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::mem_categorization::Typer;
107 use middle::pat_util;
110 use util::nodemap::NodeMap;
114 use std::mem::transmute;
120 use syntax::codemap::{BytePos, original_sp, Span};
121 use syntax::parse::token::special_idents;
122 use syntax::parse::token;
123 use syntax::print::pprust::{expr_to_string, block_to_string};
125 use syntax::{visit, ast_util};
126 use syntax::visit::{Visitor, FnKind};
128 /// For use with `propagate_through_loop`.
130 /// An endless `loop` loop.
132 /// A `while` loop, with the given expression as condition.
134 /// A `for` loop, with the given pattern to bind.
138 #[deriving(PartialEq)]
139 struct Variable(uint);
140 #[deriving(PartialEq)]
141 struct LiveNode(uint);
144 fn get(&self) -> uint { let Variable(v) = *self; v }
148 fn get(&self) -> uint { let LiveNode(v) = *self; v }
151 impl Clone for LiveNode {
152 fn clone(&self) -> LiveNode {
157 #[deriving(PartialEq)]
165 fn live_node_kind_to_string(lnk: LiveNodeKind, cx: &ty::ctxt) -> String {
166 let cm = cx.sess.codemap();
169 format!("Free var node [{}]", cm.span_to_string(s))
172 format!("Expr node [{}]", cm.span_to_string(s))
175 format!("Var def node [{}]", cm.span_to_string(s))
177 ExitNode => "Exit node".to_string(),
181 impl<'a, 'tcx, 'v> Visitor<'v> for IrMaps<'a, 'tcx> {
182 fn visit_fn(&mut self, fk: FnKind<'v>, fd: &'v FnDecl,
183 b: &'v Block, s: Span, n: NodeId) {
184 visit_fn(self, fk, fd, b, s, n);
186 fn visit_local(&mut self, l: &ast::Local) { visit_local(self, l); }
187 fn visit_expr(&mut self, ex: &Expr) { visit_expr(self, ex); }
188 fn visit_arm(&mut self, a: &Arm) { visit_arm(self, a); }
191 pub fn check_crate(tcx: &ty::ctxt) {
192 visit::walk_crate(&mut IrMaps::new(tcx), tcx.map.krate());
193 tcx.sess.abort_if_errors();
196 impl fmt::Show for LiveNode {
197 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
198 write!(f, "ln({})", self.get())
202 impl fmt::Show for Variable {
203 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
204 write!(f, "v({})", self.get())
208 // ______________________________________________________________________
211 // This is the first pass and the one that drives the main
212 // computation. It walks up and down the IR once. On the way down,
213 // we count for each function the number of variables as well as
214 // liveness nodes. A liveness node is basically an expression or
215 // capture clause that does something of interest: either it has
216 // interesting control flow or it uses/defines a local variable.
218 // On the way back up, at each function node we create liveness sets
219 // (we now know precisely how big to make our various vectors and so
220 // forth) and then do the data-flow propagation to compute the set
221 // of live variables at each program point.
223 // Finally, we run back over the IR one last time and, using the
224 // computed liveness, check various safety conditions. For example,
225 // there must be no live nodes at the definition site for a variable
226 // unless it has an initializer. Similarly, each non-mutable local
227 // variable must not be assigned if there is some successor
228 // assignment. And so forth.
231 fn is_valid(&self) -> bool {
232 self.get() != uint::MAX
236 fn invalid_node() -> LiveNode { LiveNode(uint::MAX) }
254 struct IrMaps<'a, 'tcx: 'a> {
255 tcx: &'a ty::ctxt<'tcx>,
257 num_live_nodes: uint,
259 live_node_map: NodeMap<LiveNode>,
260 variable_map: NodeMap<Variable>,
261 capture_info_map: NodeMap<Rc<Vec<CaptureInfo>>>,
262 var_kinds: Vec<VarKind>,
263 lnks: Vec<LiveNodeKind>,
266 impl<'a, 'tcx> IrMaps<'a, 'tcx> {
267 fn new(tcx: &'a ty::ctxt<'tcx>) -> IrMaps<'a, 'tcx> {
272 live_node_map: NodeMap::new(),
273 variable_map: NodeMap::new(),
274 capture_info_map: NodeMap::new(),
275 var_kinds: Vec::new(),
280 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
281 let ln = LiveNode(self.num_live_nodes);
283 self.num_live_nodes += 1;
285 debug!("{} is of kind {}", ln.to_string(),
286 live_node_kind_to_string(lnk, self.tcx));
291 fn add_live_node_for_node(&mut self, node_id: NodeId, lnk: LiveNodeKind) {
292 let ln = self.add_live_node(lnk);
293 self.live_node_map.insert(node_id, ln);
295 debug!("{} is node {}", ln.to_string(), node_id);
298 fn add_variable(&mut self, vk: VarKind) -> Variable {
299 let v = Variable(self.num_vars);
300 self.var_kinds.push(vk);
304 Local(LocalInfo { id: node_id, .. }) | Arg(node_id, _) => {
305 self.variable_map.insert(node_id, v);
310 debug!("{} is {:?}", v.to_string(), vk);
315 fn variable(&self, node_id: NodeId, span: Span) -> Variable {
316 match self.variable_map.find(&node_id) {
321 .span_bug(span, format!("no variable registered for id {}",
322 node_id).as_slice());
327 fn variable_name(&self, var: Variable) -> String {
328 match self.var_kinds.get(var.get()) {
329 &Local(LocalInfo { ident: nm, .. }) | &Arg(_, nm) => {
330 token::get_ident(nm).get().to_string()
332 &ImplicitRet => "<implicit-ret>".to_string()
336 fn set_captures(&mut self, node_id: NodeId, cs: Vec<CaptureInfo>) {
