918 lines
37 KiB
Rust
918 lines
37 KiB
Rust
//! Propagates assignment destinations backwards in the CFG to eliminate redundant assignments.
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//!
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//! # Motivation
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//!
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//! MIR building can insert a lot of redundant copies, and Rust code in general often tends to move
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//! values around a lot. The result is a lot of assignments of the form `dest = {move} src;` in MIR.
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//! MIR building for constants in particular tends to create additional locals that are only used
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//! inside a single block to shuffle a value around unnecessarily.
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//!
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//! LLVM by itself is not good enough at eliminating these redundant copies (eg. see
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//! <https://github.com/rust-lang/rust/issues/32966>), so this leaves some performance on the table
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//! that we can regain by implementing an optimization for removing these assign statements in rustc
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//! itself. When this optimization runs fast enough, it can also speed up the constant evaluation
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//! and code generation phases of rustc due to the reduced number of statements and locals.
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//!
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//! # The Optimization
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//!
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//! Conceptually, this optimization is "destination propagation". It is similar to the Named Return
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//! Value Optimization, or NRVO, known from the C++ world, except that it isn't limited to return
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//! values or the return place `_0`. On a very high level, independent of the actual implementation
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//! details, it does the following:
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//!
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//! 1) Identify `dest = src;` statements that can be soundly eliminated.
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//! 2) Replace all mentions of `src` with `dest` ("unifying" them and propagating the destination
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//! backwards).
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//! 3) Delete the `dest = src;` statement (by making it a `nop`).
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//!
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//! Step 1) is by far the hardest, so it is explained in more detail below.
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//!
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//! ## Soundness
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//!
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//! Given an `Assign` statement `dest = src;`, where `dest` is a `Place` and `src` is an `Rvalue`,
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//! there are a few requirements that must hold for the optimization to be sound:
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//!
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//! * `dest` must not contain any *indirection* through a pointer. It must access part of the base
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//! local. Otherwise it might point to arbitrary memory that is hard to track.
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//!
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//! It must also not contain any indexing projections, since those take an arbitrary `Local` as
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//! the index, and that local might only be initialized shortly before `dest` is used.
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//!
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//! * `src` must be a bare `Local` without any indirections or field projections (FIXME: Is this a
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//! fundamental restriction or just current impl state?). It can be copied or moved by the
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//! assignment.
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//!
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//! * The `dest` and `src` locals must never be [*live*][liveness] at the same time. If they are, it
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//! means that they both hold a (potentially different) value that is needed by a future use of
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//! the locals. Unifying them would overwrite one of the values.
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//!
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//! Note that computing liveness of locals that have had their address taken is more difficult:
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//! Short of doing full escape analysis on the address/pointer/reference, the pass would need to
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//! assume that any operation that can potentially involve opaque user code (such as function
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//! calls, destructors, and inline assembly) may access any local that had its address taken
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//! before that point.
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//!
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//! Here, the first two conditions are simple structural requirements on the `Assign` statements
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//! that can be trivially checked. The liveness requirement however is more difficult and costly to
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//! check.
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//!
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//! ## Previous Work
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//!
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//! A [previous attempt] at implementing an optimization like this turned out to be a significant
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//! regression in compiler performance. Fixing the regressions introduced a lot of undesirable
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//! complexity to the implementation.
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//!
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//! A [subsequent approach] tried to avoid the costly computation by limiting itself to acyclic
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//! CFGs, but still turned out to be far too costly to run due to suboptimal performance within
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//! individual basic blocks, requiring a walk across the entire block for every assignment found
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//! within the block. For the `tuple-stress` benchmark, which has 458745 statements in a single
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//! block, this proved to be far too costly.
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//!
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//! Since the first attempt at this, the compiler has improved dramatically, and new analysis
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//! frameworks have been added that should make this approach viable without requiring a limited
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//! approach that only works for some classes of CFGs:
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//! - rustc now has a powerful dataflow analysis framework that can handle forwards and backwards
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//! analyses efficiently.
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//! - Layout optimizations for generators have been added to improve code generation for
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//! async/await, which are very similar in spirit to what this optimization does. Both walk the
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//! MIR and record conflicting uses of locals in a `BitMatrix`.
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//!
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//! Also, rustc now has a simple NRVO pass (see `nrvo.rs`), which handles a subset of the cases that
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//! this destination propagation pass handles, proving that similar optimizations can be performed
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//! on MIR.
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//!
