std::hash;
use std::iter;
use std::ops::Range;
use rustc_serialize::{Decodable, Encodable};
use rustc_target::abi::Size;
use rustc_type_ir::{TyDecoder, TyEncoder};
use super::AllocRange;
type Block = u64;
/// A bitmask where each bit refers to the byte with the same index. If the bit is `true`, the byte
/// is initialized. If it is `false` the byte is uninitialized.
/// The actual bits are only materialized when needed, and we try to keep this data lazy as long as
/// possible. Currently, if all the blocks have the same value, then the mask represents either a
/// fully initialized or fully uninitialized const allocation, so we can only store that single
/// value.
#[derive(Clone, Debug, Eq, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]
pub struct InitMask {
blocks: InitMaskBlocks,
len: Size,
}
#[derive(Clone, Debug, Eq, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]
enum InitMaskBlocks {
Lazy {
/// Whether the lazy init mask is fully initialized or uninitialized.
state: bool,
},
Materialized(InitMaskMaterialized),
}
impl InitMask {
pub fn new(size: Size, state: bool) -> Self {
// Blocks start lazily allocated, until we have to materialize them.
let blocks = InitMaskBlocks::Lazy { state };
InitMask { len: size, blocks }
}
/// Checks whether the `range` is entirely initialized.
///
/// Returns `Ok(())` if it's initialized. Otherwise returns a range of byte
/// indexes for the first contiguous span of the uninitialized access.
#[inline]
pub fn is_range_initialized(&self, range: AllocRange) -> Result<(), AllocRange> {
let end = range.end();
if end > self.len {
return Err(AllocRange::from(self.len..end));
}
match self.blocks {
InitMaskBlocks::Lazy { state } => {
// Lazily allocated blocks represent the full mask, and cover the requested range by
// definition.
if state { Ok(()) } else { Err(range) }
}
InitMaskBlocks::Materialized(ref blocks) => {
blocks.is_range_initialized(range.start, end)
}
}
}
/// Sets a specified range to a value. If the range is out-of-bounds, the mask will grow to
/// accommodate it entirely.
pub fn set_range(&mut self, range: AllocRange, new_state: bool) {
let start = range.start;
let end = range.end();
let is_full_overwrite = start == Size::ZERO && end >= self.len;
// Optimize the cases of a full init/uninit state, while handling growth if needed.
match self.blocks {
InitMaskBlocks::Lazy { ref mut state } if is_full_overwrite => {
// This is fully overwriting the mask, and we'll still have a single initialization
// state: the blocks can stay lazy.
*state = new_state;
self.len = end;
}
InitMaskBlocks::Materialized(_) if is_full_overwrite => {
// This is also fully overwriting materialized blocks with a single initialization
// state: we'll have no need for these blocks anymore and can make them lazy.
self.blocks = InitMaskBlocks::Lazy { state: new_state };
self.len = end;
}
InitMaskBlocks::Lazy { state } if state == new_state => {
// Here we're partially overwriting the mask but the initialization state doesn't
// change: the blocks can stay lazy.
if end > self.len {
self.len = end;
}
}
_ => {
// Otherwise, we have a partial overwrite that can result in a mix of initialization
// states, so we'll need materialized blocks.
let len = self.len;
let blocks = self.materialize_blocks();
// There are 3 cases of interest here, if we have:
//
// [--------]
// ^ ^
// 0 len
//
// 1) the range to set can be in-bounds:
//
// xxxx = [start, end]
// [--------]
// ^ ^
// 0 len
//
// Here, we'll simply set the single `start` to `end` range.
//
// 2) the range to set can be partially out-of-bounds:
//
// xxxx = [start, end]
// [--------]
// ^ ^
// 0 len
//
// We have 2 subranges to handle:
// - we'll set the existing `start` to `len` range.
// - we'll grow and set the `len` to `end` range.
//
// 3) the range to set can be fully out-of-bounds:
//
// ---xxxx = [start, end]
// [--------]
// ^ ^
// 0 len
//
// Since we're growing the mask to a single `new_state` value, we consider the gap
// from `len` to `start` to be part of the range, and have a single subrange to
// handle: we'll grow and set the `len` to `end` range.