337 self.capture_info_map.insert(node_id, Rc::new(cs));
340 fn lnk(&self, ln: LiveNode) -> LiveNodeKind {
341 *self.lnks.get(ln.get())
345 impl<'a, 'tcx, 'v> Visitor<'v> for Liveness<'a, 'tcx> {
346 fn visit_fn(&mut self, fk: FnKind<'v>, fd: &'v FnDecl, b: &'v Block, s: Span, n: NodeId) {
347 check_fn(self, fk, fd, b, s, n);
349 fn visit_local(&mut self, l: &ast::Local) {
350 check_local(self, l);
352 fn visit_expr(&mut self, ex: &Expr) {
353 check_expr(self, ex);
355 fn visit_arm(&mut self, a: &Arm) {
360 fn visit_fn(ir: &mut IrMaps,
366 debug!("visit_fn: id={}", id);
367 let _i = ::util::common::indenter();
369 // swap in a new set of IR maps for this function body:
370 let mut fn_maps = IrMaps::new(ir.tcx);
373 debug!("creating fn_maps: {}",
374 transmute::<&IrMaps, *const IrMaps>(&fn_maps));
377 for arg in decl.inputs.iter() {
378 pat_util::pat_bindings(&ir.tcx.def_map,
380 |_bm, arg_id, _x, path1| {
381 debug!("adding argument {}", arg_id);
382 let ident = path1.node;
383 fn_maps.add_variable(Arg(arg_id, ident));
387 // gather up the various local variables, significant expressions,
389 visit::walk_fn(&mut fn_maps, fk, decl, body, sp);
391 // Special nodes and variables:
392 // - exit_ln represents the end of the fn, either by return or fail
393 // - implicit_ret_var is a pseudo-variable that represents
394 // an implicit return
395 let specials = Specials {
396 exit_ln: fn_maps.add_live_node(ExitNode),
397 fallthrough_ln: fn_maps.add_live_node(ExitNode),
398 no_ret_var: fn_maps.add_variable(ImplicitRet)
402 let mut lsets = Liveness::new(&mut fn_maps, specials);
403 let entry_ln = lsets.compute(decl, body);
405 // check for various error conditions
406 lsets.visit_block(body);
407 lsets.check_ret(id, sp, fk, entry_ln, body);
408 lsets.warn_about_unused_args(decl, entry_ln);
411 fn visit_local(ir: &mut IrMaps, local: &ast::Local) {
412 pat_util::pat_bindings(&ir.tcx.def_map, &*local.pat, |_, p_id, sp, path1| {
413 debug!("adding local variable {}", p_id);
414 let name = path1.node;
415 ir.add_live_node_for_node(p_id, VarDefNode(sp));
416 ir.add_variable(Local(LocalInfo {
421 visit::walk_local(ir, local);
424 fn visit_arm(ir: &mut IrMaps, arm: &Arm) {
425 for pat in arm.pats.iter() {
426 pat_util::pat_bindings(&ir.tcx.def_map, &**pat, |bm, p_id, sp, path1| {
427 debug!("adding local variable {} from match with bm {:?}",
429 let name = path1.node;
430 ir.add_live_node_for_node(p_id, VarDefNode(sp));
431 ir.add_variable(Local(LocalInfo {
437 visit::walk_arm(ir, arm);
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);
447 DefLocal(..) => ir.add_live_node_for_node(expr.id, ExprNode(expr.span)),
450 visit::walk_expr(ir, expr);
452 ExprFnBlock(..) | ExprProc(..) | ExprUnboxedFn(..) => {
453 // Interesting control flow (for loops can contain labeled
454 // breaks or continues)
455 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
457 // Make a live_node for each captured variable, with the span
458 // being the location that the variable is used. This results
459 // in better error messages than just pointing at the closure
460 // construction site.
461 let mut call_caps = Vec::new();
462 ty::with_freevars(ir.tcx, expr.id, |freevars| {
463 for fv in freevars.iter() {
466 let fv_ln = ir.add_live_node(FreeVarNode(fv.span));
467 call_caps.push(CaptureInfo {ln: fv_ln,
474 ir.set_captures(expr.id, call_caps);
476 visit::walk_expr(ir, expr);
479 // live nodes required for interesting control flow:
480 ExprIf(..) | ExprMatch(..) | ExprWhile(..) | ExprLoop(..) => {
481 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
482 visit::walk_expr(ir, expr);
485 ir.tcx.sess.span_bug(expr.span, "non-desugared ExprIfLet");
487 ExprWhileLet(..) => {
488 ir.tcx.sess.span_bug(expr.span, "non-desugared ExprWhileLet");
490 ExprForLoop(ref pat, _, _, _) => {
491 pat_util::pat_bindings(&ir.tcx.def_map, &**pat, |bm, p_id, sp, path1| {
492 debug!("adding local variable {} from for loop with bm {:?}",
494 let name = path1.node;
495 ir.add_live_node_for_node(p_id, VarDefNode(sp));
496 ir.add_variable(Local(LocalInfo {
501 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
502 visit::walk_expr(ir, expr);
504 ExprBinary(op, _, _) if ast_util::lazy_binop(op) => {
505 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
506 visit::walk_expr(ir, expr);
509 // otherwise, live nodes are not required:
510 ExprIndex(..) | ExprField(..) | ExprTupField(..) | ExprVec(..) |
511 ExprCall(..) | ExprMethodCall(..) | ExprTup(..) | ExprSlice(..) |
512 ExprBinary(..) | ExprAddrOf(..) |
513 ExprCast(..) | ExprUnary(..) | ExprBreak(_) |
514 ExprAgain(_) | ExprLit(_) | ExprRet(..) | ExprBlock(..) |
515 ExprAssign(..) | ExprAssignOp(..) | ExprMac(..) |
516 ExprStruct(..) | ExprRepeat(..) | ExprParen(..) |
517 ExprInlineAsm(..) | ExprBox(..) => {
518 visit::walk_expr(ir, expr);
523 // ______________________________________________________________________
524 // Computing liveness sets
526 // Actually we compute just a bit more than just liveness, but we use
527 // the same basic propagation framework in all cases.