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//! ## Pre/Post Optimization
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//!
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//! It is recommended to run `SimplifyCfg` and then `SimplifyLocals` some time after this pass, as
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//! it replaces the eliminated assign statements with `nop`s and leaves unused locals behind.
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//!
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//! [liveness]: https://en.wikipedia.org/wiki/Live_variable_analysis
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//! [previous attempt]: https://github.com/rust-lang/rust/pull/47954
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//! [subsequent approach]: https://github.com/rust-lang/rust/pull/71003
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use crate::MirPass;
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use itertools::Itertools;
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use rustc_data_structures::unify::{InPlaceUnificationTable, UnifyKey};
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use rustc_index::{
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bit_set::{BitMatrix, BitSet},
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vec::IndexVec,
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};
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use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor};
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use rustc_middle::mir::{dump_mir, PassWhere};
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use rustc_middle::mir::{
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traversal, Body, InlineAsmOperand, Local, LocalKind, Location, Operand, Place, PlaceElem,
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Rvalue, Statement, StatementKind, Terminator, TerminatorKind,
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};
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use rustc_middle::ty::TyCtxt;
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use rustc_mir_dataflow::impls::{borrowed_locals, MaybeInitializedLocals, MaybeLiveLocals};
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use rustc_mir_dataflow::Analysis;
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// Empirical measurements have resulted in some observations:
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// - Running on a body with a single block and 500 locals takes barely any time
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// - Running on a body with ~400 blocks and ~300 relevant locals takes "too long"
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// ...so we just limit both to somewhat reasonable-ish looking values.
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const MAX_LOCALS: usize = 500;
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const MAX_BLOCKS: usize = 250;
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pub struct DestinationPropagation;
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impl<'tcx> MirPass<'tcx> for DestinationPropagation {
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fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
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// FIXME(#79191, #82678): This is unsound.
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//
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// Only run at mir-opt-level=3 or higher for now (we don't fix up debuginfo and remove
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// storage statements at the moment).
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sess.opts.unstable_opts.unsound_mir_opts && sess.mir_opt_level() >= 3
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}
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fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
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let def_id = body.source.def_id();
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let candidates = find_candidates(body);
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if candidates.is_empty() {
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debug!("{:?}: no dest prop candidates, done", def_id);
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return;
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}
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// Collect all locals we care about. We only compute conflicts for these to save time.
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let mut relevant_locals = BitSet::new_empty(body.local_decls.len());
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for CandidateAssignment { dest, src, loc: _ } in &candidates {
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relevant_locals.insert(dest.local);
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relevant_locals.insert(*src);
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}
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// This pass unfortunately has `O(l² * s)` performance, where `l` is the number of locals
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// and `s` is the number of statements and terminators in the function.
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// To prevent blowing up compile times too much, we bail out when there are too many locals.
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let relevant = relevant_locals.count();
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debug!(
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"{:?}: {} locals ({} relevant), {} blocks",
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def_id,
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body.local_decls.len(),
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relevant,
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body.basic_blocks.len()
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);
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if relevant > MAX_LOCALS {
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warn!(
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"too many candidate locals in {:?} ({}, max is {}), not optimizing",
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def_id, relevant, MAX_LOCALS
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);
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return;
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}
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if body.basic_blocks.len() > MAX_BLOCKS {
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warn!(
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"too many blocks in {:?} ({}, max is {}), not optimizing",
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def_id,
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body.basic_blocks.len(),
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MAX_BLOCKS
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);
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return;
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}
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let mut conflicts = Conflicts::build(tcx, body, &relevant_locals);
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let mut replacements = Replacements::new(body.local_decls.len());
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for candidate @ CandidateAssignment { dest, src, loc } in candidates {
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// Merge locals that don't conflict.
