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Move RWUTable to a separate module #79844

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Dec 10, 2020
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Move RWUTable to a separate module #79844

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151 changes: 5 additions & 146 deletions compiler/rustc_passes/src/liveness.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -105,6 +105,8 @@ use std::io;
use std::io::prelude::*;
use std::rc::Rc;

mod rwu_table;

rustc_index::newtype_index! {
pub struct Variable {
DEBUG_FORMAT = "v({})",
Expand Down Expand Up @@ -468,149 +470,6 @@ impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
// Actually we compute just a bit more than just liveness, but we use
// the same basic propagation framework in all cases.

#[derive(Clone, Copy)]
struct RWU {
reader: bool,
writer: bool,
used: bool,
}

/// Conceptually, this is like a `Vec<Vec<RWU>>`. But the number of
/// RWU`s can get very large, so it uses a more compact representation.
struct RWUTable {
/// Total number of live nodes.
live_nodes: usize,
/// Total number of variables.
vars: usize,

/// A compressed representation of `RWU`s.
///
/// Each word represents 2 different `RWU`s packed together. Each packed RWU
/// is stored in 4 bits: a reader bit, a writer bit, a used bit and a
/// padding bit.
///
/// The data for each live node is contiguous and starts at a word boundary,
/// so there might be an unused space left.
words: Vec<u8>,
/// Number of words per each live node.
live_node_words: usize,
}

impl RWUTable {
const RWU_READER: u8 = 0b0001;
const RWU_WRITER: u8 = 0b0010;
const RWU_USED: u8 = 0b0100;
const RWU_MASK: u8 = 0b1111;

/// Size of packed RWU in bits.
const RWU_BITS: usize = 4;
/// Size of a word in bits.
const WORD_BITS: usize = std::mem::size_of::<u8>() * 8;
/// Number of packed RWUs that fit into a single word.
const WORD_RWU_COUNT: usize = Self::WORD_BITS / Self::RWU_BITS;

fn new(live_nodes: usize, vars: usize) -> RWUTable {
let live_node_words = (vars + Self::WORD_RWU_COUNT - 1) / Self::WORD_RWU_COUNT;
Self { live_nodes, vars, live_node_words, words: vec![0u8; live_node_words * live_nodes] }
}

fn word_and_shift(&self, ln: LiveNode, var: Variable) -> (usize, u32) {
assert!(ln.index() < self.live_nodes);
assert!(var.index() < self.vars);

let var = var.index();
let word = var / Self::WORD_RWU_COUNT;
let shift = Self::RWU_BITS * (var % Self::WORD_RWU_COUNT);
(ln.index() * self.live_node_words + word, shift as u32)
}

fn pick2_rows_mut(&mut self, a: LiveNode, b: LiveNode) -> (&mut [u8], &mut [u8]) {
assert!(a.index() < self.live_nodes);
assert!(b.index() < self.live_nodes);
assert!(a != b);

let a_start = a.index() * self.live_node_words;
let b_start = b.index() * self.live_node_words;

unsafe {
let ptr = self.words.as_mut_ptr();
(
std::slice::from_raw_parts_mut(ptr.add(a_start), self.live_node_words),
std::slice::from_raw_parts_mut(ptr.add(b_start), self.live_node_words),
)
}
}

fn copy(&mut self, dst: LiveNode, src: LiveNode) {
if dst == src {
return;
}

let (dst_row, src_row) = self.pick2_rows_mut(dst, src);
dst_row.copy_from_slice(src_row);
}

