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WIP.

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This commit is contained in:
Chris Fallin 2021-12-19 11:45:13 -08:00
parent 973ca3833a
commit 3e67394ab1
2 changed files with 2 additions and 350 deletions
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4
src/backend/mod.rs
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@ -1,6 +1,6 @@
//! Backend: IR to Wasm.
mod stackify;
pub(crate) use stackify::*;
mod schedule;
pub(crate) use schedule::*;
mod locations;
pub(crate) use locations::*;

348
src/backend/stackify.rs
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@ -1,348 +0,0 @@
//! Stackifier-like algorithm to recover (or create) structured
//! control flow out of a CFG.
use crate::{cfg::CFGInfo, ir::*};
#[derive(Clone, Debug)]
pub enum Shape {
Block { head: BlockId, children: Vec<Shape> },
Loop { head: BlockId, children: Vec<Shape> },
Leaf { block: BlockId },
None,
}
/// Index in RPO.
type OrderedBlockId = usize;
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
enum Region {
/// Forward-branch region. Extends from end (just prior to
/// terminator) of first block to just before second block. Can be
/// extended earlier, prior to the beginning, if needed.
Forward(OrderedBlockId, OrderedBlockId),
/// Backward-branch region. Extends from start of first block to
/// end (after terminator) of second block. Can be extended past
/// the end if needed.
Backward(OrderedBlockId, OrderedBlockId),
// TODO: support irreducible CFGs by adding a `BackwardDispatch`
// region kind whose start is adjusted back to the first loop
// block. BackwardDispatch is contagious, i.e. converts adjacent
// Backward region records to BackwardDispatch.
}
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
enum BlockPoint {
Start,
End,
}
impl Shape {
/// Finds the next shape in the sequence, returning the shape and
/// the remaining starting block ID / region list.
fn get_one_shape<'a>(
start: OrderedBlockId,
order: &[BlockId],
regions: &'a [Region],
) -> Option<(Shape, OrderedBlockId, &'a [Region])> {
log::trace!("get_one_shape: start {} regions {:?}", start, regions);
if start >= order.len() {
None
} else if regions.is_empty() || start < regions[0].start() {
log::trace!(" -> leaf");
Some((
Shape::Leaf {
block: order[start],
},
start + 1,
&regions,
))
} else {
assert_eq!(start, regions[0].start());
let end = regions[0].end();
let region_end = regions
.iter()
.position(|region| region.start() >= end)
.unwrap_or(regions.len());
let subregions = &regions[1..region_end];
let (children, next_start) = Self::get_shapes(start, end, order, subregions);
let shape = if let Region::Forward(..) = &regions[0] {
Shape::Block {
head: order[start],
children,
}
} else {
Shape::Loop {
head: order[start],
children,
}
};
Some((shape, next_start, &regions[region_end..]))
}
}
fn get_shapes<'a>(
start: OrderedBlockId,
end: OrderedBlockId,
order: &[BlockId],
mut regions: &'a [Region],
) -> (Vec<Shape>, OrderedBlockId) {
log::trace!(
"get_shapes: start {} end {} regions {:?}",
start,
end,
regions
);
let mut shapes = vec![];
let mut block = start;
while block < end {
log::trace!("get_shapes: now at {}, regions {:?}", block, regions);
let (shape, next_start, next_regions) =
Self::get_one_shape(block, order, regions).unwrap();
shapes.push(shape);
block = next_start;
log::trace!(" -> next_regions = {:?}", next_regions);
regions = next_regions;
}
log::trace!("get_shapes: returning {:?}", shapes);
(shapes, block)
}
}
impl Region {
fn start(&self) -> OrderedBlockId {
match self {
&Region::Forward(a, _) | &Region::Backward(a, _) => a,
}
}
fn end(&self) -> OrderedBlockId {
match self {
&Region::Forward(_, b) | &Region::Backward(_, b) => b,
}
}
fn contains(&self, other: &Region) -> bool {
self.start() <= other.start() && self.end() >= other.end()
}
fn contains_endpoint(&self, pt: OrderedBlockId) -> bool {
self.start() <= pt && pt <= self.end()
}
fn overlaps(&self, other: &Region) -> bool {
self.end() > other.start() && other.end() > self.start()
}
fn is_forward(&self) -> bool {
match self {
&Region::Forward(..) => true,
_ => false,
}
}
fn is_backward(&self) -> bool {
match self {
&Region::Backward(..) => true,
_ => false,
}
}
}
impl Shape {
pub fn compute(f: &FunctionBody, cfg: &CFGInfo) -> Self {
// Process all non-contiguous edges in RPO block order. For
// forward and backward edges, emit Regions.
log::trace!("f = {:?}", f);
log::trace!("cfg = {:?}", cfg);
let order = cfg.rpo();
log::trace!("rpo = {:?}", order);
assert_eq!(order[0], 0); // Entry block should come first.
// Compute nest of loop headers per block.
