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 module for propagating forward dataflow information. The analysis
13 //! assumes that the items to be propagated can be represented as bits
14 //! and thus uses bitvectors. Your job is simply to specify the so-called
15 //! GEN and KILL bits for each expression.
23 use syntax::print::pprust::PrintState;
25 use rustc_data_structures::graph::OUTGOING;
27 use util::nodemap::FxHashMap;
29 use hir::intravisit::{self, IdRange};
30 use hir::print as pprust;
33 #[derive(Copy, Clone, Debug)]
34 pub enum EntryOrExit {
40 pub struct DataFlowContext<'a, 'tcx: 'a, O> {
41 tcx: TyCtxt<'a, 'tcx, 'tcx>,
43 /// a name for the analysis using this dataflow instance
44 analysis_name: &'static str,
46 /// the data flow operator
49 /// number of bits to propagate per id
52 /// number of words we will use to store bits_per_id.
53 /// equal to bits_per_id/usize::BITS rounded up.
56 // mapping from node to cfg node index
57 // FIXME (#6298): Shouldn't this go with CFG?
58 local_id_to_index: FxHashMap<hir::ItemLocalId, Vec<CFGIndex>>,
60 // Bit sets per cfg node. The following three fields (`gens`, `kills`,
61 // and `on_entry`) all have the same structure. For each id in
62 // `id_range`, there is a range of words equal to `words_per_id`.
63 // So, to access the bits for any given id, you take a slice of
64 // the full vector (see the method `compute_id_range()`).
66 /// bits generated as we exit the cfg node. Updated by `add_gen()`.
69 /// bits killed as we exit the cfg node, or non-locally jump over
70 /// it. Updated by `add_kill(KillFrom::ScopeEnd)`.
71 scope_kills: Vec<usize>,
73 /// bits killed as we exit the cfg node directly; if it is jumped
74 /// over, e.g. via `break`, the kills are not reflected in the
75 /// jump's effects. Updated by `add_kill(KillFrom::Execution)`.
76 action_kills: Vec<usize>,
78 /// bits that are valid on entry to the cfg node. Updated by
83 pub trait BitwiseOperator {
84 /// Joins two predecessor bits together, typically either `|` or `&`
85 fn join(&self, succ: usize, pred: usize) -> usize;
88 /// Parameterization for the precise form of data flow that is used.
89 pub trait DataFlowOperator : BitwiseOperator {
90 /// Specifies the initial value for each bit in the `on_entry` set
91 fn initial_value(&self) -> bool;
94 struct PropagationContext<'a, 'b: 'a, 'tcx: 'b, O: 'a> {
95 dfcx: &'a mut DataFlowContext<'b, 'tcx, O>,
99 fn get_cfg_indices<'a>(id: hir::ItemLocalId,
100 index: &'a FxHashMap<hir::ItemLocalId, Vec<CFGIndex>>)
102 index.get(&id).map_or(&[], |v| &v[..])
105 impl<'a, 'tcx, O:DataFlowOperator> DataFlowContext<'a, 'tcx, O> {
106 fn has_bitset_for_local_id(&self, n: hir::ItemLocalId) -> bool {
107 assert!(n != hir::DUMMY_ITEM_LOCAL_ID);
108 self.local_id_to_index.contains_key(&n)
112 impl<'a, 'tcx, O:DataFlowOperator> pprust::PpAnn for DataFlowContext<'a, 'tcx, O> {
113 fn nested(&self, state: &mut pprust::State, nested: pprust::Nested) -> io::Result<()> {
114 pprust::PpAnn::nested(&self.tcx.hir, state, nested)
117 ps: &mut pprust::State,
118 node: pprust::AnnNode) -> io::Result<()> {
119 let id = match node {
120 pprust::NodeName(_) => return Ok(()),
121 pprust::NodeExpr(expr) => expr.hir_id.local_id,
122 pprust::NodeBlock(blk) => blk.hir_id.local_id,
123 pprust::NodeItem(_) |
124 pprust::NodeSubItem(_) => return Ok(()),
125 pprust::NodePat(pat) => pat.hir_id.local_id
128 if !self.has_bitset_for_local_id(id) {
132 assert!(self.bits_per_id > 0);
133 let indices = get_cfg_indices(id, &self.local_id_to_index);
134 for &cfgidx in indices {
135 let (start, end) = self.compute_id_range(cfgidx);
136 let on_entry = &self.on_entry[start.. end];
137 let entry_str = bits_to_string(on_entry);
139 let gens = &self.gens[start.. end];
140 let gens_str = if gens.iter().any(|&u| u != 0) {
141 format!(" gen: {}", bits_to_string(gens))
146 let action_kills = &self.action_kills[start .. end];
147 let action_kills_str = if action_kills.iter().any(|&u| u != 0) {
148 format!(" action_kill: {}", bits_to_string(action_kills))
153 let scope_kills = &self.scope_kills[start .. end];
154 let scope_kills_str = if scope_kills.iter().any(|&u| u != 0) {
155 format!(" scope_kill: {}", bits_to_string(scope_kills))
161 format!("id {}: {}{}{}{}", id.as_usize(), entry_str,
162 gens_str, action_kills_str, scope_kills_str))?;
169 fn build_local_id_to_index(body: Option<&hir::Body>,
171 -> FxHashMap<hir::ItemLocalId, Vec<CFGIndex>> {
172 let mut index = FxHashMap();
174 // FIXME(#15020) Would it be better to fold formals from decl
175 // into cfg itself? i.e. introduce a fn-based flow-graph in
176 // addition to the current block-based flow-graph, rather than
177 // have to put traversals like this here?
