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//! Propagates assignment destinations backwards in the CFG to eliminate redundant assignments.
//!
//! # Motivation
//!
//! MIR building can insert a lot of redundant copies, and Rust code in general often tends to move
//! values around a lot. The result is a lot of assignments of the form `dest = {move} src;` in MIR.
//! MIR building for constants in particular tends to create additional locals that are only used
//! inside a single block to shuffle a value around unnecessarily.
//!
//! LLVM by itself is not good enough at eliminating these redundant copies (eg. see
//! <https://github.com/rust-lang/rust/issues/32966>), so this leaves some performance on the table
//! that we can regain by implementing an optimization for removing these assign statements in rustc
//! itself. When this optimization runs fast enough, it can also speed up the constant evaluation
//! and code generation phases of rustc due to the reduced number of statements and locals.
//!
//! # The Optimization
//!
//! Conceptually, this optimization is "destination propagation". It is similar to the Named Return
//! Value Optimization, or NRVO, known from the C++ world, except that it isn't limited to return
//! values or the return place `_0`. On a very high level, independent of the actual implementation
//! details, it does the following:
//!
//! 1) Identify `dest = src;` statements that can be soundly eliminated.
//! 2) Replace all mentions of `src` with `dest` ("unifying" them and propagating the destination
//! backwards).
//! 3) Delete the `dest = src;` statement (by making it a `nop`).
//!
//! Step 1) is by far the hardest, so it is explained in more detail below.
//!
//! ## Soundness
//!
//! Given an `Assign` statement `dest = src;`, where `dest` is a `Place` and `src` is an `Rvalue`,
//! there are a few requirements that must hold for the optimization to be sound:
//!
//! * `dest` must not contain any *indirection* through a pointer. It must access part of the base
//! local. Otherwise it might point to arbitrary memory that is hard to track.
//!
//! It must also not contain any indexing projections, since those take an arbitrary `Local` as
//! the index, and that local might only be initialized shortly before `dest` is used.
//!
//! * `src` must be a bare `Local` without any indirections or field projections (FIXME: Is this a
//! fundamental restriction or just current impl state?). It can be copied or moved by the
//! assignment.
//!
//! * The `dest` and `src` locals must never be [*live*][liveness] at the same time. If they are, it
//! means that they both hold a (potentially different) value that is needed by a future use of
//! the locals. Unifying them would overwrite one of the values.
//!
//! Note that computing liveness of locals that have had their address taken is more difficult:
//! Short of doing full escape analysis on the address/pointer/reference, the pass would need to
//! assume that any operation that can potentially involve opaque user code (such as function
//! calls, destructors, and inline assembly) may access any local that had its address taken
//! before that point.
//!
//! Here, the first two conditions are simple structural requirements on the `Assign` statements
//! that can be trivially checked. The liveness requirement however is more difficult and costly to
//! check.
//!
//! ## Previous Work
//!
//! A [previous attempt] at implementing an optimization like this turned out to be a significant
//! regression in compiler performance. Fixing the regressions introduced a lot of undesirable
//! complexity to the implementation.
//!
//! A [subsequent approach] tried to avoid the costly computation by limiting itself to acyclic
//! CFGs, but still turned out to be far too costly to run due to suboptimal performance within
//! individual basic blocks, requiring a walk across the entire block for every assignment found
//! within the block. For the `tuple-stress` benchmark, which has 458745 statements in a single
//! block, this proved to be far too costly.
//!
//! Since the first attempt at this, the compiler has improved dramatically, and new analysis
//! frameworks have been added that should make this approach viable without requiring a limited
//! approach that only works for some classes of CFGs:
//! - rustc now has a powerful dataflow analysis framework that can handle forwards and backwards
//! analyses efficiently.
//! - Layout optimizations for generators have been added to improve code generation for
//! async/await, which are very similar in spirit to what this optimization does. Both walk the
//! MIR and record conflicting uses of locals in a `BitMatrix`.
//!
//! Also, rustc now has a simple NRVO pass (see `nrvo.rs`), which handles a subset of the cases that
//! this destination propagation pass handles, proving that similar optimizations can be performed
//! on MIR.
//!
//! ## Pre/Post Optimization
//!
//! It is recommended to run `SimplifyCfg` and then `SimplifyLocals` some time after this pass, as
//! it replaces the eliminated assign statements with `nop`s and leaves unused locals behind.
