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.
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.
11 //! A classic liveness analysis based on dataflow over the AST. Computes,
12 //! for each local variable in a function, whether that variable is live
13 //! at a given point. Program execution points are identified by their
18 //! The basic model is that each local variable is assigned an index. We
19 //! represent sets of local variables using a vector indexed by this
20 //! index. The value in the vector is either 0, indicating the variable
21 //! is dead, or the id of an expression that uses the variable.
23 //! We conceptually walk over the AST in reverse execution order. If we
24 //! find a use of a variable, we add it to the set of live variables. If
25 //! we find an assignment to a variable, we remove it from the set of live
26 //! variables. When we have to merge two flows, we take the union of
27 //! those two flows---if the variable is live on both paths, we simply
28 //! pick one id. In the event of loops, we continue doing this until a
29 //! fixed point is reached.
31 //! ## Checking initialization
33 //! At the function entry point, all variables must be dead. If this is
34 //! not the case, we can report an error using the id found in the set of
35 //! live variables, which identifies a use of the variable which is not
36 //! dominated by an assignment.
40 //! After each explicit move, the variable must be dead.
42 //! ## Computing last uses
44 //! Any use of the variable where the variable is dead afterwards is a
47 //! # Implementation details
49 //! The actual implementation contains two (nested) walks over the AST.
50 //! The outer walk has the job of building up the ir_maps instance for the
51 //! enclosing function. On the way down the tree, it identifies those AST
52 //! nodes and variable IDs that will be needed for the liveness analysis
53 //! and assigns them contiguous IDs. The liveness id for an AST node is
54 //! called a `live_node` (it's a newtype'd usize) and the id for a variable
55 //! is called a `variable` (another newtype'd usize).
57 //! On the way back up the tree, as we are about to exit from a function
58 //! declaration we allocate a `liveness` instance. Now that we know
59 //! precisely how many nodes and variables we need, we can allocate all
60 //! the various arrays that we will need to precisely the right size. We then
61 //! perform the actual propagation on the `liveness` instance.
63 //! This propagation is encoded in the various `propagate_through_*()`
64 //! methods. It effectively does a reverse walk of the AST; whenever we
65 //! reach a loop node, we iterate until a fixed point is reached.
67 //! ## The `Users` struct
69 //! At each live node `N`, we track three pieces of information for each
70 //! variable `V` (these are encapsulated in the `Users` struct):
72 //! - `reader`: the `LiveNode` ID of some node which will read the value
73 //! that `V` holds on entry to `N`. Formally: a node `M` such
74 //! that there exists a path `P` from `N` to `M` where `P` does not
75 //! write `V`. If the `reader` is `invalid_node()`, then the current
76 //! value will never be read (the variable is dead, essentially).
78 //! - `writer`: the `LiveNode` ID of some node which will write the
79 //! variable `V` and which is reachable from `N`. Formally: a node `M`
80 //! such that there exists a path `P` from `N` to `M` and `M` writes
81 //! `V`. If the `writer` is `invalid_node()`, then there is no writer
82 //! of `V` that follows `N`.
84 //! - `used`: a boolean value indicating whether `V` is *used*. We
85 //! distinguish a *read* from a *use* in that a *use* is some read that
86 //! is not just used to generate a new value. For example, `x += 1` is
87 //! a read but not a use. This is used to generate better warnings.
89 //! ## Special Variables
91 //! We generate various special variables for various, well, special purposes.
92 //! These are described in the `specials` struct:
94 //! - `exit_ln`: a live node that is generated to represent every 'exit' from
95 //! the function, whether it be by explicit return, panic, or other means.
97 //! - `fallthrough_ln`: a live node that represents a fallthrough
99 //! - `no_ret_var`: a synthetic variable that is only 'read' from, the
100 //! fallthrough node. This allows us to detect functions where we fail
101 //! to return explicitly.
102 //! - `clean_exit_var`: a synthetic variable that is only 'read' from the
103 //! fallthrough node. It is only live if the function could converge
104 //! via means other than an explicit `return` expression. That is, it is
105 //! only dead if the end of the function's block can never be reached.
106 //! It is the responsibility of typeck to ensure that there are no
107 //! `return` expressions in a function declared as diverging.
108 use self::LoopKind::*;
109 use self::LiveNodeKind::*;
110 use self::VarKind::*;
112 use dep_graph::DepNode;
114 use middle::pat_util;
115 use middle::ty::{self, TyCtxt};
117 use util::nodemap::NodeMap;
119 use std::{fmt, usize};
120 use std::io::prelude::*;
123 use syntax::ast::{self, NodeId};
124 use syntax::codemap::{BytePos, original_sp, Span};
125 use syntax::parse::token::special_idents;
128 use rustc_front::hir::Expr;
129 use rustc_front::hir;
130 use rustc_front::print::pprust::{expr_to_string, block_to_string};
131 use rustc_front::intravisit::{self, Visitor, FnKind};
133 /// For use with `propagate_through_loop`.
135 /// An endless `loop` loop.
137 /// A `while` loop, with the given expression as condition.