536 fn invalid_users() -> Users {
538 reader: invalid_node(),
539 writer: invalid_node(),
546 fallthrough_ln: LiveNode,
550 static ACC_READ: uint = 1u;
551 static ACC_WRITE: uint = 2u;
552 static ACC_USE: uint = 4u;
554 struct Liveness<'a, 'tcx: 'a> {
555 ir: &'a mut IrMaps<'a, 'tcx>,
557 successors: Vec<LiveNode>,
559 // The list of node IDs for the nested loop scopes
561 loop_scope: Vec<NodeId>,
562 // mappings from loop node ID to LiveNode
563 // ("break" label should map to loop node ID,
564 // it probably doesn't now)
565 break_ln: NodeMap<LiveNode>,
566 cont_ln: NodeMap<LiveNode>
569 impl<'a, 'tcx> Liveness<'a, 'tcx> {
570 fn new(ir: &'a mut IrMaps<'a, 'tcx>, specials: Specials) -> Liveness<'a, 'tcx> {
571 let num_live_nodes = ir.num_live_nodes;
572 let num_vars = ir.num_vars;
576 successors: Vec::from_elem(num_live_nodes, invalid_node()),
577 users: Vec::from_elem(num_live_nodes * num_vars, invalid_users()),
578 loop_scope: Vec::new(),
579 break_ln: NodeMap::new(),
580 cont_ln: NodeMap::new(),
584 fn live_node(&self, node_id: NodeId, span: Span) -> LiveNode {
585 match self.ir.live_node_map.find(&node_id) {
588 // This must be a mismatch between the ir_map construction
589 // above and the propagation code below; the two sets of
590 // code have to agree about which AST nodes are worth
591 // creating liveness nodes for.
592 self.ir.tcx.sess.span_bug(
594 format!("no live node registered for node {}",
595 node_id).as_slice());
600 fn variable(&self, node_id: NodeId, span: Span) -> Variable {
601 self.ir.variable(node_id, span)
604 fn pat_bindings(&mut self,
606 f: |&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId|) {
607 pat_util::pat_bindings(&self.ir.tcx.def_map, pat, |_bm, p_id, sp, _n| {
608 let ln = self.live_node(p_id, sp);
609 let var = self.variable(p_id, sp);
610 f(self, ln, var, sp, p_id);
614 fn arm_pats_bindings(&mut self,
616 f: |&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId|) {
619 self.pat_bindings(pat, f);
625 fn define_bindings_in_pat(&mut self, pat: &Pat, succ: LiveNode)
627 self.define_bindings_in_arm_pats(Some(pat), succ)
630 fn define_bindings_in_arm_pats(&mut self, pat: Option<&Pat>, succ: LiveNode)
633 self.arm_pats_bindings(pat, |this, ln, var, _sp, _id| {
634 this.init_from_succ(ln, succ);
635 this.define(ln, var);
641 fn idx(&self, ln: LiveNode, var: Variable) -> uint {
642 ln.get() * self.ir.num_vars + var.get()
645 fn live_on_entry(&self, ln: LiveNode, var: Variable)
646 -> Option<LiveNodeKind> {
647 assert!(ln.is_valid());
648 let reader = self.users.get(self.idx(ln, var)).reader;
649 if reader.is_valid() {Some(self.ir.lnk(reader))} else {None}
653 Is this variable live on entry to any of its successor nodes?
655 fn live_on_exit(&self, ln: LiveNode, var: Variable)
656 -> Option<LiveNodeKind> {
657 let successor = *self.successors.get(ln.get());
658 self.live_on_entry(successor, var)
661 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
662 assert!(ln.is_valid());
663 self.users.get(self.idx(ln, var)).used
666 fn assigned_on_entry(&self, ln: LiveNode, var: Variable)
667 -> Option<LiveNodeKind> {
668 assert!(ln.is_valid());
669 let writer = self.users.get(self.idx(ln, var)).writer;
670 if writer.is_valid() {Some(self.ir.lnk(writer))} else {None}
673 fn assigned_on_exit(&self, ln: LiveNode, var: Variable)
674 -> Option<LiveNodeKind> {
675 let successor = *self.successors.get(ln.get());
676 self.assigned_on_entry(successor, var)
679 fn indices2(&mut self,
682 op: |&mut Liveness<'a, 'tcx>, uint, uint|) {
683 let node_base_idx = self.idx(ln, Variable(0u));
684 let succ_base_idx = self.idx(succ_ln, Variable(0u));
685 for var_idx in range(0u, self.ir.num_vars) {
686 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
693 test: |uint| -> LiveNode) -> io::IoResult<()> {
694 let node_base_idx = self.idx(ln, Variable(0));
695 for var_idx in range(0u, self.ir.num_vars) {
696 let idx = node_base_idx + var_idx;
697 if test(idx).is_valid() {
698 try!(write!(wr, " {}", Variable(var_idx).to_string()));
704 fn find_loop_scope(&self,
705 opt_label: Option<Ident>,
711 // Refers to a labeled loop. Use the results of resolve
713 match self.ir.tcx.def_map.borrow().find(&id) {
714 Some(&DefLabel(loop_id)) => loop_id,
715 _ => self.ir.tcx.sess.span_bug(sp, "label on break/loop \
716 doesn't refer to a loop")
720 // Vanilla 'break' or 'loop', so use the enclosing
722 if self.loop_scope.len() == 0 {
723 self.ir.tcx.sess.span_bug(sp, "break outside loop");
725 *self.loop_scope.last().unwrap()
731 #[allow(unused_must_use)]
732 fn ln_str(&self, ln: LiveNode) -> String {
733 let mut wr = io::MemWriter::new();
735 let wr = &mut wr as &mut io::Writer;
736 write!(wr, "[ln({}) of kind {:?} reads", ln.get(), self.ir.lnk(ln));
737 self.write_vars(wr, ln, |idx| self.users.get(idx).reader);
738 write!(wr, " writes");
739 self.write_vars(wr, ln, |idx| self.users.get(idx).writer);
740 write!(wr, " precedes {}]", self.successors.get(ln.get()).to_string());
742 str::from_utf8(wr.unwrap().as_slice()).unwrap().to_string()
745 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
746 *self.successors.get_mut(ln.get()) = succ_ln;
748 // It is not necessary to initialize the
749 // values to empty because this is the value
750 // they have when they are created, and the sets
751 // only grow during iterations.