//!
//! [liveness]: https://en.wikipedia.org/wiki/Live_variable_analysis
//! [previous attempt]: https://github.com/rust-lang/rust/pull/47954
//! [subsequent approach]: https://github.com/rust-lang/rust/pull/71003
use crate::MirPass;
use itertools::Itertools;
use rustc_data_structures::unify::{InPlaceUnificationTable, UnifyKey};
use rustc_index::{
bit_set::{BitMatrix, BitSet},
vec::IndexVec,
};
use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor};
use rustc_middle::mir::{dump_mir, PassWhere};
use rustc_middle::mir::{
traversal, Body, InlineAsmOperand, Local, LocalKind, Location, Operand, Place, PlaceElem,
Rvalue, Statement, StatementKind, Terminator, TerminatorKind,
};
use rustc_middle::ty::TyCtxt;
use rustc_mir_dataflow::impls::{borrowed_locals, MaybeInitializedLocals, MaybeLiveLocals};
use rustc_mir_dataflow::Analysis;
// Empirical measurements have resulted in some observations:
// - Running on a body with a single block and 500 locals takes barely any time
// - Running on a body with ~400 blocks and ~300 relevant locals takes "too long"
// ...so we just limit both to somewhat reasonable-ish looking values.
const MAX_LOCALS: usize = 500;
const MAX_BLOCKS: usize = 250;
pub struct DestinationPropagation;
impl<'tcx> MirPass<'tcx> for DestinationPropagation {
fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
// FIXME(#79191, #82678): This is unsound.
//
// Only run at mir-opt-level=3 or higher for now (we don't fix up debuginfo and remove
// storage statements at the moment).
sess.opts.unstable_opts.unsound_mir_opts && sess.mir_opt_level() >= 3
}
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
let def_id = body.source.def_id();
let candidates = find_candidates(body);
if candidates.is_empty() {
debug!("{:?}: no dest prop candidates, done", def_id);
return;
}
// Collect all locals we care about. We only compute conflicts for these to save time.
let mut relevant_locals = BitSet::new_empty(body.local_decls.len());
for CandidateAssignment { dest, src, loc: _ } in &candidates {
relevant_locals.insert(dest.local);
relevant_locals.insert(*src);
}
// This pass unfortunately has `O(l² * s)` performance, where `l` is the number of locals
// and `s` is the number of statements and terminators in the function.
// To prevent blowing up compile times too much, we bail out when there are too many locals.
let relevant = relevant_locals.count();
debug!(
"{:?}: {} locals ({} relevant), {} blocks",
def_id,
body.local_decls.len(),
relevant,
body.basic_blocks.len()
);
if relevant > MAX_LOCALS {
warn!(
"too many candidate locals in {:?} ({}, max is {}), not optimizing",
def_id, relevant, MAX_LOCALS
);
return;
}
if body.basic_blocks.len() > MAX_BLOCKS {
warn!(
"too many blocks in {:?} ({}, max is {}), not optimizing",
def_id,
body.basic_blocks.len(),
MAX_BLOCKS
);
return;
}
let mut conflicts = Conflicts::build(tcx, body, &relevant_locals);
let mut replacements = Replacements::new(body.local_decls.len());
for candidate @ CandidateAssignment { dest, src, loc } in candidates {
// Merge locals that don't conflict.
if !conflicts.can_unify(dest.local, src) {
debug!("at assignment {:?}, conflict {:?} vs. {:?}", loc, dest.local, src);
continue;
}
if replacements.for_src(candidate.src).is_some() {
debug!("src {:?} already has replacement", candidate.src);
continue;
}
if !tcx.consider_optimizing(|| {
format!("DestinationPropagation {:?} {:?}", def_id, candidate)
}) {
break;
}
replacements.push(candidate);
conflicts.unify(candidate.src, candidate.dest.local);
}
replacements.flatten(tcx);
debug!("replacements {:?}", replacements.map);
Replacer { tcx, replacements, place_elem_cache: Vec::new() }.visit_body(body);
// FIXME fix debug info
}
}
#[derive(Debug, Eq, PartialEq, Copy, Clone)]
struct UnifyLocal(Local);
impl From<Local> for UnifyLocal {
fn from(l: Local) -> Self {
Self(l)
}
}
impl UnifyKey for UnifyLocal {
type Value = ();
#[inline]
fn index(&self) -> u32 {
self.0.as_u32()
}
#[inline]
fn from_index(u: u32) -> Self {
Self(Local::from_u32(u))
}
fn tag() -> &'static str {
"UnifyLocal"
}
}
struct Replacements<'tcx> {
/// Maps locals to their replacement.