//
// Note that we have to materialize, set blocks, and grow the mask. We could
// therefore slightly optimize things in situations where these writes overlap.
// However, as of writing this, growing the mask doesn't happen in practice yet, so
// we don't do this micro-optimization.
if end <= len {
// Handle case 1.
blocks.set_range_inbounds(start, end, new_state);
} else {
if start < len {
// Handle the first subrange of case 2.
blocks.set_range_inbounds(start, len, new_state);
}
// Handle the second subrange of case 2, and case 3.
blocks.grow(len, end - len, new_state); // `Size` operation
self.len = end;
}
}
}
}
/// Materializes this mask's blocks when the mask is lazy.
#[inline]
fn materialize_blocks(&mut self) -> &mut InitMaskMaterialized {
if let InitMaskBlocks::Lazy { state } = self.blocks {
self.blocks = InitMaskBlocks::Materialized(InitMaskMaterialized::new(self.len, state));
}
let InitMaskBlocks::Materialized(ref mut blocks) = self.blocks else {
bug!("initmask blocks must be materialized here")
};
blocks
}
/// Returns the initialization state at the specified in-bounds index.
#[inline]
pub fn get(&self, idx: Size) -> bool {
match self.blocks {
InitMaskBlocks::Lazy { state } => state,
InitMaskBlocks::Materialized(ref blocks) => blocks.get(idx),
}
}
}
/// The actual materialized blocks of the bitmask, when we can't keep the `InitMask` lazy.
// Note: for performance reasons when interning, some of the fields can be partially
// hashed. (see the `Hash` impl below for more details), so the impl is not derived.
#[derive(Clone, Debug, Eq, PartialEq, HashStable)]
struct InitMaskMaterialized {
blocks: Vec<Block>,
}
// `Block` is a `u64`, but it is a bitmask not a numeric value. If we were to just derive
// Encodable and Decodable we would apply varint encoding to the bitmasks, which is slower
// and also produces more output when the high bits of each `u64` are occupied.
// Note: There is probably a remaining optimization for masks that do not use an entire
// `Block`.
impl<E: TyEncoder> Encodable<E> for InitMaskMaterialized {
fn encode(&self, encoder: &mut E) {
encoder.emit_usize(self.blocks.len());
for block in &self.blocks {
encoder.emit_raw_bytes(&block.to_le_bytes());
}
}
}
// This implementation is deliberately not derived, see the matching `Encodable` impl.
impl<D: TyDecoder> Decodable<D> for InitMaskMaterialized {
fn decode(decoder: &mut D) -> Self {
let num_blocks = decoder.read_usize();
let mut blocks = Vec::with_capacity(num_blocks);
for _ in 0..num_blocks {
let bytes = decoder.read_raw_bytes(8);
let block = u64::from_le_bytes(bytes.try_into().unwrap());
blocks.push(block);
}
InitMaskMaterialized { blocks }
}
}
// Const allocations are only hashed for interning. However, they can be large, making the hashing
// expensive especially since it uses `FxHash`: it's better suited to short keys, not potentially
// big buffers like the allocation's init mask. We can partially hash some fields when they're
// large.
impl hash::Hash for InitMaskMaterialized {
fn hash<H: hash::Hasher>(&self, state: &mut H) {
const MAX_BLOCKS_TO_HASH: usize = super::MAX_BYTES_TO_HASH / std::mem::size_of::<Block>();
const MAX_BLOCKS_LEN: usize = super::MAX_HASHED_BUFFER_LEN / std::mem::size_of::<Block>();
// Partially hash the `blocks` buffer when it is large. To limit collisions with common
// prefixes and suffixes, we hash the length and some slices of the buffer.
let block_count = self.blocks.len();
if block_count > MAX_BLOCKS_LEN {
// Hash the buffer's length.
block_count.hash(state);
// And its head and tail.
self.blocks[..MAX_BLOCKS_TO_HASH].hash(state);
self.blocks[block_count - MAX_BLOCKS_TO_HASH..].hash(state);
} else {
self.blocks.hash(state);
}
}
}
impl InitMaskMaterialized {
pub const BLOCK_SIZE: u64 = 64;
fn new(size: Size, state: bool) -> Self {
let mut m = InitMaskMaterialized { blocks: vec![] };
m.grow(Size::ZERO, size, state);
m
}
#[inline]
fn bit_index(bits: Size) -> (usize, usize) {
// BLOCK_SIZE is the number of bits that can fit in a `Block`.
