Dominators.cpp revision 4f4c28f75f74fe557efb63feaf5f4f8bf639dcd5 
1//===- Dominators.cpp - Dominator Calculation -----------------------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file implements simple dominator construction algorithms for finding
11// forward dominators.  Postdominators are available in libanalysis, but are not
12// included in libvmcore, because it's not needed.  Forward dominators are
13// needed to support the Verifier pass.
14//
15//===----------------------------------------------------------------------===//
16
17#include "llvm/Analysis/Dominators.h"
18#include "llvm/Support/CFG.h"
19#include "llvm/Support/Compiler.h"
20#include "llvm/ADT/DepthFirstIterator.h"
21#include "llvm/ADT/SetOperations.h"
22#include "llvm/ADT/SmallPtrSet.h"
23#include "llvm/ADT/SmallVector.h"
24#include "llvm/Analysis/DominatorInternals.h"
25#include "llvm/Instructions.h"
26#include "llvm/Support/Streams.h"
27#include <algorithm>
28using namespace llvm;
29
30namespace llvm {
31static std::ostream &operator<<(std::ostream &o,
32                                const std::set<BasicBlock*> &BBs) {
33  for (std::set<BasicBlock*>::const_iterator I = BBs.begin(), E = BBs.end();
34       I != E; ++I)
35    if (*I)
36      WriteAsOperand(o, *I, false);
37    else
38      o << " <<exit node>>";
39  return o;
40}
41}
42
43//===----------------------------------------------------------------------===//
44//  DominatorTree Implementation
45//===----------------------------------------------------------------------===//
46//
47// Provide public access to DominatorTree information.  Implementation details
48// can be found in DominatorCalculation.h.
49//
50//===----------------------------------------------------------------------===//
51
52TEMPLATE_INSTANTIATION(class DomTreeNodeBase<BasicBlock>);
53TEMPLATE_INSTANTIATION(class DominatorTreeBase<BasicBlock>);
54
55char DominatorTree::ID = 0;
56static RegisterPass<DominatorTree>
57E("domtree", "Dominator Tree Construction", false, true);
58
59bool DominatorTree::runOnFunction(Function &F) {
60  DT->recalculate(F);
61
62  return false;
63}
64
65//===----------------------------------------------------------------------===//
66//  DominanceFrontier Implementation
67//===----------------------------------------------------------------------===//
68
69char DominanceFrontier::ID = 0;
70static RegisterPass<DominanceFrontier>
71G("domfrontier", "Dominance Frontier Construction", false, true);
72
73// NewBB is split and now it has one successor. Update dominace frontier to
74// reflect this change.
75void DominanceFrontier::splitBlock(BasicBlock *NewBB) {
76  assert(NewBB->getTerminator()->getNumSuccessors() == 1
77         && "NewBB should have a single successor!");
78  BasicBlock *NewBBSucc = NewBB->getTerminator()->getSuccessor(0);
79
80  std::vector<BasicBlock*> PredBlocks;
81  for (pred_iterator PI = pred_begin(NewBB), PE = pred_end(NewBB);
82       PI != PE; ++PI)
83      PredBlocks.push_back(*PI);
84
85  if (PredBlocks.empty())
86    // If NewBB does not have any predecessors then it is a entry block.
87    // In this case, NewBB and its successor NewBBSucc dominates all
88    // other blocks.
89    return;
90
91  // NewBBSucc inherits original NewBB frontier.
92  DominanceFrontier::iterator NewBBI = find(NewBB);
93  if (NewBBI != end()) {
94    DominanceFrontier::DomSetType NewBBSet = NewBBI->second;
95    DominanceFrontier::DomSetType NewBBSuccSet;
96    NewBBSuccSet.insert(NewBBSet.begin(), NewBBSet.end());
97    addBasicBlock(NewBBSucc, NewBBSuccSet);
98  }
99
100  // If NewBB dominates NewBBSucc, then DF(NewBB) is now going to be the
101  // DF(PredBlocks[0]) without the stuff that the new block does not dominate
102  // a predecessor of.
