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.
12 * A classic liveness analysis based on dataflow over the AST. Computes,
13 * for each local variable in a function, whether that variable is live
14 * at a given point. Program execution points are identified by their
19 * The basic model is that each local variable is assigned an index. We
20 * represent sets of local variables using a vector indexed by this
21 * index. The value in the vector is either 0, indicating the variable
22 * is dead, or the id of an expression that uses the variable.
24 * We conceptually walk over the AST in reverse execution order. If we
25 * find a use of a variable, we add it to the set of live variables. If
26 * we find an assignment to a variable, we remove it from the set of live
27 * variables. When we have to merge two flows, we take the union of
28 * those two flows---if the variable is live on both paths, we simply
29 * pick one id. In the event of loops, we continue doing this until a
30 * fixed point is reached.
32 * ## Checking initialization
34 * At the function entry point, all variables must be dead. If this is
35 * not the case, we can report an error using the id found in the set of
36 * live variables, which identifies a use of the variable which is not
37 * dominated by an assignment.
41 * After each explicit move, the variable must be dead.
43 * ## Computing last uses
45 * Any use of the variable where the variable is dead afterwards is a
48 * # Implementation details
50 * The actual implementation contains two (nested) walks over the AST.
51 * The outer walk has the job of building up the ir_maps instance for the
52 * enclosing function. On the way down the tree, it identifies those AST
53 * nodes and variable IDs that will be needed for the liveness analysis
54 * and assigns them contiguous IDs. The liveness id for an AST node is
55 * called a `live_node` (it's a newtype'd uint) and the id for a variable
56 * is called a `variable` (another newtype'd uint).
58 * On the way back up the tree, as we are about to exit from a function
59 * declaration we allocate a `liveness` instance. Now that we know
60 * precisely how many nodes and variables we need, we can allocate all
61 * the various arrays that we will need to precisely the right size. We then
62 * perform the actual propagation on the `liveness` instance.
64 * This propagation is encoded in the various `propagate_through_*()`
65 * methods. It effectively does a reverse walk of the AST; whenever we
66 * reach a loop node, we iterate until a fixed point is reached.
68 * ## The `Users` struct
70 * At each live node `N`, we track three pieces of information for each
71 * variable `V` (these are encapsulated in the `Users` struct):
73 * - `reader`: the `LiveNode` ID of some node which will read the value
74 * that `V` holds on entry to `N`. Formally: a node `M` such
75 * that there exists a path `P` from `N` to `M` where `P` does not
76 * write `V`. If the `reader` is `invalid_node()`, then the current
77 * value will never be read (the variable is dead, essentially).
79 * - `writer`: the `LiveNode` ID of some node which will write the
80 * variable `V` and which is reachable from `N`. Formally: a node `M`
81 * such that there exists a path `P` from `N` to `M` and `M` writes
82 * `V`. If the `writer` is `invalid_node()`, then there is no writer
83 * of `V` that follows `N`.
85 * - `used`: a boolean value indicating whether `V` is *used*. We
86 * distinguish a *read* from a *use* in that a *use* is some read that
87 * is not just used to generate a new value. For example, `x += 1` is
88 * a read but not a use. This is used to generate better warnings.
90 * ## Special Variables
92 * We generate various special variables for various, well, special purposes.
93 * These are described in the `specials` struct:
95 * - `exit_ln`: a live node that is generated to represent every 'exit' from
96 * the function, whether it be by explicit return, fail, or other means.
98 * - `fallthrough_ln`: a live node that represents a fallthrough
100 * - `no_ret_var`: a synthetic variable that is only 'read' from, the
101 * fallthrough node. This allows us to detect functions where we fail
102 * to return explicitly.
106 use middle::freevars;
107 use middle::mem_categorization::Typer;
108 use middle::pat_util;
111 use util::nodemap::NodeMap;
116 use std::mem::transmute;
121 use syntax::codemap::{BytePos, original_sp, Span};
122 use syntax::parse::token::special_idents;
123 use syntax::parse::token;
124 use syntax::print::pprust::{expr_to_str, block_to_str};
125 use syntax::{visit, ast_util};
126 use syntax::visit::{Visitor, FnKind};
128 #[deriving(PartialEq)]
129 struct Variable(uint);
130 #[deriving(PartialEq)]
131 struct LiveNode(uint);
134 fn get(&self) -> uint { let Variable(v) = *self; v }
138 fn get(&self) -> uint { let LiveNode(v) = *self; v }
141 impl Clone for LiveNode {
142 fn clone(&self) -> LiveNode {
147 #[deriving(PartialEq)]
155 fn live_node_kind_to_str(lnk: LiveNodeKind, cx: &ty::ctxt) -> String {
156 let cm = cx.sess.codemap();
159 format!("Free var node [{}]", cm.span_to_str(s))
162 format!("Expr node [{}]", cm.span_to_str(s))
165 format!("Var def node [{}]", cm.span_to_str(s))
167 ExitNode => "Exit node".to_string(),
171 impl<'a> Visitor<()> for IrMaps<'a> {
172 fn visit_fn(&mut self, fk: &FnKind, fd: &FnDecl, b: &Block, s: Span, n: NodeId, _: ()) {
173 visit_fn(self, fk, fd, b, s, n);
175 fn visit_local(&mut self, l: &Local, _: ()) { visit_local(self, l); }
176 fn visit_expr(&mut self, ex: &Expr, _: ()) { visit_expr(self, ex); }
177 fn visit_arm(&mut self, a: &Arm, _: ()) { visit_arm(self, a); }
180 pub fn check_crate(tcx: &ty::ctxt,
182 visit::walk_crate(&mut IrMaps::new(tcx), krate, ());
183 tcx.sess.abort_if_errors();
186 impl fmt::Show for LiveNode {
187 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
188 write!(f, "ln({})", self.get())
192 impl fmt::Show for Variable {
193 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
194 write!(f, "v({})", self.get())
198 // ______________________________________________________________________
201 // This is the first pass and the one that drives the main
202 // computation. It walks up and down the IR once. On the way down,
203 // we count for each function the number of variables as well as
204 // liveness nodes. A liveness node is basically an expression or
205 // capture clause that does something of interest: either it has
206 // interesting control flow or it uses/defines a local variable.