178 if let Some(body) = body {
179 add_entries_from_fn_body(&mut index, body, cfg.entry);
182 cfg.graph.each_node(|node_idx, node| {
183 if let cfg::CFGNodeData::AST(id) = node.data {
184 index.entry(id).or_insert(vec![]).push(node_idx);
191 /// Add mappings from the ast nodes for the formal bindings to
192 /// the entry-node in the graph.
193 fn add_entries_from_fn_body(index: &mut FxHashMap<hir::ItemLocalId, Vec<CFGIndex>>,
196 use hir::intravisit::Visitor;
200 index: &'a mut FxHashMap<hir::ItemLocalId, Vec<CFGIndex>>,
202 let mut formals = Formals { entry: entry, index: index };
203 for arg in &body.arguments {
204 formals.visit_pat(&arg.pat);
206 impl<'a, 'v> Visitor<'v> for Formals<'a> {
207 fn nested_visit_map<'this>(&'this mut self) -> intravisit::NestedVisitorMap<'this, 'v> {
208 intravisit::NestedVisitorMap::None
211 fn visit_pat(&mut self, p: &hir::Pat) {
212 self.index.entry(p.hir_id.local_id).or_insert(vec![]).push(self.entry);
213 intravisit::walk_pat(self, p)
219 /// Flag used by `add_kill` to indicate whether the provided kill
220 /// takes effect only when control flows directly through the node in
221 /// question, or if the kill's effect is associated with any
222 /// control-flow directly through or indirectly over the node.
223 #[derive(Copy, Clone, PartialEq, Debug)]
225 /// A `ScopeEnd` kill is one that takes effect when any control
226 /// flow goes over the node. A kill associated with the end of the
227 /// scope of a variable declaration `let x;` is an example of a
231 /// An `Execution` kill is one that takes effect only when control
232 /// flow goes through the node to completion. A kill associated
233 /// with an assignment statement `x = expr;` is an example of an
234 /// `Execution` kill.
238 impl<'a, 'tcx, O:DataFlowOperator> DataFlowContext<'a, 'tcx, O> {
239 pub fn new(tcx: TyCtxt<'a, 'tcx, 'tcx>,
240 analysis_name: &'static str,
241 body: Option<&hir::Body>,
245 bits_per_id: usize) -> DataFlowContext<'a, 'tcx, O> {
246 let usize_bits = mem::size_of::<usize>() * 8;
247 let words_per_id = (bits_per_id + usize_bits - 1) / usize_bits;
248 let num_nodes = cfg.graph.all_nodes().len();
250 debug!("DataFlowContext::new(analysis_name: {}, id_range={:?}, \
251 bits_per_id={}, words_per_id={}) \
253 analysis_name, id_range, bits_per_id, words_per_id,
256 let entry = if oper.initial_value() { usize::MAX } else {0};
258 let zeroes = vec![0; num_nodes * words_per_id];
259 let gens = zeroes.clone();
260 let kills1 = zeroes.clone();
262 let on_entry = vec![entry; num_nodes * words_per_id];
264 let local_id_to_index = build_local_id_to_index(body, cfg);
274 action_kills: kills1,
280 pub fn add_gen(&mut self, id: hir::ItemLocalId, bit: usize) {
281 //! Indicates that `id` generates `bit`
282 debug!("{} add_gen(id={:?}, bit={})",
283 self.analysis_name, id, bit);
284 assert!(self.local_id_to_index.contains_key(&id));
285 assert!(self.bits_per_id > 0);
287 let indices = get_cfg_indices(id, &self.local_id_to_index);
288 for &cfgidx in indices {
289 let (start, end) = self.compute_id_range(cfgidx);
290 let gens = &mut self.gens[start.. end];
295 pub fn add_kill(&mut self, kind: KillFrom, id: hir::ItemLocalId, bit: usize) {
296 //! Indicates that `id` kills `bit`
