Dominators.cpp

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//===- Dominators.cpp - Dominator Calculation -----------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements simple dominator construction algorithms for finding
// forward dominators.  Postdominators are available in libanalysis, but are not
// included in libvmcore, because it's not needed.  Forward dominators are
// needed to support the Verifier pass.
//
//===----------------------------------------------------------------------===//

#include "llvm/Analysis/Dominators.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/Compiler.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SetOperations.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/DominatorInternals.h"
#include "llvm/Instructions.h"
#include "llvm/Support/Streams.h"
#include <algorithm>
using namespace llvm;

namespace llvm {
static std::ostream &operator<<(std::ostream &o,
                                const std::set<BasicBlock*> &BBs) {
  for (std::set<BasicBlock*>::const_iterator I = BBs.begin(), E = BBs.end();
       I != E; ++I)
    if (*I)
      WriteAsOperand(o, *I, false);
    else
      o << " <<exit node>>";
  return o;
}
}

//===----------------------------------------------------------------------===//
//  DominatorTree Implementation
//===----------------------------------------------------------------------===//
//
// Provide public access to DominatorTree information.  Implementation details
// can be found in DominatorCalculation.h.
//
//===----------------------------------------------------------------------===//

TEMPLATE_INSTANTIATION(class DomTreeNodeBase<BasicBlock>);
TEMPLATE_INSTANTIATION(class DominatorTreeBase<BasicBlock>);

char DominatorTree::ID = 0;
static RegisterPass<DominatorTree>
E("domtree", "Dominator Tree Construction", true, true);

bool DominatorTree::runOnFunction(Function &F) {
  DT->recalculate(F);
  return false;
}

//===----------------------------------------------------------------------===//
//  DominanceFrontier Implementation
//===----------------------------------------------------------------------===//

char DominanceFrontier::ID = 0;
static RegisterPass<DominanceFrontier>
G("domfrontier", "Dominance Frontier Construction", true, true);

// NewBB is split and now it has one successor. Update dominace frontier to
// reflect this change.
void DominanceFrontier::splitBlock(BasicBlock *NewBB) {
  assert(NewBB->getTerminator()->getNumSuccessors() == 1
         && "NewBB should have a single successor!");
  BasicBlock *NewBBSucc = NewBB->getTerminator()->getSuccessor(0);

  std::vector<BasicBlock*> PredBlocks;
  for (pred_iterator PI = pred_begin(NewBB), PE = pred_end(NewBB);
       PI != PE; ++PI)
      PredBlocks.push_back(*PI);  

  if (PredBlocks.empty())
    // If NewBB does not have any predecessors then it is a entry block.
    // In this case, NewBB and its successor NewBBSucc dominates all
    // other blocks.
    return;

  // NewBBSucc inherits original NewBB frontier.
  DominanceFrontier::iterator NewBBI = find(NewBB);
  if (NewBBI != end()) {
    DominanceFrontier::DomSetType NewBBSet = NewBBI->second;
    DominanceFrontier::DomSetType NewBBSuccSet;
    NewBBSuccSet.insert(NewBBSet.begin(), NewBBSet.end());
    addBasicBlock(NewBBSucc, NewBBSuccSet);
  }

  // If NewBB dominates NewBBSucc, then DF(NewBB) is now going to be the
  // DF(PredBlocks[0]) without the stuff that the new block does not dominate
  // a predecessor of.
  DominatorTree &DT = getAnalysis<DominatorTree>();
  if (DT.dominates(NewBB, NewBBSucc)) {
    DominanceFrontier::iterator DFI = find(PredBlocks[0]);
    if (DFI != end()) {
      DominanceFrontier::DomSetType Set = DFI->second;
      // Filter out stuff in Set that we do not dominate a predecessor of.
      for (DominanceFrontier::DomSetType::iterator SetI = Set.begin(),
             E = Set.end(); SetI != E;) {
        bool DominatesPred = false;
        for (pred_iterator PI = pred_begin(*SetI), E = pred_end(*SetI);
             PI != E; ++PI)
          if (DT.dominates(NewBB, *PI))
            DominatesPred = true;
        if (!DominatesPred)
          Set.erase(SetI++);
        else
          ++SetI;
      }

      if (NewBBI != end()) {
        for (DominanceFrontier::DomSetType::iterator SetI = Set.begin(),
               E = Set.end(); SetI != E; ++SetI) {
          BasicBlock *SB = *SetI;
          addToFrontier(NewBBI, SB);
        }
      } else 
        addBasicBlock(NewBB, Set);
    }
    
  } else {
    // DF(NewBB) is {NewBBSucc} because NewBB does not strictly dominate
    // NewBBSucc, but it does dominate itself (and there is an edge (NewBB ->
    // NewBBSucc)).  NewBBSucc is the single successor of NewBB.
    DominanceFrontier::DomSetType NewDFSet;
    NewDFSet.insert(NewBBSucc);
    addBasicBlock(NewBB, NewDFSet);
  }
  
  // Now we must loop over all of the dominance frontiers in the function,
  // replacing occurrences of NewBBSucc with NewBB in some cases.  All
  // blocks that dominate a block in PredBlocks and contained NewBBSucc in
  // their dominance frontier must be updated to contain NewBB instead.
  //
  for (Function::iterator FI = NewBB->getParent()->begin(),
         FE = NewBB->getParent()->end(); FI != FE; ++FI) {
    DominanceFrontier::iterator DFI = find(FI);
    if (DFI == end()) continue;  // unreachable block.
    
