1 //! A module for propagating forward dataflow information. The analysis
2 //! assumes that the items to be propagated can be represented as bits
3 //! and thus uses bitvectors. Your job is simply to specify the so-called
4 //! GEN and KILL bits for each expression.
7 use rustc::cfg::CFGIndex;
12 use syntax::print::pprust::PrintState;
15 use rustc_data_structures::graph::implementation::OUTGOING;
17 use rustc::util::nodemap::FxHashMap;
19 use rustc::hir::intravisit;
20 use rustc::hir::print as pprust;
23 #[derive(Copy, Clone, Debug)]
24 pub enum EntryOrExit {
30 pub struct DataFlowContext<'a, 'tcx: 'a, O> {
31 tcx: TyCtxt<'a, 'tcx, 'tcx>,
33 /// a name for the analysis using this dataflow instance
34 analysis_name: &'static str,
36 /// the data flow operator
39 /// number of bits to propagate per id
42 /// number of words we will use to store bits_per_id.
43 /// equal to bits_per_id/usize::BITS rounded up.
46 // mapping from node to cfg node index
47 // FIXME (#6298): Shouldn't this go with CFG?
48 local_id_to_index: FxHashMap<hir::ItemLocalId, Vec<CFGIndex>>,
50 // Bit sets per cfg node. The following three fields (`gens`, `kills`,
51 // and `on_entry`) all have the same structure. For each id in
52 // `id_range`, there is a range of words equal to `words_per_id`.
53 // So, to access the bits for any given id, you take a slice of
54 // the full vector (see the method `compute_id_range()`).
56 /// bits generated as we exit the cfg node. Updated by `add_gen()`.
59 /// bits killed as we exit the cfg node, or non-locally jump over
60 /// it. Updated by `add_kill(KillFrom::ScopeEnd)`.
61 scope_kills: Vec<usize>,
63 /// bits killed as we exit the cfg node directly; if it is jumped
64 /// over, e.g., via `break`, the kills are not reflected in the
65 /// jump's effects. Updated by `add_kill(KillFrom::Execution)`.
66 action_kills: Vec<usize>,
68 /// bits that are valid on entry to the cfg node. Updated by
73 pub trait BitwiseOperator {
74 /// Joins two predecessor bits together, typically either `|` or `&`
75 fn join(&self, succ: usize, pred: usize) -> usize;
78 /// Parameterization for the precise form of data flow that is used.
79 pub trait DataFlowOperator : BitwiseOperator {
80 /// Specifies the initial value for each bit in the `on_entry` set
81 fn initial_value(&self) -> bool;
84 struct PropagationContext<'a, 'b: 'a, 'tcx: 'b, O> {
85 dfcx: &'a mut DataFlowContext<'b, 'tcx, O>,
89 fn get_cfg_indices<'a>(id: hir::ItemLocalId,
90 index: &'a FxHashMap<hir::ItemLocalId, Vec<CFGIndex>>)
92 index.get(&id).map_or(&[], |v| &v[..])
95 impl<'a, 'tcx, O:DataFlowOperator> DataFlowContext<'a, 'tcx, O> {
96 fn has_bitset_for_local_id(&self, n: hir::ItemLocalId) -> bool {
97 assert!(n != hir::DUMMY_ITEM_LOCAL_ID);
98 self.local_id_to_index.contains_key(&n)
102 impl<'a, 'tcx, O:DataFlowOperator> pprust::PpAnn for DataFlowContext<'a, 'tcx, O> {
103 fn nested(&self, state: &mut pprust::State<'_>, nested: pprust::Nested) -> io::Result<()> {
104 pprust::PpAnn::nested(self.tcx.hir(), state, nested)
107 ps: &mut pprust::State<'_>,
108 node: pprust::AnnNode<'_>) -> io::Result<()> {
109 let id = match node {
110 pprust::AnnNode::Name(_) => return Ok(()),
111 pprust::AnnNode::Expr(expr) => expr.hir_id.local_id,
112 pprust::AnnNode::Block(blk) => blk.hir_id.local_id,
113 pprust::AnnNode::Item(_) |
114 pprust::AnnNode::SubItem(_) => return Ok(()),
115 pprust::AnnNode::Pat(pat) => pat.hir_id.local_id
118 if !self.has_bitset_for_local_id(id) {
122 assert!(self.bits_per_id > 0);
123 let indices = get_cfg_indices(id, &self.local_id_to_index);
124 for &cfgidx in indices {
125 let (start, end) = self.compute_id_range(cfgidx);
126 let on_entry = &self.on_entry[start.. end];
127 let entry_str = bits_to_string(on_entry);
129 let gens = &self.gens[start.. end];
130 let gens_str = if gens.iter().any(|&u| u != 0) {
131 format!(" gen: {}", bits_to_string(gens))
136 let action_kills = &self.action_kills[start .. end];
137 let action_kills_str = if action_kills.iter().any(|&u| u != 0) {
138 format!(" action_kill: {}", bits_to_string(action_kills))
143 let scope_kills = &self.scope_kills[start .. end];
144 let scope_kills_str = if scope_kills.iter().any(|&u| u != 0) {
145 format!(" scope_kill: {}", bits_to_string(scope_kills))
151 format!("id {}: {}{}{}{}", id.as_usize(), entry_str,
152 gens_str, action_kills_str, scope_kills_str))?;
159 fn build_local_id_to_index(body: Option<&hir::Body>,
161 -> FxHashMap<hir::ItemLocalId, Vec<CFGIndex>> {
162 let mut index = FxHashMap::default();
164 // FIXME(#15020) Would it be better to fold formals from decl
165 // into cfg itself? i.e., introduce a fn-based flow-graph in
166 // addition to the current block-based flow-graph, rather than
167 // have to put traversals like this here?