208 // On the way back up, at each function node we create liveness sets
209 // (we now know precisely how big to make our various vectors and so
210 // forth) and then do the data-flow propagation to compute the set
211 // of live variables at each program point.
213 // Finally, we run back over the IR one last time and, using the
214 // computed liveness, check various safety conditions. For example,
215 // there must be no live nodes at the definition site for a variable
216 // unless it has an initializer. Similarly, each non-mutable local
217 // variable must not be assigned if there is some successor
218 // assignment. And so forth.
221 fn is_valid(&self) -> bool {
222 self.get() != uint::MAX
226 fn invalid_node() -> LiveNode { LiveNode(uint::MAX) }
247 num_live_nodes: uint,
249 live_node_map: NodeMap<LiveNode>,
250 variable_map: NodeMap<Variable>,
251 capture_info_map: NodeMap<Rc<Vec<CaptureInfo>>>,
252 var_kinds: Vec<VarKind>,
253 lnks: Vec<LiveNodeKind>,
256 impl<'a> IrMaps<'a> {
257 fn new(tcx: &'a ty::ctxt) -> IrMaps<'a> {
262 live_node_map: NodeMap::new(),
263 variable_map: NodeMap::new(),
264 capture_info_map: NodeMap::new(),
265 var_kinds: Vec::new(),
270 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
271 let ln = LiveNode(self.num_live_nodes);
273 self.num_live_nodes += 1;
275 debug!("{} is of kind {}", ln.to_str(),
276 live_node_kind_to_str(lnk, self.tcx));
281 fn add_live_node_for_node(&mut self, node_id: NodeId, lnk: LiveNodeKind) {
282 let ln = self.add_live_node(lnk);
283 self.live_node_map.insert(node_id, ln);
285 debug!("{} is node {}", ln.to_str(), node_id);
288 fn add_variable(&mut self, vk: VarKind) -> Variable {
289 let v = Variable(self.num_vars);
290 self.var_kinds.push(vk);
294 Local(LocalInfo { id: node_id, .. }) | Arg(node_id, _) => {
295 self.variable_map.insert(node_id, v);
300 debug!("{} is {:?}", v.to_str(), vk);
305 fn variable(&self, node_id: NodeId, span: Span) -> Variable {
306 match self.variable_map.find(&node_id) {
311 .span_bug(span, format!("no variable registered for id {}",
312 node_id).as_slice());
317 fn variable_name(&self, var: Variable) -> String {
318 match self.var_kinds.get(var.get()) {
319 &Local(LocalInfo { ident: nm, .. }) | &Arg(_, nm) => {
320 token::get_ident(nm).get().to_str()
322 &ImplicitRet => "<implicit-ret>".to_string()
326 fn set_captures(&mut self, node_id: NodeId, cs: Vec<CaptureInfo>) {
327 self.capture_info_map.insert(node_id, Rc::new(cs));
330 fn lnk(&self, ln: LiveNode) -> LiveNodeKind {
331 *self.lnks.get(ln.get())
335 impl<'a> Visitor<()> for Liveness<'a> {
336 fn visit_fn(&mut self, fk: &FnKind, fd: &FnDecl, b: &Block, s: Span, n: NodeId, _: ()) {
337 check_fn(self, fk, fd, b, s, n);
339 fn visit_local(&mut self, l: &Local, _: ()) {
340 check_local(self, l);
342 fn visit_expr(&mut self, ex: &Expr, _: ()) {
343 check_expr(self, ex);
345 fn visit_arm(&mut self, a: &Arm, _: ()) {
350 fn visit_fn(ir: &mut IrMaps,
356 debug!("visit_fn: id={}", id);
357 let _i = ::util::common::indenter();
359 // swap in a new set of IR maps for this function body:
360 let mut fn_maps = IrMaps::new(ir.tcx);
363 debug!("creating fn_maps: {}",
364 transmute::<&IrMaps, *const IrMaps>(&fn_maps));
367 for arg in decl.inputs.iter() {
368 pat_util::pat_bindings(&ir.tcx.def_map,
370 |_bm, arg_id, _x, path| {
371 debug!("adding argument {}", arg_id);
372 let ident = ast_util::path_to_ident(path);
373 fn_maps.add_variable(Arg(arg_id, ident));
377 // gather up the various local variables, significant expressions,
379 visit::walk_fn(&mut fn_maps, fk, decl, body, sp, ());
381 // Special nodes and variables:
382 // - exit_ln represents the end of the fn, either by return or fail
383 // - implicit_ret_var is a pseudo-variable that represents
384 // an implicit return
385 let specials = Specials {
386 exit_ln: fn_maps.add_live_node(ExitNode),
387 fallthrough_ln: fn_maps.add_live_node(ExitNode),
388 no_ret_var: fn_maps.add_variable(ImplicitRet)
392 let mut lsets = Liveness::new(&mut fn_maps, specials);
393 let entry_ln = lsets.compute(decl, body);
395 // check for various error conditions
396 lsets.visit_block(body, ());
397 lsets.check_ret(id, sp, fk, entry_ln, body);
398 lsets.warn_about_unused_args(decl, entry_ln);
401 fn visit_local(ir: &mut IrMaps, local: &Local) {
402 pat_util::pat_bindings(&ir.tcx.def_map, &*local.pat, |_, p_id, sp, path| {
403 debug!("adding local variable {}", p_id);
404 let name = ast_util::path_to_ident(path);
405 ir.add_live_node_for_node(p_id, VarDefNode(sp));
406 ir.add_variable(Local(LocalInfo {
411 visit::walk_local(ir, local, ());
414 fn visit_arm(ir: &mut IrMaps, arm: &Arm) {
415 for pat in arm.pats.iter() {
416 pat_util::pat_bindings(&ir.tcx.def_map, &**pat, |bm, p_id, sp, path| {
417 debug!("adding local variable {} from match with bm {:?}",
419 let name = ast_util::path_to_ident(path);
420 ir.add_live_node_for_node(p_id, VarDefNode(sp));
421 ir.add_variable(Local(LocalInfo {
427 visit::walk_arm(ir, arm, ());
430 fn moved_variable_node_id_from_def(def: Def) -> Option<NodeId> {
434 DefLocal(nid, _) => Some(nid),
440 fn visit_expr(ir: &mut IrMaps, expr: &Expr) {
442 // live nodes required for uses or definitions of variables:
444 let def = ir.tcx.def_map.borrow().get_copy(&expr.id);
445 debug!("expr {}: path that leads to {:?}", expr.id, def);
446 if moved_variable_node_id_from_def(def).is_some() {
447 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
449 visit::walk_expr(ir, expr, ());
451 ExprFnBlock(..) | ExprProc(..) => {
452 // Interesting control flow (for loops can contain labeled
453 // breaks or continues)
454 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
456 // Make a live_node for each captured variable, with the span
457 // being the location that the variable is used. This results
458 // in better error messages than just pointing at the closure
459 // construction site.