297 debug!("{} add_kill(id={:?}, bit={})",
298 self.analysis_name, id, bit);
299 assert!(self.local_id_to_index.contains_key(&id));
300 assert!(self.bits_per_id > 0);
302 let indices = get_cfg_indices(id, &self.local_id_to_index);
303 for &cfgidx in indices {
304 let (start, end) = self.compute_id_range(cfgidx);
305 let kills = match kind {
306 KillFrom::Execution => &mut self.action_kills[start.. end],
307 KillFrom::ScopeEnd => &mut self.scope_kills[start.. end],
313 fn apply_gen_kill(&self, cfgidx: CFGIndex, bits: &mut [usize]) {
314 //! Applies the gen and kill sets for `cfgidx` to `bits`
315 debug!("{} apply_gen_kill(cfgidx={:?}, bits={}) [before]",
316 self.analysis_name, cfgidx, mut_bits_to_string(bits));
317 assert!(self.bits_per_id > 0);
319 let (start, end) = self.compute_id_range(cfgidx);
320 let gens = &self.gens[start.. end];
321 bitwise(bits, gens, &Union);
322 let kills = &self.action_kills[start.. end];
323 bitwise(bits, kills, &Subtract);
324 let kills = &self.scope_kills[start.. end];
325 bitwise(bits, kills, &Subtract);
327 debug!("{} apply_gen_kill(cfgidx={:?}, bits={}) [after]",
328 self.analysis_name, cfgidx, mut_bits_to_string(bits));
331 fn compute_id_range(&self, cfgidx: CFGIndex) -> (usize, usize) {
332 let n = cfgidx.node_id();
333 let start = n * self.words_per_id;
334 let end = start + self.words_per_id;
336 assert!(start < self.gens.len());
337 assert!(end <= self.gens.len());
338 assert!(self.gens.len() == self.action_kills.len());
339 assert!(self.gens.len() == self.scope_kills.len());
340 assert!(self.gens.len() == self.on_entry.len());
346 pub fn each_bit_on_entry<F>(&self, id: hir::ItemLocalId, mut f: F) -> bool where
347 F: FnMut(usize) -> bool,
349 //! Iterates through each bit that is set on entry to `id`.
350 //! Only useful after `propagate()` has been called.
351 if !self.has_bitset_for_local_id(id) {
354 let indices = get_cfg_indices(id, &self.local_id_to_index);
355 for &cfgidx in indices {
356 if !self.each_bit_for_node(EntryOrExit::Entry, cfgidx, |i| f(i)) {
363 pub fn each_bit_for_node<F>(&self, e: EntryOrExit, cfgidx: CFGIndex, f: F) -> bool where
364 F: FnMut(usize) -> bool,
366 //! Iterates through each bit that is set on entry/exit to `cfgidx`.
367 //! Only useful after `propagate()` has been called.
369 if self.bits_per_id == 0 {
370 // Skip the surprisingly common degenerate case. (Note
371 // compute_id_range requires self.words_per_id > 0.)
375 let (start, end) = self.compute_id_range(cfgidx);
376 let on_entry = &self.on_entry[start.. end];
378 let slice = match e {
379 EntryOrExit::Entry => on_entry,
380 EntryOrExit::Exit => {
381 let mut t = on_entry.to_vec();
382 self.apply_gen_kill(cfgidx, &mut t);
387 debug!("{} each_bit_for_node({:?}, cfgidx={:?}) bits={}",
388 self.analysis_name, e, cfgidx, bits_to_string(slice));
389 self.each_bit(slice, f)
392 pub fn each_gen_bit<F>(&self, id: hir::ItemLocalId, mut f: F) -> bool where
393 F: FnMut(usize) -> bool,
395 //! Iterates through each bit in the gen set for `id`.
396 if !self.has_bitset_for_local_id(id) {
400 if self.bits_per_id == 0 {
401 // Skip the surprisingly common degenerate case. (Note
402 // compute_id_range requires self.words_per_id > 0.)