168 if let Some(body) = body {
169 add_entries_from_fn_body(&mut index, body, cfg.entry);
172 cfg.graph.each_node(|node_idx, node| {
173 if let cfg::CFGNodeData::AST(id) = node.data {
174 index.entry(id).or_default().push(node_idx);
181 /// Adds mappings from the ast nodes for the formal bindings to
182 /// the entry-node in the graph.
183 fn add_entries_from_fn_body(index: &mut FxHashMap<hir::ItemLocalId, Vec<CFGIndex>>,
186 use rustc::hir::intravisit::Visitor;
190 index: &'a mut FxHashMap<hir::ItemLocalId, Vec<CFGIndex>>,
192 let mut formals = Formals { entry: entry, index: index };
193 for arg in &body.arguments {
194 formals.visit_pat(&arg.pat);
196 impl<'a, 'v> Visitor<'v> for Formals<'a> {
197 fn nested_visit_map<'this>(&'this mut self) -> intravisit::NestedVisitorMap<'this, 'v> {
198 intravisit::NestedVisitorMap::None
201 fn visit_pat(&mut self, p: &hir::Pat) {
202 self.index.entry(p.hir_id.local_id).or_default().push(self.entry);
203 intravisit::walk_pat(self, p)
209 /// Flag used by `add_kill` to indicate whether the provided kill
210 /// takes effect only when control flows directly through the node in
211 /// question, or if the kill's effect is associated with any
212 /// control-flow directly through or indirectly over the node.
213 #[derive(Copy, Clone, PartialEq, Debug)]
215 /// A `ScopeEnd` kill is one that takes effect when any control
216 /// flow goes over the node. A kill associated with the end of the
217 /// scope of a variable declaration `let x;` is an example of a
221 /// An `Execution` kill is one that takes effect only when control
222 /// flow goes through the node to completion. A kill associated
223 /// with an assignment statement `x = expr;` is an example of an
224 /// `Execution` kill.