460 let mut call_caps = Vec::new();
461 freevars::with_freevars(ir.tcx, expr.id, |freevars| {
462 for fv in freevars.iter() {
463 match moved_variable_node_id_from_def(fv.def) {
465 let fv_ln = ir.add_live_node(FreeVarNode(fv.span));
466 call_caps.push(CaptureInfo {ln: fv_ln,
473 ir.set_captures(expr.id, call_caps);
475 visit::walk_expr(ir, expr, ());
478 // live nodes required for interesting control flow:
479 ExprIf(..) | ExprMatch(..) | ExprWhile(..) | ExprLoop(..) => {
480 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
481 visit::walk_expr(ir, expr, ());
483 ExprForLoop(..) => fail!("non-desugared expr_for_loop"),
484 ExprBinary(op, _, _) if ast_util::lazy_binop(op) => {
485 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
486 visit::walk_expr(ir, expr, ());
489 // otherwise, live nodes are not required:
490 ExprIndex(..) | ExprField(..) | ExprVstore(..) | ExprVec(..) |
491 ExprCall(..) | ExprMethodCall(..) | ExprTup(..) |
492 ExprBinary(..) | ExprAddrOf(..) |
493 ExprCast(..) | ExprUnary(..) | ExprBreak(_) |
494 ExprAgain(_) | ExprLit(_) | ExprRet(..) | ExprBlock(..) |
495 ExprAssign(..) | ExprAssignOp(..) | ExprMac(..) |
496 ExprStruct(..) | ExprRepeat(..) | ExprParen(..) |
497 ExprInlineAsm(..) | ExprBox(..) => {
498 visit::walk_expr(ir, expr, ());
503 // ______________________________________________________________________
504 // Computing liveness sets
506 // Actually we compute just a bit more than just liveness, but we use
507 // the same basic propagation framework in all cases.
516 fn invalid_users() -> Users {
518 reader: invalid_node(),
519 writer: invalid_node(),
526 fallthrough_ln: LiveNode,
530 static ACC_READ: uint = 1u;
531 static ACC_WRITE: uint = 2u;
532 static ACC_USE: uint = 4u;
534 struct Liveness<'a> {
535 ir: &'a mut IrMaps<'a>,
537 successors: Vec<LiveNode>,
539 // The list of node IDs for the nested loop scopes
541 loop_scope: Vec<NodeId>,
542 // mappings from loop node ID to LiveNode
543 // ("break" label should map to loop node ID,
544 // it probably doesn't now)
545 break_ln: NodeMap<LiveNode>,
546 cont_ln: NodeMap<LiveNode>
549 impl<'a> Liveness<'a> {
550 fn new(ir: &'a mut IrMaps<'a>, specials: Specials) -> Liveness<'a> {
551 let num_live_nodes = ir.num_live_nodes;
552 let num_vars = ir.num_vars;
556 successors: Vec::from_elem(num_live_nodes, invalid_node()),
557 users: Vec::from_elem(num_live_nodes * num_vars, invalid_users()),
558 loop_scope: Vec::new(),
559 break_ln: NodeMap::new(),
560 cont_ln: NodeMap::new(),
564 fn live_node(&self, node_id: NodeId, span: Span) -> LiveNode {
565 match self.ir.live_node_map.find(&node_id) {
568 // This must be a mismatch between the ir_map construction
569 // above and the propagation code below; the two sets of
570 // code have to agree about which AST nodes are worth
571 // creating liveness nodes for.