// Each bit in a `Block` represents the initialization state of one byte of an allocation,
// so we use `.bytes()` here.
let bits = bits.bytes();
let a = bits / Self::BLOCK_SIZE;
let b = bits % Self::BLOCK_SIZE;
(usize::try_from(a).unwrap(), usize::try_from(b).unwrap())
}
#[inline]
fn size_from_bit_index(block: impl TryInto<u64>, bit: impl TryInto<u64>) -> Size {
let block = block.try_into().ok().unwrap();
let bit = bit.try_into().ok().unwrap();
Size::from_bytes(block * Self::BLOCK_SIZE + bit)
}
/// Checks whether the `range` is entirely initialized.
///
/// Returns `Ok(())` if it's initialized. Otherwise returns a range of byte
/// indexes for the first contiguous span of the uninitialized access.
#[inline]
fn is_range_initialized(&self, start: Size, end: Size) -> Result<(), AllocRange> {
let uninit_start = self.find_bit(start, end, false);
match uninit_start {
Some(uninit_start) => {
let uninit_end = self.find_bit(uninit_start, end, true).unwrap_or(end);
Err(AllocRange::from(uninit_start..uninit_end))
}
None => Ok(()),
}
}
fn set_range_inbounds(&mut self, start: Size, end: Size, new_state: bool) {
let (block_a, bit_a) = Self::bit_index(start);
let (block_b, bit_b) = Self::bit_index(end);
if block_a == block_b {
// First set all bits except the first `bit_a`,
// then unset the last `64 - bit_b` bits.
let range = if bit_b == 0 {
u64::MAX << bit_a
} else {
(u64::MAX << bit_a) & (u64::MAX >> (64 - bit_b))
};
if new_state {
self.blocks[block_a] |= range;
} else {
self.blocks[block_a] &= !range;
}
return;
}
// across block boundaries
if new_state {
// Set `bit_a..64` to `1`.
self.blocks[block_a] |= u64::MAX << bit_a;
// Set `0..bit_b` to `1`.
if bit_b != 0 {
self.blocks[block_b] |= u64::MAX >> (64 - bit_b);
}
// Fill in all the other blocks (much faster than one bit at a time).
for block in (block_a + 1)..block_b {
self.blocks[block] = u64::MAX;
}
} else {
// Set `bit_a..64` to `0`.
self.blocks[block_a] &= !(u64::MAX << bit_a);
// Set `0..bit_b` to `0`.
if bit_b != 0 {
self.blocks[block_b] &= !(u64::MAX >> (64 - bit_b));
}
// Fill in all the other blocks (much faster than one bit at a time).
for block in (block_a + 1)..block_b {
self.blocks[block] = 0;
}
}
}
#[inline]
fn get(&self, i: Size) -> bool {
let (block, bit) = Self::bit_index(i);
(self.blocks[block] & (1 << bit)) != 0
}
fn grow(&mut self, len: Size, amount: Size, new_state: bool) {
if amount.bytes() == 0 {
return;
}
let unused_trailing_bits =
u64::try_from(self.blocks.len()).unwrap() * Self::BLOCK_SIZE - len.bytes();
// If there's not enough capacity in the currently allocated blocks, allocate some more.
if amount.bytes() > unused_trailing_bits {
let additional_blocks = amount.bytes() / Self::BLOCK_SIZE + 1;
// We allocate the blocks to the correct value for the requested init state, so we won't
// have to manually set them with another write.
let block = if new_state { u64::MAX } else { 0 };
self.blocks
.extend(iter::repeat(block).take(usize::try_from(additional_blocks).unwrap()));
}
// New blocks have already been set here, so we only need to set the unused trailing bits,
// if any.