228 impl<'a, 'tcx, O:DataFlowOperator> DataFlowContext<'a, 'tcx, O> {
229 pub fn new(tcx: TyCtxt<'a, 'tcx, 'tcx>,
230 analysis_name: &'static str,
231 body: Option<&hir::Body>,
234 bits_per_id: usize) -> DataFlowContext<'a, 'tcx, O> {
235 let usize_bits = mem::size_of::<usize>() * 8;
236 let words_per_id = (bits_per_id + usize_bits - 1) / usize_bits;
237 let num_nodes = cfg.graph.all_nodes().len();
239 debug!("DataFlowContext::new(analysis_name: {}, \
240 bits_per_id={}, words_per_id={}) \
242 analysis_name, bits_per_id, words_per_id,
245 let entry = if oper.initial_value() { usize::MAX } else {0};
247 let zeroes = vec![0; num_nodes * words_per_id];
248 let gens = zeroes.clone();
249 let kills1 = zeroes.clone();
251 let on_entry = vec![entry; num_nodes * words_per_id];
253 let local_id_to_index = build_local_id_to_index(body, cfg);
263 action_kills: kills1,
269 pub fn add_gen(&mut self, id: hir::ItemLocalId, bit: usize) {
270 //! Indicates that `id` generates `bit`
271 debug!("{} add_gen(id={:?}, bit={})",
272 self.analysis_name, id, bit);
273 assert!(self.local_id_to_index.contains_key(&id));
274 assert!(self.bits_per_id > 0);
276 let indices = get_cfg_indices(id, &self.local_id_to_index);
277 for &cfgidx in indices {
278 let (start, end) = self.compute_id_range(cfgidx);
279 let gens = &mut self.gens[start.. end];
284 pub fn add_kill(&mut self, kind: KillFrom, id: hir::ItemLocalId, bit: usize) {
285 //! Indicates that `id` kills `bit`
286 debug!("{} add_kill(id={:?}, bit={})",
287 self.analysis_name, id, bit);
288 assert!(self.local_id_to_index.contains_key(&id));
289 assert!(self.bits_per_id > 0);
291 let indices = get_cfg_indices(id, &self.local_id_to_index);
292 for &cfgidx in indices {
293 let (start, end) = self.compute_id_range(cfgidx);
294 let kills = match kind {
295 KillFrom::Execution => &mut self.action_kills[start.. end],
296 KillFrom::ScopeEnd => &mut self.scope_kills[start.. end],
302 fn apply_gen_kill(&self, cfgidx: CFGIndex, bits: &mut [usize]) {
303 //! Applies the gen and kill sets for `cfgidx` to `bits`
304 debug!("{} apply_gen_kill(cfgidx={:?}, bits={}) [before]",
305 self.analysis_name, cfgidx, mut_bits_to_string(bits));
306 assert!(self.bits_per_id > 0);
308 let (start, end) = self.compute_id_range(cfgidx);
309 let gens = &self.gens[start.. end];
310 bitwise(bits, gens, &Union);
311 let kills = &self.action_kills[start.. end];
312 bitwise(bits, kills, &Subtract);
313 let kills = &self.scope_kills[start.. end];
314 bitwise(bits, kills, &Subtract);
316 debug!("{} apply_gen_kill(cfgidx={:?}, bits={}) [after]",
317 self.analysis_name, cfgidx, mut_bits_to_string(bits));
320 fn compute_id_range(&self, cfgidx: CFGIndex) -> (usize, usize) {
321 let n = cfgidx.node_id();
322 let start = n * self.words_per_id;
323 let end = start + self.words_per_id;
325 assert!(start < self.gens.len());
326 assert!(end <= self.gens.len());
327 assert!(self.gens.len() == self.action_kills.len());
328 assert!(self.gens.len() == self.scope_kills.len());
329 assert!(self.gens.len() == self.on_entry.len());
335 pub fn each_bit_on_entry<F>(&self, id: hir::ItemLocalId, mut f: F) -> bool where
336 F: FnMut(usize) -> bool,
338 //! Iterates through each bit that is set on entry to `id`.
339 //! Only useful after `propagate()` has been called.
340 if !self.has_bitset_for_local_id(id) {
343 let indices = get_cfg_indices(id, &self.local_id_to_index);
344 for &cfgidx in indices {
345 if !self.each_bit_for_node(EntryOrExit::Entry, cfgidx, |i| f(i)) {
352 pub fn each_bit_for_node<F>(&self, e: EntryOrExit, cfgidx: CFGIndex, f: F) -> bool where
353 F: FnMut(usize) -> bool,
355 //! Iterates through each bit that is set on entry/exit to `cfgidx`.
356 //! Only useful after `propagate()` has been called.
358 if self.bits_per_id == 0 {
359 // Skip the surprisingly common degenerate case. (Note
360 // compute_id_range requires self.words_per_id > 0.)
364 let (start, end) = self.compute_id_range(cfgidx);
365 let on_entry = &self.on_entry[start.. end];
367 let slice = match e {
368 EntryOrExit::Entry => on_entry,
369 EntryOrExit::Exit => {
370 let mut t = on_entry.to_vec();
371 self.apply_gen_kill(cfgidx, &mut t);
376 debug!("{} each_bit_for_node({:?}, cfgidx={:?}) bits={}",
377 self.analysis_name, e, cfgidx, bits_to_string(slice));
378 self.each_bit(slice, f)
381 pub fn each_gen_bit<F>(&self, id: hir::ItemLocalId, mut f: F) -> bool where
382 F: FnMut(usize) -> bool,
384 //! Iterates through each bit in the gen set for `id`.
385 if !self.has_bitset_for_local_id(id) {
389 if self.bits_per_id == 0 {
390 // Skip the surprisingly common degenerate case. (Note
391 // compute_id_range requires self.words_per_id > 0.)