141 #[derive(Copy, Clone, PartialEq)]
142 struct Variable(usize);
144 #[derive(Copy, PartialEq)]
145 struct LiveNode(usize);
148 fn get(&self) -> usize { let Variable(v) = *self; v }
152 fn get(&self) -> usize { let LiveNode(v) = *self; v }
155 impl Clone for LiveNode {
156 fn clone(&self) -> LiveNode {
161 #[derive(Copy, Clone, PartialEq, Debug)]
169 fn live_node_kind_to_string(lnk: LiveNodeKind, cx: &TyCtxt) -> String {
170 let cm = cx.sess.codemap();
173 format!("Free var node [{}]", cm.span_to_string(s))
176 format!("Expr node [{}]", cm.span_to_string(s))
179 format!("Var def node [{}]", cm.span_to_string(s))
181 ExitNode => "Exit node".to_string(),
185 impl<'a, 'tcx, 'v> Visitor<'v> for IrMaps<'a, 'tcx> {
186 fn visit_fn(&mut self, fk: FnKind<'v>, fd: &'v hir::FnDecl,
187 b: &'v hir::Block, s: Span, id: NodeId) {
188 visit_fn(self, fk, fd, b, s, id);
190 fn visit_local(&mut self, l: &hir::Local) { visit_local(self, l); }
191 fn visit_expr(&mut self, ex: &Expr) { visit_expr(self, ex); }
192 fn visit_arm(&mut self, a: &hir::Arm) { visit_arm(self, a); }
195 pub fn check_crate(tcx: &TyCtxt) {
196 let _task = tcx.dep_graph.in_task(DepNode::Liveness);
197 tcx.map.krate().visit_all_items(&mut IrMaps::new(tcx));
198 tcx.sess.abort_if_errors();
201 impl fmt::Debug for LiveNode {
202 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
203 write!(f, "ln({})", self.get())
207 impl fmt::Debug for Variable {
208 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
209 write!(f, "v({})", self.get())
213 // ______________________________________________________________________
216 // This is the first pass and the one that drives the main
217 // computation. It walks up and down the IR once. On the way down,
218 // we count for each function the number of variables as well as
219 // liveness nodes. A liveness node is basically an expression or
220 // capture clause that does something of interest: either it has
221 // interesting control flow or it uses/defines a local variable.
223 // On the way back up, at each function node we create liveness sets
224 // (we now know precisely how big to make our various vectors and so
225 // forth) and then do the data-flow propagation to compute the set
226 // of live variables at each program point.
228 // Finally, we run back over the IR one last time and, using the
229 // computed liveness, check various safety conditions. For example,
230 // there must be no live nodes at the definition site for a variable
231 // unless it has an initializer. Similarly, each non-mutable local
232 // variable must not be assigned if there is some successor
233 // assignment. And so forth.
236 fn is_valid(&self) -> bool {
237 self.get() != usize::MAX
241 fn invalid_node() -> LiveNode { LiveNode(usize::MAX) }
248 #[derive(Copy, Clone, Debug)]
254 #[derive(Copy, Clone, Debug)]
256 Arg(NodeId, ast::Name),
262 struct IrMaps<'a, 'tcx: 'a> {
263 tcx: &'a TyCtxt<'tcx>,
265 num_live_nodes: usize,
267 live_node_map: NodeMap<LiveNode>,
268 variable_map: NodeMap<Variable>,
269 capture_info_map: NodeMap<Rc<Vec<CaptureInfo>>>,
270 var_kinds: Vec<VarKind>,
271 lnks: Vec<LiveNodeKind>,
274 impl<'a, 'tcx> IrMaps<'a, 'tcx> {
275 fn new(tcx: &'a TyCtxt<'tcx>) -> IrMaps<'a, 'tcx> {
280 live_node_map: NodeMap(),
281 variable_map: NodeMap(),
282 capture_info_map: NodeMap(),
283 var_kinds: Vec::new(),
288 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
289 let ln = LiveNode(self.num_live_nodes);
291 self.num_live_nodes += 1;
293 debug!("{:?} is of kind {}", ln,
294 live_node_kind_to_string(lnk, self.tcx));
299 fn add_live_node_for_node(&mut self, node_id: NodeId, lnk: LiveNodeKind) {
300 let ln = self.add_live_node(lnk);
301 self.live_node_map.insert(node_id, ln);
303 debug!("{:?} is node {}", ln, node_id);
306 fn add_variable(&mut self, vk: VarKind) -> Variable {
307 let v = Variable(self.num_vars);
308 self.var_kinds.push(vk);
312 Local(LocalInfo { id: node_id, .. }) | Arg(node_id, _) => {
313 self.variable_map.insert(node_id, v);
315 ImplicitRet | CleanExit => {}
318 debug!("{:?} is {:?}", v, vk);
323 fn variable(&self, node_id: NodeId, span: Span) -> Variable {
324 match self.variable_map.get(&node_id) {
329 .span_bug(span, &format!("no variable registered for id {}",
335 fn variable_name(&self, var: Variable) -> String {
336 match self.var_kinds[var.get()] {
337 Local(LocalInfo { name, .. }) | Arg(_, name) => {
340 ImplicitRet => "<implicit-ret>".to_string(),
341 CleanExit => "<clean-exit>".to_string()
345 fn set_captures(&mut self, node_id: NodeId, cs: Vec<CaptureInfo>) {
346 self.capture_info_map.insert(node_id, Rc::new(cs));
349 fn lnk(&self, ln: LiveNode) -> LiveNodeKind {
354 impl<'a, 'tcx, 'v> Visitor<'v> for Liveness<'a, 'tcx> {
355 fn visit_fn(&mut self, fk: FnKind<'v>, fd: &'v hir::FnDecl,
356 b: &'v hir::Block, s: Span, n: NodeId) {
357 check_fn(self, fk, fd, b, s, n);
359 fn visit_local(&mut self, l: &hir::Local) {
360 check_local(self, l);
362 fn visit_expr(&mut self, ex: &Expr) {
363 check_expr(self, ex);
365 fn visit_arm(&mut self, a: &hir::Arm) {
370 fn visit_fn(ir: &mut IrMaps,
378 // swap in a new set of IR maps for this function body:
379 let mut fn_maps = IrMaps::new(ir.tcx);
381 debug!("creating fn_maps: {:?}", &fn_maps as *const IrMaps);
383 for arg in &decl.inputs {
384 pat_util::pat_bindings(&ir.tcx.def_map,
386 |_bm, arg_id, _x, path1| {
