src/librustc/middle/cfg/construct.rs

   1 // Copyright 2012-2014 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 use rustc_data_structures::graph;
  12 use middle::cfg::*;
  13 use middle::def::Def;
  14 use middle::pat_util;
  15 use middle::ty;
  16 use syntax::ast;
  17 use syntax::ptr::P;
  18
  19 use rustc_front::hir::{self, PatKind};
  20
  21 struct CFGBuilder<'a, 'tcx: 'a> {
  22     tcx: &'a ty::ctxt<'tcx>,
  23     graph: CFGGraph,
  24     fn_exit: CFGIndex,
  25     loop_scopes: Vec<LoopScope>,
  26 }
  27
  28 #[derive(Copy, Clone)]
  29 struct LoopScope {
  30     loop_id: ast::NodeId,     // id of loop/while node
  31     continue_index: CFGIndex, // where to go on a `loop`
  32     break_index: CFGIndex,    // where to go on a `break
  33 }
  34
  35 pub fn construct(tcx: &ty::ctxt,
  36                  blk: &hir::Block) -> CFG {
  37     let mut graph = graph::Graph::new();
  38     let entry = graph.add_node(CFGNodeData::Entry);
  39
  40     // `fn_exit` is target of return exprs, which lies somewhere
  41     // outside input `blk`. (Distinguishing `fn_exit` and `block_exit`
  42     // also resolves chicken-and-egg problem that arises if you try to
  43     // have return exprs jump to `block_exit` during construction.)
  44     let fn_exit = graph.add_node(CFGNodeData::Exit);
  45     let block_exit;
  46
  47     let mut cfg_builder = CFGBuilder {
  48         graph: graph,
  49         fn_exit: fn_exit,
  50         tcx: tcx,
  51         loop_scopes: Vec::new()
  52     };
  53     block_exit = cfg_builder.block(blk, entry);
  54     cfg_builder.add_contained_edge(block_exit, fn_exit);
  55     let CFGBuilder {graph, ..} = cfg_builder;
  56     CFG {graph: graph,
  57          entry: entry,
  58          exit: fn_exit}
  59 }
  60
  61 impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
  62     fn block(&mut self, blk: &hir::Block, pred: CFGIndex) -> CFGIndex {
  63         let mut stmts_exit = pred;
  64         for stmt in &blk.stmts {
  65             stmts_exit = self.stmt(stmt, stmts_exit);
  66         }
  67
  68         let expr_exit = self.opt_expr(&blk.expr, stmts_exit);
  69
  70         self.add_ast_node(blk.id, &[expr_exit])
  71     }
  72
  73     fn stmt(&mut self, stmt: &hir::Stmt, pred: CFGIndex) -> CFGIndex {
  74         match stmt.node {
  75             hir::StmtDecl(ref decl, id) => {
  76                 let exit = self.decl(&decl, pred);
  77                 self.add_ast_node(id, &[exit])
  78             }
  79
  80             hir::StmtExpr(ref expr, id) | hir::StmtSemi(ref expr, id) => {
  81                 let exit = self.expr(&expr, pred);
  82                 self.add_ast_node(id, &[exit])
  83             }
  84         }
  85     }
  86
  87     fn decl(&mut self, decl: &hir::Decl, pred: CFGIndex) -> CFGIndex {
  88         match decl.node {
  89             hir::DeclLocal(ref local) => {
  90                 let init_exit = self.opt_expr(&local.init, pred);
  91                 self.pat(&local.pat, init_exit)
  92             }
  93
  94             hir::DeclItem(_) => {
  95                 pred
  96             }
  97         }
  98     }
  99
 100     fn pat(&mut self, pat: &hir::Pat, pred: CFGIndex) -> CFGIndex {
 101         match pat.node {
 102             PatKind::Ident(_, _, None) |
 103             PatKind::TupleStruct(_, None) |
 104             PatKind::Path(..) |
 105             PatKind::QPath(..) |
 106             PatKind::Lit(..) |
 107             PatKind::Range(..) |
 108             PatKind::Wild => {
 109                 self.add_ast_node(pat.id, &[pred])
 110             }
 111
 112             PatKind::Box(ref subpat) |
 113             PatKind::Ref(ref subpat, _) |
 114             PatKind::Ident(_, _, Some(ref subpat)) => {
 115                 let subpat_exit = self.pat(&subpat, pred);
 116                 self.add_ast_node(pat.id, &[subpat_exit])
 117             }
 118
 119             PatKind::TupleStruct(_, Some(ref subpats)) |