395 let indices = get_cfg_indices(id, &self.local_id_to_index);
396 for &cfgidx in indices {
397 let (start, end) = self.compute_id_range(cfgidx);
398 let gens = &self.gens[start.. end];
399 debug!("{} each_gen_bit(id={:?}, gens={})",
400 self.analysis_name, id, bits_to_string(gens));
401 if !self.each_bit(gens, |i| f(i)) {
408 fn each_bit<F>(&self, words: &[usize], mut f: F) -> bool where
409 F: FnMut(usize) -> bool,
411 //! Helper for iterating over the bits in a bit set.
412 //! Returns false on the first call to `f` that returns false;
413 //! if all calls to `f` return true, then returns true.
415 let usize_bits = mem::size_of::<usize>() * 8;
416 for (word_index, &word) in words.iter().enumerate() {
418 let base_index = word_index * usize_bits;
419 for offset in 0..usize_bits {
420 let bit = 1 << offset;
421 if (word & bit) != 0 {
422 // N.B., we round up the total number of bits
423 // that we store in any given bit set so that
424 // it is an even multiple of usize::BITS. This
425 // means that there may be some stray bits at
426 // the end that do not correspond to any
427 // actual value. So before we callback, check
428 // whether the bit_index is greater than the
429 // actual value the user specified and stop
431 let bit_index = base_index + offset as usize;
432 if bit_index >= self.bits_per_id {
434 } else if !f(bit_index) {
444 pub fn add_kills_from_flow_exits(&mut self, cfg: &cfg::CFG) {
445 //! Whenever you have a `break` or `continue` statement, flow
446 //! exits through any number of enclosing scopes on its way to
447 //! the new destination. This function infers the kill bits of
448 //! those control operators based on the kill bits associated
449 //! with those scopes.
451 //! This is usually called (if it is called at all), after
452 //! all add_gen and add_kill calls, but before propagate.
454 debug!("{} add_kills_from_flow_exits", self.analysis_name);
455 if self.bits_per_id == 0 {
456 // Skip the surprisingly common degenerate case. (Note
457 // compute_id_range requires self.words_per_id > 0.)
460 cfg.graph.each_edge(|_edge_index, edge| {
461 let flow_exit = edge.source();
462 let (start, end) = self.compute_id_range(flow_exit);
463 let mut orig_kills = self.scope_kills[start.. end].to_vec();
465 let mut changed = false;
466 for &id in &edge.data.exiting_scopes {
467 let opt_cfg_idx = self.local_id_to_index.get(&id);
470 for &cfg_idx in indices {
471 let (start, end) = self.compute_id_range(cfg_idx);
472 let kills = &self.scope_kills[start.. end];
473 if bitwise(&mut orig_kills, kills, &Union) {
474 debug!("scope exits: scope id={:?} \
475 (node={:?} of {:?}) added killset: {}",
476 id, cfg_idx, indices,
477 bits_to_string(kills));
483 debug!("{} add_kills_from_flow_exits flow_exit={:?} \
484 no cfg_idx for exiting_scope={:?}",
485 self.analysis_name, flow_exit, id);
491 let bits = &mut self.scope_kills[start.. end];
492 debug!("{} add_kills_from_flow_exits flow_exit={:?} bits={} [before]",
493 self.analysis_name, flow_exit, mut_bits_to_string(bits));
494 bits.copy_from_slice(&orig_kills[..]);
495 debug!("{} add_kills_from_flow_exits flow_exit={:?} bits={} [after]",
496 self.analysis_name, flow_exit, mut_bits_to_string(bits));
503 impl<'a, 'tcx, O:DataFlowOperator+Clone+'static> DataFlowContext<'a, 'tcx, O> {
504 // ^^^^^^^^^^^^^ only needed for pretty printing
505 pub fn propagate(&mut self, cfg: &cfg::CFG, body: &hir::Body) {
506 //! Performs the data flow analysis.
508 if self.bits_per_id == 0 {
509 // Optimize the surprisingly common degenerate case.
