1 // Copyright 2015 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.
11 //! A number of passes which remove various redundancies in the CFG.
13 //! The `SimplifyCfg` pass gets rid of unnecessary blocks in the CFG, whereas the `SimplifyLocals`
14 //! gets rid of all the unnecessary local variable declarations.
16 //! The `SimplifyLocals` pass is kinda expensive and therefore not very suitable to be run often.
17 //! Most of the passes should not care or be impacted in meaningful ways due to extra locals
18 //! either, so running the pass once, right before translation, should suffice.
20 //! On the other side of the spectrum, the `SimplifyCfg` pass is considerably cheap to run, thus
21 //! one should run it after every pass which may modify CFG in significant ways. This pass must
22 //! also be run before any analysis passes because it removes dead blocks, and some of these can be
25 //! The cause of this typing issue is typeck allowing most blocks whose end is not reachable have
26 //! an arbitrary return type, rather than having the usual () return type (as a note, typeck's
27 //! notion of reachability is in fact slightly weaker than MIR CFG reachability - see #31617). A
28 //! standard example of the situation is:
32 //! let _a: char = { return; };
36 //! Here the block (`{ return; }`) has the return type `char`, rather than `()`, but the MIR we
37 //! naively generate still contains the `_a = ()` write in the unreachable block "after" the
40 use rustc_data_structures::bitvec::BitVector;
41 use rustc_data_structures::indexed_vec::{Idx, IndexVec};
42 use rustc::ty::TyCtxt;
44 use rustc::mir::visit::{MutVisitor, Visitor, PlaceContext};
46 use transform::{MirPass, MirSource};
48 pub struct SimplifyCfg { label: String }
51 pub fn new(label: &str) -> Self {
52 SimplifyCfg { label: format!("SimplifyCfg-{}", label) }
56 pub fn simplify_cfg(mir: &mut Mir) {
57 CfgSimplifier::new(mir).simplify();
58 remove_dead_blocks(mir);
60 // FIXME: Should probably be moved into some kind of pass manager
61 mir.basic_blocks_mut().raw.shrink_to_fit();
64 impl MirPass for SimplifyCfg {
65 fn name<'a>(&'a self) -> Cow<'a, str> {
66 Cow::Borrowed(&self.label)
69 fn run_pass<'a, 'tcx>(&self,
70 _tcx: TyCtxt<'a, 'tcx, 'tcx>,
72 mir: &mut Mir<'tcx>) {
73 debug!("SimplifyCfg({:?}) - simplifying {:?}", self.label, mir);
78 pub struct CfgSimplifier<'a, 'tcx: 'a> {
79 basic_blocks: &'a mut IndexVec<BasicBlock, BasicBlockData<'tcx>>,
80 pred_count: IndexVec<BasicBlock, u32>
83 impl<'a, 'tcx: 'a> CfgSimplifier<'a, 'tcx> {
84 pub fn new(mir: &'a mut Mir<'tcx>) -> Self {
85 let mut pred_count = IndexVec::from_elem(0u32, mir.basic_blocks());
87 // we can't use mir.predecessors() here because that counts
88 // dead blocks, which we don't want to.
89 pred_count[START_BLOCK] = 1;
91 for (_, data) in traversal::preorder(mir) {
92 if let Some(ref term) = data.terminator {
93 for &tgt in term.successors().iter() {
99 let basic_blocks = mir.basic_blocks_mut();
107 pub fn simplify(mut self) {
111 let mut changed = false;
113 for bb in (0..self.basic_blocks.len()).map(BasicBlock::new) {
114 if self.pred_count[bb] == 0 {
118 debug!("simplifying {:?}", bb);
120 let mut terminator = self.basic_blocks[bb].terminator.take()
121 .expect("invalid terminator state");
123 for successor in terminator.successors_mut() {
124 self.collapse_goto_chain(successor, &mut changed);
127 let mut new_stmts = vec![];
128 let mut inner_changed = true;
129 while inner_changed {
130 inner_changed = false;
131 inner_changed |= self.simplify_branch(&mut terminator);
132 inner_changed |= self.merge_successor(&mut new_stmts, &mut terminator);
133 changed |= inner_changed;
136 self.basic_blocks[bb].statements.extend(new_stmts);
137 self.basic_blocks[bb].terminator = Some(terminator);
139 changed |= inner_changed;
142 if !changed { break }
146 // Collapse a goto chain starting from `start`
147 fn collapse_goto_chain(&mut self, start: &mut BasicBlock, changed: &mut bool) {
148 let mut terminator = match self.basic_blocks[*start] {
151 terminator: ref mut terminator @ Some(Terminator {
152 kind: TerminatorKind::Goto { .. }, ..
154 } if statements.is_empty() => terminator.take(),
155 // if `terminator` is None, this means we are in a loop. In that
156 // case, let all the loop collapse to its entry.
160 let target = match terminator {
161 Some(Terminator { kind: TerminatorKind::Goto { ref mut target }, .. }) => {
162 self.collapse_goto_chain(target, changed);
167 self.basic_blocks[*start].terminator = terminator;
169 debug!("collapsing goto chain from {:?} to {:?}", *start, target);
171 *changed |= *start != target;
173 if self.pred_count[*start] == 1 {
174 // This is the last reference to *start, so the pred-count to
175 // to target is moved into the current block.