/// Sets `dst` to the union of `dst` and `src`, returns true if `dst` was
/// changed.
fn union(&mut self, dst: LiveNode, src: LiveNode) -> bool {
if dst == src {
return false;
}

let mut changed = false;
let (dst_row, src_row) = self.pick2_rows_mut(dst, src);
for (dst_word, src_word) in dst_row.iter_mut().zip(src_row.iter()) {
let old = *dst_word;
let new = *dst_word | src_word;
*dst_word = new;
changed |= old != new;
}
changed
}

fn get_reader(&self, ln: LiveNode, var: Variable) -> bool {
let (word, shift) = self.word_and_shift(ln, var);
(self.words[word] >> shift) & Self::RWU_READER != 0
}

fn get_writer(&self, ln: LiveNode, var: Variable) -> bool {
let (word, shift) = self.word_and_shift(ln, var);
(self.words[word] >> shift) & Self::RWU_WRITER != 0
}

fn get_used(&self, ln: LiveNode, var: Variable) -> bool {
let (word, shift) = self.word_and_shift(ln, var);
(self.words[word] >> shift) & Self::RWU_USED != 0
}

fn get(&self, ln: LiveNode, var: Variable) -> RWU {
let (word, shift) = self.word_and_shift(ln, var);
let rwu_packed = self.words[word] >> shift;
RWU {
reader: rwu_packed & Self::RWU_READER != 0,
writer: rwu_packed & Self::RWU_WRITER != 0,
used: rwu_packed & Self::RWU_USED != 0,
}
}

fn set(&mut self, ln: LiveNode, var: Variable, rwu: RWU) {
let mut packed = 0;
if rwu.reader {
packed |= Self::RWU_READER;
}
if rwu.writer {
packed |= Self::RWU_WRITER;
}
if rwu.used {
packed |= Self::RWU_USED;
}

let (word, shift) = self.word_and_shift(ln, var);
let word = &mut self.words[word];
*word = (*word & !(Self::RWU_MASK << shift)) | (packed << shift)
}
}

const ACC_READ: u32 = 1;
const ACC_WRITE: u32 = 2;
const ACC_USE: u32 = 4;
Expand All @@ -623,7 +482,7 @@ struct Liveness<'a, 'tcx> {
upvars: Option<&'tcx FxIndexMap<hir::HirId, hir::Upvar>>,
closure_captures: Option<&'tcx FxIndexMap<hir::HirId, ty::UpvarId>>,
successors: IndexVec<LiveNode, Option<LiveNode>>,
rwu_table: RWUTable,
rwu_table: rwu_table::RWUTable,

/// A live node representing a point of execution before closure entry &
/// after closure exit. Used to calculate liveness of captured variables
Expand Down Expand Up @@ -661,7 +520,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
upvars,
closure_captures,
successors: IndexVec::from_elem_n(None, num_live_nodes),
rwu_table: RWUTable::new(num_live_nodes, num_vars),
rwu_table: rwu_table::RWUTable::new(num_live_nodes, num_vars),
closure_ln,
exit_ln,
break_ln: Default::default(),
Expand Down Expand Up @@ -802,7 +661,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
// this) so we just clear out all the data.
fn define(&mut self, writer: LiveNode, var: Variable) {
let used = self.rwu_table.get_used(writer, var);
self.rwu_table.set(writer, var, RWU { reader: false, writer: false, used });
self.rwu_table.set(writer, var, rwu_table::RWU { reader: false, writer: false, used });
debug!("{:?} defines {:?}: {}", writer, var, self.ln_str(writer));
}

Expand Down
144 changes: 144 additions & 0 deletions compiler/rustc_passes/src/liveness/rwu_table.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,144 @@
use crate::liveness::{LiveNode, Variable};

#[derive(Clone, Copy)]
pub(super) struct RWU {
pub(super) reader: bool,
pub(super) writer: bool,
pub(super) used: bool,
}

/// Conceptually, this is like a `Vec<Vec<RWU>>`. But the number of
/// RWU`s can get very large, so it uses a more compact representation.
pub(super) struct RWUTable {
/// Total number of live nodes.
live_nodes: usize,
/// Total number of variables.
vars: usize,

/// A compressed representation of `RWU`s.
///
/// Each word represents 2 different `RWU`s packed together. Each packed RWU
/// is stored in 4 bits: a reader bit, a writer bit, a used bit and a
/// padding bit.
///
/// The data for each live node is contiguous and starts at a word boundary,
/// so there might be an unused space left.
words: Vec<u8>,
/// Number of words per each live node.
live_node_words: usize,
}

impl RWUTable {
const RWU_READER: u8 = 0b0001;
const RWU_WRITER: u8 = 0b0010;
const RWU_USED: u8 = 0b0100;
const RWU_MASK: u8 = 0b1111;