 120             PatKind::Tup(ref subpats) => {
 121                 let pats_exit = self.pats_all(subpats.iter(), pred);
 122                 self.add_ast_node(pat.id, &[pats_exit])
 123             }
 124
 125             PatKind::Struct(_, ref subpats, _) => {
 126                 let pats_exit =
 127                     self.pats_all(subpats.iter().map(|f| &f.node.pat), pred);
 128                 self.add_ast_node(pat.id, &[pats_exit])
 129             }
 130
 131             PatKind::Vec(ref pre, ref vec, ref post) => {
 132                 let pre_exit = self.pats_all(pre.iter(), pred);
 133                 let vec_exit = self.pats_all(vec.iter(), pre_exit);
 134                 let post_exit = self.pats_all(post.iter(), vec_exit);
 135                 self.add_ast_node(pat.id, &[post_exit])
 136             }
 137         }
 138     }
 139
 140     fn pats_all<'b, I: Iterator<Item=&'b P<hir::Pat>>>(&mut self,
 141                                           pats: I,
 142                                           pred: CFGIndex) -> CFGIndex {
 143         //! Handles case where all of the patterns must match.
 144         pats.fold(pred, |pred, pat| self.pat(&pat, pred))
 145     }
 146
 147     fn expr(&mut self, expr: &hir::Expr, pred: CFGIndex) -> CFGIndex {
 148         match expr.node {
 149             hir::ExprBlock(ref blk) => {
 150                 let blk_exit = self.block(&blk, pred);
 151                 self.add_ast_node(expr.id, &[blk_exit])
 152             }
 153
 154             hir::ExprIf(ref cond, ref then, None) => {
 155                 //
 156                 //     [pred]
 157                 //       |
 158                 //       v 1
 159                 //     [cond]
 160                 //       |
 161                 //      / \
 162                 //     /   \
 163                 //    v 2   *
 164                 //  [then]  |
 165                 //    |     |
 166                 //    v 3   v 4
 167                 //   [..expr..]
 168                 //
 169                 let cond_exit = self.expr(&cond, pred);                // 1
 170                 let then_exit = self.block(&then, cond_exit);          // 2
 171                 self.add_ast_node(expr.id, &[cond_exit, then_exit])      // 3,4
 172             }
 173
 174             hir::ExprIf(ref cond, ref then, Some(ref otherwise)) => {
 175                 //
 176                 //     [pred]
 177                 //       |
 178                 //       v 1
 179                 //     [cond]
 180                 //       |
 181                 //      / \
 182                 //     /   \
 183                 //    v 2   v 3
 184                 //  [then][otherwise]
 185                 //    |     |
 186                 //    v 4   v 5
 187                 //   [..expr..]
 188                 //
 189                 let cond_exit = self.expr(&cond, pred);                // 1
 190                 let then_exit = self.block(&then, cond_exit);          // 2
 191                 let else_exit = self.expr(&otherwise, cond_exit);      // 3
 192                 self.add_ast_node(expr.id, &[then_exit, else_exit])      // 4, 5
 193             }
 194
 195             hir::ExprWhile(ref cond, ref body, _) => {
 196                 //
 197                 //         [pred]
 198                 //           |
 199                 //           v 1
 200                 //       [loopback] <--+ 5
 201                 //           |         |
 202                 //           v 2       |
 203                 //   +-----[cond]      |
 204                 //   |       |         |
 205                 //   |       v 4       |
 206                 //   |     [body] -----+
 207                 //   v 3
 208                 // [expr]
 209                 //
 210                 // Note that `break` and `continue` statements
 211                 // may cause additional edges.
 212
 213                 // Is the condition considered part of the loop?