176 self.pred_count[*start] = 0;
178 self.pred_count[target] += 1;
179 self.pred_count[*start] -= 1;
185 // merge a block with 1 `goto` predecessor to its parent
186 fn merge_successor(&mut self,
187 new_stmts: &mut Vec<Statement<'tcx>>,
188 terminator: &mut Terminator<'tcx>)
191 let target = match terminator.kind {
192 TerminatorKind::Goto { target }
193 if self.pred_count[target] == 1
198 debug!("merging block {:?} into {:?}", target, terminator);
199 *terminator = match self.basic_blocks[target].terminator.take() {
200 Some(terminator) => terminator,
202 // unreachable loop - this should not be possible, as we
203 // don't strand blocks, but handle it correctly.
207 new_stmts.extend(self.basic_blocks[target].statements.drain(..));
208 self.pred_count[target] = 0;
213 // turn a branch with all successors identical to a goto
214 fn simplify_branch(&mut self, terminator: &mut Terminator<'tcx>) -> bool {
215 match terminator.kind {
216 TerminatorKind::SwitchInt { .. } => {},
221 let successors = terminator.successors();
222 if let Some(&first_succ) = terminator.successors().get(0) {
223 if successors.iter().all(|s| *s == first_succ) {
224 self.pred_count[first_succ] -= (successors.len()-1) as u32;
234 debug!("simplifying branch {:?}", terminator);
235 terminator.kind = TerminatorKind::Goto { target: first_succ };
239 fn strip_nops(&mut self) {
240 for blk in self.basic_blocks.iter_mut() {
241 blk.statements.retain(|stmt| if let StatementKind::Nop = stmt.kind {
250 pub fn remove_dead_blocks(mir: &mut Mir) {
251 let mut seen = BitVector::new(mir.basic_blocks().len());
252 for (bb, _) in traversal::preorder(mir) {
253 seen.insert(bb.index());
256 let basic_blocks = mir.basic_blocks_mut();
258 let num_blocks = basic_blocks.len();
259 let mut replacements : Vec<_> = (0..num_blocks).map(BasicBlock::new).collect();
260 let mut used_blocks = 0;
261 for alive_index in seen.iter() {
262 replacements[alive_index] = BasicBlock::new(used_blocks);
263 if alive_index != used_blocks {
264 // Swap the next alive block data with the current available slot. Since alive_index is
265 // non-decreasing this is a valid operation.
266 basic_blocks.raw.swap(alive_index, used_blocks);
270 basic_blocks.raw.truncate(used_blocks);
272 for block in basic_blocks {
273 for target in block.terminator_mut().successors_mut() {
274 *target = replacements[target.index()];
280 pub struct SimplifyLocals;
282 impl MirPass for SimplifyLocals {
283 fn run_pass<'a, 'tcx>(&self,
284 _: TyCtxt<'a, 'tcx, 'tcx>,
286 mir: &mut Mir<'tcx>) {
287 let mut marker = DeclMarker { locals: BitVector::new(mir.local_decls.len()) };
288 marker.visit_mir(mir);
289 // Return pointer and arguments are always live
290 marker.locals.insert(0);
291 for idx in mir.args_iter() {
292 marker.locals.insert(idx.index());
294 let map = make_local_map(&mut mir.local_decls, marker.locals);
295 // Update references to all vars and tmps now
296 LocalUpdater { map: map }.visit_mir(mir);
297 mir.local_decls.shrink_to_fit();
301 /// Construct the mapping while swapping out unused stuff out from the `vec`.
302 fn make_local_map<'tcx, I: Idx, V>(vec: &mut IndexVec<I, V>, mask: BitVector) -> Vec<usize> {
303 let mut map: Vec<usize> = ::std::iter::repeat(!0).take(vec.len()).collect();
305 for alive_index in mask.iter() {
306 map[alive_index] = used;
307 if alive_index != used {
308 vec.swap(alive_index, used);
317 pub locals: BitVector,
320 impl<'tcx> Visitor<'tcx> for DeclMarker {
321 fn visit_local(&mut self, local: &Local, ctx: PlaceContext<'tcx>, _: Location) {
322 // ignore these altogether, they get removed along with their otherwise unused decls.
323 if ctx != PlaceContext::StorageLive && ctx != PlaceContext::StorageDead {
324 self.locals.insert(local.index());
329 struct LocalUpdater {
333 impl<'tcx> MutVisitor<'tcx> for LocalUpdater {
334 fn visit_basic_block_data(&mut self, block: BasicBlock, data: &mut BasicBlockData<'tcx>) {
335 // Remove unnecessary StorageLive and StorageDead annotations.
336 data.statements.retain(|stmt| {
338 StatementKind::StorageLive(l) | StatementKind::StorageDead(l) => {
339 self.map[l.index()] != !0
344 self.super_basic_block_data(block, data);
346 fn visit_local(&mut self, l: &mut Local, _: PlaceContext<'tcx>, _: Location) {
347 *l = Local::new(self.map[l.index()]);