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| 1 | +// Copyright 2017 The Rust Project Developers. See the COPYRIGHT |
| 2 | +// file at the top-level directory of this distribution and at |
| 3 | +// http://rust-lang.org/COPYRIGHT. |
| 4 | +// |
| 5 | +// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or |
| 6 | +// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license |
| 7 | +// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your |
| 8 | +// option. This file may not be copied, modified, or distributed |
| 9 | +// except according to those terms. |
| 10 | + |
| 11 | +//! A pass that reuses "final destinations" of values, |
| 12 | +//! propagating the lvalue back through a chain of moves. |
| 13 | +
|
| 14 | +use rustc::hir; |
| 15 | +use rustc::mir::*; |
| 16 | +use rustc::mir::visit::{LvalueContext, MutVisitor, Visitor}; |
| 17 | +use rustc::session::config::FullDebugInfo; |
| 18 | +use rustc::ty::TyCtxt; |
| 19 | +use transform::{MirPass, MirSource}; |
| 20 | + |
| 21 | +use rustc_data_structures::indexed_vec::IndexVec; |
| 22 | + |
| 23 | +pub struct ReuseLvalues; |
| 24 | + |
| 25 | +impl MirPass for ReuseLvalues { |
| 26 | + fn run_pass<'a, 'tcx>(&self, |
| 27 | + tcx: TyCtxt<'a, 'tcx, 'tcx>, |
| 28 | + source: MirSource, |
| 29 | + mir: &mut Mir<'tcx>) { |
| 30 | + // Don't run on constant MIR, because trans might not be able to |
| 31 | + // evaluate the modified MIR. |
| 32 | + // FIXME(eddyb) Remove check after miri is merged. |
| 33 | + let id = tcx.hir.as_local_node_id(source.def_id).unwrap(); |
| 34 | + match (tcx.hir.body_owner_kind(id), source.promoted) { |
| 35 | + (_, Some(_)) | |
| 36 | + (hir::BodyOwnerKind::Const, _) | |
| 37 | + (hir::BodyOwnerKind::Static(_), _) => return, |
| 38 | + |
| 39 | + (hir::BodyOwnerKind::Fn, _) => { |
| 40 | + if tcx.is_const_fn(source.def_id) { |
| 41 | + // Don't run on const functions, as, again, trans might not be able to evaluate |
| 42 | + // the optimized IR. |
| 43 | + return |
| 44 | + } |
| 45 | + } |
| 46 | + } |
| 47 | + |
| 48 | + // FIXME(eddyb) We should allow multiple user variables |
| 49 | + // per local for debuginfo instead of not optimizing. |
| 50 | + if tcx.sess.opts.debuginfo == FullDebugInfo { |
| 51 | + return; |
| 52 | + } |
| 53 | + |
| 54 | + // Collect the def and move information for all locals. |
| 55 | + let mut collector = DefMoveCollector { |
| 56 | + defs_moves: IndexVec::from_elem((Def::Never, Move::Never), &mir.local_decls), |
| 57 | + }; |
| 58 | + for arg in mir.args_iter() { |
| 59 | + // Arguments are special because they're initialized |
| 60 | + // in callers, and the collector doesn't know this. |
| 61 | + collector.defs_moves[arg].0 = Def::Other; |
| 62 | + } |
| 63 | + collector.visit_mir(mir); |
| 64 | + let mut defs_moves = collector.defs_moves; |
| 65 | + |
| 66 | + // Keep only locals with a clear initialization and move, |
| 67 | + // as they are guaranteed to have all accesses in between. |
| 68 | + // Also, the destination local of the move has to also have |
| 69 | + // a single initialization (the move itself), otherwise |
| 70 | + // there could be accesses that overlap the move chain. |
| 71 | + for local in mir.local_decls.indices() { |
| 72 | + let (def, mov) = defs_moves[local]; |
| 73 | + if let Move::Into(dest) = mov { |
| 74 | + if let Def::Once { .. } = def { |
| 75 | + if let (Def::Once { ref mut reused }, _) = defs_moves[dest] { |
| 76 | + *reused = true; |
| 77 | + continue; |
| 78 | + } |
| 79 | + } |
| 80 | + |
| 81 | + // Failed to confirm the destination. |
| 82 | + defs_moves[local].1 = Move::Other; |
| 83 | + } |
| 84 | + } |
| 85 | + |
| 86 | + // Apply the appropriate replacements. |
