// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 or the MIT license // , at your // option. This file may not be copied, modified, or distributed // except according to those terms. /*! * A classic liveness analysis based on dataflow over the AST. Computes, * for each local variable in a function, whether that variable is live * at a given point. Program execution points are identified by their * id. * * # Basic idea * * The basic model is that each local variable is assigned an index. We * represent sets of local variables using a vector indexed by this * index. The value in the vector is either 0, indicating the variable * is dead, or the id of an expression that uses the variable. * * We conceptually walk over the AST in reverse execution order. If we * find a use of a variable, we add it to the set of live variables. If * we find an assignment to a variable, we remove it from the set of live * variables. When we have to merge two flows, we take the union of * those two flows---if the variable is live on both paths, we simply * pick one id. In the event of loops, we continue doing this until a * fixed point is reached. * * ## Checking initialization * * At the function entry point, all variables must be dead. If this is * not the case, we can report an error using the id found in the set of * live variables, which identifies a use of the variable which is not * dominated by an assignment. * * ## Checking moves * * After each explicit move, the variable must be dead. * * ## Computing last uses * * Any use of the variable where the variable is dead afterwards is a * last use. * * # Implementation details * * The actual implementation contains two (nested) walks over the AST. * The outer walk has the job of building up the ir_maps instance for the * enclosing function. On the way down the tree, it identifies those AST * nodes and variable IDs that will be needed for the liveness analysis * and assigns them contiguous IDs. The liveness id for an AST node is * called a `live_node` (it's a newtype'd uint) and the id for a variable * is called a `variable` (another newtype'd uint). * * On the way back up the tree, as we are about to exit from a function * declaration we allocate a `liveness` instance. Now that we know * precisely how many nodes and variables we need, we can allocate all * the various arrays that we will need to precisely the right size. We then * perform the actual propagation on the `liveness` instance. * * This propagation is encoded in the various `propagate_through_*()` * methods. It effectively does a reverse walk of the AST; whenever we * reach a loop node, we iterate until a fixed point is reached. * * ## The `Users` struct * * At each live node `N`, we track three pieces of information for each * variable `V` (these are encapsulated in the `Users` struct): * * - `reader`: the `LiveNode` ID of some node which will read the value * that `V` holds on entry to `N`. Formally: a node `M` such * that there exists a path `P` from `N` to `M` where `P` does not * write `V`. If the `reader` is `invalid_node()`, then the current * value will never be read (the variable is dead, essentially). * * - `writer`: the `LiveNode` ID of some node which will write the * variable `V` and which is reachable from `N`. Formally: a node `M` * such that there exists a path `P` from `N` to `M` and `M` writes * `V`. If the `writer` is `invalid_node()`, then there is no writer * of `V` that follows `N`. * * - `used`: a boolean value indicating whether `V` is *used*. We * distinguish a *read* from a *use* in that a *use* is some read that * is not just used to generate a new value. For example, `x += 1` is * a read but not a use. This is used to generate better warnings. * * ## Special Variables * * We generate various special variables for various, well, special purposes. * These are described in the `specials` struct: * * - `exit_ln`: a live node that is generated to represent every 'exit' from * the function, whether it be by explicit return, fail, or other means. * * - `fallthrough_ln`: a live node that represents a fallthrough * * - `no_ret_var`: a synthetic variable that is only 'read' from, the * fallthrough node. This allows us to detect functions where we fail * to return explicitly. */ use middle::freevars; use middle::lint::{UnusedVariable, DeadAssignment}; use middle::pat_util; use middle::ty; use util::nodemap::NodeMap; use std::mem::transmute; use std::fmt; use std::io; use std::rc::Rc; use std::str; use std::uint; use syntax::ast::*; use syntax::codemap::{BytePos, original_sp, Span}; use syntax::parse::token::special_idents; use syntax::parse::token; use syntax::print::pprust::{expr_to_str, block_to_str}; use syntax::{visit, ast_util}; use syntax::visit::{Visitor, FnKind}; #[deriving(Eq)] struct Variable(uint); #[deriving(Eq)] struct LiveNode(uint); impl Variable { fn get(&self) -> uint { let Variable(v) = *self; v } } impl LiveNode { fn get(&self) -> uint { let LiveNode(v) = *self; v } } impl Clone for LiveNode { fn clone(&self) -> LiveNode { LiveNode(self.get()) } } #[deriving(Eq)] enum LiveNodeKind { FreeVarNode(Span), ExprNode(Span), VarDefNode(Span), ExitNode } fn live_node_kind_to_str(lnk: LiveNodeKind, cx: &ty::ctxt) -> String { let cm = cx.sess.codemap(); match lnk { FreeVarNode(s) => { format_strbuf!