387 debug!("adding argument {}", arg_id);
388 let name = path1.node;
389 fn_maps.add_variable(Arg(arg_id, name));
393 // gather up the various local variables, significant expressions,
395 intravisit::walk_fn(&mut fn_maps, fk, decl, body, sp);
397 // Special nodes and variables:
398 // - exit_ln represents the end of the fn, either by return or panic
399 // - implicit_ret_var is a pseudo-variable that represents
400 // an implicit return
401 let specials = Specials {
402 exit_ln: fn_maps.add_live_node(ExitNode),
403 fallthrough_ln: fn_maps.add_live_node(ExitNode),
404 no_ret_var: fn_maps.add_variable(ImplicitRet),
405 clean_exit_var: fn_maps.add_variable(CleanExit)
409 let mut lsets = Liveness::new(&mut fn_maps, specials);
410 let entry_ln = lsets.compute(decl, body);
412 // check for various error conditions
413 lsets.visit_block(body);
414 lsets.check_ret(id, sp, fk, entry_ln, body);
415 lsets.warn_about_unused_args(decl, entry_ln);
418 fn visit_local(ir: &mut IrMaps, local: &hir::Local) {
419 pat_util::pat_bindings(&ir.tcx.def_map, &local.pat, |_, p_id, sp, path1| {
420 debug!("adding local variable {}", p_id);
421 let name = path1.node;
422 ir.add_live_node_for_node(p_id, VarDefNode(sp));
423 ir.add_variable(Local(LocalInfo {
428 intravisit::walk_local(ir, local);
431 fn visit_arm(ir: &mut IrMaps, arm: &hir::Arm) {
432 for pat in &arm.pats {
433 pat_util::pat_bindings(&ir.tcx.def_map, &pat, |bm, p_id, sp, path1| {
434 debug!("adding local variable {} from match with bm {:?}",
436 let name = path1.node;
437 ir.add_live_node_for_node(p_id, VarDefNode(sp));
438 ir.add_variable(Local(LocalInfo {
444 intravisit::walk_arm(ir, arm);
447 fn visit_expr(ir: &mut IrMaps, expr: &Expr) {
449 // live nodes required for uses or definitions of variables:
450 hir::ExprPath(..) => {
451 let def = ir.tcx.def_map.borrow().get(&expr.id).unwrap().full_def();
452 debug!("expr {}: path that leads to {:?}", expr.id, def);
453 if let Def::Local(..) = def {
454 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
456 intravisit::walk_expr(ir, expr);
458 hir::ExprClosure(..) => {
459 // Interesting control flow (for loops can contain labeled
460 // breaks or continues)
461 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
463 // Make a live_node for each captured variable, with the span
464 // being the location that the variable is used. This results
465 // in better error messages than just pointing at the closure
466 // construction site.
467 let mut call_caps = Vec::new();
468 ir.tcx.with_freevars(expr.id, |freevars| {
470 if let Def::Local(_, rv) = fv.def {
471 let fv_ln = ir.add_live_node(FreeVarNode(fv.span));
472 call_caps.push(CaptureInfo {ln: fv_ln,
477 ir.set_captures(expr.id, call_caps);
479 intravisit::walk_expr(ir, expr);
482 // live nodes required for interesting control flow:
483 hir::ExprIf(..) | hir::ExprMatch(..) | hir::ExprWhile(..) | hir::ExprLoop(..) => {
484 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
485 intravisit::walk_expr(ir, expr);
487 hir::ExprBinary(op, _, _) if ::rustc_front::util::lazy_binop(op.node) => {
488 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
489 intravisit::walk_expr(ir, expr);
492 // otherwise, live nodes are not required:
493 hir::ExprIndex(..) | hir::ExprField(..) | hir::ExprTupField(..) |
494 hir::ExprVec(..) | hir::ExprCall(..) | hir::ExprMethodCall(..) |
495 hir::ExprTup(..) | hir::ExprBinary(..) | hir::ExprAddrOf(..) |
496 hir::ExprCast(..) | hir::ExprUnary(..) | hir::ExprBreak(_) |
497 hir::ExprAgain(_) | hir::ExprLit(_) | hir::ExprRet(..) |
498 hir::ExprBlock(..) | hir::ExprAssign(..) | hir::ExprAssignOp(..) |
499 hir::ExprStruct(..) | hir::ExprRepeat(..) |
500 hir::ExprInlineAsm(..) | hir::ExprBox(..) |
501 hir::ExprType(..) => {
502 intravisit::walk_expr(ir, expr);
507 // ______________________________________________________________________
508 // Computing liveness sets
510 // Actually we compute just a bit more than just liveness, but we use
511 // the same basic propagation framework in all cases.
513 #[derive(Clone, Copy)]
520 fn invalid_users() -> Users {
522 reader: invalid_node(),
523 writer: invalid_node(),
528 #[derive(Copy, Clone)]
531 fallthrough_ln: LiveNode,
532 no_ret_var: Variable,
533 clean_exit_var: Variable
536 const ACC_READ: u32 = 1;
537 const ACC_WRITE: u32 = 2;
538 const ACC_USE: u32 = 4;
540 struct Liveness<'a, 'tcx: 'a> {
541 ir: &'a mut IrMaps<'a, 'tcx>,
543 successors: Vec<LiveNode>,
545 // The list of node IDs for the nested loop scopes
547 loop_scope: Vec<NodeId>,
548 // mappings from loop node ID to LiveNode
549 // ("break" label should map to loop node ID,
550 // it probably doesn't now)
551 break_ln: NodeMap<LiveNode>,
552 cont_ln: NodeMap<LiveNode>
555 impl<'a, 'tcx> Liveness<'a, 'tcx> {
556 fn new(ir: &'a mut IrMaps<'a, 'tcx>, specials: Specials) -> Liveness<'a, 'tcx> {
557 let num_live_nodes = ir.num_live_nodes;
558 let num_vars = ir.num_vars;
562 successors: vec![invalid_node(); num_live_nodes],
563 users: vec![invalid_users(); num_live_nodes * num_vars],
564 loop_scope: Vec::new(),
570 fn live_node(&self, node_id: NodeId, span: Span) -> LiveNode {
571 match self.ir.live_node_map.get(&node_id) {
574 // This must be a mismatch between the ir_map construction
575 // above and the propagation code below; the two sets of
576 // code have to agree about which AST nodes are worth
577 // creating liveness nodes for.