753 // self.indices(ln) { |idx|
754 // self.users[idx] = invalid_users();
758 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
759 // more efficient version of init_empty() / merge_from_succ()
760 *self.successors.get_mut(ln.get()) = succ_ln;
762 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
763 *this.users.get_mut(idx) = *this.users.get(succ_idx)
765 debug!("init_from_succ(ln={}, succ={})",
766 self.ln_str(ln), self.ln_str(succ_ln));
769 fn merge_from_succ(&mut self,
774 if ln == succ_ln { return false; }
776 let mut changed = false;
777 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
778 changed |= copy_if_invalid(this.users.get(succ_idx).reader,
779 &mut this.users.get_mut(idx).reader);
780 changed |= copy_if_invalid(this.users.get(succ_idx).writer,
781 &mut this.users.get_mut(idx).writer);
782 if this.users.get(succ_idx).used && !this.users.get(idx).used {
783 this.users.get_mut(idx).used = true;
788 debug!("merge_from_succ(ln={}, succ={}, first_merge={}, changed={})",
789 ln.to_string(), self.ln_str(succ_ln), first_merge, changed);
792 fn copy_if_invalid(src: LiveNode, dst: &mut LiveNode) -> bool {
793 if src.is_valid() && !dst.is_valid() {
802 // Indicates that a local variable was *defined*; we know that no
803 // uses of the variable can precede the definition (resolve checks
804 // this) so we just clear out all the data.
805 fn define(&mut self, writer: LiveNode, var: Variable) {
806 let idx = self.idx(writer, var);
807 self.users.get_mut(idx).reader = invalid_node();
808 self.users.get_mut(idx).writer = invalid_node();
810 debug!("{} defines {} (idx={}): {}", writer.to_string(), var.to_string(),
811 idx, self.ln_str(writer));
814 // Either read, write, or both depending on the acc bitset
815 fn acc(&mut self, ln: LiveNode, var: Variable, acc: uint) {
816 debug!("{} accesses[{:x}] {}: {}",
817 ln.to_string(), acc, var.to_string(), self.ln_str(ln));
819 let idx = self.idx(ln, var);
820 let user = self.users.get_mut(idx);
822 if (acc & ACC_WRITE) != 0 {
823 user.reader = invalid_node();
827 // Important: if we both read/write, must do read second
828 // or else the write will override.
829 if (acc & ACC_READ) != 0 {
833 if (acc & ACC_USE) != 0 {
838 // _______________________________________________________________________
840 fn compute(&mut self, decl: &FnDecl, body: &Block) -> LiveNode {
841 // if there is a `break` or `again` at the top level, then it's
842 // effectively a return---this only occurs in `for` loops,
843 // where the body is really a closure.
845 debug!("compute: using id for block, {}", block_to_string(body));
847 let exit_ln = self.s.exit_ln;
848 let entry_ln: LiveNode =
849 self.with_loop_nodes(body.id, exit_ln, exit_ln,
850 |this| this.propagate_through_fn_block(decl, body));
852 // hack to skip the loop unless debug! is enabled:
853 debug!("^^ liveness computation results for body {} (entry={})",
855 for ln_idx in range(0u, self.ir.num_live_nodes) {
856 debug!("{}", self.ln_str(LiveNode(ln_idx)));
860 entry_ln.to_string());
865 fn propagate_through_fn_block(&mut self, _: &FnDecl, blk: &Block)
867 // the fallthrough exit is only for those cases where we do not
868 // explicitly return:
870 self.init_from_succ(s.fallthrough_ln, s.exit_ln);
871 if blk.expr.is_none() {
872 self.acc(s.fallthrough_ln, s.no_ret_var, ACC_READ)
875 self.propagate_through_block(blk, s.fallthrough_ln)
878 fn propagate_through_block(&mut self, blk: &Block, succ: LiveNode)
880 let succ = self.propagate_through_opt_expr(blk.expr.as_ref().map(|e| &**e), succ);
881 blk.stmts.iter().rev().fold(succ, |succ, stmt| {
882 self.propagate_through_stmt(&**stmt, succ)
886 fn propagate_through_stmt(&mut self, stmt: &Stmt, succ: LiveNode)
889 StmtDecl(ref decl, _) => {
890 self.propagate_through_decl(&**decl, succ)
893 StmtExpr(ref expr, _) | StmtSemi(ref expr, _) => {
894 self.propagate_through_expr(&**expr, succ)
898 self.ir.tcx.sess.span_bug(stmt.span, "unexpanded macro");
903 fn propagate_through_decl(&mut self, decl: &Decl, succ: LiveNode)
906 DeclLocal(ref local) => {
907 self.propagate_through_local(&**local, succ)
913 fn propagate_through_local(&mut self, local: &ast::Local, succ: LiveNode)
915 // Note: we mark the variable as defined regardless of whether
916 // there is an initializer. Initially I had thought to only mark
917 // the live variable as defined if it was initialized, and then we
918 // could check for uninit variables just by scanning what is live
919 // at the start of the function. But that doesn't work so well for
920 // immutable variables defined in a loop:
921 // loop { let x; x = 5; }
922 // because the "assignment" loops back around and generates an error.