("Free var node [{}]", cm.span_to_str(s)) } ExprNode(s) => { format_strbuf!("Expr node [{}]", cm.span_to_str(s)) } VarDefNode(s) => { format_strbuf!("Var def node [{}]", cm.span_to_str(s)) } ExitNode => "Exit node".to_strbuf(), } } impl<'a> Visitor<()> for IrMaps<'a> { fn visit_fn(&mut self, fk: &FnKind, fd: &FnDecl, b: &Block, s: Span, n: NodeId, _: ()) { visit_fn(self, fk, fd, b, s, n); } fn visit_local(&mut self, l: &Local, _: ()) { visit_local(self, l); } fn visit_expr(&mut self, ex: &Expr, _: ()) { visit_expr(self, ex); } fn visit_arm(&mut self, a: &Arm, _: ()) { visit_arm(self, a); } } pub fn check_crate(tcx: &ty::ctxt, krate: &Crate) { visit::walk_crate(&mut IrMaps(tcx), krate, ()); tcx.sess.abort_if_errors(); } impl fmt::Show for LiveNode { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { write!(f, "ln({})", self.get()) } } impl fmt::Show for Variable { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { write!(f, "v({})", self.get()) } } // ______________________________________________________________________ // Creating ir_maps // // This is the first pass and the one that drives the main // computation. It walks up and down the IR once. On the way down, // we count for each function the number of variables as well as // liveness nodes. A liveness node is basically an expression or // capture clause that does something of interest: either it has // interesting control flow or it uses/defines a local variable. // // On the way back up, at each function node we create liveness sets // (we now know precisely how big to make our various vectors and so // forth) and then do the data-flow propagation to compute the set // of live variables at each program point. // // Finally, we run back over the IR one last time and, using the // computed liveness, check various safety conditions. For example, // there must be no live nodes at the definition site for a variable // unless it has an initializer. Similarly, each non-mutable local // variable must not be assigned if there is some successor // assignment. And so forth. impl LiveNode { fn is_valid(&self) -> bool { self.get() != uint::MAX } } fn invalid_node() -> LiveNode { LiveNode(uint::MAX) } struct CaptureInfo { ln: LiveNode, is_move: bool, var_nid: NodeId } enum LocalKind { FromMatch(BindingMode), FromLetWithInitializer, FromLetNoInitializer } struct LocalInfo { id: NodeId, ident: Ident, is_mutbl: bool, kind: LocalKind, } enum VarKind { Arg(NodeId, Ident), Local(LocalInfo), ImplicitRet } struct IrMaps<'a> { tcx: &'a ty::ctxt, num_live_nodes: uint, num_vars: uint, live_node_map: NodeMap, variable_map: NodeMap, capture_info_map: NodeMap>>, var_kinds: Vec, lnks: Vec, } fn IrMaps<'a>(tcx: &'a ty::ctxt) -> IrMaps<'a> { IrMaps { tcx: tcx, num_live_nodes: 0, num_vars: 0, live_node_map: NodeMap::new(), variable_map: NodeMap::new(), capture_info_map: NodeMap::new(), var_kinds: Vec::new(), lnks: Vec::new(), } } impl<'a> IrMaps<'a> { fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode { let ln = LiveNode(self.num_live_nodes); self.lnks.push(lnk); self.num_live_nodes += 1; debug!("{} is of kind {}", ln.to_str(), live_node_kind_to_str(lnk, self.tcx)); ln } fn add_live_node_for_node(&mut self, node_id: NodeId, lnk: LiveNodeKind) { let ln = self.add_live_node(lnk); self.live_node_map.insert(node_id, ln); debug!("{} is node {}", ln.to_str(), node_id); } fn add_variable(&mut self, vk: VarKind) -> Variable { let v = Variable(self.num_vars); self.var_kinds.push(vk); self.num_vars += 1; match vk { Local(LocalInfo { id: node_id, .. }) | Arg(node_id, _) => { self.variable_map.insert(node_id, v); }, ImplicitRet => {} } debug!("{} is {:?}", v.to_str(), vk); v } fn variable(&self, node_id: NodeId, span: Span) -> Variable { match self.variable_map.find(&node_id) { Some(&var) => var, None => { self.tcx .sess .span_bug(span, format!("no variable registered for id {}", node_id).as_slice()); } } } fn variable_name(&self, var: Variable) -> String { match self.var_kinds.get(var.get()) { &Local(LocalInfo { ident: nm, .. }) | &Arg(_, nm) => { token::get_ident(nm).get().to_str().to_strbuf() }, &ImplicitRet => "".to_strbuf() } } fn set_captures(&mut self, node_id: NodeId, cs: Vec) { self.capture_info_map.insert(node_id, Rc::new(cs)); } fn lnk(&self, ln: LiveNode) -> LiveNodeKind { *self.lnks.get(ln.get()) } } impl<'a> Visitor<()> for Liveness<'a> { fn visit_fn(&mut self, fk: &FnKind, fd: &FnDecl, b: &Block, s: Span, n: NodeId, _: ()) { check_fn(self, fk, fd, b, s, n); } fn visit_local(&mut self, l: &Local, _: ()) { check_local(self, l); } fn visit_expr(&mut self, ex: &Expr, _: ()) { check_expr(self, ex); } fn visit_arm(&mut self, a: &Arm, _: ()) { check_arm(self, a); } } fn visit_fn(ir: &mut IrMaps, fk: &FnKind, decl: &FnDecl, body: &Block, sp: Span, id: NodeId) { debug!("visit_fn: id={}", id); let _i = ::util::common::indenter(); // swap in a new set of IR maps for this function body: let mut fn_maps = IrMaps(ir.tcx); unsafe { debug!("creating fn_maps: {}", transmute::<&IrMaps, *IrMaps>(&fn_maps)); } for arg in decl.inputs.iter() { pat_util::pat_bindings(&ir.tcx.def_map, arg.pat, |_bm, arg_id, _x, path| { debug!