if unused_trailing_bits > 0 {
let in_bounds_tail = Size::from_bytes(unused_trailing_bits);
self.set_range_inbounds(len, len + in_bounds_tail, new_state); // `Size` operation
}
}
/// Returns the index of the first bit in `start..end` (end-exclusive) that is equal to is_init.
fn find_bit(&self, start: Size, end: Size, is_init: bool) -> Option<Size> {
/// A fast implementation of `find_bit`,
/// which skips over an entire block at a time if it's all 0s (resp. 1s),
/// and finds the first 1 (resp. 0) bit inside a block using `trailing_zeros` instead of a loop.
///
/// Note that all examples below are written with 8 (instead of 64) bit blocks for simplicity,
/// and with the least significant bit (and lowest block) first:
/// ```text
/// 00000000|00000000
/// ^ ^ ^ ^
/// index: 0 7 8 15
/// ```
/// Also, if not stated, assume that `is_init = true`, that is, we are searching for the first 1 bit.
fn find_bit_fast(
init_mask: &InitMaskMaterialized,
start: Size,
end: Size,
is_init: bool,
) -> Option<Size> {
/// Search one block, returning the index of the first bit equal to `is_init`.
fn search_block(
bits: Block,
block: usize,
start_bit: usize,
is_init: bool,
) -> Option<Size> {
// For the following examples, assume this function was called with:
// bits = 0b00111011
// start_bit = 3
// is_init = false
// Note that, for the examples in this function, the most significant bit is written first,
// which is backwards compared to the comments in `find_bit`/`find_bit_fast`.
// Invert bits so we're always looking for the first set bit.
// ! 0b00111011
// bits = 0b11000100
let bits = if is_init { bits } else { !bits };
// Mask off unused start bits.
// 0b11000100
// & 0b11111000
// bits = 0b11000000
let bits = bits & (!0 << start_bit);
// Find set bit, if any.
// bit = trailing_zeros(0b11000000)
// bit = 6
if bits == 0 {
None
} else {
let bit = bits.trailing_zeros();
Some(InitMaskMaterialized::size_from_bit_index(block, bit))
}
}
if start >= end {
return None;
}
// Convert `start` and `end` to block indexes and bit indexes within each block.
// We must convert `end` to an inclusive bound to handle block boundaries correctly.
//
// For example:
//
// (a) 00000000|00000000 (b) 00000000|
// ^~~~~~~~~~~^ ^~~~~~~~~^
// start end start end
//
// In both cases, the block index of `end` is 1.
// But we do want to search block 1 in (a), and we don't in (b).
//
// We subtract 1 from both end positions to make them inclusive:
//
// (a) 00000000|00000000 (b) 00000000|
// ^~~~~~~~~~^ ^~~~~~~^
// start end_inclusive start end_inclusive
//
// For (a), the block index of `end_inclusive` is 1, and for (b), it's 0.
// This provides the desired behavior of searching blocks 0 and 1 for (a),
// and searching only block 0 for (b).
// There is no concern of overflows since we checked for `start >= end` above.
let (start_block, start_bit) = InitMaskMaterialized::bit_index(start);
let end_inclusive = Size::from_bytes(end.bytes() - 1);
let (end_block_inclusive, _) = InitMaskMaterialized::bit_index(end_inclusive);
// Handle first block: need to skip `start_bit` bits.
//
// We need to handle the first block separately,
// because there may be bits earlier in the block that should be ignored,
// such as the bit marked (1) in this example:
//
// (1)
// -|------
// (c) 01000000|00000000|00000001
// ^~~~~~~~~~~~~~~~~~^
// start end
if let Some(i) =
search_block(init_mask.blocks[start_block], start_block, start_bit, is_init)
{
// If the range is less than a block, we may find a matching bit after `end`.