578 self.ir.tcx.sess.span_bug(
580 &format!("no live node registered for node {}",
586 fn variable(&self, node_id: NodeId, span: Span) -> Variable {
587 self.ir.variable(node_id, span)
590 fn pat_bindings<F>(&mut self, pat: &hir::Pat, mut f: F) where
591 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId),
593 pat_util::pat_bindings(&self.ir.tcx.def_map, pat, |_bm, p_id, sp, _n| {
594 let ln = self.live_node(p_id, sp);
595 let var = self.variable(p_id, sp);
596 f(self, ln, var, sp, p_id);
600 fn arm_pats_bindings<F>(&mut self, pat: Option<&hir::Pat>, f: F) where
601 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId),
605 self.pat_bindings(pat, f);
611 fn define_bindings_in_pat(&mut self, pat: &hir::Pat, succ: LiveNode)
613 self.define_bindings_in_arm_pats(Some(pat), succ)
616 fn define_bindings_in_arm_pats(&mut self, pat: Option<&hir::Pat>, succ: LiveNode)
619 self.arm_pats_bindings(pat, |this, ln, var, _sp, _id| {
620 this.init_from_succ(ln, succ);
621 this.define(ln, var);
627 fn idx(&self, ln: LiveNode, var: Variable) -> usize {
628 ln.get() * self.ir.num_vars + var.get()
631 fn live_on_entry(&self, ln: LiveNode, var: Variable)
632 -> Option<LiveNodeKind> {
633 assert!(ln.is_valid());
634 let reader = self.users[self.idx(ln, var)].reader;
635 if reader.is_valid() {Some(self.ir.lnk(reader))} else {None}
639 Is this variable live on entry to any of its successor nodes?
641 fn live_on_exit(&self, ln: LiveNode, var: Variable)
642 -> Option<LiveNodeKind> {
643 let successor = self.successors[ln.get()];
644 self.live_on_entry(successor, var)
647 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
648 assert!(ln.is_valid());
649 self.users[self.idx(ln, var)].used
652 fn assigned_on_entry(&self, ln: LiveNode, var: Variable)
653 -> Option<LiveNodeKind> {
654 assert!(ln.is_valid());
655 let writer = self.users[self.idx(ln, var)].writer;
656 if writer.is_valid() {Some(self.ir.lnk(writer))} else {None}
659 fn assigned_on_exit(&self, ln: LiveNode, var: Variable)
660 -> Option<LiveNodeKind> {
661 let successor = self.successors[ln.get()];
662 self.assigned_on_entry(successor, var)
665 fn indices2<F>(&mut self, ln: LiveNode, succ_ln: LiveNode, mut op: F) where
666 F: FnMut(&mut Liveness<'a, 'tcx>, usize, usize),
668 let node_base_idx = self.idx(ln, Variable(0));
669 let succ_base_idx = self.idx(succ_ln, Variable(0));
670 for var_idx in 0..self.ir.num_vars {
671 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
675 fn write_vars<F>(&self,
679 -> io::Result<()> where
680 F: FnMut(usize) -> LiveNode,
682 let node_base_idx = self.idx(ln, Variable(0));
683 for var_idx in 0..self.ir.num_vars {
684 let idx = node_base_idx + var_idx;
685 if test(idx).is_valid() {
686 try!(write!(wr, " {:?}", Variable(var_idx)));
692 fn find_loop_scope(&self,
693 opt_label: Option<ast::Name>,
699 // Refers to a labeled loop. Use the results of resolve
701 match self.ir.tcx.def_map.borrow().get(&id).map(|d| d.full_def()) {
702 Some(Def::Label(loop_id)) => loop_id,
703 _ => self.ir.tcx.sess.span_bug(sp, "label on break/loop \
704 doesn't refer to a loop")
708 // Vanilla 'break' or 'loop', so use the enclosing
710 if self.loop_scope.is_empty() {
711 self.ir.tcx.sess.span_bug(sp, "break outside loop");
713 *self.loop_scope.last().unwrap()
719 #[allow(unused_must_use)]
720 fn ln_str(&self, ln: LiveNode) -> String {
721 let mut wr = Vec::new();
723 let wr = &mut wr as &mut Write;
724 write!(wr, "[ln({:?}) of kind {:?} reads", ln.get(), self.ir.lnk(ln));
725 self.write_vars(wr, ln, |idx| self.users[idx].reader);
726 write!(wr, " writes");
727 self.write_vars(wr, ln, |idx| self.users[idx].writer);
728 write!(wr, " precedes {:?}]", self.successors[ln.get()]);
730 String::from_utf8(wr).unwrap()
733 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
734 self.successors[ln.get()] = succ_ln;
736 // It is not necessary to initialize the
737 // values to empty because this is the value
738 // they have when they are created, and the sets
739 // only grow during iterations.
741 // self.indices(ln) { |idx|
742 // self.users[idx] = invalid_users();
746 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
747 // more efficient version of init_empty() / merge_from_succ()
748 self.successors[ln.get()] = succ_ln;
750 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
751 this.users[idx] = this.users[succ_idx]
753 debug!("init_from_succ(ln={}, succ={})",
754 self.ln_str(ln), self.ln_str(succ_ln));
757 fn merge_from_succ(&mut self,
762 if ln == succ_ln { return false; }
764 let mut changed = false;
765 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
766 changed |= copy_if_invalid(this.users[succ_idx].reader,
767 &mut this.users[idx].reader);
768 changed |= copy_if_invalid(this.users[succ_idx].writer,
769 &mut this.users[idx].writer);
770 if this.users[succ_idx].used && !this.users[idx].used {
771 this.users[idx].used = true;
776 debug!("merge_from_succ(ln={:?}, succ={}, first_merge={}, changed={})",
777 ln, self.ln_str(succ_ln), first_merge, changed);
780 fn copy_if_invalid(src: LiveNode, dst: &mut LiveNode) -> bool {
781 if src.is_valid() && !dst.is_valid() {
790 // Indicates that a local variable was *defined*; we know that no
791 // uses of the variable can precede the definition (resolve checks
792 // this) so we just clear out all the data.