map: IndexVec<Local, Option<Place<'tcx>>>,
/// Whose locals' live ranges to kill.
kill: BitSet<Local>,
}
impl<'tcx> Replacements<'tcx> {
fn new(locals: usize) -> Self {
Self { map: IndexVec::from_elem_n(None, locals), kill: BitSet::new_empty(locals) }
}
fn push(&mut self, candidate: CandidateAssignment<'tcx>) {
trace!("Replacements::push({:?})", candidate);
let entry = &mut self.map[candidate.src];
assert!(entry.is_none());
*entry = Some(candidate.dest);
self.kill.insert(candidate.src);
self.kill.insert(candidate.dest.local);
}
/// Applies the stored replacements to all replacements, until no replacements would result in
/// locals that need further replacements when applied.
fn flatten(&mut self, tcx: TyCtxt<'tcx>) {
// Note: This assumes that there are no cycles in the replacements, which is enforced via
// `self.unified_locals`. Otherwise this can cause an infinite loop.
for local in self.map.indices() {
if let Some(replacement) = self.map[local] {
// Substitute the base local of `replacement` until fixpoint.
let mut base = replacement.local;
let mut reversed_projection_slices = Vec::with_capacity(1);
while let Some(replacement_for_replacement) = self.map[base] {
base = replacement_for_replacement.local;
reversed_projection_slices.push(replacement_for_replacement.projection);
}
let projection: Vec<_> = reversed_projection_slices
.iter()
.rev()
.flat_map(|projs| projs.iter())
.chain(replacement.projection.iter())
.collect();
let projection = tcx.intern_place_elems(&projection);
// Replace with the final `Place`.
self.map[local] = Some(Place { local: base, projection });
}
}
}
fn for_src(&self, src: Local) -> Option<Place<'tcx>> {
self.map[src]
}
}
struct Replacer<'tcx> {
tcx: TyCtxt<'tcx>,
replacements: Replacements<'tcx>,
place_elem_cache: Vec<PlaceElem<'tcx>>,
}
impl<'tcx> MutVisitor<'tcx> for Replacer<'tcx> {
fn tcx(&self) -> TyCtxt<'tcx> {
self.tcx
}
fn visit_local(&mut self, local: &mut Local, context: PlaceContext, location: Location) {
if context.is_use() && self.replacements.for_src(*local).is_some() {
bug!(
"use of local {:?} should have been replaced by visit_place; context={:?}, loc={:?}",
local,
context,
location,
);
}
}
fn visit_place(&mut self, place: &mut Place<'tcx>, context: PlaceContext, location: Location) {
if let Some(replacement) = self.replacements.for_src(place.local) {
// Rebase `place`s projections onto `replacement`'s.
self.place_elem_cache.clear();
self.place_elem_cache.extend(replacement.projection.iter().chain(place.projection));
let projection = self.tcx.intern_place_elems(&self.place_elem_cache);
let new_place = Place { local: replacement.local, projection };
debug!("Replacer: {:?} -> {:?}", place, new_place);
*place = new_place;
}
self.super_place(place, context, location);
}
fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
self.super_statement(statement, location);
match &statement.kind {
// FIXME: Don't delete storage statements, merge the live ranges instead
StatementKind::StorageDead(local) | StatementKind::StorageLive(local)
if self.replacements.kill.contains(*local) =>
{
statement.make_nop()
}
StatementKind::Assign(box (dest, rvalue)) => {
match rvalue {
Rvalue::Use(Operand::Copy(place) | Operand::Move(place)) => {
// These might've been turned into self-assignments by the replacement
// (this includes the original statement we wanted to eliminate).
if dest == place {
debug!("{:?} turned into self-assignment, deleting", location);
statement.make_nop();
}
}
_ => {}
}
}
_ => {}
}
}
}
struct Conflicts<'a> {
relevant_locals: &'a BitSet<Local>,
/// The conflict matrix. It is always symmetric and the adjacency matrix of the corresponding
/// conflict graph.