924 // So now we just check that variables defined w/o an
925 // initializer are not live at the point of their
926 // initialization, which is mildly more complex than checking
927 // once at the func header but otherwise equivalent.
929 let succ = self.propagate_through_opt_expr(local.init.as_ref().map(|e| &**e), succ);
930 self.define_bindings_in_pat(&*local.pat, succ)
933 fn propagate_through_exprs(&mut self, exprs: &[P<Expr>], succ: LiveNode)
935 exprs.iter().rev().fold(succ, |succ, expr| {
936 self.propagate_through_expr(&**expr, succ)
940 fn propagate_through_opt_expr(&mut self,
941 opt_expr: Option<&Expr>,
944 opt_expr.iter().fold(succ, |succ, expr| {
945 self.propagate_through_expr(&**expr, succ)
949 fn propagate_through_expr(&mut self, expr: &Expr, succ: LiveNode)
951 debug!("propagate_through_expr: {}", expr_to_string(expr));
954 // Interesting cases with control flow or which gen/kill
957 self.access_path(expr, succ, ACC_READ | ACC_USE)
960 ExprField(ref e, _, _) => {
961 self.propagate_through_expr(&**e, succ)
964 ExprTupField(ref e, _, _) => {
965 self.propagate_through_expr(&**e, succ)
968 ExprFnBlock(_, _, ref blk) |
969 ExprProc(_, ref blk) |
970 ExprUnboxedFn(_, _, _, ref blk) => {
971 debug!("{} is an ExprFnBlock, ExprProc, or ExprUnboxedFn",
972 expr_to_string(expr));
975 The next-node for a break is the successor of the entire
976 loop. The next-node for a continue is the top of this loop.
978 let node = self.live_node(expr.id, expr.span);
979 self.with_loop_nodes(blk.id, succ, node, |this| {
981 // the construction of a closure itself is not important,
982 // but we have to consider the closed over variables.
983 let caps = match this.ir.capture_info_map.find(&expr.id) {
984 Some(caps) => caps.clone(),
986 this.ir.tcx.sess.span_bug(expr.span, "no registered caps");
989 caps.iter().rev().fold(succ, |succ, cap| {
990 this.init_from_succ(cap.ln, succ);
991 let var = this.variable(cap.var_nid, expr.span);
992 this.acc(cap.ln, var, ACC_READ | ACC_USE);
998 ExprIf(ref cond, ref then, ref els) => {
1012 let else_ln = self.propagate_through_opt_expr(els.as_ref().map(|e| &**e), succ);
1013 let then_ln = self.propagate_through_block(&**then, succ);
1014 let ln = self.live_node(expr.id, expr.span);
1015 self.init_from_succ(ln, else_ln);
1016 self.merge_from_succ(ln, then_ln, false);
1017 self.propagate_through_expr(&**cond, ln)
1021 self.ir.tcx.sess.span_bug(expr.span, "non-desugared ExprIfLet");
1024 ExprWhile(ref cond, ref blk, _) => {
1025 self.propagate_through_loop(expr, WhileLoop(&**cond), &**blk, succ)
1028 ExprWhileLet(..) => {
1029 self.ir.tcx.sess.span_bug(expr.span, "non-desugared ExprWhileLet");
1032 ExprForLoop(ref pat, ref head, ref blk, _) => {
1033 let ln = self.propagate_through_loop(expr, ForLoop(&**pat), &**blk, succ);
1034 self.propagate_through_expr(&**head, ln)
1037 // Note that labels have been resolved, so we don't need to look
1038 // at the label ident
1039 ExprLoop(ref blk, _) => {
1040 self.propagate_through_loop(expr, LoopLoop, &**blk, succ)
1043 ExprMatch(ref e, ref arms, _) => {
1058 let ln = self.live_node(expr.id, expr.span);
1059 self.init_empty(ln, succ);
1060 let mut first_merge = true;
1061 for arm in arms.iter() {
1063 self.propagate_through_expr(&*arm.body, succ);
1065 self.propagate_through_opt_expr(arm.guard.as_ref().map(|e| &**e), body_succ);
1066 // only consider the first pattern; any later patterns must have
1067 // the same bindings, and we also consider the first pattern to be
1068 // the "authoritative" set of ids
1070 self.define_bindings_in_arm_pats(arm.pats.as_slice().head().map(|p| &**p),
1072 self.merge_from_succ(ln, arm_succ, first_merge);
1073 first_merge = false;
1075 self.propagate_through_expr(&**e, ln)
1078 ExprRet(ref o_e) => {
1079 // ignore succ and subst exit_ln:
1080 let exit_ln = self.s.exit_ln;
1081 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), exit_ln)
1084 ExprBreak(opt_label) => {
1085 // Find which label this break jumps to
1086 let sc = self.find_loop_scope(opt_label, expr.id, expr.span);
1088 // Now that we know the label we're going to,
1089 // look it up in the break loop nodes table
1091 match self.break_ln.find(&sc) {
1093 None => self.ir.tcx.sess.span_bug(expr.span,
1094 "break to unknown label")