103  DominatorTree &DT = getAnalysis<DominatorTree>();
104  if (DT.dominates(NewBB, NewBBSucc)) {
105    DominanceFrontier::iterator DFI = find(PredBlocks[0]);
106    if (DFI != end()) {
107      DominanceFrontier::DomSetType Set = DFI->second;
108      // Filter out stuff in Set that we do not dominate a predecessor of.
109      for (DominanceFrontier::DomSetType::iterator SetI = Set.begin(),
110             E = Set.end(); SetI != E;) {
111        bool DominatesPred = false;
112        for (pred_iterator PI = pred_begin(*SetI), E = pred_end(*SetI);
113             PI != E; ++PI)
114          if (DT.dominates(NewBB, *PI))
115            DominatesPred = true;
116        if (!DominatesPred)
117          Set.erase(SetI++);
118        else
119          ++SetI;
120      }
121
122      if (NewBBI != end()) {
123        for (DominanceFrontier::DomSetType::iterator SetI = Set.begin(),
124               E = Set.end(); SetI != E; ++SetI) {
125          BasicBlock *SB = *SetI;
126          addToFrontier(NewBBI, SB);
127        }
128      } else
129        addBasicBlock(NewBB, Set);
130    }
131
132  } else {
133    // DF(NewBB) is {NewBBSucc} because NewBB does not strictly dominate
134    // NewBBSucc, but it does dominate itself (and there is an edge (NewBB ->
135    // NewBBSucc)).  NewBBSucc is the single successor of NewBB.
136    DominanceFrontier::DomSetType NewDFSet;
137    NewDFSet.insert(NewBBSucc);
138    addBasicBlock(NewBB, NewDFSet);
139  }
140
141  // Now we must loop over all of the dominance frontiers in the function,
142  // replacing occurrences of NewBBSucc with NewBB in some cases.  All
143  // blocks that dominate a block in PredBlocks and contained NewBBSucc in
144  // their dominance frontier must be updated to contain NewBB instead.
145  //
146  for (Function::iterator FI = NewBB->getParent()->begin(),
147         FE = NewBB->getParent()->end(); FI != FE; ++FI) {
148    DominanceFrontier::iterator DFI = find(FI);
149    if (DFI == end()) continue;  // unreachable block.
150
151    // Only consider nodes that have NewBBSucc in their dominator frontier.
152    if (!DFI->second.count(NewBBSucc)) continue;
153
154    // Verify whether this block dominates a block in predblocks.  If not, do
155    // not update it.
156    bool BlockDominatesAny = false;
157    for (std::vector<BasicBlock*>::const_iterator BI = PredBlocks.begin(),
158           BE = PredBlocks.end(); BI != BE; ++BI) {
159      if (DT.dominates(FI, *BI)) {
160        BlockDominatesAny = true;
161        break;
162      }
163    }
164
165    if (!BlockDominatesAny)
166      continue;
167
168    // If NewBBSucc should not stay in our dominator frontier, remove it.
169    // We remove it unless there is a predecessor of NewBBSucc that we
170    // dominate, but we don't strictly dominate NewBBSucc.
171    bool ShouldRemove = true;
172    if ((BasicBlock*)FI == NewBBSucc || !DT.dominates(FI, NewBBSucc)) {
173      // Okay, we know that PredDom does not strictly dominate NewBBSucc.
174      // Check to see if it dominates any predecessors of NewBBSucc.