("adding argument {}", arg_id); let ident = ast_util::path_to_ident(path); fn_maps.add_variable(Arg(arg_id, ident)); }) }; // gather up the various local variables, significant expressions, // and so forth: visit::walk_fn(&mut fn_maps, fk, decl, body, sp, ()); // Special nodes and variables: // - exit_ln represents the end of the fn, either by return or fail // - implicit_ret_var is a pseudo-variable that represents // an implicit return let specials = Specials { exit_ln: fn_maps.add_live_node(ExitNode), fallthrough_ln: fn_maps.add_live_node(ExitNode), no_ret_var: fn_maps.add_variable(ImplicitRet) }; // compute liveness let mut lsets = Liveness(&mut fn_maps, specials); let entry_ln = lsets.compute(decl, body); // check for various error conditions lsets.visit_block(body, ()); lsets.check_ret(id, sp, fk, entry_ln, body); lsets.warn_about_unused_args(decl, entry_ln); } fn visit_local(ir: &mut IrMaps, local: &Local) { pat_util::pat_bindings(&ir.tcx.def_map, local.pat, |bm, p_id, sp, path| { debug!("adding local variable {}", p_id); let name = ast_util::path_to_ident(path); ir.add_live_node_for_node(p_id, VarDefNode(sp)); let kind = match local.init { Some(_) => FromLetWithInitializer, None => FromLetNoInitializer }; let mutbl = match bm { BindByValue(MutMutable) => true, _ => false }; ir.add_variable(Local(LocalInfo { id: p_id, ident: name, is_mutbl: mutbl, kind: kind })); }); visit::walk_local(ir, local, ()); } fn visit_arm(ir: &mut IrMaps, arm: &Arm) { for pat in arm.pats.iter() { pat_util::pat_bindings(&ir.tcx.def_map, *pat, |bm, p_id, sp, path| { debug!("adding local variable {} from match with bm {:?}", p_id, bm); let name = ast_util::path_to_ident(path); let mutbl = match bm { BindByValue(MutMutable) => true, _ => false }; ir.add_live_node_for_node(p_id, VarDefNode(sp)); ir.add_variable(Local(LocalInfo { id: p_id, ident: name, is_mutbl: mutbl, kind: FromMatch(bm) })); }) } visit::walk_arm(ir, arm, ()); } fn moved_variable_node_id_from_def(def: Def) -> Option { match def { DefBinding(nid, _) | DefArg(nid, _) | DefLocal(nid, _) => Some(nid), _ => None } } fn visit_expr(ir: &mut IrMaps, expr: &Expr) { match expr.node { // live nodes required for uses or definitions of variables: ExprPath(_) => { let def = ir.tcx.def_map.borrow().get_copy(&expr.id); debug!("expr {}: path that leads to {:?}", expr.id, def); if moved_variable_node_id_from_def(def).is_some() { ir.add_live_node_for_node(expr.id, ExprNode(expr.span)); } visit::walk_expr(ir, expr, ()); } ExprFnBlock(..) | ExprProc(..) => { // Interesting control flow (for loops can contain labeled // breaks or continues) ir.add_live_node_for_node(expr.id, ExprNode(expr.span)); // Make a live_node for each captured variable, with the span // being the location that the variable is used. This results // in better error messages than just pointing at the closure // construction site. let mut call_caps = Vec::new(); let fv_mode = freevars::get_capture_mode(ir.tcx, expr.id); freevars::with_freevars(ir.tcx, expr.id, |freevars| { for fv in freevars.iter() { match moved_variable_node_id_from_def(fv.def) { Some(rv) => { let fv_ln = ir.add_live_node(FreeVarNode(fv.span)); let fv_id = ast_util::def_id_of_def(fv.def).node; let fv_ty = ty::node_id_to_type(ir.tcx, fv_id); let is_move = match fv_mode { // var must be dead afterwards freevars::CaptureByValue => { ty::type_moves_by_default(ir.tcx, fv_ty) } // var can still be used freevars::CaptureByRef => { false } }; call_caps.push(CaptureInfo {ln: fv_ln, is_move: is_move, var_nid: rv}); } None => {} } } }); ir.set_captures(expr.id, call_caps); visit::walk_expr(ir, expr, ()); } // live nodes required for interesting control flow: ExprIf(..) | ExprMatch(..) | ExprWhile(..) | ExprLoop(..) => { ir.add_live_node_for_node(expr.id, ExprNode(expr.span)); visit::walk_expr(ir, expr, ()); } ExprForLoop(..) => fail!("non-desugared expr_for_loop"), ExprBinary(op, _, _) if ast_util::lazy_binop(op) => { ir.add_live_node_for_node(expr.id, ExprNode(expr.span)); visit::walk_expr(ir, expr, ()); } // otherwise, live nodes are not required: ExprIndex(..) | ExprField(..) | ExprVstore(..) | ExprVec(..) | ExprCall(..) | ExprMethodCall(..) | ExprTup(..) | ExprBinary(..) | ExprAddrOf(..) | ExprCast(..) | ExprUnary(..) | ExprBreak(_) | ExprAgain(_) | ExprLit(_) | ExprRet(..) | ExprBlock(..) | ExprAssign(..) | ExprAssignOp(..) | ExprMac(..) | ExprStruct(..) | ExprRepeat(..) | ExprParen(..) | ExprInlineAsm(..) | ExprBox(..) => { visit::walk_expr(ir, expr, ()); } } } // ______________________________________________________________________ // Computing liveness sets // // Actually we compute just a bit more than just liveness, but we use // the same basic propagation framework in all cases. #[deriving(Clone)] struct Users { reader: LiveNode, writer: LiveNode, used: bool } fn invalid_users() -> Users { Users { reader: invalid_node(), writer: invalid_node(), used: false } } struct Specials { exit_ln: LiveNode, fallthrough_ln: LiveNode, no_ret_var: Variable } static ACC_READ: uint = 1u; static ACC_WRITE: uint = 2u; static ACC_USE: uint = 4u; struct Liveness<'a> { ir: &'a mut IrMaps<'a>, s: Specials, successors: Vec, users: Vec, // The list of node IDs for the nested loop scopes // we're in. loop_scope: Vec, // mappings from loop node ID to LiveNode // ("break" label should map to loop node ID, // it probably doesn't now) break_ln: NodeMap, cont_ln: NodeMap } fn Liveness<'a>(ir: &'a mut IrMaps<'a>, specials: Specials) -> Liveness<'a> { Liveness { ir: ir, s: specials, successors: Vec::from_elem(ir.num_live_nodes, invalid_node()), users: Vec::from_elem(ir.num_live_nodes * ir.num_vars, invalid_users()), loop_scope: Vec::new(), break_ln: NodeMap::new(), cont_ln: NodeMap::new(), } } impl<'a> Liveness<'a> { fn live_node(&self, node_id: NodeId, span: Span) -> LiveNode { match self.ir.live_node_map.find(&node_id) { Some(&ln) => ln, None => { // This must be a mismatch between the ir_map construction // above and the propagation code below; the two sets of // code have to agree about which AST nodes are worth // creating liveness nodes for. self.ir.tcx.sess.span_bug( span, format!("no live node registered for node {}", node_id).as_slice()); } } } fn variable(&self, node_id: NodeId, span: Span) -> Variable { self.ir.variable(node_id, span) } fn pat_bindings(&mut self, pat: &Pat, f: |&mut Liveness<'a>, LiveNode, Variable, Span, NodeId|) { pat_util::pat_bindings(&self.ir.tcx.def_map, pat, |_bm, p_id, sp, _n| { let ln = self.live_node(p_id, sp); let var = self.variable(p_id, sp); f(self, ln, var, sp, p_id); }) } fn arm_pats_bindings(&mut self, pats: &[@Pat], f: |&mut Liveness<'a>, LiveNode, Variable, Span, NodeId|) { // only consider the first pattern; any later patterns must have // the same bindings, and we also consider the first pattern to be // the "authoritative" set of ids if !pats.is_empty() { self.pat_bindings(pats[0], f) } } fn define_bindings_in_pat(&mut self, pat: @Pat, succ: LiveNode) -> LiveNode { self.define_bindings_in_arm_pats([pat], succ) } fn define_bindings_in_arm_pats(&mut self, pats: &[@Pat], succ: LiveNode) -> LiveNode { let mut succ = succ; self.arm_pats_bindings(pats, |this, ln, var, _sp, _id| { this.init_from_succ(ln, succ); this.define(ln, var); succ = ln; }); succ } fn idx(&self, ln: LiveNode, var: Variable) -> uint { ln.get() * self.ir.num_vars + var.get() } fn live_on_entry(&self, ln: LiveNode, var: Variable) -> Option { assert!(ln.is_valid()); let reader = self.users.get(self.idx(ln, var)).reader; if reader.is_valid() {Some(self.ir.lnk(reader))} else {None} } /* Is this variable live on entry to any of its successor nodes? */ fn live_on_exit(&self, ln: LiveNode, var: Variable) -> Option { let successor = *self.successors.get(ln.get()); self.live_on_entry(successor, var) } fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool { assert!(ln.is_valid()); self.users.get(self.idx(ln, var)).used } fn assigned_on_entry(&self, ln: LiveNode, var: Variable) -> Option { assert!(ln.is_valid()); let writer = self.users.get(self.idx(ln, var)).writer; if writer.is_valid() {Some(self.ir.lnk(writer))} else {None} } fn assigned_on_exit(&self, ln: LiveNode, var: Variable) -> Option { let successor = *self.successors.get(ln.get()); self.assigned_on_entry(successor, var) } fn indices2(&mut self, ln: LiveNode, succ_ln: LiveNode, op: |&mut Liveness<'a>, uint, uint|) { let node_base_idx = self.idx(ln, Variable(0u)); let succ_base_idx = self.idx(succ_ln, Variable(0u)); for var_idx in range(0u, self.ir.num_vars) { op(self, node_base_idx + var_idx, succ_base_idx + var_idx); } } fn write_vars(&self, wr: &mut io::Writer, ln: LiveNode, test: |uint| -> LiveNode) -> io::IoResult<()> { let node_base_idx = self.idx(ln, Variable(0)); for var_idx in range(0u, self.ir.num_vars) { let idx = node_base_idx + var_idx; if test(idx).is_valid() { try!(write!(wr, " {}", Variable(var_idx).to_str())); } } Ok(()) } fn find_loop_scope(&self, opt_label: Option, id: NodeId, sp: Span) -> NodeId { match opt_label { Some(_) => { // Refers to a labeled loop. Use the results of resolve // to find with one match self.ir.tcx.def_map.borrow().find(&id) { Some(&DefLabel(loop_id)) => loop_id, _ => self.ir.tcx.sess.span_bug(sp, "label on break/loop \ doesn't refer to a loop") } } None => { // Vanilla 'break' or 'loop', so use the enclosing // loop scope if self.loop_scope.len() == 0 { self.ir.tcx.sess.span_bug(sp, "break outside loop"); } else { // FIXME(#5275): this shouldn't have to be a method... self.last_loop_scope() } } } } fn last_loop_scope(&self) -> NodeId { *self.loop_scope.last().unwrap() } #[allow(unused_must_use)] fn ln_str(&self, ln: LiveNode) -> String { let mut wr = io::MemWriter::new(); { let wr = &mut wr as &mut io::Writer; write!(wr, "[ln({}) of kind {:?} reads", ln.get(), self.ir.lnk(ln)); self.write_vars(wr, ln, |idx| self.users.get(idx).reader); write!(wr, " writes"); self.write_vars(wr, ln, |idx| self.users.get(idx).writer); write!(wr, " precedes {}]", self.successors.get(ln.get()).to_str()); } str::from_utf8(wr.unwrap().as_slice()).unwrap().to_strbuf() } fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) { *self.successors.get_mut(ln.get()) = succ_ln; // It is not necessary to initialize the // values to empty because this is the value // they have when they are created, and the sets // only grow during iterations. // // self.indices(ln) { |idx| // self.users[idx] = invalid_users(); // } } fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) { // more efficient version of init_empty() / merge_from_succ() *self.successors.get_mut(ln.get()) = succ_ln; self.indices2(ln, succ_ln, |this, idx, succ_idx| { *this.users.get_mut(idx) = *this.users.get(succ_idx) }); debug!