1098 ExprAgain(opt_label) => {
1099 // Find which label this expr continues to
1100 let sc = self.find_loop_scope(opt_label, expr.id, expr.span);
1102 // Now that we know the label we're going to,
1103 // look it up in the continue loop nodes table
1105 match self.cont_ln.find(&sc) {
1107 None => self.ir.tcx.sess.span_bug(expr.span,
1108 "loop to unknown label")
1112 ExprAssign(ref l, ref r) => {
1113 // see comment on lvalues in
1114 // propagate_through_lvalue_components()
1115 let succ = self.write_lvalue(&**l, succ, ACC_WRITE);
1116 let succ = self.propagate_through_lvalue_components(&**l, succ);
1117 self.propagate_through_expr(&**r, succ)
1120 ExprAssignOp(_, ref l, ref r) => {
1121 // see comment on lvalues in
1122 // propagate_through_lvalue_components()
1123 let succ = self.write_lvalue(&**l, succ, ACC_WRITE|ACC_READ);
1124 let succ = self.propagate_through_expr(&**r, succ);
1125 self.propagate_through_lvalue_components(&**l, succ)
1128 // Uninteresting cases: just propagate in rev exec order
1130 ExprVec(ref exprs) => {
1131 self.propagate_through_exprs(exprs.as_slice(), succ)
1134 ExprRepeat(ref element, ref count) => {
1135 let succ = self.propagate_through_expr(&**count, succ);
1136 self.propagate_through_expr(&**element, succ)
1139 ExprStruct(_, ref fields, ref with_expr) => {
1140 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1141 fields.iter().rev().fold(succ, |succ, field| {
1142 self.propagate_through_expr(&*field.expr, succ)
1146 ExprCall(ref f, ref args) => {
1147 // calling a fn with bot return type means that the fn
1148 // will fail, and hence the successors can be ignored
1149 let is_bot = !self.ir.tcx.is_method_call(expr.id) && {
1150 let t_ret = ty::ty_fn_ret(ty::expr_ty(self.ir.tcx, &**f));
1151 ty::type_is_bot(t_ret)
1153 let succ = if is_bot {
1158 let succ = self.propagate_through_exprs(args.as_slice(), succ);
1159 self.propagate_through_expr(&**f, succ)
1162 ExprMethodCall(_, _, ref args) => {
1163 // calling a method with bot return type means that the method
1164 // will fail, and hence the successors can be ignored
1165 let t_ret = ty::node_id_to_type(self.ir.tcx, expr.id);
1166 let succ = if ty::type_is_bot(t_ret) {self.s.exit_ln}
1168 self.propagate_through_exprs(args.as_slice(), succ)
1171 ExprTup(ref exprs) => {
1172 self.propagate_through_exprs(exprs.as_slice(), succ)
1175 ExprBinary(op, ref l, ref r) if ast_util::lazy_binop(op) => {
1176 let r_succ = self.propagate_through_expr(&**r, succ);
1178 let ln = self.live_node(expr.id, expr.span);
1179 self.init_from_succ(ln, succ);
1180 self.merge_from_succ(ln, r_succ, false);
1182 self.propagate_through_expr(&**l, ln)
1185 ExprIndex(ref l, ref r) |
1186 ExprBinary(_, ref l, ref r) |
1187 ExprBox(ref l, ref r) => {
1188 let r_succ = self.propagate_through_expr(&**r, succ);
1189 self.propagate_through_expr(&**l, r_succ)
1192 ExprSlice(ref e1, ref e2, ref e3, _) => {
1193 let succ = e3.as_ref().map_or(succ, |e| self.propagate_through_expr(&**e, succ));
1194 let succ = e2.as_ref().map_or(succ, |e| self.propagate_through_expr(&**e, succ));
1195 self.propagate_through_expr(&**e1, succ)
1198 ExprAddrOf(_, ref e) |
1199 ExprCast(ref e, _) |
1200 ExprUnary(_, ref e) |
1201 ExprParen(ref e) => {
1202 self.propagate_through_expr(&**e, succ)
1205 ExprInlineAsm(ref ia) => {
1207 let succ = ia.outputs.iter().rev().fold(succ, |succ, &(_, ref expr, _)| {
1208 // see comment on lvalues
1209 // in propagate_through_lvalue_components()
1210 let succ = self.write_lvalue(&**expr, succ, ACC_WRITE);
1211 self.propagate_through_lvalue_components(&**expr, succ)
1213 // Inputs are executed first. Propagate last because of rev order
1214 ia.inputs.iter().rev().fold(succ, |succ, &(_, ref expr)| {
1215 self.propagate_through_expr(&**expr, succ)
1223 ExprBlock(ref blk) => {
1224 self.propagate_through_block(&**blk, succ)
1228 self.ir.tcx.sess.span_bug(expr.span, "unexpanded macro");
1233 fn propagate_through_lvalue_components(&mut self,
1239 // In general, the full flow graph structure for an
1240 // assignment/move/etc can be handled in one of two ways,
1241 // depending on whether what is being assigned is a "tracked
1242 // value" or not. A tracked value is basically a local
1243 // variable or argument.