572 self.ir.tcx.sess.span_bug(
574 format!("no live node registered for node {}",
575 node_id).as_slice());
580 fn variable(&self, node_id: NodeId, span: Span) -> Variable {
581 self.ir.variable(node_id, span)
584 fn pat_bindings(&mut self,
586 f: |&mut Liveness<'a>, LiveNode, Variable, Span, NodeId|) {
587 pat_util::pat_bindings(&self.ir.tcx.def_map, pat, |_bm, p_id, sp, _n| {
588 let ln = self.live_node(p_id, sp);
589 let var = self.variable(p_id, sp);
590 f(self, ln, var, sp, p_id);
594 fn arm_pats_bindings(&mut self,
596 f: |&mut Liveness<'a>, LiveNode, Variable, Span, NodeId|) {
597 // only consider the first pattern; any later patterns must have
598 // the same bindings, and we also consider the first pattern to be
599 // the "authoritative" set of ids
600 if !pats.is_empty() {
601 self.pat_bindings(&*pats[0], f)
605 fn define_bindings_in_pat(&mut self, pat: Gc<Pat>, succ: LiveNode)
607 self.define_bindings_in_arm_pats([pat], succ)
610 fn define_bindings_in_arm_pats(&mut self, pats: &[Gc<Pat>], succ: LiveNode)
613 self.arm_pats_bindings(pats, |this, ln, var, _sp, _id| {
614 this.init_from_succ(ln, succ);
615 this.define(ln, var);
621 fn idx(&self, ln: LiveNode, var: Variable) -> uint {
622 ln.get() * self.ir.num_vars + var.get()
625 fn live_on_entry(&self, ln: LiveNode, var: Variable)
626 -> Option<LiveNodeKind> {
627 assert!(ln.is_valid());
628 let reader = self.users.get(self.idx(ln, var)).reader;
629 if reader.is_valid() {Some(self.ir.lnk(reader))} else {None}
633 Is this variable live on entry to any of its successor nodes?
635 fn live_on_exit(&self, ln: LiveNode, var: Variable)
636 -> Option<LiveNodeKind> {
637 let successor = *self.successors.get(ln.get());
638 self.live_on_entry(successor, var)
641 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
642 assert!(ln.is_valid());
643 self.users.get(self.idx(ln, var)).used
646 fn assigned_on_entry(&self, ln: LiveNode, var: Variable)
647 -> Option<LiveNodeKind> {
648 assert!(ln.is_valid());
649 let writer = self.users.get(self.idx(ln, var)).writer;
650 if writer.is_valid() {Some(self.ir.lnk(writer))} else {None}
653 fn assigned_on_exit(&self, ln: LiveNode, var: Variable)
654 -> Option<LiveNodeKind> {
655 let successor = *self.successors.get(ln.get());
656 self.assigned_on_entry(successor, var)
659 fn indices2(&mut self,
662 op: |&mut Liveness<'a>, uint, uint|) {
663 let node_base_idx = self.idx(ln, Variable(0u));
664 let succ_base_idx = self.idx(succ_ln, Variable(0u));
665 for var_idx in range(0u, self.ir.num_vars) {
666 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
673 test: |uint| -> LiveNode) -> io::IoResult<()> {
674 let node_base_idx = self.idx(ln, Variable(0));
675 for var_idx in range(0u, self.ir.num_vars) {
676 let idx = node_base_idx + var_idx;
677 if test(idx).is_valid() {
678 try!(write!(wr, " {}", Variable(var_idx).to_str()));
684 fn find_loop_scope(&self,
685 opt_label: Option<Ident>,
691 // Refers to a labeled loop. Use the results of resolve
693 match self.ir.tcx.def_map.borrow().find(&id) {
694 Some(&DefLabel(loop_id)) => loop_id,
695 _ => self.ir.tcx.sess.span_bug(sp, "label on break/loop \
696 doesn't refer to a loop")
700 // Vanilla 'break' or 'loop', so use the enclosing
702 if self.loop_scope.len() == 0 {
703 self.ir.tcx.sess.span_bug(sp, "break outside loop");
705 *self.loop_scope.last().unwrap()
711 #[allow(unused_must_use)]
712 fn ln_str(&self, ln: LiveNode) -> String {
713 let mut wr = io::MemWriter::new();
715 let wr = &mut wr as &mut io::Writer;
716 write!(wr, "[ln({}) of kind {:?} reads", ln.get(), self.ir.lnk(ln));
717 self.write_vars(wr, ln, |idx| self.users.get(idx).reader);
718 write!(wr, " writes");
719 self.write_vars(wr, ln, |idx| self.users.get(idx).writer);
720 write!(wr, " precedes {}]", self.successors.get(ln.get()).to_str());
722 str::from_utf8(wr.unwrap().as_slice()).unwrap().to_string()
725 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
726 *self.successors.get_mut(ln.get()) = succ_ln;
728 // It is not necessary to initialize the
729 // values to empty because this is the value
730 // they have when they are created, and the sets
731 // only grow during iterations.
733 // self.indices(ln) { |idx|
734 // self.users[idx] = invalid_users();
738 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
739 // more efficient version of init_empty() / merge_from_succ()
740 *self.successors.get_mut(ln.get()) = succ_ln;
742 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
743 *this.users.get_mut(idx) = *this.users.get(succ_idx)
745 debug!("init_from_succ(ln={}, succ={})",
746 self.ln_str(ln), self.ln_str(succ_ln));
749 fn merge_from_succ(&mut self,
754 if ln == succ_ln { return false; }
756 let mut changed = false;
757 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
758 changed |= copy_if_invalid(this.users.get(succ_idx).reader,
759 &mut this.users.get_mut(idx).reader);
760 changed |= copy_if_invalid(this.users.get(succ_idx).writer,
761 &mut this.users.get_mut(idx).writer);
762 if this.users.get(succ_idx).used && !this.users.get(idx).used {
763 this.users.get_mut(idx).used = true;
768 debug!("merge_from_succ(ln={}, succ={}, first_merge={}, changed={})",
769 ln.to_str(), self.ln_str(succ_ln), first_merge, changed);
772 fn copy_if_invalid(src: LiveNode, dst: &mut LiveNode) -> bool {
773 if src.is_valid() && !dst.is_valid() {
782 // Indicates that a local variable was *defined*; we know that no
783 // uses of the variable can precede the definition (resolve checks
784 // this) so we just clear out all the data.
