1 //! This pass finds basic blocks that are completely equal,
2 //! and replaces all uses with just one of them.
4 use std::{collections::hash_map::Entry, hash::Hash, hash::Hasher};
6 use crate::transform::MirPass;
8 use rustc_data_structures::fx::FxHashMap;
9 use rustc_middle::mir::visit::MutVisitor;
10 use rustc_middle::mir::*;
11 use rustc_middle::ty::TyCtxt;
13 use super::simplify::simplify_cfg;
15 pub struct DeduplicateBlocks;
17 impl<'tcx> MirPass<'tcx> for DeduplicateBlocks {
18 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
19 if tcx.sess.mir_opt_level() < 4 {
22 debug!("Running DeduplicateBlocks on `{:?}`", body.source);
23 let duplicates = find_duplicates(body);
24 let has_opts_to_apply = !duplicates.is_empty();
26 if has_opts_to_apply {
27 let mut opt_applier = OptApplier { tcx, duplicates };
28 opt_applier.visit_body(body);
34 struct OptApplier<'tcx> {
36 duplicates: FxHashMap<BasicBlock, BasicBlock>,
39 impl<'tcx> MutVisitor<'tcx> for OptApplier<'tcx> {
40 fn tcx(&self) -> TyCtxt<'tcx> {
44 fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
45 for target in terminator.successors_mut() {
46 if let Some(replacement) = self.duplicates.get(target) {
47 debug!("SUCCESS: Replacing: `{:?}` with `{:?}`", target, replacement);
48 *target = *replacement;
52 self.super_terminator(terminator, location);
56 fn find_duplicates<'a, 'tcx>(body: &'a Body<'tcx>) -> FxHashMap<BasicBlock, BasicBlock> {
57 let mut duplicates = FxHashMap::default();
59 let bbs_to_go_through =
60 body.basic_blocks().iter_enumerated().filter(|(_, bbd)| !bbd.is_cleanup).count();
63 FxHashMap::with_capacity_and_hasher(bbs_to_go_through, Default::default());
65 // Go through the basic blocks backwards. This means that in case of duplicates,
66 // we can use the basic block with the highest index as the replacement for all lower ones.
67 // For example, if bb1, bb2 and bb3 are duplicates, we will first insert bb3 in same_hashes.
68 // Then we will see that bb2 is a duplicate of bb3,
69 // and insert bb2 with the replacement bb3 in the duplicates list.
70 // When we see bb1, we see that it is a duplicate of bb3, and therefore insert it in the duplicates list
71 // with replacement bb3.
72 // When the duplicates are removed, we will end up with only bb3.
73 for (bb, bbd) in body.basic_blocks().iter_enumerated().rev().filter(|(_, bbd)| !bbd.is_cleanup)
75 // Basic blocks can get really big, so to avoid checking for duplicates in basic blocks
76 // that are unlikely to have duplicates, we stop early. The early bail number has been
77 // found experimentally by eprintln while compiling the crates in the rustc-perf suite.
78 if bbd.statements.len() > 10 {
82 let to_hash = BasicBlockHashable { basic_block_data: bbd };
83 let entry = same_hashes.entry(to_hash);
85 Entry::Occupied(occupied) => {
86 // The basic block was already in the hashmap, which means we have a duplicate
87 let value = *occupied.get();
88 debug!("Inserting {:?} -> {:?}", bb, value);
89 duplicates.insert(bb, value).expect_none("key was already inserted");
91 Entry::Vacant(vacant) => {
100 struct BasicBlockHashable<'tcx, 'a> {
101 basic_block_data: &'a BasicBlockData<'tcx>,
104 impl<'tcx, 'a> Hash for BasicBlockHashable<'tcx, 'a> {
105 fn hash<H: Hasher>(&self, state: &mut H) {
106 hash_statements(state, self.basic_block_data.statements.iter());
107 // Note that since we only hash the kind, we lose span information if we deduplicate the blocks
108 self.basic_block_data.terminator().kind.hash(state);
112 impl<'tcx, 'a> Eq for BasicBlockHashable<'tcx, 'a> {}
114 impl<'tcx, 'a> PartialEq for BasicBlockHashable<'tcx, 'a> {
115 fn eq(&self, other: &Self) -> bool {
116 self.basic_block_data.statements.len() == other.basic_block_data.statements.len()
117 && &self.basic_block_data.terminator().kind == &other.basic_block_data.terminator().kind
122 .zip(&other.basic_block_data.statements)
123 .all(|(x, y)| statement_eq(&x.kind, &y.kind))
127 fn hash_statements<'a, 'tcx, H: Hasher>(
129 iter: impl Iterator<Item = &'a Statement<'tcx>>,
134 statement_hash(hasher, &stmt.kind);
138 fn statement_hash<'tcx, H: Hasher>(hasher: &mut H, stmt: &StatementKind<'tcx>) {
140 StatementKind::Assign(box (place, rvalue)) => {
142 rvalue_hash(hasher, rvalue)
148 fn rvalue_hash<H: Hasher>(hasher: &mut H, rvalue: &Rvalue<'tcx>) {
150 Rvalue::Use(op) => operand_hash(hasher, op),
155 fn operand_hash<H: Hasher>(hasher: &mut H, operand: &Operand<'tcx>) {
157 Operand::Constant(box Constant { user_ty: _, literal, span: _ }) => literal.hash(hasher),
162 fn statement_eq<'tcx>(lhs: &StatementKind<'tcx>, rhs: &StatementKind<'tcx>) -> bool {
163 let res = match (lhs, rhs) {
165 StatementKind::Assign(box (place, rvalue)),
166 StatementKind::Assign(box (place2, rvalue2)),
167 ) => place == place2 && rvalue_eq(rvalue, rvalue2),
170 debug!("statement_eq lhs: `{:?}` rhs: `{:?}` result: {:?}", lhs, rhs, res);
174 fn rvalue_eq(lhs: &Rvalue<'tcx>, rhs: &Rvalue<'tcx>) -> bool {
175 let res = match (lhs, rhs) {
176 (Rvalue::Use(op1), Rvalue::Use(op2)) => operand_eq(op1, op2),
179 debug!("rvalue_eq lhs: `{:?}` rhs: `{:?}` result: {:?}", lhs, rhs, res);
183 fn operand_eq(lhs: &Operand<'tcx>, rhs: &Operand<'tcx>) -> bool {
184 let res = match (lhs, rhs) {
186 Operand::Constant(box Constant { user_ty: _, literal, span: _ }),
187 Operand::Constant(box Constant { user_ty: _, literal: literal2, span: _ }),
188 ) => literal == literal2,
191 debug!("operand_eq lhs: `{:?}` rhs: `{:?}` result: {:?}", lhs, rhs, res);