1245 // The two kinds of graphs are:
1247 // Tracked lvalue Untracked lvalue
1248 // ----------------------++-----------------------
1252 // (rvalue) || (rvalue)
1255 // (write of lvalue) || (lvalue components)
1260 // ----------------------++-----------------------
1262 // I will cover the two cases in turn:
1264 // # Tracked lvalues
1266 // A tracked lvalue is a local variable/argument `x`. In
1267 // these cases, the link_node where the write occurs is linked
1268 // to node id of `x`. The `write_lvalue()` routine generates
1269 // the contents of this node. There are no subcomponents to
1272 // # Non-tracked lvalues
1274 // These are lvalues like `x[5]` or `x.f`. In that case, we
1275 // basically ignore the value which is written to but generate
1276 // reads for the components---`x` in these two examples. The
1277 // components reads are generated by
1278 // `propagate_through_lvalue_components()` (this fn).
1280 // # Illegal lvalues
1282 // It is still possible to observe assignments to non-lvalues;
1283 // these errors are detected in the later pass borrowck. We
1284 // just ignore such cases and treat them as reads.
1287 ExprPath(_) => succ,
1288 ExprField(ref e, _, _) => self.propagate_through_expr(&**e, succ),
1289 ExprTupField(ref e, _, _) => self.propagate_through_expr(&**e, succ),
1290 _ => self.propagate_through_expr(expr, succ)
1294 // see comment on propagate_through_lvalue()
1295 fn write_lvalue(&mut self, expr: &Expr, succ: LiveNode, acc: uint)
1298 ExprPath(_) => self.access_path(expr, succ, acc),
1300 // We do not track other lvalues, so just propagate through
1301 // to their subcomponents. Also, it may happen that
1302 // non-lvalues occur here, because those are detected in the
1303 // later pass borrowck.
1308 fn access_path(&mut self, expr: &Expr, succ: LiveNode, acc: uint)
1310 match self.ir.tcx.def_map.borrow().get_copy(&expr.id) {
1312 let ln = self.live_node(expr.id, expr.span);
1314 self.init_from_succ(ln, succ);
1315 let var = self.variable(nid, expr.span);
1316 self.acc(ln, var, acc);
1324 fn propagate_through_loop(&mut self,
1333 We model control flow like this:
1351 let mut first_merge = true;
1352 let ln = self.live_node(expr.id, expr.span);
1353 self.init_empty(ln, succ);
1357 // If this is not a `loop` loop, then it's possible we bypass
1358 // the body altogether. Otherwise, the only way is via a `break`
1359 // in the loop body.
1360 self.merge_from_succ(ln, succ, first_merge);
1361 first_merge = false;
1364 debug!("propagate_through_loop: using id for loop body {} {}",
1365 expr.id, block_to_string(body));
1367 let cond_ln = match kind {
1369 ForLoop(ref pat) => self.define_bindings_in_pat(*pat, ln),
1370 WhileLoop(ref cond) => self.propagate_through_expr(&**cond, ln),
1372 let body_ln = self.with_loop_nodes(expr.id, succ, ln, |this| {
1373 this.propagate_through_block(body, cond_ln)
1376 // repeat until fixed point is reached:
1377 while self.merge_from_succ(ln, body_ln, first_merge) {
1378 first_merge = false;
1380 let new_cond_ln = match kind {
1382 ForLoop(ref pat) => {
1383 self.define_bindings_in_pat(*pat, ln)
1385 WhileLoop(ref cond) => {
1386 self.propagate_through_expr(&**cond, ln)
1389 assert!(cond_ln == new_cond_ln);
1390 assert!(body_ln == self.with_loop_nodes(expr.id, succ, ln,
1391 |this| this.propagate_through_block(body, cond_ln)));
1397 fn with_loop_nodes<R>(&mut self,
1398 loop_node_id: NodeId,
1401 f: |&mut Liveness<'a, 'tcx>| -> R)
1403 debug!("with_loop_nodes: {} {}", loop_node_id, break_ln.get());
1404 self.loop_scope.push(loop_node_id);
1405 self.break_ln.insert(loop_node_id, break_ln);
1406 self.cont_ln.insert(loop_node_id, cont_ln);
1408 self.loop_scope.pop();
1413 // _______________________________________________________________________
1414 // Checking for error conditions
1416 fn check_local(this: &mut Liveness, local: &ast::Local) {
1419 this.warn_about_unused_or_dead_vars_in_pat(&*local.pat);
1422 this.pat_bindings(&*local.pat, |this, ln, var, sp, id| {
1423 this.warn_about_unused(sp, id, ln, var);
1428 visit::walk_local(this, local);
1431 fn check_arm(this: &mut Liveness, arm: &Arm) {
1432 // only consider the first pattern; any later patterns must have
1433 // the same bindings, and we also consider the first pattern to be
1434 // the "authoritative" set of ids
1435 this.arm_pats_bindings(arm.pats.as_slice().head().map(|p| &**p), |this, ln, var, sp, id| {
1436 this.warn_about_unused(sp, id, ln, var);
1438 visit::walk_arm(this, arm);
1441 fn check_expr(this: &mut Liveness, expr: &Expr) {
1443 ExprAssign(ref l, ref r) => {
1444 this.check_lvalue(&**l);
1445 this.visit_expr(&**r);
1447 visit::walk_expr(this, expr);
1450 ExprAssignOp(_, ref l, _) => {
1451 this.check_lvalue(&**l);
1453 visit::walk_expr(this, expr);
1456 ExprInlineAsm(ref ia) => {
1457 for &(_, ref input) in ia.inputs.iter() {
1458 this.visit_expr(&**input);