793 fn define(&mut self, writer: LiveNode, var: Variable) {
794 let idx = self.idx(writer, var);
795 self.users[idx].reader = invalid_node();
796 self.users[idx].writer = invalid_node();
798 debug!("{:?} defines {:?} (idx={}): {}", writer, var,
799 idx, self.ln_str(writer));
802 // Either read, write, or both depending on the acc bitset
803 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
804 debug!("{:?} accesses[{:x}] {:?}: {}",
805 ln, acc, var, self.ln_str(ln));
807 let idx = self.idx(ln, var);
808 let user = &mut self.users[idx];
810 if (acc & ACC_WRITE) != 0 {
811 user.reader = invalid_node();
815 // Important: if we both read/write, must do read second
816 // or else the write will override.
817 if (acc & ACC_READ) != 0 {
821 if (acc & ACC_USE) != 0 {
826 // _______________________________________________________________________
828 fn compute(&mut self, decl: &hir::FnDecl, body: &hir::Block) -> LiveNode {
829 // if there is a `break` or `again` at the top level, then it's
830 // effectively a return---this only occurs in `for` loops,
831 // where the body is really a closure.
833 debug!("compute: using id for block, {}", block_to_string(body));
835 let exit_ln = self.s.exit_ln;
836 let entry_ln: LiveNode =
837 self.with_loop_nodes(body.id, exit_ln, exit_ln,
838 |this| this.propagate_through_fn_block(decl, body));
840 // hack to skip the loop unless debug! is enabled:
841 debug!("^^ liveness computation results for body {} (entry={:?})",
843 for ln_idx in 0..self.ir.num_live_nodes {
844 debug!("{:?}", self.ln_str(LiveNode(ln_idx)));
853 fn propagate_through_fn_block(&mut self, _: &hir::FnDecl, blk: &hir::Block)
855 // the fallthrough exit is only for those cases where we do not
856 // explicitly return:
858 self.init_from_succ(s.fallthrough_ln, s.exit_ln);
859 if blk.expr.is_none() {
860 self.acc(s.fallthrough_ln, s.no_ret_var, ACC_READ)
862 self.acc(s.fallthrough_ln, s.clean_exit_var, ACC_READ);
864 self.propagate_through_block(blk, s.fallthrough_ln)
867 fn propagate_through_block(&mut self, blk: &hir::Block, succ: LiveNode)
869 let succ = self.propagate_through_opt_expr(blk.expr.as_ref().map(|e| &**e), succ);
870 blk.stmts.iter().rev().fold(succ, |succ, stmt| {
871 self.propagate_through_stmt(stmt, succ)
875 fn propagate_through_stmt(&mut self, stmt: &hir::Stmt, succ: LiveNode)
878 hir::StmtDecl(ref decl, _) => {
879 self.propagate_through_decl(&decl, succ)
882 hir::StmtExpr(ref expr, _) | hir::StmtSemi(ref expr, _) => {
883 self.propagate_through_expr(&expr, succ)
888 fn propagate_through_decl(&mut self, decl: &hir::Decl, succ: LiveNode)
891 hir::DeclLocal(ref local) => {
892 self.propagate_through_local(&local, succ)
894 hir::DeclItem(_) => succ,
898 fn propagate_through_local(&mut self, local: &hir::Local, succ: LiveNode)
900 // Note: we mark the variable as defined regardless of whether
901 // there is an initializer. Initially I had thought to only mark
902 // the live variable as defined if it was initialized, and then we
903 // could check for uninit variables just by scanning what is live
904 // at the start of the function. But that doesn't work so well for
905 // immutable variables defined in a loop:
906 // loop { let x; x = 5; }
907 // because the "assignment" loops back around and generates an error.
909 // So now we just check that variables defined w/o an
910 // initializer are not live at the point of their
911 // initialization, which is mildly more complex than checking
912 // once at the func header but otherwise equivalent.
914 let succ = self.propagate_through_opt_expr(local.init.as_ref().map(|e| &**e), succ);
915 self.define_bindings_in_pat(&local.pat, succ)
918 fn propagate_through_exprs(&mut self, exprs: &[P<Expr>], succ: LiveNode)
920 exprs.iter().rev().fold(succ, |succ, expr| {
921 self.propagate_through_expr(&expr, succ)
925 fn propagate_through_opt_expr(&mut self,
926 opt_expr: Option<&Expr>,
929 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
932 fn propagate_through_expr(&mut self, expr: &Expr, succ: LiveNode)
934 debug!("propagate_through_expr: {}", expr_to_string(expr));
937 // Interesting cases with control flow or which gen/kill
939 hir::ExprPath(..) => {
940 self.access_path(expr, succ, ACC_READ | ACC_USE)
943 hir::ExprField(ref e, _) => {
944 self.propagate_through_expr(&e, succ)
947 hir::ExprTupField(ref e, _) => {
948 self.propagate_through_expr(&e, succ)
951 hir::ExprClosure(_, _, ref blk) => {
952 debug!("{} is an ExprClosure",
953 expr_to_string(expr));
956 The next-node for a break is the successor of the entire
957 loop. The next-node for a continue is the top of this loop.
959 let node = self.live_node(expr.id, expr.span);
960 self.with_loop_nodes(blk.id, succ, node, |this| {
962 // the construction of a closure itself is not important,
963 // but we have to consider the closed over variables.