514 let words_per_id = self.words_per_id;
515 let mut propcx = PropagationContext {
520 let nodes_po = cfg.graph.nodes_in_postorder(OUTGOING, cfg.entry);
521 let mut temp = vec![0; words_per_id];
522 let mut num_passes = 0;
523 while propcx.changed {
525 propcx.changed = false;
526 propcx.reset(&mut temp);
527 propcx.walk_cfg(cfg, &nodes_po, &mut temp);
529 debug!("finished in {} iterations", num_passes);
532 debug!("Dataflow result for {}:", self.analysis_name);
533 debug!("{}", pprust::to_string(self, |s| {
534 s.cbox(pprust::indent_unit)?;
536 s.print_expr(&body.value)
541 impl<'a, 'b, 'tcx, O:DataFlowOperator> PropagationContext<'a, 'b, 'tcx, O> {
542 fn walk_cfg(&mut self,
544 nodes_po: &[CFGIndex],
545 in_out: &mut [usize]) {
546 debug!("DataFlowContext::walk_cfg(in_out={}) {}",
547 bits_to_string(in_out), self.dfcx.analysis_name);
548 assert!(self.dfcx.bits_per_id > 0);
550 // Iterate over nodes in reverse postorder
551 for &node_index in nodes_po.iter().rev() {
552 let node = cfg.graph.node(node_index);
553 debug!("DataFlowContext::walk_cfg idx={:?} id={:?} begin in_out={}",
554 node_index, node.data.id(), bits_to_string(in_out));
556 let (start, end) = self.dfcx.compute_id_range(node_index);
558 // Initialize local bitvector with state on-entry.
559 in_out.copy_from_slice(&self.dfcx.on_entry[start.. end]);
561 // Compute state on-exit by applying transfer function to
563 self.dfcx.apply_gen_kill(node_index, in_out);
565 // Propagate state on-exit from node into its successors.
566 self.propagate_bits_into_graph_successors_of(in_out, cfg, node_index);
570 fn reset(&mut self, bits: &mut [usize]) {
571 let e = if self.dfcx.oper.initial_value() {usize::MAX} else {0};
577 fn propagate_bits_into_graph_successors_of(&mut self,
581 for (_, edge) in cfg.graph.outgoing_edges(cfgidx) {
582 self.propagate_bits_into_entry_set_for(pred_bits, edge);
586 fn propagate_bits_into_entry_set_for(&mut self,
588 edge: &cfg::CFGEdge) {
589 let source = edge.source();
590 let cfgidx = edge.target();
591 debug!("{} propagate_bits_into_entry_set_for(pred_bits={}, {:?} to {:?})",
592 self.dfcx.analysis_name, bits_to_string(pred_bits), source, cfgidx);
593 assert!(self.dfcx.bits_per_id > 0);
595 let (start, end) = self.dfcx.compute_id_range(cfgidx);
597 // (scoping mutable borrow of self.dfcx.on_entry)
598 let on_entry = &mut self.dfcx.on_entry[start.. end];
599 bitwise(on_entry, pred_bits, &self.dfcx.oper)
602 debug!("{} changed entry set for {:?} to {}",
603 self.dfcx.analysis_name, cfgidx,
604 bits_to_string(&self.dfcx.on_entry[start.. end]));
610 fn mut_bits_to_string(words: &mut [usize]) -> String {
611 bits_to_string(words)
614 fn bits_to_string(words: &[usize]) -> String {
615 let mut result = String::new();
618 // Note: this is a little endian printout of bytes.
622 for _ in 0..mem::size_of::<usize>() {
624 result.push_str(&format!("{:02x}", v & 0xFF));
634 fn bitwise<Op:BitwiseOperator>(out_vec: &mut [usize],
637 assert_eq!(out_vec.len(), in_vec.len());
638 let mut changed = false;
639 for (out_elt, in_elt) in out_vec.iter_mut().zip(in_vec) {
640 let old_val = *out_elt;
641 let new_val = op.join(old_val, *in_elt);
643 changed |= old_val != new_val;
648 fn set_bit(words: &mut [usize], bit: usize) -> bool {
649 debug!("set_bit: words={} bit={}",
650 mut_bits_to_string(words), bit_str(bit));
651 let usize_bits = mem::size_of::<usize>() * 8;
652 let word = bit / usize_bits;
653 let bit_in_word = bit % usize_bits;
654 let bit_mask = 1 << bit_in_word;
655 debug!("word={} bit_in_word={} bit_mask={}", word, bit_in_word, bit_mask);
656 let oldv = words[word];
657 let newv = oldv | bit_mask;
662 fn bit_str(bit: usize) -> String {
664 let lobits = 1 << (bit & 0b111);
665 format!("[{}:{}-{:02x}]", bit, byte, lobits)
669 impl BitwiseOperator for Union {
670 fn join(&self, a: usize, b: usize) -> usize { a | b }
673 impl BitwiseOperator for Subtract {
674 fn join(&self, a: usize, b: usize) -> usize { a & !b }