matrix: BitMatrix<Local, Local>,
/// Preallocated `BitSet` used by `unify`.
unify_cache: BitSet<Local>,
/// Tracks locals that have been merged together to prevent cycles and propagate conflicts.
unified_locals: InPlaceUnificationTable<UnifyLocal>,
}
impl<'a> Conflicts<'a> {
fn build<'tcx>(
tcx: TyCtxt<'tcx>,
body: &'_ Body<'tcx>,
relevant_locals: &'a BitSet<Local>,
) -> Self {
// We don't have to look out for locals that have their address taken, since
// `find_candidates` already takes care of that.
let conflicts = BitMatrix::from_row_n(
&BitSet::new_empty(body.local_decls.len()),
body.local_decls.len(),
);
let mut init = MaybeInitializedLocals
.into_engine(tcx, body)
.iterate_to_fixpoint()
.into_results_cursor(body);
let mut live =
MaybeLiveLocals.into_engine(tcx, body).iterate_to_fixpoint().into_results_cursor(body);
let mut reachable = None;
dump_mir(tcx, None, "DestinationPropagation-dataflow", &"", body, |pass_where, w| {
let reachable = reachable.get_or_insert_with(|| traversal::reachable_as_bitset(body));
match pass_where {
PassWhere::BeforeLocation(loc) if reachable.contains(loc.block) => {
init.seek_before_primary_effect(loc);
live.seek_after_primary_effect(loc);
writeln!(w, " // init: {:?}", init.get())?;
writeln!(w, " // live: {:?}", live.get())?;
}
PassWhere::AfterTerminator(bb) if reachable.contains(bb) => {
let loc = body.terminator_loc(bb);
init.seek_after_primary_effect(loc);
live.seek_before_primary_effect(loc);
writeln!(w, " // init: {:?}", init.get())?;
writeln!(w, " // live: {:?}", live.get())?;
}
PassWhere::BeforeBlock(bb) if reachable.contains(bb) => {
init.seek_to_block_start(bb);
live.seek_to_block_start(bb);
writeln!(w, " // init: {:?}", init.get())?;
writeln!(w, " // live: {:?}", live.get())?;
}
PassWhere::BeforeCFG | PassWhere::AfterCFG | PassWhere::AfterLocation(_) => {}
PassWhere::BeforeLocation(_) | PassWhere::AfterTerminator(_) => {
writeln!(w, " // init: <unreachable>")?;
writeln!(w, " // live: <unreachable>")?;
}
PassWhere::BeforeBlock(_) => {
writeln!(w, " // init: <unreachable>")?;
writeln!(w, " // live: <unreachable>")?;
}
}
Ok(())
});
let mut this = Self {
relevant_locals,
matrix: conflicts,
unify_cache: BitSet::new_empty(body.local_decls.len()),
unified_locals: {
let mut table = InPlaceUnificationTable::new();
// Pre-fill table with all locals (this creates N nodes / "connected" components,
// "graph"-ically speaking).
for local in 0..body.local_decls.len() {
assert_eq!(table.new_key(()), UnifyLocal(Local::from_usize(local)));
}
table
},
};
let mut live_and_init_locals = Vec::new();
// Visit only reachable basic blocks. The exact order is not important.
for (block, data) in traversal::preorder(body) {
// We need to observe the dataflow state *before* all possible locations (statement or
// terminator) in each basic block, and then observe the state *after* the terminator
// effect is applied. As long as neither `init` nor `borrowed` has a "before" effect,
// we will observe all possible dataflow states.
// Since liveness is a backwards analysis, we need to walk the results backwards. To do
// that, we first collect in the `MaybeInitializedLocals` results in a forwards
// traversal.
live_and_init_locals.resize_with(data.statements.len() + 1, || {
BitSet::new_empty(body.local_decls.len())
});
// First, go forwards for `MaybeInitializedLocals` and apply intra-statement/terminator
// conflicts.
for (i, statement) in data.statements.iter().enumerate() {
this.record_statement_conflicts(statement);
let loc = Location { block, statement_index: i };
init.seek_before_primary_effect(loc);
live_and_init_locals[i].clone_from(init.get());
}
this.record_terminator_conflicts(data.terminator());
let term_loc = Location { block, statement_index: data.statements.len() };
init.seek_before_primary_effect(term_loc);
live_and_init_locals[term_loc.statement_index].clone_from(init.get());
// Now, go backwards and union with the liveness results.