| 87 | + let mut replacer = Replacer { |
| 88 | + defs_moves |
| 89 | + }; |
| 90 | + replacer.visit_mir(mir); |
| 91 | + } |
| 92 | +} |
| 93 | + |
| 94 | +/// How (many times) was a local written? |
| 95 | +/// Note that borrows are completely ignored, |
| 96 | +/// as they get invalidated by moves regardless. |
| 97 | +#[derive(Copy, Clone, Debug)] |
| 98 | +enum Def { |
| 99 | + /// No writes to this local. |
| 100 | + Never, |
| 101 | + |
| 102 | + /// Only one direct initialization, from `Assign` or `Call`. |
| 103 | + /// |
| 104 | + /// If `reused` is `true`, this local is a destination |
| 105 | + /// in a chain of moves and should have all of its |
| 106 | + /// `StorageLive` statements removed. |
| 107 | + Once { |
| 108 | + reused: bool |
| 109 | + }, |
| 110 | + |
| 111 | + /// Any other number or kind of mutating accesses. |
| 112 | + Other, |
| 113 | +} |
| 114 | + |
| 115 | +/// How (many times) was a local moved? |
| 116 | +#[derive(Copy, Clone, Debug)] |
| 117 | +enum Move { |
| 118 | + /// No moves of this local. |
| 119 | + Never, |
| 120 | + |
| 121 | + /// Only one move, assigning another local. |
| 122 | + /// |
| 123 | + /// Ends up replaced by its destination and should |
| 124 | + /// have all of its `StorageDead` statements removed. |
| 125 | + Into(Local), |
| 126 | + |
| 127 | + /// Any other number or kind of moves. |
| 128 | + Other, |
| 129 | +} |
| 130 | + |
| 131 | + |
| 132 | +struct DefMoveCollector { |
| 133 | + defs_moves: IndexVec<Local, (Def, Move)>, |
| 134 | +} |
| 135 | + |
| 136 | +impl<'tcx> Visitor<'tcx> for DefMoveCollector { |
| 137 | + fn visit_local(&mut self, |
| 138 | + &local: &Local, |
| 139 | + context: LvalueContext<'tcx>, |
| 140 | + _location: Location) { |
| 141 | + let (ref mut def, ref mut mov) = self.defs_moves[local]; |
| 142 | + match context { |
| 143 | + // We specifically want the first direct initialization. |
| 144 | + LvalueContext::Store | |
| 145 | + LvalueContext::Call => { |
| 146 | + if let Def::Never = *def { |
| 147 | + *def = Def::Once { reused: false }; |
| 148 | + } else { |
| 149 | + *def = Def::Other; |
| 150 | + } |
| 151 | + } |
| 152 | + |
| 153 | + // Initialization of a field or similar. |
| 154 | + LvalueContext::Projection(Mutability::Mut) => { |
| 155 | + *def = Def::Other; |
| 156 | + } |
| 157 | + |
| 158 | + // Both of these count as moved, and not the kind |
| 159 | + // we want for `Move::Into` (see `visit_assign`). |
| 160 | + LvalueContext::Drop | |
| 161 | + LvalueContext::Move => *mov = Move::Other, |
| 162 | + |
| 163 | + // We can ignore everything else. |
| 164 | + LvalueContext::Inspect | |
| 165 | + LvalueContext::Copy | |
| 166 | + LvalueContext::Projection(Mutability::Not) | |
| 167 | + LvalueContext::Borrow { .. } | |
| 168 | + LvalueContext::StorageLive | |
| 169 | + LvalueContext::StorageDead | |
| 170 | + LvalueContext::Validate => {} |
| 171 | + } |
| 172 | + } |
| 173 | + |
| 174 | + fn visit_projection(&mut self, |
| 175 | + proj: &LvalueProjection<'tcx>, |
| 176 | + context: LvalueContext<'tcx>, |
| 177 | + location: Location) { |
| 178 | + // Pretend derefs copy the underlying pointer, as we don't |
| 179 | + // need to treat the base local as being mutated or moved. |
| 180 | + let context = if let ProjectionElem::Deref = proj.elem { |
| 181 | + LvalueContext::Copy |
| 182 | + } else { |
| 183 | + match context { |
| 184 | + // Pass down the kinds of contexts for which we don't |
| 185 | + // need to disambigutate between direct and projected. |
| 186 | + LvalueContext::Borrow { .. } | |
| 187 | + LvalueContext::Copy | |
| 188 | + LvalueContext::Move | |
| 189 | + LvalueContext::Drop => context, |