175      for (pred_iterator PI = pred_begin(NewBBSucc),
176           E = pred_end(NewBBSucc); PI != E; ++PI)
177        if (DT.dominates(FI, *PI)) {
178          ShouldRemove = false;
179          break;
180        }
181    }
182
183    if (ShouldRemove)
184      removeFromFrontier(DFI, NewBBSucc);
185    addToFrontier(DFI, NewBB);
186  }
187}
188
189namespace {
190  class DFCalculateWorkObject {
191  public:
192    DFCalculateWorkObject(BasicBlock *B, BasicBlock *P,
193                          const DomTreeNode *N,
194                          const DomTreeNode *PN)
195    : currentBB(B), parentBB(P), Node(N), parentNode(PN) {}
196    BasicBlock *currentBB;
197    BasicBlock *parentBB;
198    const DomTreeNode *Node;
199    const DomTreeNode *parentNode;
200  };
201}
202
203const DominanceFrontier::DomSetType &
204DominanceFrontier::calculate(const DominatorTree &DT,
205                             const DomTreeNode *Node) {
206  BasicBlock *BB = Node->getBlock();
207  DomSetType *Result = NULL;
208
209  std::vector<DFCalculateWorkObject> workList;
210  SmallPtrSet<BasicBlock *, 32> visited;
211
212  workList.push_back(DFCalculateWorkObject(BB, NULL, Node, NULL));
213  do {
214    DFCalculateWorkObject *currentW = &workList.back();
215    assert (currentW && "Missing work object.");
216
217    BasicBlock *currentBB = currentW->currentBB;
218    BasicBlock *parentBB = currentW->parentBB;
219    const DomTreeNode *currentNode = currentW->Node;
220    const DomTreeNode *parentNode = currentW->parentNode;
221    assert (currentBB && "Invalid work object. Missing current Basic Block");
222    assert (currentNode && "Invalid work object. Missing current Node");
223    DomSetType &S = Frontiers[currentBB];
224
225    // Visit each block only once.
226    if (visited.count(currentBB) == 0) {
227      visited.insert(currentBB);
228
229      // Loop over CFG successors to calculate DFlocal[currentNode]
230      for (succ_iterator SI = succ_begin(currentBB), SE = succ_end(currentBB);
231           SI != SE; ++SI) {
232        // Does Node immediately dominate this successor?
233        if (DT[*SI]->getIDom() != currentNode)
234          S.insert(*SI);
235      }
236    }
237
238    // At this point, S is DFlocal.  Now we union in DFup's of our children...
239    // Loop through and visit the nodes that Node immediately dominates (Node's
240    // children in the IDomTree)
241    bool visitChild = false;
242    for (DomTreeNode::const_iterator NI = currentNode->begin(),
243           NE = currentNode->end(); NI != NE; ++NI) {
244      DomTreeNode *IDominee = *NI;
245      BasicBlock *childBB = IDominee->getBlock();
246      if (visited.count(childBB) == 0) {
247        workList.push_back(DFCalculateWorkObject(childBB, currentBB,
248                                                 IDominee, currentNode));
249        visitChild = true;
250      }
251    }
252
253    // If all children are visited or there is any child then pop this block
254    // from the workList.
255    if (!visitChild) {
256
257      if (!parentBB) {
258        Result = &S;
259        break;
260      }
261
262      DomSetType::const_iterator CDFI = S.begin(), CDFE = S.end();
263      DomSetType &parentSet = Frontiers[parentBB];
264      for (; CDFI != CDFE; ++CDFI) {
265        if (!DT.properlyDominates(parentNode, DT[*CDFI]))
266          parentSet.insert(*CDFI);
267      }
268      workList.pop_back();
269    }
270
271  } while (!workList.empty());
272
273  return *Result;
274}
275
276void DominanceFrontierBase::print(std::ostream &o, const Module* ) const {
277  for (const_iterator I = begin(), E = end(); I != E; ++I) {
278    o << "  DomFrontier for BB";
279    if (I->first)
280      WriteAsOperand(o, I->first, false);
281    else
282      o << " <<exit node>>";
283    o << " is:\t" << I->second << "\n";
284  }
285}
286
287void DominanceFrontierBase::dump() {
288  print (llvm::cerr);
289}
290