964 let caps = match this.ir.capture_info_map.get(&expr.id) {
965 Some(caps) => caps.clone(),
967 this.ir.tcx.sess.span_bug(expr.span, "no registered caps");
970 caps.iter().rev().fold(succ, |succ, cap| {
971 this.init_from_succ(cap.ln, succ);
972 let var = this.variable(cap.var_nid, expr.span);
973 this.acc(cap.ln, var, ACC_READ | ACC_USE);
979 hir::ExprIf(ref cond, ref then, ref els) => {
993 let else_ln = self.propagate_through_opt_expr(els.as_ref().map(|e| &**e), succ);
994 let then_ln = self.propagate_through_block(&then, succ);
995 let ln = self.live_node(expr.id, expr.span);
996 self.init_from_succ(ln, else_ln);
997 self.merge_from_succ(ln, then_ln, false);
998 self.propagate_through_expr(&cond, ln)
1001 hir::ExprWhile(ref cond, ref blk, _) => {
1002 self.propagate_through_loop(expr, WhileLoop(&cond), &blk, succ)
1005 // Note that labels have been resolved, so we don't need to look
1006 // at the label ident
1007 hir::ExprLoop(ref blk, _) => {
1008 self.propagate_through_loop(expr, LoopLoop, &blk, succ)
1011 hir::ExprMatch(ref e, ref arms, _) => {
1026 let ln = self.live_node(expr.id, expr.span);
1027 self.init_empty(ln, succ);
1028 let mut first_merge = true;
1031 self.propagate_through_expr(&arm.body, succ);
1033 self.propagate_through_opt_expr(arm.guard.as_ref().map(|e| &**e), body_succ);
1034 // only consider the first pattern; any later patterns must have
1035 // the same bindings, and we also consider the first pattern to be
1036 // the "authoritative" set of ids
1038 self.define_bindings_in_arm_pats(arm.pats.first().map(|p| &**p),
1040 self.merge_from_succ(ln, arm_succ, first_merge);
1041 first_merge = false;
1043 self.propagate_through_expr(&e, ln)
1046 hir::ExprRet(ref o_e) => {
1047 // ignore succ and subst exit_ln:
1048 let exit_ln = self.s.exit_ln;
1049 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), exit_ln)
1052 hir::ExprBreak(opt_label) => {
1053 // Find which label this break jumps to
1054 let sc = self.find_loop_scope(opt_label.map(|l| l.node.name), expr.id, expr.span);
1056 // Now that we know the label we're going to,
1057 // look it up in the break loop nodes table
1059 match self.break_ln.get(&sc) {
1061 None => self.ir.tcx.sess.span_bug(expr.span,
1062 "break to unknown label")
1066 hir::ExprAgain(opt_label) => {
1067 // Find which label this expr continues to
1068 let sc = self.find_loop_scope(opt_label.map(|l| l.node.name), expr.id, expr.span);
1070 // Now that we know the label we're going to,
1071 // look it up in the continue loop nodes table
1073 match self.cont_ln.get(&sc) {
1075 None => self.ir.tcx.sess.span_bug(expr.span,
1076 "loop to unknown label")
1080 hir::ExprAssign(ref l, ref r) => {
1081 // see comment on lvalues in
1082 // propagate_through_lvalue_components()
1083 let succ = self.write_lvalue(&l, succ, ACC_WRITE);
1084 let succ = self.propagate_through_lvalue_components(&l, succ);
1085 self.propagate_through_expr(&r, succ)
1088 hir::ExprAssignOp(_, ref l, ref r) => {
1089 // see comment on lvalues in
1090 // propagate_through_lvalue_components()
1091 let succ = self.write_lvalue(&l, succ, ACC_WRITE|ACC_READ);
1092 let succ = self.propagate_through_expr(&r, succ);
1093 self.propagate_through_lvalue_components(&l, succ)
1096 // Uninteresting cases: just propagate in rev exec order
1098 hir::ExprVec(ref exprs) => {
1099 self.propagate_through_exprs(&exprs[..], succ)
1102 hir::ExprRepeat(ref element, ref count) => {
1103 let succ = self.propagate_through_expr(&count, succ);
1104 self.propagate_through_expr(&element, succ)
1107 hir::ExprStruct(_, ref fields, ref with_expr) => {
1108 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1109 fields.iter().rev().fold(succ, |succ, field| {
1110 self.propagate_through_expr(&field.expr, succ)
1114 hir::ExprCall(ref f, ref args) => {
1115 let diverges = !self.ir.tcx.is_method_call(expr.id) &&
1116 self.ir.tcx.expr_ty_adjusted(&f).fn_ret().diverges();
1117 let succ = if diverges {
1122 let succ = self.propagate_through_exprs(&args[..], succ);
1123 self.propagate_through_expr(&f, succ)
1126 hir::ExprMethodCall(_, _, ref args) => {
1127 let method_call = ty::MethodCall::expr(expr.id);
1128 let method_ty = self.ir.tcx.tables.borrow().method_map[&method_call].ty;
1129 let succ = if method_ty.fn_ret().diverges() {
1134 self.propagate_through_exprs(&args[..], succ)
1137 hir::ExprTup(ref exprs) => {
1138 self.propagate_through_exprs(&exprs[..], succ)
1141 hir::ExprBinary(op, ref l, ref r) if ::rustc_front::util::lazy_binop(op.node) => {
1142 let r_succ = self.propagate_through_expr(&r, succ);
1144 let ln = self.live_node(expr.id, expr.span);
1145 self.init_from_succ(ln, succ);
1146 self.merge_from_succ(ln, r_succ, false);
1148 self.propagate_through_expr(&l, ln)
1151 hir::ExprIndex(ref l, ref r) |
1152 hir::ExprBinary(_, ref l, ref r) => {
1153 let r_succ = self.propagate_through_expr(&r, succ);
1154 self.propagate_through_expr(&l, r_succ)
1157 hir::ExprBox(ref e) |
1158 hir::ExprAddrOf(_, ref e) |
1159 hir::ExprCast(ref e, _) |
1160 hir::ExprType(ref e, _) |
1161 hir::ExprUnary(_, ref e) => {
1162 self.propagate_through_expr(&e, succ)
1165 hir::ExprInlineAsm(ref ia) => {
1167 let succ = ia.outputs.iter().rev().fold(succ,
1169 // see comment on lvalues
1170 // in propagate_through_lvalue_components()
1171 if out.is_indirect {
1172 self.propagate_through_expr(&out.expr, succ)
1174 let acc = if out.is_rw { ACC_WRITE|ACC_READ } else { ACC_WRITE };
1175 let succ = self.write_lvalue(&out.expr, succ, acc);
1176 self.propagate_through_lvalue_components(&out.expr, succ)
1180 // Inputs are executed first. Propagate last because of rev order
1181 ia.inputs.iter().rev().fold(succ, |succ, &(_, ref expr)| {
1182 self.propagate_through_expr(&expr, succ)
1186 hir::ExprLit(..) => {
1190 hir::ExprBlock(ref blk) => {
1191 self.propagate_through_block(&blk, succ)
1196 fn propagate_through_lvalue_components(&mut self,
1202 // In general, the full flow graph structure for an
1203 // assignment/move/etc can be handled in one of two ways,
1204 // depending on whether what is being assigned is a "tracked
1205 // value" or not. A tracked value is basically a local
1206 // variable or argument.
