lib/VMCore/Dominators.cpp

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