 214                 let loopback = self.add_dummy_node(&[pred]);              // 1
 215                 let cond_exit = self.expr(&cond, loopback);             // 2
 216                 let expr_exit = self.add_ast_node(expr.id, &[cond_exit]); // 3
 217                 self.loop_scopes.push(LoopScope {
 218                     loop_id: expr.id,
 219                     continue_index: loopback,
 220                     break_index: expr_exit
 221                 });
 222                 let body_exit = self.block(&body, cond_exit);          // 4
 223                 self.add_contained_edge(body_exit, loopback);            // 5
 224                 self.loop_scopes.pop();
 225                 expr_exit
 226             }
 227
 228             hir::ExprLoop(ref body, _) => {
 229                 //
 230                 //     [pred]
 231                 //       |
 232                 //       v 1
 233                 //   [loopback] <---+
 234                 //       |      4   |
 235                 //       v 3        |
 236                 //     [body] ------+
 237                 //
 238                 //     [expr] 2
 239                 //
 240                 // Note that `break` and `loop` statements
 241                 // may cause additional edges.
 242
 243                 let loopback = self.add_dummy_node(&[pred]);              // 1
 244                 let expr_exit = self.add_ast_node(expr.id, &[]);          // 2
 245                 self.loop_scopes.push(LoopScope {
 246                     loop_id: expr.id,
 247                     continue_index: loopback,
 248                     break_index: expr_exit,
 249                 });
 250                 let body_exit = self.block(&body, loopback);           // 3
 251                 self.add_contained_edge(body_exit, loopback);            // 4
 252                 self.loop_scopes.pop();
 253                 expr_exit
 254             }
 255
 256             hir::ExprMatch(ref discr, ref arms, _) => {
 257                 self.match_(expr.id, &discr, &arms, pred)
 258             }
 259
 260             hir::ExprBinary(op, ref l, ref r) if ::rustc_front::util::lazy_binop(op.node) => {
 261                 //
 262                 //     [pred]
 263                 //       |
 264                 //       v 1
 265                 //      [l]
 266                 //       |
 267                 //      / \
 268                 //     /   \
 269                 //    v 2  *
 270                 //   [r]   |
 271                 //    |    |
 272                 //    v 3  v 4
 273                 //   [..exit..]
 274                 //
 275                 let l_exit = self.expr(&l, pred);                      // 1
 276                 let r_exit = self.expr(&r, l_exit);                    // 2
 277                 self.add_ast_node(expr.id, &[l_exit, r_exit])            // 3,4
 278             }
 279
 280             hir::ExprRet(ref v) => {
 281                 let v_exit = self.opt_expr(v, pred);
 282                 let b = self.add_ast_node(expr.id, &[v_exit]);
 283                 self.add_returning_edge(expr, b);
 284                 self.add_unreachable_node()
 285             }
 286
 287             hir::ExprBreak(label) => {
 288                 let loop_scope = self.find_scope(expr, label.map(|l| l.node.name));
 289                 let b = self.add_ast_node(expr.id, &[pred]);
 290                 self.add_exiting_edge(expr, b,
 291                                       loop_scope, loop_scope.break_index);
 292                 self.add_unreachable_node()
 293             }
 294
 295             hir::ExprAgain(label) => {
 296                 let loop_scope = self.find_scope(expr, label.map(|l| l.node.name));
 297                 let a = self.add_ast_node(expr.id, &[pred]);
 298                 self.add_exiting_edge(expr, a,
 299                                       loop_scope, loop_scope.continue_index);
 300                 self.add_unreachable_node()
 301             }
 302
 303             hir::ExprVec(ref elems) => {
 304                 self.straightline(expr, pred, elems.iter().map(|e| &**e))
 305             }
 306
 307             hir::ExprCall(ref func, ref args) => {
 308                 self.call(expr, pred, &func, args.iter().map(|e| &**e))
 309             }
 310
 311             hir::ExprMethodCall(_, _, ref args) => {
 312                 self.call(expr, pred, &args[0], args[1..].iter().map(|e| &**e))
 313             }
 314
 315             hir::ExprIndex(ref l, ref r) |
 316             hir::ExprBinary(_, ref l, ref r) if self.tcx.is_method_call(expr.id) => {
 317                 self.call(expr, pred, &l, Some(&**r).into_iter())
 318             }
 319
 320             hir::ExprRange(ref start, ref end) => {
 321                 let fields = start.as_ref().map(|e| &**e).into_iter()