for statement_index in (0..=data.statements.len()).rev() {
let loc = Location { block, statement_index };
live.seek_after_primary_effect(loc);
live_and_init_locals[statement_index].intersect(live.get());
trace!("record conflicts at {:?}", loc);
this.record_dataflow_conflicts(&mut live_and_init_locals[statement_index]);
}
init.seek_to_block_end(block);
live.seek_to_block_end(block);
let mut conflicts = init.get().clone();
conflicts.intersect(live.get());
trace!("record conflicts at end of {:?}", block);
this.record_dataflow_conflicts(&mut conflicts);
}
this
}
fn record_dataflow_conflicts(&mut self, new_conflicts: &mut BitSet<Local>) {
// Remove all locals that are not candidates.
new_conflicts.intersect(self.relevant_locals);
for local in new_conflicts.iter() {
self.matrix.union_row_with(&new_conflicts, local);
}
}
fn record_local_conflict(&mut self, a: Local, b: Local, why: &str) {
trace!("conflict {:?} <-> {:?} due to {}", a, b, why);
self.matrix.insert(a, b);
self.matrix.insert(b, a);
}
/// Records locals that must not overlap during the evaluation of `stmt`. These locals conflict
/// and must not be merged.
fn record_statement_conflicts(&mut self, stmt: &Statement<'_>) {
match &stmt.kind {
// While the left and right sides of an assignment must not overlap, we do not mark
// conflicts here as that would make this optimization useless. When we optimize, we
// eliminate the resulting self-assignments automatically.
StatementKind::Assign(_) => {}
StatementKind::SetDiscriminant { .. }
| StatementKind::Deinit(..)
| StatementKind::StorageLive(..)
| StatementKind::StorageDead(..)
| StatementKind::Retag(..)
| StatementKind::FakeRead(..)
| StatementKind::AscribeUserType(..)
| StatementKind::Coverage(..)
| StatementKind::Intrinsic(..)
| StatementKind::Nop => {}
}
}
fn record_terminator_conflicts(&mut self, term: &Terminator<'_>) {
match &term.kind {
TerminatorKind::DropAndReplace {
place: dropped_place,
value,
target: _,
unwind: _,
} => {
if let Some(place) = value.place()
&& !place.is_indirect()
&& !dropped_place.is_indirect()
{
self.record_local_conflict(
place.local,
dropped_place.local,
"DropAndReplace operand overlap",
);
}
}
TerminatorKind::Yield { value, resume: _, resume_arg, drop: _ } => {
if let Some(place) = value.place() {
if !place.is_indirect() && !resume_arg.is_indirect() {
self.record_local_conflict(
place.local,
resume_arg.local,
"Yield operand overlap",
);
}
}
}
TerminatorKind::Call {
func,
args,
destination,
target: _,
cleanup: _,
from_hir_call: _,
fn_span: _,
} => {
// No arguments may overlap with the destination.
for arg in args.iter().chain(Some(func)) {
if let Some(place) = arg.place() {
if !place.is_indirect() && !destination.is_indirect() {
self.record_local_conflict(
destination.local,
place.local,
"call dest/arg overlap",
);
}
}
}
}
TerminatorKind::InlineAsm {
template: _,
operands,
options: _,
line_spans: _,
destination: _,
cleanup: _,
} => {
// The intended semantics here aren't documented, we just assume that nothing that
// could be written to by the assembly may overlap with any other operands.
for op in operands {
match op {
InlineAsmOperand::Out { reg: _, late: _, place: Some(dest_place) }
| InlineAsmOperand::InOut {
reg: _,
late: _,
in_value: _,
out_place: Some(dest_place),
} => {
// For output place `place`, add all places accessed by the inline asm.