/// Size of packed RWU in bits.
const RWU_BITS: usize = 4;
/// Size of a word in bits.
const WORD_BITS: usize = std::mem::size_of::<u8>() * 8;
/// Number of packed RWUs that fit into a single word.
const WORD_RWU_COUNT: usize = Self::WORD_BITS / Self::RWU_BITS;

pub(super) fn new(live_nodes: usize, vars: usize) -> RWUTable {
let live_node_words = (vars + Self::WORD_RWU_COUNT - 1) / Self::WORD_RWU_COUNT;
Self { live_nodes, vars, live_node_words, words: vec![0u8; live_node_words * live_nodes] }
}

fn word_and_shift(&self, ln: LiveNode, var: Variable) -> (usize, u32) {
assert!(ln.index() < self.live_nodes);
assert!(var.index() < self.vars);

let var = var.index();
let word = var / Self::WORD_RWU_COUNT;
let shift = Self::RWU_BITS * (var % Self::WORD_RWU_COUNT);
(ln.index() * self.live_node_words + word, shift as u32)
}

fn pick2_rows_mut(&mut self, a: LiveNode, b: LiveNode) -> (&mut [u8], &mut [u8]) {
assert!(a.index() < self.live_nodes);
assert!(b.index() < self.live_nodes);
assert!(a != b);

let a_start = a.index() * self.live_node_words;
let b_start = b.index() * self.live_node_words;

unsafe {
let ptr = self.words.as_mut_ptr();
(
std::slice::from_raw_parts_mut(ptr.add(a_start), self.live_node_words),
std::slice::from_raw_parts_mut(ptr.add(b_start), self.live_node_words),
)
}
}

pub(super) fn copy(&mut self, dst: LiveNode, src: LiveNode) {
if dst == src {
return;
}

let (dst_row, src_row) = self.pick2_rows_mut(dst, src);
dst_row.copy_from_slice(src_row);
}

/// Sets `dst` to the union of `dst` and `src`, returns true if `dst` was
/// changed.
pub(super) fn union(&mut self, dst: LiveNode, src: LiveNode) -> bool {
if dst == src {
return false;
}

let mut changed = false;
let (dst_row, src_row) = self.pick2_rows_mut(dst, src);
for (dst_word, src_word) in dst_row.iter_mut().zip(src_row.iter()) {
let old = *dst_word;
let new = *dst_word | src_word;
*dst_word = new;
changed |= old != new;
}
changed
}

pub(super) fn get_reader(&self, ln: LiveNode, var: Variable) -> bool {
let (word, shift) = self.word_and_shift(ln, var);
(self.words[word] >> shift) & Self::RWU_READER != 0
}

pub(super) fn get_writer(&self, ln: LiveNode, var: Variable) -> bool {
let (word, shift) = self.word_and_shift(ln, var);
(self.words[word] >> shift) & Self::RWU_WRITER != 0
}

pub(super) fn get_used(&self, ln: LiveNode, var: Variable) -> bool {
let (word, shift) = self.word_and_shift(ln, var);
(self.words[word] >> shift) & Self::RWU_USED != 0
}

pub(super) fn get(&self, ln: LiveNode, var: Variable) -> RWU {
let (word, shift) = self.word_and_shift(ln, var);
let rwu_packed = self.words[word] >> shift;
RWU {
reader: rwu_packed & Self::RWU_READER != 0,
writer: rwu_packed & Self::RWU_WRITER != 0,
used: rwu_packed & Self::RWU_USED != 0,
}
}

pub(super) fn set(&mut self, ln: LiveNode, var: Variable, rwu: RWU) {
let mut packed = 0;
if rwu.reader {
packed |= Self::RWU_READER;
}
if rwu.writer {
packed |= Self::RWU_WRITER;
}
if rwu.used {
packed |= Self::RWU_USED;
}

let (word, shift) = self.word_and_shift(ln, var);
let word = &mut self.words[word];
*word = (*word & !(Self::RWU_MASK << shift)) | (packed << shift)
}
}