 322                     .chain(end.as_ref().map(|e| &**e));
 323                 self.straightline(expr, pred, fields)
 324             }
 325
 326             hir::ExprUnary(_, ref e) if self.tcx.is_method_call(expr.id) => {
 327                 self.call(expr, pred, &e, None::<hir::Expr>.iter())
 328             }
 329
 330             hir::ExprTup(ref exprs) => {
 331                 self.straightline(expr, pred, exprs.iter().map(|e| &**e))
 332             }
 333
 334             hir::ExprStruct(_, ref fields, ref base) => {
 335                 let field_cfg = self.straightline(expr, pred, fields.iter().map(|f| &*f.expr));
 336                 self.opt_expr(base, field_cfg)
 337             }
 338
 339             hir::ExprRepeat(ref elem, ref count) => {
 340                 self.straightline(expr, pred, [elem, count].iter().map(|&e| &**e))
 341             }
 342
 343             hir::ExprAssign(ref l, ref r) |
 344             hir::ExprAssignOp(_, ref l, ref r) => {
 345                 self.straightline(expr, pred, [r, l].iter().map(|&e| &**e))
 346             }
 347
 348             hir::ExprIndex(ref l, ref r) |
 349             hir::ExprBinary(_, ref l, ref r) => { // NB: && and || handled earlier
 350                 self.straightline(expr, pred, [l, r].iter().map(|&e| &**e))
 351             }
 352
 353             hir::ExprBox(ref e) |
 354             hir::ExprAddrOf(_, ref e) |
 355             hir::ExprCast(ref e, _) |
 356             hir::ExprType(ref e, _) |
 357             hir::ExprUnary(_, ref e) |
 358             hir::ExprField(ref e, _) |
 359             hir::ExprTupField(ref e, _) => {
 360                 self.straightline(expr, pred, Some(&**e).into_iter())
 361             }
 362
 363             hir::ExprInlineAsm(ref inline_asm) => {
 364                 let inputs = inline_asm.inputs.iter();
 365                 let outputs = inline_asm.outputs.iter();
 366                 let post_inputs = self.exprs(inputs.map(|a| {
 367                     debug!("cfg::construct InlineAsm id:{} input:{:?}", expr.id, a);
 368                     let &(_, ref expr) = a;
 369                     &**expr
 370                 }), pred);
 371                 let post_outputs = self.exprs(outputs.map(|a| {
 372                     debug!("cfg::construct InlineAsm id:{} output:{:?}", expr.id, a);
 373                     &*a.expr
 374                 }), post_inputs);
 375                 self.add_ast_node(expr.id, &[post_outputs])
 376             }
 377
 378             hir::ExprClosure(..) |
 379             hir::ExprLit(..) |
 380             hir::ExprPath(..) => {
 381                 self.straightline(expr, pred, None::<hir::Expr>.iter())
 382             }
 383         }
 384     }
 385
 386     fn call<'b, I: Iterator<Item=&'b hir::Expr>>(&mut self,
 387             call_expr: &hir::Expr,
 388             pred: CFGIndex,
 389             func_or_rcvr: &hir::Expr,
 390             args: I) -> CFGIndex {
 391         let method_call = ty::MethodCall::expr(call_expr.id);
 392         let fn_ty = match self.tcx.tables.borrow().method_map.get(&method_call) {
 393             Some(method) => method.ty,
 394             None => self.tcx.expr_ty_adjusted(func_or_rcvr)
 395         };
 396
 397         let func_or_rcvr_exit = self.expr(func_or_rcvr, pred);
 398         let ret = self.straightline(call_expr, func_or_rcvr_exit, args);
 399         if fn_ty.fn_ret().diverges() {
 400             self.add_unreachable_node()
 401         } else {
 402             ret
 403         }
 404     }
 405
 406     fn exprs<'b, I: Iterator<Item=&'b hir::Expr>>(&mut self,
 407                                              exprs: I,
 408                                              pred: CFGIndex) -> CFGIndex {
 409         //! Constructs graph for `exprs` evaluated in order
 410         exprs.fold(pred, |p, e| self.expr(e, p))
 411     }
 412
 413     fn opt_expr(&mut self,
 414                 opt_expr: &Option<P<hir::Expr>>,
 415                 pred: CFGIndex) -> CFGIndex {
 416         //! Constructs graph for `opt_expr` evaluated, if Some
 417         opt_expr.iter().fold(pred, |p, e| self.expr(&e, p))
 418     }
 419
 420     fn straightline<'b, I: Iterator<Item=&'b hir::Expr>>(&mut self,
 421                     expr: &hir::Expr,
 422                     pred: CFGIndex,
 423                     subexprs: I) -> CFGIndex {
 424         //! Handles case of an expression that evaluates `subexprs` in order
 425
 426         let subexprs_exit = self.exprs(subexprs, pred);
 427         self.add_ast_node(expr.id, &[subexprs_exit])
 428     }
 429
 430     fn match_(&mut self, id: ast::NodeId, discr: &hir::Expr,
 431               arms: &[hir::Arm], pred: CFGIndex) -> CFGIndex {
 432         // The CFG for match expression is quite complex, so no ASCII
 433         // art for it (yet).