//
// For example, we shouldn't successfully find bit (2), because it's after `end`:
//
// (2)
// -------|
// (d) 00000001|00000000|00000001
// ^~~~~^
// start end
//
// An alternative would be to mask off end bits in the same way as we do for start bits,
// but performing this check afterwards is faster and simpler to implement.
if i < end {
return Some(i);
} else {
return None;
}
}
// Handle remaining blocks.
//
// We can skip over an entire block at once if it's all 0s (resp. 1s).
// The block marked (3) in this example is the first block that will be handled by this loop,
// and it will be skipped for that reason:
//
// (3)
// --------
// (e) 01000000|00000000|00000001
// ^~~~~~~~~~~~~~~~~~^
// start end
if start_block < end_block_inclusive {
// This loop is written in a specific way for performance.
// Notably: `..end_block_inclusive + 1` is used for an inclusive range instead of `..=end_block_inclusive`,
// and `.zip(start_block + 1..)` is used to track the index instead of `.enumerate().skip().take()`,
// because both alternatives result in significantly worse codegen.
// `end_block_inclusive + 1` is guaranteed not to wrap, because `end_block_inclusive <= end / BLOCK_SIZE`,
// and `BLOCK_SIZE` (the number of bits per block) will always be at least 8 (1 byte).
for (&bits, block) in init_mask.blocks[start_block + 1..end_block_inclusive + 1]
.iter()
.zip(start_block + 1..)
{
if let Some(i) = search_block(bits, block, 0, is_init) {
// If this is the last block, we may find a matching bit after `end`.
//
// For example, we shouldn't successfully find bit (4), because it's after `end`:
//
// (4)
// -------|
// (f) 00000001|00000000|00000001
// ^~~~~~~~~~~~~~~~~~^
// start end
//
// As above with example (d), we could handle the end block separately and mask off end bits,
// but unconditionally searching an entire block at once and performing this check afterwards
// is faster and much simpler to implement.
if i < end {
return Some(i);
} else {
return None;
}
}
}
}
None
}
#[cfg_attr(not(debug_assertions), allow(dead_code))]
fn find_bit_slow(
init_mask: &InitMaskMaterialized,
start: Size,
end: Size,
is_init: bool,
) -> Option<Size> {
(start..end).find(|&i| init_mask.get(i) == is_init)
}
let result = find_bit_fast(self, start, end, is_init);
debug_assert_eq!(
result,
find_bit_slow(self, start, end, is_init),
"optimized implementation of find_bit is wrong for start={start:?} end={end:?} is_init={is_init} init_mask={self:#?}"
);
result
}
}
/// A contiguous chunk of initialized or uninitialized memory.
pub enum InitChunk {
Init(Range<Size>),
Uninit(Range<Size>),
}
impl InitChunk {
#[inline]
pub fn is_init(&self) -> bool {
match self {
Self::Init(_) => true,
Self::Uninit(_) => false,
}
}
#[inline]
pub fn range(&self) -> Range<Size> {
match self {
Self::Init(r) => r.clone(),
Self::Uninit(r) => r.clone(),
}
}
}
impl InitMask {
/// Returns an iterator, yielding a range of byte indexes for each contiguous region
/// of initialized or uninitialized bytes inside the range `start..end` (end-exclusive).
///
/// The iterator guarantees the following:
/// - Chunks are nonempty.
/// - Chunks are adjacent (each range's start is equal to the previous range's end).
/// - Chunks span exactly `start..end` (the first starts at `start`, the last ends at `end`).
/// - Chunks alternate between [`InitChunk::Init`] and [`InitChunk::Uninit`].
#[inline]
pub fn range_as_init_chunks(&self, range: AllocRange) -> InitChunkIter<'_> {
let start = range.start;
let end = range.end();
assert!(end <= self.len);
let is_init = if start < end {
self.get(start)
} else {
// `start..end` is empty: there are no chunks, so use some arbitrary value
false
};
InitChunkIter { init_mask: self, is_init, start, end }
}
}
/// Yields [`InitChunk`]s. See [`InitMask::range_as_init_chunks`].
#[derive(Clone)]
pub struct InitChunkIter<'a> {
init_mask: &'a InitMask,
/// Whether the next chunk we will return is initialized.