// If a given block is a loop header, then
// `loop_end[block_rpo_index]` will be
// Some(last_loop_body_rpo_index)`.
let mut loop_header_to_end: Vec<Option<OrderedBlockId>> = vec![None; order.len()];
// Record forward edges as tuples of RPO-block-indices for
// processing below.
let mut forward_edges: Vec<(OrderedBlockId, OrderedBlockId)> = vec![];
for (block_pos, &block) in order.iter().enumerate() {
for &succ in cfg.succs(block) {
let succ_pos = cfg
.rpo_pos(succ)
.expect("if block is reachable then succ should be too");
if succ_pos < block_pos {
let end = loop_header_to_end[succ_pos].unwrap_or(block_pos);
let end = std::cmp::max(end, block_pos);
log::trace!("loop backedge: header RPO {} latch RPO {} (header block {} latch block {})",
succ_pos, block_pos, order[succ_pos], order[block_pos]);
loop_header_to_end[succ_pos] = Some(end);
} else if succ_pos > block_pos + 1 {
log::trace!(
"forward edge: to RPO {} from RPO {} (to block {} from block {})",
succ_pos,
block_pos,
order[succ_pos],
order[block_pos]
);
forward_edges.push((block_pos, succ_pos));
}
}
}
// Extend loop ends to fully nest subloops. Also build loop
// nest info for each block.
let mut stack = vec![];
let mut loop_nest = vec![];
for block in 0..order.len() {
while let Some(&(_first, last)) = stack.last() {
if block > last {
stack.pop();
} else {
break;
}
}
if let Some(end) = loop_header_to_end[block] {
stack.push((block, end));
}
for &mut (start, ref mut end) in &mut stack {
if block > *end {
loop_header_to_end[start] = Some(block);
*end = block;
}
}
loop_nest.push(stack.clone());
}
log::trace!("loop_header_to_end = {:?}", loop_header_to_end);
log::trace!("loop_nest = {:?}", loop_nest);
// Look for irreducible edges.
for &(from, to) in &forward_edges {
let from_loop_nest = &loop_nest[from];
let to_loop_nest = &loop_nest[to];
for i in 0..to_loop_nest.len() {
if i >= from_loop_nest.len() || from_loop_nest[i] != to_loop_nest[i] {
// Entering a loop; the `to` must be the header.
let header_block = to_loop_nest[i].0;
if to != header_block {
panic!(
"Irreducible edge from RPO {} block {} to RPO {} block {}: jumps into loop with header RPO {} block {}",
from, order[from], to, order[to], header_block, order[header_block]
);
}
}
}
}
log::trace!("forward_edges = {:?}", forward_edges);
// Process forward edges: add "block-start count" to
// containing scopes, and mark block-end points.
let mut block_ends = vec![false; order.len()];
let mut block_end_to_start = vec![None; order.len()];
let mut block_starts = vec![vec![]; order.len()];
for &(from, to) in &forward_edges {
if !block_ends[to] {
block_ends[to] = true;
}
let start = block_end_to_start[to].unwrap_or(from);
let start = std::cmp::min(start, from);
block_end_to_start[to] = Some(start);
}
for block in 0..order.len() {
if let Some(start) = block_end_to_start[block] {
// Examine loop nest of endpoint. Must be a prefix of
// loop nest of startpoint if control flow is
// reducible. If start is inside any additional loops,
// we need to move the startpoint back to the start of
// those loops.
let start_loopnest = &loop_nest[start];
let end_loopnest = &loop_nest[block];
// Find common prefix of loopnests at start and end
// points, and put a block start at the top of that
// loop. In other words, we put the block around the
// innermost loop that surrounds the whole forward
// edge (or around the whole body if not). As long as
// control flow is reducible, this will not result in
// an edge into a loop.
let start_idx = start_loopnest
.iter()
.zip(end_loopnest.iter())
.take_while(|(a, b)| a == b)
.count();
let start = if start_idx > 0 {
start_loopnest[start_idx - 1].0
} else {
0
};
block_starts[start].push(block);
}
}
log::trace!("block_starts = {:?}", block_starts);
log::trace!("block_end_to_start = {:?}", block_end_to_start);
// We can't have a loop header at block 0; otherwise there is
// no way to express a forward edge outside the outermost loop
// nest.
assert!(loop_header_to_end[0].is_none());
// Now generate the region list which we use to produce the "shape".
let mut regions = vec![];
for block in 0..order.len() {
if let Some(loop_end) = loop_header_to_end[block] {
regions.push(Region::Backward(block, loop_end));
}
for &block_end in block_starts[block].iter().rev() {
regions.push(Region::Forward(block, block_end));
}
}
log::trace!("after stackifying: {:?}", regions);
// Ensure the regions properly nest.
#[cfg(debug_assertions)]
{
let mut stack: Vec<Region> = vec![];
for region in &regions {
while let Some(top) = stack.last() {
if top.contains(region) {
stack.push(region.clone());
break;
} else if region.overlaps(top) {
panic!(
"Non-nested region: {:?} (overlaps {:?}) in nest: {:?} (overall: {:?})",
region, top, stack, regions
);
} else {
stack.pop();
}
}
if stack.is_empty() {
stack.push(region.clone());
}
}
}
// Build the final shape description.
let (shapes, _) = Shape::get_shapes(0, order.len(), &order[..], &regions[..]);
let root = if shapes.len() == 1 {
shapes.into_iter().next().unwrap()
} else {
Shape::Block {
head: 0,
children: shapes,
}
};
log::trace!("shape: {:?}", root);
root
}
}