785 fn define(&mut self, writer: LiveNode, var: Variable) {
786 let idx = self.idx(writer, var);
787 self.users.get_mut(idx).reader = invalid_node();
788 self.users.get_mut(idx).writer = invalid_node();
790 debug!("{} defines {} (idx={}): {}", writer.to_str(), var.to_str(),
791 idx, self.ln_str(writer));
794 // Either read, write, or both depending on the acc bitset
795 fn acc(&mut self, ln: LiveNode, var: Variable, acc: uint) {
796 debug!("{} accesses[{:x}] {}: {}",
797 ln.to_str(), acc, var.to_str(), self.ln_str(ln));
799 let idx = self.idx(ln, var);
800 let user = self.users.get_mut(idx);
802 if (acc & ACC_WRITE) != 0 {
803 user.reader = invalid_node();
807 // Important: if we both read/write, must do read second
808 // or else the write will override.
809 if (acc & ACC_READ) != 0 {
813 if (acc & ACC_USE) != 0 {
818 // _______________________________________________________________________
820 fn compute(&mut self, decl: &FnDecl, body: &Block) -> LiveNode {
821 // if there is a `break` or `again` at the top level, then it's
822 // effectively a return---this only occurs in `for` loops,
823 // where the body is really a closure.
825 debug!("compute: using id for block, {}", block_to_str(body));
827 let exit_ln = self.s.exit_ln;
828 let entry_ln: LiveNode =
829 self.with_loop_nodes(body.id, exit_ln, exit_ln,
830 |this| this.propagate_through_fn_block(decl, body));
832 // hack to skip the loop unless debug! is enabled:
833 debug!("^^ liveness computation results for body {} (entry={})",
835 for ln_idx in range(0u, self.ir.num_live_nodes) {
836 debug!("{}", self.ln_str(LiveNode(ln_idx)));
845 fn propagate_through_fn_block(&mut self, _: &FnDecl, blk: &Block)
847 // the fallthrough exit is only for those cases where we do not
848 // explicitly return:
850 self.init_from_succ(s.fallthrough_ln, s.exit_ln);
851 if blk.expr.is_none() {
852 self.acc(s.fallthrough_ln, s.no_ret_var, ACC_READ)
855 self.propagate_through_block(blk, s.fallthrough_ln)
858 fn propagate_through_block(&mut self, blk: &Block, succ: LiveNode)
860 let succ = self.propagate_through_opt_expr(blk.expr, succ);
861 blk.stmts.iter().rev().fold(succ, |succ, stmt| {
862 self.propagate_through_stmt(&**stmt, succ)
866 fn propagate_through_stmt(&mut self, stmt: &Stmt, succ: LiveNode)
869 StmtDecl(ref decl, _) => {
870 self.propagate_through_decl(&**decl, succ)
873 StmtExpr(ref expr, _) | StmtSemi(ref expr, _) => {
874 self.propagate_through_expr(&**expr, succ)
878 self.ir.tcx.sess.span_bug(stmt.span, "unexpanded macro");
883 fn propagate_through_decl(&mut self, decl: &Decl, succ: LiveNode)
886 DeclLocal(ref local) => {
887 self.propagate_through_local(&**local, succ)
893 fn propagate_through_local(&mut self, local: &Local, succ: LiveNode)
895 // Note: we mark the variable as defined regardless of whether
896 // there is an initializer. Initially I had thought to only mark
897 // the live variable as defined if it was initialized, and then we
898 // could check for uninit variables just by scanning what is live
899 // at the start of the function. But that doesn't work so well for
900 // immutable variables defined in a loop:
901 // loop { let x; x = 5; }
902 // because the "assignment" loops back around and generates an error.
904 // So now we just check that variables defined w/o an
905 // initializer are not live at the point of their
906 // initialization, which is mildly more complex than checking
907 // once at the func header but otherwise equivalent.
909 let succ = self.propagate_through_opt_expr(local.init, succ);
910 self.define_bindings_in_pat(local.pat, succ)
913 fn propagate_through_exprs(&mut self, exprs: &[Gc<Expr>], succ: LiveNode)
915 exprs.iter().rev().fold(succ, |succ, expr| {
916 self.propagate_through_expr(&**expr, succ)
920 fn propagate_through_opt_expr(&mut self,
921 opt_expr: Option<Gc<Expr>>,
924 opt_expr.iter().fold(succ, |succ, expr| {
925 self.propagate_through_expr(&**expr, succ)
929 fn propagate_through_expr(&mut self, expr: &Expr, succ: LiveNode)
931 debug!("propagate_through_expr: {}", expr_to_str(expr));
934 // Interesting cases with control flow or which gen/kill
937 self.access_path(expr, succ, ACC_READ | ACC_USE)
940 ExprField(ref e, _, _) => {
941 self.propagate_through_expr(&**e, succ)
944 ExprFnBlock(_, ref blk) | ExprProc(_, ref blk) => {
945 debug!("{} is an ExprFnBlock or ExprProc", expr_to_str(expr));
948 The next-node for a break is the successor of the entire
949 loop. The next-node for a continue is the top of this loop.
951 let node = self.live_node(expr.id, expr.span);
952 self.with_loop_nodes(blk.id, succ, node, |this| {
954 // the construction of a closure itself is not important,
955 // but we have to consider the closed over variables.