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if !conflicts.can_unify(dest.local, src) {
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debug!("at assignment {:?}, conflict {:?} vs. {:?}", loc, dest.local, src);
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continue;
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}
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if replacements.for_src(candidate.src).is_some() {
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debug!("src {:?} already has replacement", candidate.src);
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continue;
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}
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if !tcx.consider_optimizing(|| {
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format!("DestinationPropagation {:?} {:?}", def_id, candidate)
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}) {
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break;
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}
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replacements.push(candidate);
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conflicts.unify(candidate.src, candidate.dest.local);
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}
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replacements.flatten(tcx);
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debug!("replacements {:?}", replacements.map);
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Replacer { tcx, replacements, place_elem_cache: Vec::new() }.visit_body(body);
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// FIXME fix debug info
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}
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}
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#[derive(Debug, Eq, PartialEq, Copy, Clone)]
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struct UnifyLocal(Local);
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impl From<Local> for UnifyLocal {
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fn from(l: Local) -> Self {
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Self(l)
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}
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}
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impl UnifyKey for UnifyLocal {
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type Value = ();
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#[inline]
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fn index(&self) -> u32 {
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self.0.as_u32()
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}
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#[inline]
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fn from_index(u: u32) -> Self {
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Self(Local::from_u32(u))
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}
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fn tag() -> &'static str {
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"UnifyLocal"
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}
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}
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struct Replacements<'tcx> {
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/// Maps locals to their replacement.
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map: IndexVec<Local, Option<Place<'tcx>>>,
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/// Whose locals' live ranges to kill.
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kill: BitSet<Local>,
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}
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impl<'tcx> Replacements<'tcx> {
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fn new(locals: usize) -> Self {
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Self { map: IndexVec::from_elem_n(None, locals), kill: BitSet::new_empty(locals) }
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}
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fn push(&mut self, candidate: CandidateAssignment<'tcx>) {
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trace!("Replacements::push({:?})", candidate);
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let entry = &mut self.map[candidate.src];
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assert!(entry.is_none());
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*entry = Some(candidate.dest);
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self.kill.insert(candidate.src);
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self.kill.insert(candidate.dest.local);
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}
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/// Applies the stored replacements to all replacements, until no replacements would result in
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/// locals that need further replacements when applied.
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fn flatten(&mut self, tcx: TyCtxt<'tcx>) {
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// Note: This assumes that there are no cycles in the replacements, which is enforced via
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// `self.unified_locals`. Otherwise this can cause an infinite loop.
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for local in self.map.indices() {
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if let Some(replacement) = self.map[local] {
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// Substitute the base local of `replacement` until fixpoint.
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let mut base = replacement.local;
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let mut reversed_projection_slices = Vec::with_capacity(1);
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while let Some(replacement_for_replacement) = self.map[base] {
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base = replacement_for_replacement.local;
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reversed_projection_slices.push(replacement_for_replacement.projection);
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}
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let projection: Vec<_> = reversed_projection_slices
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.iter()
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.rev()
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.flat_map(|projs| projs.iter())
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.chain(replacement.projection.iter())
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.collect();
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let projection = tcx.intern_place_elems(&projection);
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// Replace with the final `Place`.
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self.map[local] = Some(Place { local: base, projection });
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}
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}
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}
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fn for_src(&self, src: Local) -> Option<Place<'tcx>> {
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self.map[src]
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}
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}
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struct Replacer<'tcx> {
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tcx: TyCtxt<'tcx>,
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replacements: Replacements<'tcx>,
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place_elem_cache: Vec<PlaceElem<'tcx>>,
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}
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impl<'tcx> MutVisitor<'tcx> for Replacer<'tcx> {
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fn tcx(&self) -> TyCtxt<'tcx> {
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self.tcx
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}
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fn visit_local(&mut self, local: &mut Local, context: PlaceContext, location: Location) {
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if context.is_use() && self.replacements.for_src(*local).is_some() {
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bug!(
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"use of local {:?} should have been replaced by visit_place; context={:?}, loc={:?}",
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local,
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context,
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location,
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);
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}
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}
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fn visit_place(&mut self, place: &mut Place<'tcx>, context: PlaceContext, location: Location) {
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if let Some(replacement) = self.replacements.for_src(place.local) {
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// Rebase `place`s projections onto `replacement`'s.
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self.place_elem_cache.clear();
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self.place_elem_cache.extend(replacement.projection.iter().chain(place.projection));
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let projection = self.tcx.intern_place_elems(&self.place_elem_cache);
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let new_place = Place { local: replacement.local, projection };
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debug!("Replacer: {:?} -> {:?}", place, new_place);
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*place = new_place;
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}
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self.super_place(place, context, location);
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}
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fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
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self.super_statement(statement, location);
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match &statement.kind {
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// FIXME: Don't delete storage statements, merge the live ranges instead
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StatementKind::StorageDead(local) | StatementKind::StorageLive(local)
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if self.replacements.kill.contains(*local) =>
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{
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statement.make_nop()
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}
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StatementKind::Assign(box (dest, rvalue)) => {
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match rvalue {
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Rvalue::Use(Operand::Copy(place) | Operand::Move(place)) => {
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// These might've been turned into self-assignments by the replacement
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// (this includes the original statement we wanted to eliminate).
