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// 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 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, 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::lint::{UnusedVariable, DeadAssignment};
use middle::pat_util;
use middle::ty;
use middle::typeck;
use middle::moves;
use util::nodemap::NodeMap;
use std::cast::transmute;
use std::fmt;
use std::io;
use std::rc::Rc;
use std::str;
use std::uint;
use syntax::ast::*;
use syntax::codemap::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) -> ~str {
let cm = cx.sess.codemap();
match lnk {
FreeVarNode(s) => format!("Free var node [{}]", cm.span_to_str(s)),
ExprNode(s) => format!("Expr node [{}]", cm.span_to_str(s)),
VarDefNode(s) => format!("Var def node [{}]", cm.span_to_str(s)),
ExitNode => ~"Exit node"
}
}
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,
method_map: typeck::MethodMap,
capture_map: &moves::CaptureMap,
krate: &Crate) {
visit::walk_crate(&mut IrMaps(tcx, method_map, capture_map), krate, ());
tcx.sess.abort_if_errors();
}
impl fmt::Show for LiveNode {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f.buf, "ln({})", self.get())
}
}
impl fmt::Show for Variable {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f.buf, "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,
method_map: typeck::MethodMap,
capture_map: &'a moves::CaptureMap,
num_live_nodes: uint,
num_vars: uint,
live_node_map: NodeMap<LiveNode>,
variable_map: NodeMap<Variable>,
capture_info_map: NodeMap<Rc<Vec<CaptureInfo>>>,
var_kinds: Vec<VarKind>,
lnks: Vec<LiveNodeKind>,
}
fn IrMaps<'a>(tcx: &'a ty::ctxt,
method_map: typeck::MethodMap,
capture_map: &'a moves::CaptureMap)
-> IrMaps<'a> {
IrMaps {
tcx: tcx,
method_map: method_map,
capture_map: capture_map,
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));
}
}
}
fn variable_name(&self, var: Variable) -> ~str {
match self.var_kinds.get(var.get()) {
&Local(LocalInfo { ident: nm, .. }) | &Arg(_, nm) => {
token::get_ident(nm).get().to_str()
},
&ImplicitRet => ~"<implicit-ret>"
}
}
fn set_captures(&mut self, node_id: NodeId, cs: Vec<CaptureInfo>) {
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, ir.method_map, ir.capture_map);
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, id, ());
// 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 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 moves::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();
for cv in ir.capture_map.get(&expr.id).iter() {
match moves::moved_variable_node_id_from_def(cv.def) {
Some(rv) => {
let cv_ln = ir.add_live_node(FreeVarNode(cv.span));
let is_move = match cv.mode {
// var must be dead afterwards
moves::CapMove => true,
// var can stil be used
moves::CapCopy | moves::CapRef => false
};
call_caps.push(CaptureInfo {ln: cv_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<LiveNode>,
users: Vec<Users>,
// The list of node IDs for the nested loop scopes
// we're in.
loop_scope: Vec<NodeId>,
// mappings from loop node ID to LiveNode
// ("break" label should map to loop node ID,
// it probably doesn't now)
break_ln: NodeMap<LiveNode>,
cont_ln: NodeMap<LiveNode>
}
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));
}
}
}
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 "authoratative" 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<LiveNodeKind> {
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<LiveNodeKind> {
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<LiveNodeKind> {
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<LiveNodeKind> {
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<Ident>,
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) -> ~str {
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_owned(wr.unwrap()).unwrap()
}
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.rev_iter().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 moves::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<R>(&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 span_semicolon = Span {
lo: last_stmt.span.hi,
hi: last_stmt.span.hi,
expn_info: last_stmt.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 moves::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<~str> {
let name = self.ir.variable_name(var);
if name.len() == 0 || name[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!("variable `{}` is assigned to, \
but never used", *name));
} else {
self.ir.tcx.sess.add_lint(UnusedVariable, id, sp,
format!("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!("value assigned to `{}` is never read", *name));
}
}
}
}
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