/// If there are no more chunks, contains some arbitrary value.
is_init: bool,
/// The current byte index into `init_mask`.
start: Size,
/// The end byte index into `init_mask`.
end: Size,
}
impl<'a> Iterator for InitChunkIter<'a> {
type Item = InitChunk;
#[inline]
fn next(&mut self) -> Option<Self::Item> {
if self.start >= self.end {
return None;
}
let end_of_chunk = match self.init_mask.blocks {
InitMaskBlocks::Lazy { .. } => {
// If we're iterating over the chunks of lazy blocks, we just emit a single
// full-size chunk.
self.end
}
InitMaskBlocks::Materialized(ref blocks) => {
let end_of_chunk =
blocks.find_bit(self.start, self.end, !self.is_init).unwrap_or(self.end);
end_of_chunk
}
};
let range = self.start..end_of_chunk;
let ret =
Some(if self.is_init { InitChunk::Init(range) } else { InitChunk::Uninit(range) });
self.is_init = !self.is_init;
self.start = end_of_chunk;
ret
}
}
/// Run-length encoding of the uninit mask.
/// Used to copy parts of a mask multiple times to another allocation.
pub struct InitCopy {
/// Whether the first range is initialized.
initial: bool,
/// The lengths of ranges that are run-length encoded.
/// The initialization state of the ranges alternate starting with `initial`.
ranges: smallvec::SmallVec<[u64; 1]>,
}
impl InitCopy {
pub fn no_bytes_init(&self) -> bool {
// The `ranges` are run-length encoded and of alternating initialization state.
// So if `ranges.len() > 1` then the second block is an initialized range.
!self.initial && self.ranges.len() == 1
}
}
/// Transferring the initialization mask to other allocations.
impl InitMask {
/// Creates a run-length encoding of the initialization mask; panics if range is empty.
///
/// This is essentially a more space-efficient version of
/// `InitMask::range_as_init_chunks(...).collect::<Vec<_>>()`.
pub fn prepare_copy(&self, range: AllocRange) -> InitCopy {
// Since we are copying `size` bytes from `src` to `dest + i * size` (`for i in 0..repeat`),
// a naive initialization mask copying algorithm would repeatedly have to read the initialization mask from
// the source and write it to the destination. Even if we optimized the memory accesses,
// we'd be doing all of this `repeat` times.
// Therefore we precompute a compressed version of the initialization mask of the source value and
// then write it back `repeat` times without computing any more information from the source.
// A precomputed cache for ranges of initialized / uninitialized bits
// 0000010010001110 will become
// `[5, 1, 2, 1, 3, 3, 1]`,
// where each element toggles the state.
let mut ranges = smallvec::SmallVec::<[u64; 1]>::new();
let mut chunks = self.range_as_init_chunks(range).peekable();
let initial = chunks.peek().expect("range should be nonempty").is_init();
// Here we rely on `range_as_init_chunks` to yield alternating init/uninit chunks.
for chunk in chunks {
let len = chunk.range().end.bytes() - chunk.range().start.bytes();
ranges.push(len);
}
InitCopy { ranges, initial }
}
/// Applies multiple instances of the run-length encoding to the initialization mask.
pub fn apply_copy(&mut self, defined: InitCopy, range: AllocRange, repeat: u64) {
// An optimization where we can just overwrite an entire range of initialization bits if
// they are going to be uniformly `1` or `0`. If this happens to be a full-range overwrite,
// we won't need materialized blocks either.
if defined.ranges.len() <= 1 {
let start = range.start;
let end = range.start + range.size * repeat; // `Size` operations
self.set_range(AllocRange::from(start..end), defined.initial);
return;
}
// We're about to do one or more partial writes, so we ensure the blocks are materialized.
let blocks = self.materialize_blocks();
for mut j in 0..repeat {
j *= range.size.bytes();
j += range.start.bytes();
let mut cur = defined.initial;
for range in &defined.ranges {
let old_j = j;
j += range;
blocks.set_range_inbounds(Size::from_bytes(old_j), Size::from_bytes(j), cur);
cur = !cur;
}
}
}
}