for op in operands {
match op {
InlineAsmOperand::In { reg: _, value } => {
if let Some(p) = value.place()
&& !p.is_indirect()
&& !dest_place.is_indirect()
{
self.record_local_conflict(
p.local,
dest_place.local,
"asm! operand overlap",
);
}
}
InlineAsmOperand::Out {
reg: _,
late: _,
place: Some(place),
} => {
if !place.is_indirect() && !dest_place.is_indirect() {
self.record_local_conflict(
place.local,
dest_place.local,
"asm! operand overlap",
);
}
}
InlineAsmOperand::InOut {
reg: _,
late: _,
in_value,
out_place,
} => {
if let Some(place) = in_value.place()
&& !place.is_indirect()
&& !dest_place.is_indirect()
{
self.record_local_conflict(
place.local,
dest_place.local,
"asm! operand overlap",
);
}
if let Some(place) = out_place
&& !place.is_indirect()
&& !dest_place.is_indirect()
{
self.record_local_conflict(
place.local,
dest_place.local,
"asm! operand overlap",
);
}
}
InlineAsmOperand::Out { reg: _, late: _, place: None }
| InlineAsmOperand::Const { value: _ }
| InlineAsmOperand::SymFn { value: _ }
| InlineAsmOperand::SymStatic { def_id: _ } => {}
}
}
}
InlineAsmOperand::InOut {
reg: _,
late: _,
in_value: _,
out_place: None,
}
| InlineAsmOperand::In { reg: _, value: _ }
| InlineAsmOperand::Out { reg: _, late: _, place: None }
| InlineAsmOperand::Const { value: _ }
| InlineAsmOperand::SymFn { value: _ }
| InlineAsmOperand::SymStatic { def_id: _ } => {}
}
}
}
TerminatorKind::Goto { .. }
| TerminatorKind::SwitchInt { .. }
| TerminatorKind::Resume
| TerminatorKind::Abort
| TerminatorKind::Return
| TerminatorKind::Unreachable
| TerminatorKind::Drop { .. }
| TerminatorKind::Assert { .. }
| TerminatorKind::GeneratorDrop
| TerminatorKind::FalseEdge { .. }
| TerminatorKind::FalseUnwind { .. } => {}
}
}
/// Checks whether `a` and `b` may be merged. Returns `false` if there's a conflict.
fn can_unify(&mut self, a: Local, b: Local) -> bool {
// After some locals have been unified, their conflicts are only tracked in the root key,
// so look that up.
let a = self.unified_locals.find(a).0;
let b = self.unified_locals.find(b).0;
if a == b {
// Already merged (part of the same connected component).
return false;
}
if self.matrix.contains(a, b) {
// Conflict (derived via dataflow, intra-statement conflicts, or inherited from another
// local during unification).
return false;
}
true
}
/// Merges the conflicts of `a` and `b`, so that each one inherits all conflicts of the other.
///
/// `can_unify` must have returned `true` for the same locals, or this may panic or lead to
/// miscompiles.
///
/// This is called when the pass makes the decision to unify `a` and `b` (or parts of `a` and
/// `b`) and is needed to ensure that future unification decisions take potentially newly
/// introduced conflicts into account.
///
/// For an example, assume we have locals `_0`, `_1`, `_2`, and `_3`. There are these conflicts:
///
/// * `_0` <-> `_1`
/// * `_1` <-> `_2`
/// * `_3` <-> `_0`
///
/// We then decide to merge `_2` with `_3` since they don't conflict. Then we decide to merge
/// `_2` with `_0`, which also doesn't have a conflict in the above list. However `_2` is now
/// `_3`, which does conflict with `_0`.
fn unify(&mut self, a: Local, b: Local) {
trace!("unify({:?}, {:?})", a, b);
// Get the root local of the connected components. The root local stores the conflicts of
// all locals in the connected component (and *is stored* as the conflicting local of other
// locals).
let a = self.unified_locals.find(a).0;
let b = self.unified_locals.find(b).0;
assert_ne!(a, b);
trace!("roots: a={:?}, b={:?}", a, b);
trace!("{:?} conflicts: {:?}", a, self.matrix.iter(a).format(", "));
trace!("{:?} conflicts: {:?}", b, self.matrix.iter(b).format(", "));
self.unified_locals.union(a, b);
let root = self.unified_locals.find(a).0;
assert!(root == a || root == b);
// Make all locals that conflict with `a` also conflict with `b`, and vice versa.
self.unify_cache.clear();
for conflicts_with_a in self.matrix.iter(a) {
self.unify_cache.insert(conflicts_with_a);
}
for conflicts_with_b in self.matrix.iter(b) {
self.unify_cache.insert(conflicts_with_b);
}
for conflicts_with_a_or_b in self.unify_cache.iter() {
// Set both `a` and `b` for this local's row.
self.matrix.insert(conflicts_with_a_or_b, a);
self.matrix.insert(conflicts_with_a_or_b, b);
}
// Write the locals `a` conflicts with to `b`'s row.
self.matrix.union_rows(a, b);
// Write the locals `b` conflicts with to `a`'s row.
self.matrix.union_rows(b, a);
}
}
/// A `dest = {move} src;` statement at `loc`.