("init_from_succ(ln={}, succ={})", self.ln_str(ln), self.ln_str(succ_ln)); } fn merge_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode, first_merge: bool) -> bool { if ln == succ_ln { return false; } let mut changed = false; self.indices2(ln, succ_ln, |this, idx, succ_idx| { changed |= copy_if_invalid(this.users.get(succ_idx).reader, &mut this.users.get_mut(idx).reader); changed |= copy_if_invalid(this.users.get(succ_idx).writer, &mut this.users.get_mut(idx).writer); if this.users.get(succ_idx).used && !this.users.get(idx).used { this.users.get_mut(idx).used = true; changed = true; } }); debug!("merge_from_succ(ln={}, succ={}, first_merge={}, changed={})", ln.to_str(), self.ln_str(succ_ln), first_merge, changed); return changed; fn copy_if_invalid(src: LiveNode, dst: &mut LiveNode) -> bool { if src.is_valid() && !dst.is_valid() { *dst = src; true } else { false } } } // Indicates that a local variable was *defined*; we know that no // uses of the variable can precede the definition (resolve checks // this) so we just clear out all the data. fn define(&mut self, writer: LiveNode, var: Variable) { let idx = self.idx(writer, var); self.users.get_mut(idx).reader = invalid_node(); self.users.get_mut(idx).writer = invalid_node(); debug!("{} defines {} (idx={}): {}", writer.to_str(), var.to_str(), idx, self.ln_str(writer)); } // Either read, write, or both depending on the acc bitset fn acc(&mut self, ln: LiveNode, var: Variable, acc: uint) { debug!("{} accesses[{:x}] {}: {}", ln.to_str(), acc, var.to_str(), self.ln_str(ln)); let idx = self.idx(ln, var); let user = self.users.get_mut(idx); if (acc & ACC_WRITE) != 0 { user.reader = invalid_node(); user.writer = ln; } // Important: if we both read/write, must do read second // or else the write will override. if (acc & ACC_READ) != 0 { user.reader = ln; } if (acc & ACC_USE) != 0 { user.used = true; } } // _______________________________________________________________________ fn compute(&mut self, decl: &FnDecl, body: &Block) -> LiveNode { // if there is a `break` or `again` at the top level, then it's // effectively a return---this only occurs in `for` loops, // where the body is really a closure. debug!("compute: using id for block, {}", block_to_str(body)); let entry_ln: LiveNode = self.with_loop_nodes(body.id, self.s.exit_ln, self.s.exit_ln, |this| this.propagate_through_fn_block(decl, body)); // hack to skip the loop unless debug! is enabled: debug!("^^ liveness computation results for body {} (entry={})", { for ln_idx in range(0u, self.ir.num_live_nodes) { debug!("{}", self.ln_str(LiveNode(ln_idx))); } body.id }, entry_ln.to_str()); entry_ln } fn propagate_through_fn_block(&mut self, _: &FnDecl, blk: &Block) -> LiveNode { // the fallthrough exit is only for those cases where we do not // explicitly return: self.init_from_succ(self.s.fallthrough_ln, self.s.exit_ln); if blk.expr.is_none() { self.acc(self.s.fallthrough_ln, self.s.no_ret_var, ACC_READ) } self.propagate_through_block(blk, self.s.fallthrough_ln) } fn propagate_through_block(&mut self, blk: &Block, succ: LiveNode) -> LiveNode { let succ = self.propagate_through_opt_expr(blk.expr, succ); blk.stmts.iter().rev().fold(succ, |succ, stmt| { self.propagate_through_stmt(*stmt, succ) }) } fn propagate_through_stmt(&mut self, stmt: &Stmt, succ: LiveNode) -> LiveNode { match stmt.node { StmtDecl(decl, _) => { self.propagate_through_decl(decl, succ) } StmtExpr(expr, _) | StmtSemi(expr, _) => { self.propagate_through_expr(expr, succ) } StmtMac(..) => { self.ir.tcx.sess.span_bug(stmt.span, "unexpanded macro"); } } } fn propagate_through_decl(&mut self, decl: &Decl, succ: LiveNode) -> LiveNode { match decl.node { DeclLocal(ref local) => { self.propagate_through_local(*local, succ) } DeclItem(_) => succ, } } fn propagate_through_local(&mut self, local: &Local, succ: LiveNode) -> LiveNode { // Note: we mark the variable as defined regardless of whether // there is an initializer. Initially I had thought to only mark // the live variable as defined if it was initialized, and then we // could check for uninit variables just by scanning what is live // at the start of the function. But that doesn't work so well for // immutable variables defined in a loop: // loop { let x; x = 5; } // because the "assignment" loops back around and generates an error. // // So now we just check that variables defined w/o an // initializer are not live at the point of their // initialization, which is mildly more complex than checking // once at the func header but otherwise equivalent. let succ = self.propagate_through_opt_expr(local.init, succ); self.define_bindings_in_pat(local.pat, succ) } fn propagate_through_exprs(&mut self, exprs: &[@Expr], succ: LiveNode) -> LiveNode { exprs.iter().rev().fold(succ, |succ, expr| { self.propagate_through_expr(*expr, succ) }) } fn propagate_through_opt_expr(&mut self, opt_expr: Option<@Expr>, succ: LiveNode) -> LiveNode { opt_expr.iter().fold(succ, |succ, expr| { self.propagate_through_expr(*expr, succ) }) } fn propagate_through_expr(&mut self, expr: &Expr, succ: LiveNode) -> LiveNode { debug!