956 let caps = match this.ir.capture_info_map.find(&expr.id) {
957 Some(caps) => caps.clone(),
959 this.ir.tcx.sess.span_bug(expr.span, "no registered caps");
962 caps.iter().rev().fold(succ, |succ, cap| {
963 this.init_from_succ(cap.ln, succ);
964 let var = this.variable(cap.var_nid, expr.span);
965 this.acc(cap.ln, var, ACC_READ | ACC_USE);
971 ExprIf(ref cond, ref then, ref els) => {
985 let else_ln = self.propagate_through_opt_expr(els.clone(), succ);
986 let then_ln = self.propagate_through_block(&**then, succ);
987 let ln = self.live_node(expr.id, expr.span);
988 self.init_from_succ(ln, else_ln);
989 self.merge_from_succ(ln, then_ln, false);
990 self.propagate_through_expr(&**cond, ln)
993 ExprWhile(ref cond, ref blk) => {
994 self.propagate_through_loop(expr, Some(cond.clone()), &**blk, succ)
997 ExprForLoop(..) => fail!("non-desugared expr_for_loop"),
999 // Note that labels have been resolved, so we don't need to look
1000 // at the label ident
1001 ExprLoop(ref blk, _) => {
1002 self.propagate_through_loop(expr, None, &**blk, succ)
1005 ExprMatch(ref e, ref arms) => {
1020 let ln = self.live_node(expr.id, expr.span);
1021 self.init_empty(ln, succ);
1022 let mut first_merge = true;
1023 for arm in arms.iter() {
1025 self.propagate_through_expr(&*arm.body, succ);
1027 self.propagate_through_opt_expr(arm.guard, body_succ);
1029 self.define_bindings_in_arm_pats(arm.pats.as_slice(),
1031 self.merge_from_succ(ln, arm_succ, first_merge);
1032 first_merge = false;
1034 self.propagate_through_expr(&**e, ln)
1038 // ignore succ and subst exit_ln:
1039 let exit_ln = self.s.exit_ln;
1040 self.propagate_through_opt_expr(o_e, exit_ln)
1043 ExprBreak(opt_label) => {
1044 // Find which label this break jumps to
1045 let sc = self.find_loop_scope(opt_label, expr.id, expr.span);
1047 // Now that we know the label we're going to,
1048 // look it up in the break loop nodes table
1050 match self.break_ln.find(&sc) {
1052 None => self.ir.tcx.sess.span_bug(expr.span,
1053 "break to unknown label")
1057 ExprAgain(opt_label) => {
1058 // Find which label this expr continues to
1059 let sc = self.find_loop_scope(opt_label, expr.id, expr.span);
1061 // Now that we know the label we're going to,
1062 // look it up in the continue loop nodes table
1064 match self.cont_ln.find(&sc) {
1066 None => self.ir.tcx.sess.span_bug(expr.span,
1067 "loop to unknown label")
1071 ExprAssign(ref l, ref r) => {
1072 // see comment on lvalues in
1073 // propagate_through_lvalue_components()
1074 let succ = self.write_lvalue(&**l, succ, ACC_WRITE);
1075 let succ = self.propagate_through_lvalue_components(&**l, succ);
1076 self.propagate_through_expr(&**r, succ)
1079 ExprAssignOp(_, ref l, ref r) => {
1080 // see comment on lvalues in
1081 // propagate_through_lvalue_components()
1082 let succ = self.write_lvalue(&**l, succ, ACC_WRITE|ACC_READ);
1083 let succ = self.propagate_through_expr(&**r, succ);
1084 self.propagate_through_lvalue_components(&**l, succ)
1087 // Uninteresting cases: just propagate in rev exec order
1089 ExprVstore(ref expr, _) => {
1090 self.propagate_through_expr(&**expr, succ)
1093 ExprVec(ref exprs) => {
1094 self.propagate_through_exprs(exprs.as_slice(), succ)
1097 ExprRepeat(ref element, ref count) => {
1098 let succ = self.propagate_through_expr(&**count, succ);
1099 self.propagate_through_expr(&**element, succ)
1102 ExprStruct(_, ref fields, ref with_expr) => {
1103 let succ = self.propagate_through_opt_expr(with_expr.clone(), succ);
1104 fields.iter().rev().fold(succ, |succ, field| {
1105 self.propagate_through_expr(&*field.expr, succ)
1109 ExprCall(ref f, ref args) => {
1110 // calling a fn with bot return type means that the fn
1111 // will fail, and hence the successors can be ignored
1112 let is_bot = !self.ir.tcx.is_method_call(expr.id) && {
1113 let t_ret = ty::ty_fn_ret(ty::expr_ty(self.ir.tcx, &**f));
1114 ty::type_is_bot(t_ret)
1116 let succ = if is_bot {
1121 let succ = self.propagate_through_exprs(args.as_slice(), succ);
1122 self.propagate_through_expr(&**f, succ)
1125 ExprMethodCall(_, _, ref args) => {
1126 // calling a method with bot return type means that the method
1127 // will fail, and hence the successors can be ignored
1128 let t_ret = ty::node_id_to_type(self.ir.tcx, expr.id);
1129 let succ = if ty::type_is_bot(t_ret) {self.s.exit_ln}
1131 self.propagate_through_exprs(args.as_slice(), succ)
1134 ExprTup(ref exprs) => {
1135 self.propagate_through_exprs(exprs.as_slice(), succ)
1138 ExprBinary(op, ref l, ref r) if ast_util::lazy_binop(op) => {
1139 let r_succ = self.propagate_through_expr(&**r, succ);
1141 let ln = self.live_node(expr.id, expr.span);
1142 self.init_from_succ(ln, succ);
1143 self.merge_from_succ(ln, r_succ, false);
1145 self.propagate_through_expr(&**l, ln)
1148 ExprIndex(ref l, ref r) |
1149 ExprBinary(_, ref l, ref r) |
1150 ExprBox(ref l, ref r) => {
1151 self.propagate_through_exprs([l.clone(), r.clone()], succ)
1154 ExprAddrOf(_, ref e) |
1155 ExprCast(ref e, _) |
1156 ExprUnary(_, ref e) |
1157 ExprParen(ref e) => {
1158 self.propagate_through_expr(&**e, succ)
1161 ExprInlineAsm(ref ia) => {
1162 let succ = ia.outputs.iter().rev().fold(succ, |succ, &(_, ref expr)| {
1163 // see comment on lvalues in
1164 // propagate_through_lvalue_components()
1165 let succ = self.write_lvalue(&**expr, succ, ACC_WRITE);
1166 self.propagate_through_lvalue_components(&**expr, succ)
1168 // Inputs are executed first. Propagate last because of rev order
1169 ia.inputs.iter().rev().fold(succ, |succ, &(_, ref expr)| {
1170 self.propagate_through_expr(&**expr, succ)
1178 ExprBlock(ref blk) => {
1179 self.propagate_through_block(&**blk, succ)
1183 self.ir.tcx.sess.span_bug(expr.span, "unexpanded macro");
1188 fn propagate_through_lvalue_components(&mut self,
1194 // In general, the full flow graph structure for an
1195 // assignment/move/etc can be handled in one of two ways,
1196 // depending on whether what is being assigned is a "tracked
1197 // value" or not. A tracked value is basically a local
1198 // variable or argument.