406 let indices = get_cfg_indices(id, &self.local_id_to_index);
407 for &cfgidx in indices {
408 let (start, end) = self.compute_id_range(cfgidx);
409 let gens = &self.gens[start.. end];
410 debug!("{} each_gen_bit(id={:?}, gens={})",
411 self.analysis_name, id, bits_to_string(gens));
412 if !self.each_bit(gens, |i| f(i)) {
419 fn each_bit<F>(&self, words: &[usize], mut f: F) -> bool where
420 F: FnMut(usize) -> bool,
422 //! Helper for iterating over the bits in a bit set.
423 //! Returns false on the first call to `f` that returns false;
424 //! if all calls to `f` return true, then returns true.
426 let usize_bits = mem::size_of::<usize>() * 8;
427 for (word_index, &word) in words.iter().enumerate() {
429 let base_index = word_index * usize_bits;
430 for offset in 0..usize_bits {
431 let bit = 1 << offset;
432 if (word & bit) != 0 {
433 // NB: we round up the total number of bits
434 // that we store in any given bit set so that
435 // it is an even multiple of usize::BITS. This
436 // means that there may be some stray bits at
437 // the end that do not correspond to any
438 // actual value. So before we callback, check
439 // whether the bit_index is greater than the
440 // actual value the user specified and stop
442 let bit_index = base_index + offset as usize;
443 if bit_index >= self.bits_per_id {
445 } else if !f(bit_index) {
455 pub fn add_kills_from_flow_exits(&mut self, cfg: &cfg::CFG) {
456 //! Whenever you have a `break` or `continue` statement, flow
457 //! exits through any number of enclosing scopes on its way to
458 //! the new destination. This function infers the kill bits of
459 //! those control operators based on the kill bits associated
460 //! with those scopes.
462 //! This is usually called (if it is called at all), after
463 //! all add_gen and add_kill calls, but before propagate.
465 debug!("{} add_kills_from_flow_exits", self.analysis_name);
466 if self.bits_per_id == 0 {
467 // Skip the surprisingly common degenerate case. (Note
468 // compute_id_range requires self.words_per_id > 0.)
471 cfg.graph.each_edge(|_edge_index, edge| {
472 let flow_exit = edge.source();
473 let (start, end) = self.compute_id_range(flow_exit);
474 let mut orig_kills = self.scope_kills[start.. end].to_vec();
476 let mut changed = false;
477 for &id in &edge.data.exiting_scopes {
478 let opt_cfg_idx = self.local_id_to_index.get(&id);
481 for &cfg_idx in indices {
482 let (start, end) = self.compute_id_range(cfg_idx);
483 let kills = &self.scope_kills[start.. end];
484 if bitwise(&mut orig_kills, kills, &Union) {
485 debug!("scope exits: scope id={:?} \
486 (node={:?} of {:?}) added killset: {}",
487 id, cfg_idx, indices,
488 bits_to_string(kills));
494 debug!("{} add_kills_from_flow_exits flow_exit={:?} \
495 no cfg_idx for exiting_scope={:?}",
496 self.analysis_name, flow_exit, id);
502 let bits = &mut self.scope_kills[start.. end];
503 debug!("{} add_kills_from_flow_exits flow_exit={:?} bits={} [before]",
504 self.analysis_name, flow_exit, mut_bits_to_string(bits));
505 bits.copy_from_slice(&orig_kills[..]);
506 debug!("{} add_kills_from_flow_exits flow_exit={:?} bits={} [after]",
507 self.analysis_name, flow_exit, mut_bits_to_string(bits));
514 impl<'a, 'tcx, O:DataFlowOperator+Clone+'static> DataFlowContext<'a, 'tcx, O> {
515 // ^^^^^^^^^^^^^ only needed for pretty printing
516 pub fn propagate(&mut self, cfg: &cfg::CFG, body: &hir::Body) {
517 //! Performs the data flow analysis.
519 if self.bits_per_id == 0 {
520 // Optimize the surprisingly common degenerate case.