("propagate_through_expr: {}", expr_to_str(expr)); match expr.node { // Interesting cases with control flow or which gen/kill ExprPath(_) => { self.access_path(expr, succ, ACC_READ | ACC_USE) } ExprField(e, _, _) => { self.propagate_through_expr(e, succ) } ExprFnBlock(_, blk) | ExprProc(_, blk) => { debug!("{} is an ExprFnBlock or ExprProc", expr_to_str(expr)); /* The next-node for a break is the successor of the entire loop. The next-node for a continue is the top of this loop. */ let node = self.live_node(expr.id, expr.span); self.with_loop_nodes(blk.id, succ, node, |this| { // the construction of a closure itself is not important, // but we have to consider the closed over variables. let caps = match this.ir.capture_info_map.find(&expr.id) { Some(caps) => caps.clone(), None => { this.ir.tcx.sess.span_bug(expr.span, "no registered caps"); } }; caps.iter().rev().fold(succ, |succ, cap| { this.init_from_succ(cap.ln, succ); let var = this.variable(cap.var_nid, expr.span); this.acc(cap.ln, var, ACC_READ | ACC_USE); cap.ln }) }) } ExprIf(cond, then, els) => { // // (cond) // | // v // (expr) // / \ // | | // v v // (then)(els) // | | // v v // ( succ ) // let else_ln = self.propagate_through_opt_expr(els, succ); let then_ln = self.propagate_through_block(then, succ); let ln = self.live_node(expr.id, expr.span); self.init_from_succ(ln, else_ln); self.merge_from_succ(ln, then_ln, false); self.propagate_through_expr(cond, ln) } ExprWhile(cond, blk) => { self.propagate_through_loop(expr, Some(cond), blk, succ) } ExprForLoop(..) => fail!("non-desugared expr_for_loop"), // Note that labels have been resolved, so we don't need to look // at the label ident ExprLoop(blk, _) => { self.propagate_through_loop(expr, None, blk, succ) } ExprMatch(e, ref arms) => { // // (e) // | // v // (expr) // / | \ // | | | // v v v // (..arms..) // | | | // v v v // ( succ ) // // let ln = self.live_node(expr.id, expr.span); self.init_empty(ln, succ); let mut first_merge = true; for arm in arms.iter() { let body_succ = self.propagate_through_expr(arm.body, succ); let guard_succ = self.propagate_through_opt_expr(arm.guard, body_succ); let arm_succ = self.define_bindings_in_arm_pats(arm.pats.as_slice(), guard_succ); self.merge_from_succ(ln, arm_succ, first_merge); first_merge = false; }; self.propagate_through_expr(e, ln) } ExprRet(o_e) => { // ignore succ and subst exit_ln: self.propagate_through_opt_expr(o_e, self.s.exit_ln) } ExprBreak(opt_label) => { // Find which label this break jumps to let sc = self.find_loop_scope(opt_label, expr.id, expr.span); // Now that we know the label we're going to, // look it up in the break loop nodes table match self.break_ln.find(&sc) { Some(&b) => b, None => self.ir.tcx.sess.span_bug(expr.span, "break to unknown label") } } ExprAgain(opt_label) => { // Find which label this expr continues to let sc = self.find_loop_scope(opt_label, expr.id, expr.span); // Now that we know the label we're going to, // look it up in the continue loop nodes table match self.cont_ln.find(&sc) { Some(&b) => b, None => self.ir.tcx.sess.span_bug(expr.span, "loop to unknown label") } } ExprAssign(l, r) => { // see comment on lvalues in // propagate_through_lvalue_components() let succ = self.write_lvalue(l, succ, ACC_WRITE); let succ = self.propagate_through_lvalue_components(l, succ); self.propagate_through_expr(r, succ) } ExprAssignOp(_, l, r) => { // see comment on lvalues in // propagate_through_lvalue_components() let succ = self.write_lvalue(l, succ, ACC_WRITE|ACC_READ); let succ = self.propagate_through_expr(r, succ); self.propagate_through_lvalue_components(l, succ) } // Uninteresting cases: just propagate in rev exec order ExprVstore(expr, _) => { self.propagate_through_expr(expr, succ) } ExprVec(ref exprs) => { self.propagate_through_exprs(exprs.as_slice(), succ) } ExprRepeat(element, count) => { let succ = self.propagate_through_expr(count, succ); self.propagate_through_expr(element, succ) } ExprStruct(_, ref fields, with_expr) => { let succ = self.propagate_through_opt_expr(with_expr, succ); fields.iter().rev().fold(succ, |succ, field| { self.propagate_through_expr(field.expr, succ) }) } ExprCall(f, ref args) => { // calling a fn with bot return type means that the fn // will fail, and hence the successors can be ignored let t_ret = ty::ty_fn_ret(ty::expr_ty(self.ir.tcx, f)); let succ = if ty::type_is_bot(t_ret) {self.s.exit_ln} else {succ}; let succ = self.propagate_through_exprs(args.as_slice(), succ); self.propagate_through_expr(f, succ) } ExprMethodCall(_, _, ref args) => { // calling a method with bot return type means that the method // will fail, and hence the successors can be ignored let t_ret = ty::node_id_to_type(self.ir.tcx, expr.id); let succ = if ty::type_is_bot(t_ret) {self.s.exit_ln} else {succ}; self.propagate_through_exprs(args.as_slice(), succ) } ExprTup(ref exprs) => { self.propagate_through_exprs(exprs.as_slice(), succ) } ExprBinary(op, l, r) if ast_util::lazy_binop(op) => { let r_succ = self.propagate_through_expr(r, succ); let ln = self.live_node(expr.id, expr.span); self.init_from_succ(ln, succ); self.merge_from_succ(ln, r_succ, false); self.propagate_through_expr(l, ln) } ExprIndex(l, r) | ExprBinary(_, l, r) | ExprBox(l, r) => { self.propagate_through_exprs([l, r], succ) } ExprAddrOf(_, e) | ExprCast(e, _) | ExprUnary(_, e) | ExprParen(e) => { self.propagate_through_expr(e, succ) } ExprInlineAsm(ref