1208 // The two kinds of graphs are:
1210 // Tracked lvalue Untracked lvalue
1211 // ----------------------++-----------------------
1215 // (rvalue) || (rvalue)
1218 // (write of lvalue) || (lvalue components)
1223 // ----------------------++-----------------------
1225 // I will cover the two cases in turn:
1227 // # Tracked lvalues
1229 // A tracked lvalue is a local variable/argument `x`. In
1230 // these cases, the link_node where the write occurs is linked
1231 // to node id of `x`. The `write_lvalue()` routine generates
1232 // the contents of this node. There are no subcomponents to
1235 // # Non-tracked lvalues
1237 // These are lvalues like `x[5]` or `x.f`. In that case, we
1238 // basically ignore the value which is written to but generate
1239 // reads for the components---`x` in these two examples. The
1240 // components reads are generated by
1241 // `propagate_through_lvalue_components()` (this fn).
1243 // # Illegal lvalues
1245 // It is still possible to observe assignments to non-lvalues;
1246 // these errors are detected in the later pass borrowck. We
1247 // just ignore such cases and treat them as reads.
1250 hir::ExprPath(..) => succ,
1251 hir::ExprField(ref e, _) => self.propagate_through_expr(&e, succ),
1252 hir::ExprTupField(ref e, _) => self.propagate_through_expr(&e, succ),
1253 _ => self.propagate_through_expr(expr, succ)
1257 // see comment on propagate_through_lvalue()
1258 fn write_lvalue(&mut self, expr: &Expr, succ: LiveNode, acc: u32)
1261 hir::ExprPath(..) => {
1262 self.access_path(expr, succ, acc)
1265 // We do not track other lvalues, so just propagate through
1266 // to their subcomponents. Also, it may happen that
1267 // non-lvalues occur here, because those are detected in the
1268 // later pass borrowck.
1273 fn access_path(&mut self, expr: &Expr, succ: LiveNode, acc: u32)
1275 match self.ir.tcx.def_map.borrow().get(&expr.id).unwrap().full_def() {
1276 Def::Local(_, nid) => {
1277 let ln = self.live_node(expr.id, expr.span);
1279 self.init_from_succ(ln, succ);
1280 let var = self.variable(nid, expr.span);
1281 self.acc(ln, var, acc);
1289 fn propagate_through_loop(&mut self,
1298 We model control flow like this:
1316 let mut first_merge = true;
1317 let ln = self.live_node(expr.id, expr.span);
1318 self.init_empty(ln, succ);
1322 // If this is not a `loop` loop, then it's possible we bypass
1323 // the body altogether. Otherwise, the only way is via a `break`
1324 // in the loop body.
1325 self.merge_from_succ(ln, succ, first_merge);
1326 first_merge = false;
1329 debug!("propagate_through_loop: using id for loop body {} {}",
1330 expr.id, block_to_string(body));
1332 let cond_ln = match kind {
1334 WhileLoop(ref cond) => self.propagate_through_expr(&cond, ln),
1336 let body_ln = self.with_loop_nodes(expr.id, succ, ln, |this| {
1337 this.propagate_through_block(body, cond_ln)
1340 // repeat until fixed point is reached:
1341 while self.merge_from_succ(ln, body_ln, first_merge) {
1342 first_merge = false;
1344 let new_cond_ln = match kind {
1346 WhileLoop(ref cond) => {
1347 self.propagate_through_expr(&cond, ln)
1350 assert!(cond_ln == new_cond_ln);
1351 assert!(body_ln == self.with_loop_nodes(expr.id, succ, ln,
1352 |this| this.propagate_through_block(body, cond_ln)));
1358 fn with_loop_nodes<R, F>(&mut self,
1359 loop_node_id: NodeId,
1364 F: FnOnce(&mut Liveness<'a, 'tcx>) -> R,
1366 debug!("with_loop_nodes: {} {}", loop_node_id, break_ln.get());
1367 self.loop_scope.push(loop_node_id);
1368 self.break_ln.insert(loop_node_id, break_ln);
1369 self.cont_ln.insert(loop_node_id, cont_ln);
1371 self.loop_scope.pop();
1376 // _______________________________________________________________________
1377 // Checking for error conditions
1379 fn check_local(this: &mut Liveness, local: &hir::Local) {
1382 this.warn_about_unused_or_dead_vars_in_pat(&local.pat);
1385 this.pat_bindings(&local.pat, |this, ln, var, sp, id| {
1386 this.warn_about_unused(sp, id, ln, var);
1391 intravisit::walk_local(this, local);
1394 fn check_arm(this: &mut Liveness, arm: &hir::Arm) {
1395 // only consider the first pattern; any later patterns must have
1396 // the same bindings, and we also consider the first pattern to be
1397 // the "authoritative" set of ids
1398 this.arm_pats_bindings(arm.pats.first().map(|p| &**p), |this, ln, var, sp, id| {
1399 this.warn_about_unused(sp, id, ln, var);
1401 intravisit::walk_arm(this, arm);
1404 fn check_expr(this: &mut Liveness, expr: &Expr) {
1406 hir::ExprAssign(ref l, _) => {
1407 this.check_lvalue(&l);
1409 intravisit::walk_expr(this, expr);
1412 hir::ExprAssignOp(_, ref l, _) => {
1413 this.check_lvalue(&l);
1415 intravisit::walk_expr(this, expr);
1418 hir::ExprInlineAsm(ref ia) => {
1419 for &(_, ref input) in &ia.inputs {
1420 this.visit_expr(&input);
1423 // Output operands must be lvalues
1424 for out in &ia.outputs {
1425 if !out.is_indirect {
1426 this.check_lvalue(&out.expr);
1428 this.visit_expr(&out.expr);
1431 intravisit::walk_expr(this, expr);
1434 // no correctness conditions related to liveness
1435 hir::ExprCall(..) | hir::ExprMethodCall(..) | hir::ExprIf(..) |
1436 hir::ExprMatch(..) | hir::ExprWhile(..) | hir::ExprLoop(..) |
1437 hir::ExprIndex(..) | hir::ExprField(..) | hir::ExprTupField(..) |
1438 hir::ExprVec(..) | hir::ExprTup(..) | hir::ExprBinary(..) |
1439 hir::ExprCast(..) | hir::ExprUnary(..) | hir::ExprRet(..) |
1440 hir::ExprBreak(..) | hir::ExprAgain(..) | hir::ExprLit(_) |
1441 hir::ExprBlock(..) | hir::ExprAddrOf(..) |
1442 hir::ExprStruct(..) | hir::ExprRepeat(..) |
1443 hir::ExprClosure(..) | hir::ExprPath(..) | hir::ExprBox(..) |
1444 hir::ExprType(..) => {
1445 intravisit::walk_expr(this, expr);
1450 fn check_fn(_v: &Liveness,
1452 _decl: &hir::FnDecl,
1456 // do not check contents of nested fns
1459 impl<'a, 'tcx> Liveness<'a, 'tcx> {
1460 fn fn_ret(&self, id: NodeId) -> ty::PolyFnOutput<'tcx> {
1461 let fn_ty = self.ir.tcx.node_id_to_type(id);
1463 ty::TyClosure(closure_def_id, ref substs) =>
1464 self.ir.tcx.closure_type(closure_def_id, substs).sig.output(),
1476 // within the fn body, late-bound regions are liberated
1477 // and must outlive the *call-site* of the function.