 434         //
 435         // The CFG generated below matches roughly what trans puts
 436         // out. Each pattern and guard is visited in parallel, with
 437         // arms containing multiple patterns generating multiple nodes
 438         // for the same guard expression. The guard expressions chain
 439         // into each other from top to bottom, with a specific
 440         // exception to allow some additional valid programs
 441         // (explained below). Trans differs slightly in that the
 442         // pattern matching may continue after a guard but the visible
 443         // behaviour should be the same.
 444         //
 445         // What is going on is explained in further comments.
 446
 447         // Visit the discriminant expression
 448         let discr_exit = self.expr(discr, pred);
 449
 450         // Add a node for the exit of the match expression as a whole.
 451         let expr_exit = self.add_ast_node(id, &[]);
 452
 453         // Keep track of the previous guard expressions
 454         let mut prev_guards = Vec::new();
 455         // Track if the previous pattern contained bindings or wildcards
 456         let mut prev_has_bindings = false;
 457
 458         for arm in arms {
 459             // Add an exit node for when we've visited all the
 460             // patterns and the guard (if there is one) in the arm.
 461             let arm_exit = self.add_dummy_node(&[]);
 462
 463             for pat in &arm.pats {
 464                 // Visit the pattern, coming from the discriminant exit
 465                 let mut pat_exit = self.pat(&pat, discr_exit);
 466
 467                 // If there is a guard expression, handle it here
 468                 if let Some(ref guard) = arm.guard {
 469                     // Add a dummy node for the previous guard
 470                     // expression to target
 471                     let guard_start = self.add_dummy_node(&[pat_exit]);
 472                     // Visit the guard expression
 473                     let guard_exit = self.expr(&guard, guard_start);
 474
 475                     let this_has_bindings = pat_util::pat_contains_bindings_or_wild(
 476                         &self.tcx.def_map.borrow(), &pat);
 477
 478                     // If both this pattern and the previous pattern
 479                     // were free of bindings, they must consist only
 480                     // of "constant" patterns. Note we cannot match an
 481                     // all-constant pattern, fail the guard, and then
 482                     // match *another* all-constant pattern. This is
 483                     // because if the previous pattern matches, then
 484                     // we *cannot* match this one, unless all the
 485                     // constants are the same (which is rejected by
 486                     // `check_match`).
 487                     //
 488                     // We can use this to be smarter about the flow
 489                     // along guards. If the previous pattern matched,
 490                     // then we know we will not visit the guard in
 491                     // this one (whether or not the guard succeeded),
 492                     // if the previous pattern failed, then we know
 493                     // the guard for that pattern will not have been
 494                     // visited. Thus, it is not possible to visit both
 495                     // the previous guard and the current one when
 496                     // both patterns consist only of constant
 497                     // sub-patterns.
 498                     //
 499                     // However, if the above does not hold, then all
 500                     // previous guards need to be wired to visit the
 501                     // current guard pattern.