ia) => { let succ = ia.outputs.iter().rev().fold(succ, |succ, &(_, expr)| { // see comment on lvalues in // propagate_through_lvalue_components() let succ = self.write_lvalue(expr, succ, ACC_WRITE); self.propagate_through_lvalue_components(expr, succ) }); // Inputs are executed first. Propagate last because of rev order ia.inputs.iter().rev().fold(succ, |succ, &(_, expr)| { self.propagate_through_expr(expr, succ) }) } ExprLit(..) => { succ } ExprBlock(blk) => { self.propagate_through_block(blk, succ) } ExprMac(..) => { self.ir.tcx.sess.span_bug(expr.span, "unexpanded macro"); } } } fn propagate_through_lvalue_components(&mut self, expr: &Expr, succ: LiveNode) -> LiveNode { // # Lvalues // // In general, the full flow graph structure for an // assignment/move/etc can be handled in one of two ways, // depending on whether what is being assigned is a "tracked // value" or not. A tracked value is basically a local // variable or argument. // // The two kinds of graphs are: // // Tracked lvalue Untracked lvalue // ----------------------++----------------------- // || // | || | // v || v // (rvalue) || (rvalue) // | || | // v || v // (write of lvalue) || (lvalue components) // | || | // v || v // (succ) || (succ) // || // ----------------------++----------------------- // // I will cover the two cases in turn: // // # Tracked lvalues // // A tracked lvalue is a local variable/argument `x`. In // these cases, the link_node where the write occurs is linked // to node id of `x`. The `write_lvalue()` routine generates // the contents of this node. There are no subcomponents to // consider. // // # Non-tracked lvalues // // These are lvalues like `x[5]` or `x.f`. In that case, we // basically ignore the value which is written to but generate // reads for the components---`x` in these two examples. The // components reads are generated by // `propagate_through_lvalue_components()` (this fn). // // # Illegal lvalues // // It is still possible to observe assignments to non-lvalues; // these errors are detected in the later pass borrowck. We // just ignore such cases and treat them as reads. match expr.node { ExprPath(_) => succ, ExprField(e, _, _) => self.propagate_through_expr(e, succ), _ => self.propagate_through_expr(expr, succ) } } // see comment on propagate_through_lvalue() fn write_lvalue(&mut self, expr: &Expr, succ: LiveNode, acc: uint) -> LiveNode { match expr.node { ExprPath(_) => self.access_path(expr, succ, acc), // We do not track other lvalues, so just propagate through // to their subcomponents. Also, it may happen that // non-lvalues occur here, because those are detected in the // later pass borrowck. _ => succ } } fn access_path(&mut self, expr: &Expr, succ: LiveNode, acc: uint) -> LiveNode { let def = self.ir.tcx.def_map.borrow().get_copy(&expr.id); match moved_variable_node_id_from_def(def) { Some(nid) => { let ln = self.live_node(expr.id, expr.span); if acc != 0u { self.init_from_succ(ln, succ); let var = self.variable(nid, expr.span); self.acc(ln, var, acc); } ln } None => succ } } fn propagate_through_loop(&mut self, expr: &Expr, cond: Option<@Expr>, body: &Block, succ: LiveNode) -> LiveNode { /* We model control flow like this: (cond) <--+ | | v | +-- (expr) | | | | | v | | (body) ---+ | | v (succ) */ // first iteration: let mut first_merge = true; let ln = self.live_node(expr.id, expr.span); self.init_empty(ln, succ); if cond.is_some() { // if there is a condition, then it's possible we bypass // the body altogether. otherwise, the only way is via a // break in the loop body. self.merge_from_succ(ln, succ, first_merge); first_merge = false; } debug!("propagate_through_loop: using id for loop body {} {}", expr.id, block_to_str(body)); let cond_ln = self.propagate_through_opt_expr(cond, ln); let body_ln = self.with_loop_nodes(expr.id, succ, ln, |this| { this.propagate_through_block(body, cond_ln) }); // repeat until fixed point is reached: while self.merge_from_succ(ln, body_ln, first_merge) { first_merge = false; assert!(cond_ln == self.propagate_through_opt_expr(cond, ln)); assert!(body_ln == self.with_loop_nodes(expr.id, succ, ln, |this| this.propagate_through_block(body, cond_ln))); } cond_ln } fn with_loop_nodes(&mut self, loop_node_id: NodeId, break_ln: LiveNode, cont_ln: LiveNode, f: |&mut Liveness<'a>| -> R) -> R { debug!