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Various cleanups to dest prop #105683

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Dec 15, 2022
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Various cleanups to dest prop #105683

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171 changes: 88 additions & 83 deletions compiler/rustc_mir_transform/src/dest_prop.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -132,15 +132,12 @@ use std::collections::hash_map::{Entry, OccupiedEntry};
use crate::MirPass;
use rustc_data_structures::fx::FxHashMap;
use rustc_index::bit_set::BitSet;
use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor};
use rustc_middle::mir::{dump_mir, PassWhere};
use rustc_middle::mir::{
traversal, BasicBlock, Body, InlineAsmOperand, Local, LocalKind, Location, Operand, Place,
Rvalue, Statement, StatementKind, TerminatorKind,
};
use rustc_middle::mir::{
visit::{MutVisitor, PlaceContext, Visitor},
ProjectionElem,
};
use rustc_middle::ty::TyCtxt;
use rustc_mir_dataflow::impls::MaybeLiveLocals;
use rustc_mir_dataflow::{Analysis, ResultsCursor};
Expand Down Expand Up @@ -359,40 +356,45 @@ struct FilterInformation<'a, 'body, 'alloc, 'tcx> {
// through these methods, and not directly.
impl<'alloc> Candidates<'alloc> {
/// Just `Vec::retain`, but the condition is inverted and we add debugging output
fn vec_remove_debug(
fn vec_filter_candidates(
src: Local,
v: &mut Vec<Local>,
mut f: impl FnMut(Local) -> bool,
mut f: impl FnMut(Local) -> CandidateFilter,
at: Location,
) {
v.retain(|dest| {
let remove = f(*dest);
if remove {
if remove == CandidateFilter::Remove {
trace!("eliminating {:?} => {:?} due to conflict at {:?}", src, dest, at);
}
!remove
remove == CandidateFilter::Keep
});
}

/// `vec_remove_debug` but for an `Entry`
fn entry_remove(
/// `vec_filter_candidates` but for an `Entry`
fn entry_filter_candidates(
mut entry: OccupiedEntry<'_, Local, Vec<Local>>,
p: Local,
f: impl FnMut(Local) -> bool,
f: impl FnMut(Local) -> CandidateFilter,
at: Location,
) {
let candidates = entry.get_mut();
Self::vec_remove_debug(p, candidates, f, at);
Self::vec_filter_candidates(p, candidates, f, at);
if candidates.len() == 0 {
entry.remove();
}
}

/// Removes all candidates `(p, q)` or `(q, p)` where `p` is the indicated local and `f(q)` is true.
fn remove_candidates_if(&mut self, p: Local, mut f: impl FnMut(Local) -> bool, at: Location) {
/// For all candidates `(p, q)` or `(q, p)` removes the candidate if `f(q)` says to do so
fn filter_candidates_by(
&mut self,
p: Local,
mut f: impl FnMut(Local) -> CandidateFilter,
at: Location,
) {
// Cover the cases where `p` appears as a `src`
if let Entry::Occupied(entry) = self.c.entry(p) {
Self::entry_remove(entry, p, &mut f, at);
Self::entry_filter_candidates(entry, p, &mut f, at);
}
// And the cases where `p` appears as a `dest`
let Some(srcs) = self.reverse.get_mut(&p) else {
Expand All @@ -401,18 +403,31 @@ impl<'alloc> Candidates<'alloc> {
// We use `retain` here to remove the elements from the reverse set if we've removed the
// matching candidate in the forward set.
srcs.retain(|src| {
if !f(*src) {
if f(*src) == CandidateFilter::Keep {
return true;
}
let Entry::Occupied(entry) = self.c.entry(*src) else {
return false;
};
Self::entry_remove(entry, *src, |dest| dest == p, at);
Self::entry_filter_candidates(
entry,
*src,
|dest| {
if dest == p { CandidateFilter::Remove } else { CandidateFilter::Keep }
},
at,
);
false
});
}
}

#[derive(Copy, Clone, PartialEq, Eq)]
enum CandidateFilter {
Keep,
Remove,
}

impl<'a, 'body, 'alloc, 'tcx> FilterInformation<'a, 'body, 'alloc, 'tcx> {
/// Filters the set of candidates to remove those that conflict.
///
Expand Down Expand Up @@ -460,7 +475,7 @@ impl<'a, 'body, 'alloc, 'tcx> FilterInformation<'a, 'body, 'alloc, 'tcx> {
for (i, statement) in data.statements.iter().enumerate().rev() {
self.at = Location { block, statement_index: i };
self.live.seek_after_primary_effect(self.at);
self.get_statement_write_info(&statement.kind);
self.write_info.for_statement(&statement.kind, self.body);
self.apply_conflicts();
}
}
Expand All @@ -469,80 +484,59 @@ impl<'a, 'body, 'alloc, 'tcx> FilterInformation<'a, 'body, 'alloc, 'tcx> {
fn apply_conflicts(&mut self) {
let writes = &self.write_info.writes;
for p in writes {
self.candidates.remove_candidates_if(
let other_skip = self.write_info.skip_pair.and_then(|(a, b)| {
if a == *p {
Some(b)
} else if b == *p {
Some(a)
} else {
None
}
});
self.candidates.filter_candidates_by(
*p,
// It is possible that a local may be live for less than the
// duration of a statement This happens in the case of function
// calls or inline asm. Because of this, we also mark locals as
// conflicting when both of them are written to in the same
// statement.
|q| self.live.contains(q) || writes.contains(&q),
|q| {
if Some(q) == other_skip {
return CandidateFilter::Keep;
}
// It is possible that a local may be live for less than the
// duration of a statement This happens in the case of function
// calls or inline asm. Because of this, we also mark locals as
// conflicting when both of them are written to in the same
// statement.
if self.live.contains(q) || writes.contains(&q) {
CandidateFilter::Remove
} else {
CandidateFilter::Keep
}
},
self.at,
);
}
}