1461 // Output operands must be lvalues
1462 for &(_, ref out, _) in ia.outputs.iter() {
1463 this.check_lvalue(&**out);
1464 this.visit_expr(&**out);
1467 visit::walk_expr(this, expr);
1470 ExprForLoop(ref pat, _, _, _) => {
1471 this.pat_bindings(&**pat, |this, ln, var, sp, id| {
1472 this.warn_about_unused(sp, id, ln, var);
1475 visit::walk_expr(this, expr);
1478 // no correctness conditions related to liveness
1479 ExprCall(..) | ExprMethodCall(..) | ExprIf(..) | ExprMatch(..) |
1480 ExprWhile(..) | ExprLoop(..) | ExprIndex(..) | ExprField(..) |
1481 ExprTupField(..) | ExprVec(..) | ExprTup(..) | ExprBinary(..) |
1482 ExprCast(..) | ExprUnary(..) | ExprRet(..) | ExprBreak(..) |
1483 ExprAgain(..) | ExprLit(_) | ExprBlock(..) | ExprSlice(..) |
1484 ExprMac(..) | ExprAddrOf(..) | ExprStruct(..) | ExprRepeat(..) |
1485 ExprParen(..) | ExprFnBlock(..) | ExprProc(..) | ExprUnboxedFn(..) |
1486 ExprPath(..) | ExprBox(..) => {
1487 visit::walk_expr(this, expr);
1490 this.ir.tcx.sess.span_bug(expr.span, "non-desugared ExprIfLet");
1492 ExprWhileLet(..) => {
1493 this.ir.tcx.sess.span_bug(expr.span, "non-desugared ExprWhileLet");
1498 fn check_fn(_v: &Liveness,
1504 // do not check contents of nested fns
1507 impl<'a, 'tcx> Liveness<'a, 'tcx> {
1514 if self.live_on_entry(entry_ln, self.s.no_ret_var).is_some() {
1515 // if no_ret_var is live, then we fall off the end of the
1516 // function without any kind of return expression:
1518 let t_ret = ty::ty_fn_ret(ty::node_id_to_type(self.ir.tcx, id));
1519 if ty::type_is_nil(t_ret) {
1520 // for nil return types, it is ok to not return a value expl.
1521 } else if ty::type_is_bot(t_ret) {
1522 // for bot return types, not ok. Function should fail.
1523 self.ir.tcx.sess.span_err(
1524 sp, "some control paths may return");
1526 let ends_with_stmt = match body.expr {
1527 None if body.stmts.len() > 0 =>
1528 match body.stmts.last().unwrap().node {
1529 StmtSemi(ref e, _) => {
1530 let t_stmt = ty::expr_ty(self.ir.tcx, &**e);
1531 ty::get(t_stmt).sty == ty::get(t_ret).sty
1537 self.ir.tcx.sess.span_err(
1538 sp, "not all control paths return a value");
1540 let last_stmt = body.stmts.last().unwrap();
1541 let original_span = original_sp(self.ir.tcx.sess.codemap(),
1542 last_stmt.span, sp);
1543 let span_semicolon = Span {
1544 lo: original_span.hi - BytePos(1),
1545 hi: original_span.hi,
1546 expn_id: original_span.expn_id
1548 self.ir.tcx.sess.span_note(
1549 span_semicolon, "consider removing this semicolon:");
1555 fn check_lvalue(&mut self, expr: &Expr) {
1558 match self.ir.tcx.def_map.borrow().get_copy(&expr.id) {
1560 // Assignment to an immutable variable or argument: only legal
1561 // if there is no later assignment. If this local is actually
1562 // mutable, then check for a reassignment to flag the mutability
1564 let ln = self.live_node(expr.id, expr.span);
1565 let var = self.variable(nid, expr.span);
1566 self.warn_about_dead_assign(expr.span, expr.id, ln, var);
1573 // For other kinds of lvalues, no checks are required,
1574 // and any embedded expressions are actually rvalues
1575 visit::walk_expr(self, expr);
1580 fn should_warn(&self, var: Variable) -> Option<String> {
1581 let name = self.ir.variable_name(var);
1582 if name.len() == 0 || name.as_bytes()[0] == ('_' as u8) {
1589 fn warn_about_unused_args(&self, decl: &FnDecl, entry_ln: LiveNode) {
1590 for arg in decl.inputs.iter() {
1591 pat_util::pat_bindings(&self.ir.tcx.def_map,
1593 |_bm, p_id, sp, path1| {
1594 let var = self.variable(p_id, sp);
1595 // Ignore unused self.
1596 let ident = path1.node;
1597 if ident.name != special_idents::self_.name {
1598 self.warn_about_unused(sp, p_id, entry_ln, var);
1604 fn warn_about_unused_or_dead_vars_in_pat(&mut self, pat: &Pat) {
1605 self.pat_bindings(pat, |this, ln, var, sp, id| {
1606 if !this.warn_about_unused(sp, id, ln, var) {
1607 this.warn_about_dead_assign(sp, id, ln, var);
1612 fn warn_about_unused(&self,
1618 if !self.used_on_entry(ln, var) {
1619 let r = self.should_warn(var);
1620 for name in r.iter() {
1622 // annoying: for parameters in funcs like `fn(x: int)
1623 // {ret}`, there is only one node, so asking about
1624 // assigned_on_exit() is not meaningful.
1625 let is_assigned = if ln == self.s.exit_ln {
1628 self.assigned_on_exit(ln, var).is_some()
1632 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_VARIABLES, id, sp,
1633 format!("variable `{}` is assigned to, but never used",
1636 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_VARIABLES, id, sp,
1637 format!("unused variable: `{}`", *name));
1646 fn warn_about_dead_assign(&self,
1651 if self.live_on_exit(ln, var).is_none() {
1652 let r = self.should_warn(var);
1653 for name in r.iter() {
1654 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_ASSIGNMENTS, id, sp,
1655 format!("value assigned to `{}` is never read", *name));