1200 // The two kinds of graphs are:
1202 // Tracked lvalue Untracked lvalue
1203 // ----------------------++-----------------------
1207 // (rvalue) || (rvalue)
1210 // (write of lvalue) || (lvalue components)
1215 // ----------------------++-----------------------
1217 // I will cover the two cases in turn:
1219 // # Tracked lvalues
1221 // A tracked lvalue is a local variable/argument `x`. In
1222 // these cases, the link_node where the write occurs is linked
1223 // to node id of `x`. The `write_lvalue()` routine generates
1224 // the contents of this node. There are no subcomponents to
1227 // # Non-tracked lvalues
1229 // These are lvalues like `x[5]` or `x.f`. In that case, we
1230 // basically ignore the value which is written to but generate
1231 // reads for the components---`x` in these two examples. The
1232 // components reads are generated by
1233 // `propagate_through_lvalue_components()` (this fn).
1235 // # Illegal lvalues
1237 // It is still possible to observe assignments to non-lvalues;
1238 // these errors are detected in the later pass borrowck. We
1239 // just ignore such cases and treat them as reads.
1242 ExprPath(_) => succ,
1243 ExprField(ref e, _, _) => self.propagate_through_expr(&**e, succ),
1244 _ => self.propagate_through_expr(expr, succ)
1248 // see comment on propagate_through_lvalue()
1249 fn write_lvalue(&mut self, expr: &Expr, succ: LiveNode, acc: uint)
1252 ExprPath(_) => self.access_path(expr, succ, acc),
1254 // We do not track other lvalues, so just propagate through
1255 // to their subcomponents. Also, it may happen that
1256 // non-lvalues occur here, because those are detected in the
1257 // later pass borrowck.
1262 fn access_path(&mut self, expr: &Expr, succ: LiveNode, acc: uint)
1264 let def = self.ir.tcx.def_map.borrow().get_copy(&expr.id);
1265 match moved_variable_node_id_from_def(def) {
1267 let ln = self.live_node(expr.id, expr.span);
1269 self.init_from_succ(ln, succ);
1270 let var = self.variable(nid, expr.span);
1271 self.acc(ln, var, acc);
1279 fn propagate_through_loop(&mut self,
1281 cond: Option<Gc<Expr>>,
1288 We model control flow like this:
1306 let mut first_merge = true;
1307 let ln = self.live_node(expr.id, expr.span);
1308 self.init_empty(ln, succ);
1310 // if there is a condition, then it's possible we bypass
1311 // the body altogether. otherwise, the only way is via a
1312 // break in the loop body.
1313 self.merge_from_succ(ln, succ, first_merge);
1314 first_merge = false;
1316 debug!("propagate_through_loop: using id for loop body {} {}",
1317 expr.id, block_to_str(body));
1319 let cond_ln = self.propagate_through_opt_expr(cond, ln);
1320 let body_ln = self.with_loop_nodes(expr.id, succ, ln, |this| {
1321 this.propagate_through_block(body, cond_ln)
1324 // repeat until fixed point is reached:
1325 while self.merge_from_succ(ln, body_ln, first_merge) {
1326 first_merge = false;
1327 assert!(cond_ln == self.propagate_through_opt_expr(cond,
1329 assert!(body_ln == self.with_loop_nodes(expr.id, succ, ln,
1330 |this| this.propagate_through_block(body, cond_ln)));
1336 fn with_loop_nodes<R>(&mut self,
1337 loop_node_id: NodeId,
1340 f: |&mut Liveness<'a>| -> R)
1342 debug!("with_loop_nodes: {} {}", loop_node_id, break_ln.get());
1343 self.loop_scope.push(loop_node_id);
1344 self.break_ln.insert(loop_node_id, break_ln);
1345 self.cont_ln.insert(loop_node_id, cont_ln);
1347 self.loop_scope.pop();
1352 // _______________________________________________________________________
1353 // Checking for error conditions
1355 fn check_local(this: &mut Liveness, local: &Local) {
1358 this.warn_about_unused_or_dead_vars_in_pat(&*local.pat);
1361 this.pat_bindings(&*local.pat, |this, ln, var, sp, id| {
1362 this.warn_about_unused(sp, id, ln, var);
1367 visit::walk_local(this, local, ());
1370 fn check_arm(this: &mut Liveness, arm: &Arm) {
1371 this.arm_pats_bindings(arm.pats.as_slice(), |this, ln, var, sp, id| {
1372 this.warn_about_unused(sp, id, ln, var);
1374 visit::walk_arm(this, arm, ());
1377 fn check_expr(this: &mut Liveness, expr: &Expr) {
1379 ExprAssign(ref l, ref r) => {
1380 this.check_lvalue(&**l);
1381 this.visit_expr(&**r, ());
1383 visit::walk_expr(this, expr, ());
1386 ExprAssignOp(_, ref l, _) => {
1387 this.check_lvalue(&**l);
1389 visit::walk_expr(this, expr, ());
1392 ExprInlineAsm(ref ia) => {
1393 for &(_, ref input) in ia.inputs.iter() {
1394 this.visit_expr(&**input, ());
1397 // Output operands must be lvalues
1398 for &(_, ref out) in ia.outputs.iter() {
1399 this.check_lvalue(&**out);
1400 this.visit_expr(&**out, ());