("with_loop_nodes: {} {}", loop_node_id, break_ln.get()); self.loop_scope.push(loop_node_id); self.break_ln.insert(loop_node_id, break_ln); self.cont_ln.insert(loop_node_id, cont_ln); let r = f(self); self.loop_scope.pop(); r } } // _______________________________________________________________________ // Checking for error conditions fn check_local(this: &mut Liveness, local: &Local) { match local.init { Some(_) => { this.warn_about_unused_or_dead_vars_in_pat(local.pat); }, None => { this.pat_bindings(local.pat, |this, ln, var, sp, id| { this.warn_about_unused(sp, id, ln, var); }) } } visit::walk_local(this, local, ()); } fn check_arm(this: &mut Liveness, arm: &Arm) { this.arm_pats_bindings(arm.pats.as_slice(), |this, ln, var, sp, id| { this.warn_about_unused(sp, id, ln, var); }); visit::walk_arm(this, arm, ()); } fn check_expr(this: &mut Liveness, expr: &Expr) { match expr.node { ExprAssign(l, r) => { this.check_lvalue(l); this.visit_expr(r, ()); visit::walk_expr(this, expr, ()); } ExprAssignOp(_, l, _) => { this.check_lvalue(l); visit::walk_expr(this, expr, ()); } ExprInlineAsm(ref ia) => { for &(_, input) in ia.inputs.iter() { this.visit_expr(input, ()); } // Output operands must be lvalues for &(_, out) in ia.outputs.iter() { this.check_lvalue(out); this.visit_expr(out, ()); } visit::walk_expr(this, expr, ()); } // no correctness conditions related to liveness ExprCall(..) | ExprMethodCall(..) | ExprIf(..) | ExprMatch(..) | ExprWhile(..) | ExprLoop(..) | ExprIndex(..) | ExprField(..) | ExprVstore(..) | ExprVec(..) | ExprTup(..) | ExprBinary(..) | ExprCast(..) | ExprUnary(..) | ExprRet(..) | ExprBreak(..) | ExprAgain(..) | ExprLit(_) | ExprBlock(..) | ExprMac(..) | ExprAddrOf(..) | ExprStruct(..) | ExprRepeat(..) | ExprParen(..) | ExprFnBlock(..) | ExprProc(..) | ExprPath(..) | ExprBox(..) => { visit::walk_expr(this, expr, ()); } ExprForLoop(..) => fail!("non-desugared expr_for_loop") } } fn check_fn(_v: &Liveness, _fk: &FnKind, _decl: &FnDecl, _body: &Block, _sp: Span, _id: NodeId) { // do not check contents of nested fns } impl<'a> Liveness<'a> { fn check_ret(&self, id: NodeId, sp: Span, _fk: &FnKind, entry_ln: LiveNode, body: &Block) { if self.live_on_entry(entry_ln, self.s.no_ret_var).is_some() { // if no_ret_var is live, then we fall off the end of the // function without any kind of return expression: let t_ret = ty::ty_fn_ret(ty::node_id_to_type(self.ir.tcx, id)); if ty::type_is_nil(t_ret) { // for nil return types, it is ok to not return a value expl. } else if ty::type_is_bot(t_ret) { // for bot return types, not ok. Function should fail. self.ir.tcx.sess.span_err( sp, "some control paths may return"); } else { let ends_with_stmt = match body.expr { None if body.stmts.len() > 0 => match body.stmts.last().unwrap().node { StmtSemi(e, _) => { let t_stmt = ty::expr_ty(self.ir.tcx, e); ty::get(t_stmt).sty == ty::get(t_ret).sty }, _ => false }, _ => false }; if ends_with_stmt { let last_stmt = body.stmts.last().unwrap(); let original_span = original_sp(last_stmt.span, sp); let span_semicolon = Span { lo: original_span.hi - BytePos(1), hi: original_span.hi, expn_info: original_span.expn_info }; self.ir.tcx.sess.span_note( span_semicolon, "consider removing this semicolon:"); } self.ir.tcx.sess.span_err( sp, "not all control paths return a value"); } } } fn check_lvalue(&mut self, expr: &Expr) { match expr.node { ExprPath(_) => { match self.ir.tcx.def_map.borrow().get_copy(&expr.id) { DefLocal(nid, _) => { // Assignment to an immutable variable or argument: only legal // if there is no later assignment. If this local is actually // mutable, then check for a reassignment to flag the mutability // as being used. let ln = self.live_node(expr.id, expr.span); let var = self.variable(nid, expr.span); self.warn_about_dead_assign(expr.span, expr.id, ln, var); } def => { match moved_variable_node_id_from_def(def) { Some(nid) => { let ln = self.live_node(expr.id, expr.span); let var = self.variable(nid, expr.span); self.warn_about_dead_assign(expr.span, expr.id, ln, var); } None => {} } } } } _ => { // For other kinds of lvalues, no checks are required, // and any embedded expressions are actually rvalues visit::walk_expr(self, expr, ()); } } } fn should_warn(&self, var: Variable) -> Option { let name = self.ir.variable_name(var); if name.len() == 0 || name.as_slice()[0] == ('_' as u8) { None } else { Some(name) } } fn warn_about_unused_args(&self, decl: &FnDecl, entry_ln: LiveNode) { for arg in decl.inputs.iter() { pat_util::pat_bindings(&self.ir.tcx.def_map, arg.pat, |_bm, p_id, sp, path| { let var = self.variable(p_id, sp); // Ignore unused self. let ident = ast_util::path_to_ident(path); if ident.name != special_idents::self_.name { self.warn_about_unused(sp, p_id, entry_ln, var); } }) } } fn warn_about_unused_or_dead_vars_in_pat(&mut self, pat: &Pat) { self.pat_bindings(pat, |this, ln, var, sp, id| { if !this.warn_about_unused(sp, id, ln, var) { this.warn_about_dead_assign(sp, id, ln, var); } }) } fn warn_about_unused(&self, sp: Span, id: NodeId, ln: LiveNode, var: Variable) -> bool { if !self.used_on_entry(ln, var) { let r = self.should_warn(var); for name in r.iter() { // annoying: for parameters in funcs like `fn(x: int) // {ret}`, there is only one node, so asking about // assigned_on_exit() is not meaningful. let is_assigned = if ln == self.s.exit_ln { false } else { self.assigned_on_exit(ln, var).is_some() }; if is_assigned { self.ir.tcx.sess.add_lint(UnusedVariable, id, sp, format_strbuf!("variable `{}` is assigned to, \ but never used", *name)); } else { self.ir.tcx.sess.add_lint(UnusedVariable, id, sp, format_strbuf!("unused variable: `{}`", *name)); } } true } else { false } } fn warn_about_dead_assign(&self, sp: Span, id: NodeId, ln: LiveNode, var: Variable) { if self.live_on_exit(ln, var).is_none() { let r = self.should_warn(var); for name in r.iter() { self.ir.tcx.sess.add_lint(DeadAssignment, id, sp, format_strbuf!("value assigned to `{}` is never read", *name)); } } } }