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Erik Desjardins 2021-08-14 13:54:35 -04:00
parent 3c2b706da6
commit 5bef23d0fa
1 changed files with 118 additions and 18 deletions
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136
compiler/rustc_middle/src/mir/interpret/allocation.rs
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@ -548,6 +548,9 @@ impl InitMaskCompressed {
/// Transferring the initialization mask to other allocations.
impl<Tag, Extra> Allocation<Tag, Extra> {
/// Creates a run-length encoding of the initialization mask.
///
/// This is essentially a more space-efficient version of
/// `InitMask::range_as_init_chunks(...).collect::<Vec<_>>()`.
pub fn compress_uninit_range(&self, range: AllocRange) -> InitMaskCompressed {
// 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
@ -723,6 +726,12 @@ 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, start: Size, end: Size) -> InitChunkIter<'_> {
InitChunkIter::new(self, start, end)
@ -839,7 +848,7 @@ impl InitChunk {
/// Yields [`InitChunk`]s. See [`InitMask::range_as_init_chunks`].
pub struct InitChunkIter<'a> {
init_mask: &'a InitMask,
/// Whether the last chunk was initialized.
/// Whether the next chunk we will return is initialized.
is_init: bool,
/// The current byte index into `init_mask`.
start: Size,
@ -884,18 +893,45 @@ impl<'a> Iterator for InitChunkIter<'a> {
/// Returns the index of the first bit in `start..end` (end-exclusive) that is equal to is_init.
fn find_bit(init_mask: &InitMask, 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:
///
/// 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: &InitMask, 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> {
// invert bits so we're always looking for the first set bit
// For the following examples, assume this function was called with:
// bits = 11011100
// start_bit = 3
// is_init = false
// Note again that the least significant bit is written first,
// which is backwards compared to how we normally write numbers.
// Invert bits so we're always looking for the first set bit.
// ! 11011100
// bits = 00100011
let bits = if is_init { bits } else { !bits };
// mask off unused start bits
// Mask off unused start bits.
// 00100011
// & 00011111
// bits = 00000011
let bits = bits & (!0 << start_bit);
// find set bit, if any
// Find set bit, if any.
// bit = trailing_zeros(00000011)
// bit = 6
if bits == 0 {
None
} else {
@ -908,31 +944,83 @@ fn find_bit(init_mask: &InitMask, start: Size, end: Size, is_init: bool) -> Opti
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).
//
// If 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).
let (start_block, start_bit) = bit_index(start);
let (end_block, end_bit) = bit_index(end);
let end_inclusive = Size::from_bytes(end.bytes() - 1);
let (end_block_inclusive, _) = bit_index(end_inclusive);
// handle first block: need to skip `start_bit` bits
// 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 i < end {
return Some(i);
} else {
// if the range is less than a block, we may find a matching bit after `end`
// 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.
return None;
}
}
let one_block_past_the_end = if end_bit > 0 {
// if `end_bit` > 0, then the range overlaps `end_block`
end_block + 1
} else {
end_block
};
// handle remaining blocks
if start_block < one_block_past_the_end {
for (&bits, block) in init_mask.blocks[start_block + 1..one_block_past_the_end]
// 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..)
{
@ -940,7 +1028,19 @@ fn find_bit(init_mask: &InitMask, start: Size, end: Size, is_init: bool) -> Opti
if i < end {
return Some(i);
} else {
// if this is the last block, we may find a matching bit after `end`
// 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.
return None;
}
}