525 let words_per_id = self.words_per_id;
526 let mut propcx = PropagationContext {
531 let nodes_po = cfg.graph.nodes_in_postorder(OUTGOING, cfg.entry);
532 let mut temp = vec![0; words_per_id];
533 let mut num_passes = 0;
534 while propcx.changed {
536 propcx.changed = false;
537 propcx.reset(&mut temp);
538 propcx.walk_cfg(cfg, &nodes_po, &mut temp);
540 debug!("finished in {} iterations", num_passes);
543 debug!("Dataflow result for {}:", self.analysis_name);
544 debug!("{}", pprust::to_string(self, |s| {
545 s.cbox(pprust::indent_unit)?;
547 s.print_expr(&body.value)
552 impl<'a, 'b, 'tcx, O:DataFlowOperator> PropagationContext<'a, 'b, 'tcx, O> {
553 fn walk_cfg(&mut self,
555 nodes_po: &[CFGIndex],
556 in_out: &mut [usize]) {
557 debug!("DataFlowContext::walk_cfg(in_out={}) {}",
558 bits_to_string(in_out), self.dfcx.analysis_name);
559 assert!(self.dfcx.bits_per_id > 0);
561 // Iterate over nodes in reverse postorder
562 for &node_index in nodes_po.iter().rev() {
563 let node = cfg.graph.node(node_index);
564 debug!("DataFlowContext::walk_cfg idx={:?} id={:?} begin in_out={}",
565 node_index, node.data.id(), bits_to_string(in_out));
567 let (start, end) = self.dfcx.compute_id_range(node_index);
569 // Initialize local bitvector with state on-entry.
570 in_out.copy_from_slice(&self.dfcx.on_entry[start.. end]);
572 // Compute state on-exit by applying transfer function to
574 self.dfcx.apply_gen_kill(node_index, in_out);
576 // Propagate state on-exit from node into its successors.
577 self.propagate_bits_into_graph_successors_of(in_out, cfg, node_index);
581 fn reset(&mut self, bits: &mut [usize]) {
582 let e = if self.dfcx.oper.initial_value() {usize::MAX} else {0};
588 fn propagate_bits_into_graph_successors_of(&mut self,
592 for (_, edge) in cfg.graph.outgoing_edges(cfgidx) {
593 self.propagate_bits_into_entry_set_for(pred_bits, edge);
597 fn propagate_bits_into_entry_set_for(&mut self,
599 edge: &cfg::CFGEdge) {
600 let source = edge.source();
601 let cfgidx = edge.target();
602 debug!("{} propagate_bits_into_entry_set_for(pred_bits={}, {:?} to {:?})",
603 self.dfcx.analysis_name, bits_to_string(pred_bits), source, cfgidx);
604 assert!(self.dfcx.bits_per_id > 0);
606 let (start, end) = self.dfcx.compute_id_range(cfgidx);
608 // (scoping mutable borrow of self.dfcx.on_entry)
609 let on_entry = &mut self.dfcx.on_entry[start.. end];
610 bitwise(on_entry, pred_bits, &self.dfcx.oper)
613 debug!("{} changed entry set for {:?} to {}",
614 self.dfcx.analysis_name, cfgidx,
615 bits_to_string(&self.dfcx.on_entry[start.. end]));
621 fn mut_bits_to_string(words: &mut [usize]) -> String {
622 bits_to_string(words)
625 fn bits_to_string(words: &[usize]) -> String {
626 let mut result = String::new();
629 // Note: this is a little endian printout of bytes.
633 for _ in 0..mem::size_of::<usize>() {
635 result.push_str(&format!("{:02x}", v & 0xFF));
645 fn bitwise<Op:BitwiseOperator>(out_vec: &mut [usize],
648 assert_eq!(out_vec.len(), in_vec.len());
649 let mut changed = false;
650 for (out_elt, in_elt) in out_vec.iter_mut().zip(in_vec) {
651 let old_val = *out_elt;
652 let new_val = op.join(old_val, *in_elt);
654 changed |= old_val != new_val;
659 fn set_bit(words: &mut [usize], bit: usize) -> bool {
660 debug!("set_bit: words={} bit={}",
661 mut_bits_to_string(words), bit_str(bit));
662 let usize_bits = mem::size_of::<usize>() * 8;
663 let word = bit / usize_bits;
664 let bit_in_word = bit % usize_bits;
665 let bit_mask = 1 << bit_in_word;
666 debug!("word={} bit_in_word={} bit_mask={}", word, bit_in_word, bit_mask);
667 let oldv = words[word];
668 let newv = oldv | bit_mask;
673 fn bit_str(bit: usize) -> String {
675 let lobits = 1 << (bit & 0b111);
676 format!("[{}:{}-{:02x}]", bit, byte, lobits)
680 impl BitwiseOperator for Union {
681 fn join(&self, a: usize, b: usize) -> usize { a | b }
684 impl BitwiseOperator for Subtract {
685 fn join(&self, a: usize, b: usize) -> usize { a & !b }