| 190 | + |
| 191 | + _ if context.is_mutating_use() => { |
| 192 | + LvalueContext::Projection(Mutability::Mut) |
| 193 | + } |
| 194 | + _ => { |
| 195 | + LvalueContext::Projection(Mutability::Not) |
| 196 | + } |
| 197 | + } |
| 198 | + }; |
| 199 | + self.visit_lvalue(&proj.base, context, location); |
| 200 | + self.visit_projection_elem(&proj.elem, context, location); |
| 201 | + } |
| 202 | + |
| 203 | + fn visit_assign(&mut self, |
| 204 | + _: BasicBlock, |
| 205 | + lvalue: &Lvalue<'tcx>, |
| 206 | + rvalue: &Rvalue<'tcx>, |
| 207 | + location: Location) { |
| 208 | + self.visit_lvalue(lvalue, LvalueContext::Store, location); |
| 209 | + |
| 210 | + // Handle `dest = move src`, and skip the `visit_local` |
| 211 | + // for `src`, which would always set it to `Move::Other`. |
| 212 | + match (lvalue, rvalue) { |
| 213 | + (&Lvalue::Local(dest), &Rvalue::Use(Operand::Move(Lvalue::Local(src)))) => { |
| 214 | + let (_, ref mut mov) = self.defs_moves[src]; |
| 215 | + // We specifically want the first whole move into another local. |
| 216 | + if let Move::Never = *mov { |
| 217 | + *mov = Move::Into(dest); |
| 218 | + } else { |
| 219 | + *mov = Move::Other; |
| 220 | + } |
| 221 | + } |
| 222 | + _ => { |
| 223 | + self.visit_rvalue(rvalue, location); |
| 224 | + } |
| 225 | + } |
| 226 | + } |
| 227 | +} |
| 228 | + |
| 229 | +struct Replacer { |
| 230 | + defs_moves: IndexVec<Local, (Def, Move)>, |
| 231 | +} |
| 232 | + |
| 233 | +impl<'a, 'tcx> MutVisitor<'tcx> for Replacer { |
| 234 | + fn visit_local(&mut self, |
| 235 | + local: &mut Local, |
| 236 | + context: LvalueContext<'tcx>, |
| 237 | + location: Location) { |
| 238 | + let original = *local; |
| 239 | + if let (_, Move::Into(dest)) = self.defs_moves[original] { |
| 240 | + *local = dest; |
| 241 | + |
| 242 | + // Keep going, in case the move chain doesn't stop here. |
| 243 | + self.visit_local(local, context, location); |
| 244 | + |
| 245 | + // Cache the final result, in a similar way to union-find. |
| 246 | + let final_dest = *local; |
| 247 | + if final_dest != dest { |
| 248 | + self.defs_moves[original].1 = Move::Into(final_dest); |
| 249 | + } |
| 250 | + } |
| 251 | + } |
| 252 | + |
| 253 | + fn visit_statement(&mut self, |
| 254 | + block: BasicBlock, |
| 255 | + statement: &mut Statement<'tcx>, |
| 256 | + location: Location) { |
| 257 | + // Fuse storage liveness ranges of move chains, by removing |
| 258 | + // `StorageLive` of destinations and `StorageDead` of sources. |
| 259 | + match statement.kind { |
| 260 | + StatementKind::StorageLive(local) | |
| 261 | + StatementKind::StorageDead(local) => { |
| 262 | + // FIXME(eddyb) We also have to remove `StorageLive` of |
| 263 | + // sources and `StorageDead` of destinations to avoid |
| 264 | + // creating invalid storage liveness (half-)ranges. |
| 265 | + // The proper solution might involve recomputing them. |
| 266 | + match self.defs_moves[local] { |
| 267 | + (Def::Once { reused: true }, _) | |
| 268 | + (_, Move::Into(_)) => { |
| 269 | + statement.kind = StatementKind::Nop; |
| 270 | + } |
| 271 | + _ => {} |
| 272 | + } |
| 273 | + } |
| 274 | + _ => {} |
| 275 | + } |
| 276 | + |
| 277 | + self.super_statement(block, statement, location); |
| 278 | + |
| 279 | + // Remove self-assignments resulting from replaced move chains. |
| 280 | + match statement.kind { |
| 281 | + StatementKind::Assign(Lvalue::Local(dest), |
| 282 | + Rvalue::Use(Operand::Move(Lvalue::Local(src)))) if dest == src => { |
| 283 | + statement.kind = StatementKind::Nop; |
| 284 | + } |
| 285 | + _ => {} |
| 286 | + } |
| 287 | + } |
| 288 | +} |
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