    // Only consider nodes that have NewBBSucc in their dominator frontier.
    if (!DFI->second.count(NewBBSucc)) continue;

    // Verify whether this block dominates a block in predblocks.  If not, do
    // not update it.
    bool BlockDominatesAny = false;
    for (std::vector<BasicBlock*>::const_iterator BI = PredBlocks.begin(), 
           BE = PredBlocks.end(); BI != BE; ++BI) {
      if (DT.dominates(FI, *BI)) {
        BlockDominatesAny = true;
        break;
      }
    }
    
    if (!BlockDominatesAny)
      continue;
    
    // If NewBBSucc should not stay in our dominator frontier, remove it.
    // We remove it unless there is a predecessor of NewBBSucc that we
    // dominate, but we don't strictly dominate NewBBSucc.
    bool ShouldRemove = true;
    if ((BasicBlock*)FI == NewBBSucc || !DT.dominates(FI, NewBBSucc)) {
      // Okay, we know that PredDom does not strictly dominate NewBBSucc.
      // Check to see if it dominates any predecessors of NewBBSucc.
      for (pred_iterator PI = pred_begin(NewBBSucc),
           E = pred_end(NewBBSucc); PI != E; ++PI)
        if (DT.dominates(FI, *PI)) {
          ShouldRemove = false;
          break;
        }
    }
    
    if (ShouldRemove)
      removeFromFrontier(DFI, NewBBSucc);
    addToFrontier(DFI, NewBB);
  }
}

namespace {
  class DFCalculateWorkObject {
  public:
    DFCalculateWorkObject(BasicBlock *B, BasicBlock *P, 
                          const DomTreeNode *N,
                          const DomTreeNode *PN)
    : currentBB(B), parentBB(P), Node(N), parentNode(PN) {}
    BasicBlock *currentBB;
    BasicBlock *parentBB;
    const DomTreeNode *Node;
    const DomTreeNode *parentNode;
  };
}

const DominanceFrontier::DomSetType &
DominanceFrontier::calculate(const DominatorTree &DT,
                             const DomTreeNode *Node) {
  BasicBlock *BB = Node->getBlock();
  DomSetType *Result = NULL;

  std::vector<DFCalculateWorkObject> workList;
  SmallPtrSet<BasicBlock *, 32> visited;

  workList.push_back(DFCalculateWorkObject(BB, NULL, Node, NULL));
  do {
    DFCalculateWorkObject *currentW = &workList.back();
    assert (currentW && "Missing work object.");

    BasicBlock *currentBB = currentW->currentBB;
    BasicBlock *parentBB = currentW->parentBB;
    const DomTreeNode *currentNode = currentW->Node;
    const DomTreeNode *parentNode = currentW->parentNode;
    assert (currentBB && "Invalid work object. Missing current Basic Block");
    assert (currentNode && "Invalid work object. Missing current Node");
    DomSetType &S = Frontiers[currentBB];

    // Visit each block only once.
    if (visited.count(currentBB) == 0) {
      visited.insert(currentBB);

      // Loop over CFG successors to calculate DFlocal[currentNode]
      for (succ_iterator SI = succ_begin(currentBB), SE = succ_end(currentBB);
           SI != SE; ++SI) {
        // Does Node immediately dominate this successor?
        if (DT[*SI]->getIDom() != currentNode)
          S.insert(*SI);
      }
    }

    // At this point, S is DFlocal.  Now we union in DFup's of our children...
    // Loop through and visit the nodes that Node immediately dominates (Node's
    // children in the IDomTree)
    bool visitChild = false;
    for (DomTreeNode::const_iterator NI = currentNode->begin(), 
           NE = currentNode->end(); NI != NE; ++NI) {
      DomTreeNode *IDominee = *NI;
      BasicBlock *childBB = IDominee->getBlock();
      if (visited.count(childBB) == 0) {
        workList.push_back(DFCalculateWorkObject(childBB, currentBB,
                                                 IDominee, currentNode));
        visitChild = true;
      }
    }

    // If all children are visited or there is any child then pop this block
    // from the workList.
    if (!visitChild) {

      if (!parentBB) {
        Result = &S;
        break;
      }

      DomSetType::const_iterator CDFI = S.begin(), CDFE = S.end();
      DomSetType &parentSet = Frontiers[parentBB];
      for (; CDFI != CDFE; ++CDFI) {
        if (!DT.properlyDominates(parentNode, DT[*CDFI]))
          parentSet.insert(*CDFI);
      }
      workList.pop_back();
    }

  } while (!workList.empty());

  return *Result;
}

void DominanceFrontierBase::print(std::ostream &o, const Module* ) const {
  for (const_iterator I = begin(), E = end(); I != E; ++I) {
    o << "  DomFrontier for BB";
    if (I->first)
      WriteAsOperand(o, I->first, false);
    else
      o << " <<exit node>>";
    o << " is:\t" << I->second << "\n";
  }
}

void DominanceFrontierBase::dump() {
  print (llvm::cerr);
}