1479 self.ir.tcx.liberate_late_bound_regions(
1480 self.ir.tcx.region_maps.call_site_extent(id, body.id),
1484 ty::FnConverging(t_ret)
1485 if self.live_on_entry(entry_ln, self.s.no_ret_var).is_some() => {
1488 // for nil return types, it is ok to not return a value expl.
1490 let ends_with_stmt = match body.expr {
1491 None if !body.stmts.is_empty() =>
1492 match body.stmts.first().unwrap().node {
1493 hir::StmtSemi(ref e, _) => {
1494 self.ir.tcx.expr_ty(&e) == t_ret
1500 let mut err = struct_span_err!(self.ir.tcx.sess,
1503 "not all control paths return a value");
1505 let last_stmt = body.stmts.first().unwrap();
1506 let original_span = original_sp(self.ir.tcx.sess.codemap(),
1507 last_stmt.span, sp);
1508 let span_semicolon = Span {
1509 lo: original_span.hi - BytePos(1),
1510 hi: original_span.hi,
1511 expn_id: original_span.expn_id
1513 err.span_help(span_semicolon, "consider removing this semicolon:");
1519 if self.live_on_entry(entry_ln, self.s.clean_exit_var).is_some() => {
1520 span_err!(self.ir.tcx.sess, sp, E0270,
1521 "computation may converge in a function marked as diverging");
1528 fn check_lvalue(&mut self, expr: &Expr) {
1530 hir::ExprPath(..) => {
1531 if let Def::Local(_, nid) = self.ir.tcx.def_map.borrow().get(&expr.id)
1534 // Assignment to an immutable variable or argument: only legal
1535 // if there is no later assignment. If this local is actually
1536 // mutable, then check for a reassignment to flag the mutability
1538 let ln = self.live_node(expr.id, expr.span);
1539 let var = self.variable(nid, expr.span);
1540 self.warn_about_dead_assign(expr.span, expr.id, ln, var);
1544 // For other kinds of lvalues, no checks are required,
1545 // and any embedded expressions are actually rvalues
1546 intravisit::walk_expr(self, expr);
1551 fn should_warn(&self, var: Variable) -> Option<String> {
1552 let name = self.ir.variable_name(var);
1553 if name.is_empty() || name.as_bytes()[0] == ('_' as u8) {
1560 fn warn_about_unused_args(&self, decl: &hir::FnDecl, entry_ln: LiveNode) {
1561 for arg in &decl.inputs {
1562 pat_util::pat_bindings(&self.ir.tcx.def_map,
1564 |_bm, p_id, sp, path1| {
1565 let var = self.variable(p_id, sp);
1566 // Ignore unused self.
1567 let name = path1.node;
1568 if name != special_idents::self_.name {
1569 if !self.warn_about_unused(sp, p_id, entry_ln, var) {
1570 if self.live_on_entry(entry_ln, var).is_none() {
1571 self.report_dead_assign(p_id, sp, var, true);
1579 fn warn_about_unused_or_dead_vars_in_pat(&mut self, pat: &hir::Pat) {
1580 self.pat_bindings(pat, |this, ln, var, sp, id| {
1581 if !this.warn_about_unused(sp, id, ln, var) {
1582 this.warn_about_dead_assign(sp, id, ln, var);
1587 fn warn_about_unused(&self,
1593 if !self.used_on_entry(ln, var) {
1594 let r = self.should_warn(var);
1595 if let Some(name) = r {
1597 // annoying: for parameters in funcs like `fn(x: i32)
1598 // {ret}`, there is only one node, so asking about
1599 // assigned_on_exit() is not meaningful.
1600 let is_assigned = if ln == self.s.exit_ln {
1603 self.assigned_on_exit(ln, var).is_some()
1607 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_VARIABLES, id, sp,
1608 format!("variable `{}` is assigned to, but never used",
1610 } else if name != "self" {
1611 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_VARIABLES, id, sp,
1612 format!("unused variable: `{}`", name));
1621 fn warn_about_dead_assign(&self,
1626 if self.live_on_exit(ln, var).is_none() {
1627 self.report_dead_assign(id, sp, var, false);
1631 fn report_dead_assign(&self, id: NodeId, sp: Span, var: Variable, is_argument: bool) {
1632 if let Some(name) = self.should_warn(var) {
1634 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_ASSIGNMENTS, id, sp,
1635 format!("value passed to `{}` is never read", name));
1637 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_ASSIGNMENTS, id, sp,
1638 format!("value assigned to `{}` is never read", name));