1403 visit::walk_expr(this, expr, ());
1406 // no correctness conditions related to liveness
1407 ExprCall(..) | ExprMethodCall(..) | ExprIf(..) | ExprMatch(..) |
1408 ExprWhile(..) | ExprLoop(..) | ExprIndex(..) | ExprField(..) |
1409 ExprVstore(..) | ExprVec(..) | ExprTup(..) |
1411 ExprCast(..) | ExprUnary(..) | ExprRet(..) | ExprBreak(..) |
1412 ExprAgain(..) | ExprLit(_) | ExprBlock(..) |
1413 ExprMac(..) | ExprAddrOf(..) | ExprStruct(..) | ExprRepeat(..) |
1414 ExprParen(..) | ExprFnBlock(..) | ExprProc(..) | ExprPath(..) |
1416 visit::walk_expr(this, expr, ());
1418 ExprForLoop(..) => fail!("non-desugared expr_for_loop")
1422 fn check_fn(_v: &Liveness,
1428 // do not check contents of nested fns
1431 impl<'a> Liveness<'a> {
1438 if self.live_on_entry(entry_ln, self.s.no_ret_var).is_some() {
1439 // if no_ret_var is live, then we fall off the end of the
1440 // function without any kind of return expression:
1442 let t_ret = ty::ty_fn_ret(ty::node_id_to_type(self.ir.tcx, id));
1443 if ty::type_is_nil(t_ret) {
1444 // for nil return types, it is ok to not return a value expl.
1445 } else if ty::type_is_bot(t_ret) {
1446 // for bot return types, not ok. Function should fail.
1447 self.ir.tcx.sess.span_err(
1448 sp, "some control paths may return");
1450 let ends_with_stmt = match body.expr {
1451 None if body.stmts.len() > 0 =>
1452 match body.stmts.last().unwrap().node {
1453 StmtSemi(ref e, _) => {
1454 let t_stmt = ty::expr_ty(self.ir.tcx, &**e);
1455 ty::get(t_stmt).sty == ty::get(t_ret).sty
1462 let last_stmt = body.stmts.last().unwrap();
1463 let original_span = original_sp(last_stmt.span, sp);
1464 let span_semicolon = Span {
1465 lo: original_span.hi - BytePos(1),
1466 hi: original_span.hi,
1467 expn_info: original_span.expn_info
1469 self.ir.tcx.sess.span_note(
1470 span_semicolon, "consider removing this semicolon:");
1472 self.ir.tcx.sess.span_err(
1473 sp, "not all control paths return a value");
1478 fn check_lvalue(&mut self, expr: &Expr) {
1481 match self.ir.tcx.def_map.borrow().get_copy(&expr.id) {
1482 DefLocal(nid, _) => {
1483 // Assignment to an immutable variable or argument: only legal
1484 // if there is no later assignment. If this local is actually
1485 // mutable, then check for a reassignment to flag the mutability
1487 let ln = self.live_node(expr.id, expr.span);
1488 let var = self.variable(nid, expr.span);
1489 self.warn_about_dead_assign(expr.span, expr.id, ln, var);
1492 match moved_variable_node_id_from_def(def) {
1494 let ln = self.live_node(expr.id, expr.span);
1495 let var = self.variable(nid, expr.span);
1496 self.warn_about_dead_assign(expr.span, expr.id, ln, var);
1505 // For other kinds of lvalues, no checks are required,
1506 // and any embedded expressions are actually rvalues
1507 visit::walk_expr(self, expr, ());
1512 fn should_warn(&self, var: Variable) -> Option<String> {
1513 let name = self.ir.variable_name(var);
1514 if name.len() == 0 || name.as_slice()[0] == ('_' as u8) {
1521 fn warn_about_unused_args(&self, decl: &FnDecl, entry_ln: LiveNode) {
1522 for arg in decl.inputs.iter() {
1523 pat_util::pat_bindings(&self.ir.tcx.def_map,
1525 |_bm, p_id, sp, path| {
1526 let var = self.variable(p_id, sp);
1527 // Ignore unused self.
1528 let ident = ast_util::path_to_ident(path);
1529 if ident.name != special_idents::self_.name {
1530 self.warn_about_unused(sp, p_id, entry_ln, var);
1536 fn warn_about_unused_or_dead_vars_in_pat(&mut self, pat: &Pat) {
1537 self.pat_bindings(pat, |this, ln, var, sp, id| {
1538 if !this.warn_about_unused(sp, id, ln, var) {
1539 this.warn_about_dead_assign(sp, id, ln, var);
1544 fn warn_about_unused(&self,
1550 if !self.used_on_entry(ln, var) {
1551 let r = self.should_warn(var);
1552 for name in r.iter() {
1554 // annoying: for parameters in funcs like `fn(x: int)
1555 // {ret}`, there is only one node, so asking about
1556 // assigned_on_exit() is not meaningful.
1557 let is_assigned = if ln == self.s.exit_ln {
1560 self.assigned_on_exit(ln, var).is_some()
1564 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_VARIABLE, id, sp,
1565 format!("variable `{}` is assigned to, but never used",
1568 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_VARIABLE, id, sp,
1569 format!("unused variable: `{}`", *name));
1578 fn warn_about_dead_assign(&self,
1583 if self.live_on_exit(ln, var).is_none() {
1584 let r = self.should_warn(var);
1585 for name in r.iter() {
1586 self.ir.tcx.sess.add_lint(lint::builtin::DEAD_ASSIGNMENT, id, sp,
1587 format!("value assigned to `{}` is never read", *name));