///
/// We want to consider merging `dest` and `src` due to this assignment.
#[derive(Debug, Copy, Clone)]
struct CandidateAssignment<'tcx> {
/// Does not contain indirection or indexing (so the only local it contains is the place base).
dest: Place<'tcx>,
src: Local,
loc: Location,
}
/// Scans the MIR for assignments between locals that we might want to consider merging.
///
/// This will filter out assignments that do not match the right form (as described in the top-level
/// comment) and also throw out assignments that involve a local that has its address taken or is
/// otherwise ineligible (eg. locals used as array indices are ignored because we cannot propagate
/// arbitrary places into array indices).
fn find_candidates<'tcx>(body: &Body<'tcx>) -> Vec<CandidateAssignment<'tcx>> {
let mut visitor = FindAssignments {
body,
candidates: Vec::new(),
ever_borrowed_locals: borrowed_locals(body),
locals_used_as_array_index: locals_used_as_array_index(body),
};
visitor.visit_body(body);
visitor.candidates
}
struct FindAssignments<'a, 'tcx> {
body: &'a Body<'tcx>,
candidates: Vec<CandidateAssignment<'tcx>>,
ever_borrowed_locals: BitSet<Local>,
locals_used_as_array_index: BitSet<Local>,
}
impl<'tcx> Visitor<'tcx> for FindAssignments<'_, 'tcx> {
fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
if let StatementKind::Assign(box (
dest,
Rvalue::Use(Operand::Copy(src) | Operand::Move(src)),
)) = &statement.kind
{
// `dest` must not have pointer indirection.
if dest.is_indirect() {
return;
}
// `src` must be a plain local.
if !src.projection.is_empty() {
return;
}
// Since we want to replace `src` with `dest`, `src` must not be required.
if is_local_required(src.local, self.body) {
return;
}
// Can't optimize if either local ever has their address taken. This optimization does
// liveness analysis only based on assignments, and a local can be live even if its
// never assigned to again, because a reference to it might be live.
// FIXME: This can be smarter and take `StorageDead` into account (which invalidates
// borrows).
if self.ever_borrowed_locals.contains(dest.local)
|| self.ever_borrowed_locals.contains(src.local)
{
return;
}
assert_ne!(dest.local, src.local, "self-assignments are UB");
// We can't replace locals occurring in `PlaceElem::Index` for now.
if self.locals_used_as_array_index.contains(src.local) {
return;
}
for elem in dest.projection {
if let PlaceElem::Index(_) = elem {
// `dest` contains an indexing projection.
return;
}
}
self.candidates.push(CandidateAssignment {
dest: *dest,
src: src.local,
loc: location,
});
}
}
}
/// Some locals are part of the function's interface and can not be removed.
///
/// Note that these locals *can* still be merged with non-required locals by removing that other
/// local.
fn is_local_required(local: Local, body: &Body<'_>) -> bool {
match body.local_kind(local) {
LocalKind::Arg | LocalKind::ReturnPointer => true,
LocalKind::Var | LocalKind::Temp => false,
}
}
/// `PlaceElem::Index` only stores a `Local`, so we can't replace that with a full `Place`.
///
/// Collect locals used as indices so we don't generate candidates that are impossible to apply
/// later.
fn locals_used_as_array_index(body: &Body<'_>) -> BitSet<Local> {
let mut visitor = IndexCollector { locals: BitSet::new_empty(body.local_decls.len()) };
visitor.visit_body(body);
visitor.locals
}
struct IndexCollector {
locals: BitSet<Local>,
}
impl<'tcx> Visitor<'tcx> for IndexCollector {
fn visit_projection_elem(
&mut self,
local: Local,
proj_base: &[PlaceElem<'tcx>],
elem: PlaceElem<'tcx>,
context: PlaceContext,
location: Location,
) {
if let PlaceElem::Index(i) = elem {
self.locals.insert(i);
}
self.super_projection_elem(local, proj_base, elem, context, location);
}
}