 502                     if prev_has_bindings || this_has_bindings {
 503                         while let Some(prev) = prev_guards.pop() {
 504                             self.add_contained_edge(prev, guard_start);
 505                         }
 506                     }
 507
 508                     prev_has_bindings = this_has_bindings;
 509
 510                     // Push the guard onto the list of previous guards
 511                     prev_guards.push(guard_exit);
 512
 513                     // Update the exit node for the pattern
 514                     pat_exit = guard_exit;
 515                 }
 516
 517                 // Add an edge from the exit of this pattern to the
 518                 // exit of the arm
 519                 self.add_contained_edge(pat_exit, arm_exit);
 520             }
 521
 522             // Visit the body of this arm
 523             let body_exit = self.expr(&arm.body, arm_exit);
 524
 525             // Link the body to the exit of the expression
 526             self.add_contained_edge(body_exit, expr_exit);
 527         }
 528
 529         expr_exit
 530     }
 531
 532     fn add_dummy_node(&mut self, preds: &[CFGIndex]) -> CFGIndex {
 533         self.add_node(CFGNodeData::Dummy, preds)
 534     }
 535
 536     fn add_ast_node(&mut self, id: ast::NodeId, preds: &[CFGIndex]) -> CFGIndex {
 537         assert!(id != ast::DUMMY_NODE_ID);
 538         self.add_node(CFGNodeData::AST(id), preds)
 539     }
 540
 541     fn add_unreachable_node(&mut self) -> CFGIndex {
 542         self.add_node(CFGNodeData::Unreachable, &[])
 543     }
 544
 545     fn add_node(&mut self, data: CFGNodeData, preds: &[CFGIndex]) -> CFGIndex {
 546         let node = self.graph.add_node(data);
 547         for &pred in preds {
 548             self.add_contained_edge(pred, node);
 549         }
 550         node
 551     }
 552
 553     fn add_contained_edge(&mut self,
 554                           source: CFGIndex,
 555                           target: CFGIndex) {
 556         let data = CFGEdgeData {exiting_scopes: vec!() };
 557         self.graph.add_edge(source, target, data);
 558     }
 559
 560     fn add_exiting_edge(&mut self,
 561                         from_expr: &hir::Expr,
 562                         from_index: CFGIndex,
 563                         to_loop: LoopScope,
 564                         to_index: CFGIndex) {
 565         let mut data = CFGEdgeData {exiting_scopes: vec!() };
 566         let mut scope = self.tcx.region_maps.node_extent(from_expr.id);
 567         let target_scope = self.tcx.region_maps.node_extent(to_loop.loop_id);
 568         while scope != target_scope {
 569             data.exiting_scopes.push(scope.node_id(&self.tcx.region_maps));
 570             scope = self.tcx.region_maps.encl_scope(scope);
 571         }
 572         self.graph.add_edge(from_index, to_index, data);
 573     }
 574
 575     fn add_returning_edge(&mut self,
 576                           _from_expr: &hir::Expr,
 577                           from_index: CFGIndex) {
 578         let mut data = CFGEdgeData {
 579             exiting_scopes: vec!(),
 580         };
 581         for &LoopScope { loop_id: id, .. } in self.loop_scopes.iter().rev() {
 582             data.exiting_scopes.push(id);
 583         }
 584         self.graph.add_edge(from_index, self.fn_exit, data);
 585     }
 586
 587     fn find_scope(&self,
 588                   expr: &hir::Expr,
 589                   label: Option<ast::Name>) -> LoopScope {
 590         if label.is_none() {
 591             return *self.loop_scopes.last().unwrap();
 592         }
 593
 594         match self.tcx.def_map.borrow().get(&expr.id).map(|d| d.full_def()) {
 595             Some(Def::Label(loop_id)) => {
 596                 for l in &self.loop_scopes {
 597                     if l.loop_id == loop_id {
 598                         return *l;
 599                     }
 600                 }
 601                 self.tcx.sess.span_bug(expr.span,
 602                     &format!("no loop scope for id {}", loop_id));
 603             }
 604
 605             r => {
 606                 self.tcx.sess.span_bug(expr.span,
 607                     &format!("bad entry `{:?}` in def_map for label", r));
 608             }
 609         }
 610     }
 611 }