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if dest == place {
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debug!("{:?} turned into self-assignment, deleting", location);
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statement.make_nop();
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}
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}
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_ => {}
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}
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}
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_ => {}
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}
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}
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}
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struct Conflicts<'a> {
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relevant_locals: &'a BitSet<Local>,
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/// The conflict matrix. It is always symmetric and the adjacency matrix of the corresponding
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/// conflict graph.
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matrix: BitMatrix<Local, Local>,
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/// Preallocated `BitSet` used by `unify`.
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unify_cache: BitSet<Local>,
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/// Tracks locals that have been merged together to prevent cycles and propagate conflicts.
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unified_locals: InPlaceUnificationTable<UnifyLocal>,
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}
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impl<'a> Conflicts<'a> {
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fn build<'tcx>(
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tcx: TyCtxt<'tcx>,
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body: &'_ Body<'tcx>,
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relevant_locals: &'a BitSet<Local>,
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) -> Self {
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// We don't have to look out for locals that have their address taken, since
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// `find_candidates` already takes care of that.
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let conflicts = BitMatrix::from_row_n(
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&BitSet::new_empty(body.local_decls.len()),
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body.local_decls.len(),
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);
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let mut init = MaybeInitializedLocals
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.into_engine(tcx, body)
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.iterate_to_fixpoint()
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.into_results_cursor(body);
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let mut live =
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MaybeLiveLocals.into_engine(tcx, body).iterate_to_fixpoint().into_results_cursor(body);
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let mut reachable = None;
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dump_mir(tcx, None, "DestinationPropagation-dataflow", &"", body, |pass_where, w| {
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let reachable = reachable.get_or_insert_with(|| traversal::reachable_as_bitset(body));
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match pass_where {
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PassWhere::BeforeLocation(loc) if reachable.contains(loc.block) => {
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init.seek_before_primary_effect(loc);
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live.seek_after_primary_effect(loc);
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writeln!(w, " // init: {:?}", init.get())?;
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writeln!(w, " // live: {:?}", live.get())?;
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}
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PassWhere::AfterTerminator(bb) if reachable.contains(bb) => {
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let loc = body.terminator_loc(bb);
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init.seek_after_primary_effect(loc);
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live.seek_before_primary_effect(loc);
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writeln!(w, " // init: {:?}", init.get())?;
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writeln!(w, " // live: {:?}", live.get())?;
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}
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PassWhere::BeforeBlock(bb) if reachable.contains(bb) => {
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init.seek_to_block_start(bb);
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live.seek_to_block_start(bb);
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|
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writeln!(w, " // init: {:?}", init.get())?;
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writeln!(w, " // live: {:?}", live.get())?;
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}
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PassWhere::BeforeCFG | PassWhere::AfterCFG | PassWhere::AfterLocation(_) => {}
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|
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|
PassWhere::BeforeLocation(_) | PassWhere::AfterTerminator(_) => {
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writeln!(w, " // init: <unreachable>")?;
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writeln!(w, " // live: <unreachable>")?;
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}
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PassWhere::BeforeBlock(_) => {
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writeln!(w, " // init: <unreachable>")?;
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writeln!(w, " // live: <unreachable>")?;
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}
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}
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|
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Ok(())
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});
|
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|
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let mut this = Self {
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relevant_locals,
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matrix: conflicts,
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unify_cache: BitSet::new_empty(body.local_decls.len()),
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unified_locals: {
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let mut table = InPlaceUnificationTable::new();
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// Pre-fill table with all locals (this creates N nodes / "connected" components,
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// "graph"-ically speaking).
|
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for local in 0..body.local_decls.len() {
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assert_eq!(table.new_key(()), UnifyLocal(Local::from_usize(local)));
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}
|
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table
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},
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};
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|
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let mut live_and_init_locals = Vec::new();
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|
|
|
// 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
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|
// terminator) in each basic block, and then observe the state *after* the terminator
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// effect is applied. As long as neither `init` nor `borrowed` has a "before" effect,
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// 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
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|
// traversal.
|
|
|
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live_and_init_locals.resize_with(data.statements.len() + 1, || {
|
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BitSet::new_empty(body.local_decls.len())
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|
});
|
|
|
|
// 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::CopyNonOverlapping(..)
|
|
| 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);
|
|
}
|
|
}
|