/// Gets the write info for the `statement`.
fn get_statement_write_info(&mut self, statement: &StatementKind<'tcx>) {
self.write_info.writes.clear();
match statement {
StatementKind::Assign(box (lhs, rhs)) => match rhs {
Rvalue::Use(op) => {
if !lhs.is_indirect() {
self.get_assign_use_write_info(*lhs, op);
return;
}
}
_ => (),
},
_ => (),
}

self.write_info.for_statement(statement);
}

fn get_assign_use_write_info(&mut self, lhs: Place<'tcx>, rhs: &Operand<'tcx>) {
// We register the writes for the operand unconditionally
self.write_info.add_operand(rhs);
// However, we cannot do the same thing for the `lhs` as that would always block the
// optimization. Instead, we consider removing candidates manually.
let Some(rhs) = rhs.place() else {
self.write_info.add_place(lhs);
return;
};
// Find out which candidate pair we should skip, if any
let Some((src, dest)) = places_to_candidate_pair(lhs, rhs, self.body) else {
self.write_info.add_place(lhs);
return;
};
self.candidates.remove_candidates_if(
lhs.local,
|other| {
// Check if this is the candidate pair that should not be removed
if (lhs.local == src && other == dest) || (lhs.local == dest && other == src) {
return false;
}
// Otherwise, do the "standard" thing
self.live.contains(other)
},
self.at,
)
}
}

/// Describes where a statement/terminator writes to
#[derive(Default, Debug)]
struct WriteInfo {
writes: Vec<Local>,
/// If this pair of locals is a candidate pair, completely skip processing it during this
/// statement. All other candidates are unaffected.
skip_pair: Option<(Local, Local)>,
}

impl WriteInfo {
fn for_statement<'tcx>(&mut self, statement: &StatementKind<'tcx>) {
fn for_statement<'tcx>(&mut self, statement: &StatementKind<'tcx>, body: &Body<'tcx>) {
self.reset();
match statement {
StatementKind::Assign(box (lhs, rhs)) => {
self.add_place(*lhs);
match rhs {
Rvalue::Use(op) | Rvalue::Repeat(op, _) => {
Rvalue::Use(op) => {
self.add_operand(op);
self.consider_skipping_for_assign_use(*lhs, op, body);
}
Rvalue::Repeat(op, _) => {
self.add_operand(op);
}
Rvalue::Cast(_, op, _)
Expand Down Expand Up @@ -586,8 +580,22 @@ impl WriteInfo {
}
}

fn consider_skipping_for_assign_use<'tcx>(
&mut self,
lhs: Place<'tcx>,
rhs: &Operand<'tcx>,
body: &Body<'tcx>,
) {
let Some(rhs) = rhs.place() else {
return
};
if let Some(pair) = places_to_candidate_pair(lhs, rhs, body) {
self.skip_pair = Some(pair);
}
}

fn for_terminator<'tcx>(&mut self, terminator: &TerminatorKind<'tcx>) {
self.writes.clear();
self.reset();
match terminator {
TerminatorKind::SwitchInt { discr: op, .. }
| TerminatorKind::Assert { cond: op, .. } => {
Expand Down Expand Up @@ -657,15 +665,16 @@ impl WriteInfo {
Operand::Copy(_) | Operand::Constant(_) => (),
}
}

fn reset(&mut self) {
self.writes.clear();
self.skip_pair = None;
}
}

/////////////////////////////////////////////////////
// Candidate accumulation

fn is_constant<'tcx>(place: Place<'tcx>) -> bool {
place.projection.iter().all(|p| !matches!(p, ProjectionElem::Deref | ProjectionElem::Index(_)))
}

/// If the pair of places is being considered for merging, returns the candidate which would be
/// merged in order to accomplish this.
///
Expand Down Expand Up @@ -741,10 +750,6 @@ impl<'tcx> Visitor<'tcx> for FindAssignments<'_, '_, 'tcx> {
Rvalue::Use(Operand::Copy(rhs) | Operand::Move(rhs)),
)) = &statement.kind
{
if !is_constant(*lhs) || !is_constant(*rhs) {
return;
}

let Some((src, dest)) = places_to_candidate_pair(*lhs, *rhs, self.body) else {
return;
};
Expand Down