path: root/lib/VMCore/Dominators.cpp

//===- Dominators.cpp - Dominator Calculation -----------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and 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/Assembly/Writer.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SetOperations.h"
#include "llvm/ADT/SmallPtrSet.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
//===----------------------------------------------------------------------===//
//
// DominatorTree construction - This pass constructs immediate dominator
// information for a flow-graph based on the algorithm described in this
// document:
//
//   A Fast Algorithm for Finding Dominators in a Flowgraph
//   T. Lengauer & R. Tarjan, ACM TOPLAS July 1979, pgs 121-141.
//
// This implements both the O(n*ack(n)) and the O(n*log(n)) versions of EVAL and
// LINK, but it turns out that the theoretically slower O(n*log(n))
// implementation is actually faster than the "efficient" algorithm (even for
// large CFGs) because the constant overheads are substantially smaller.  The
// lower-complexity version can be enabled with the following #define:
//
#define BALANCE_IDOM_TREE 0
//
//===----------------------------------------------------------------------===//

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

unsigned DominatorTree::DFSPass(BasicBlock *V, InfoRec &VInfo,
                                      unsigned N) {
  // This is more understandable as a recursive algorithm, but we can't use the
  // recursive algorithm due to stack depth issues.  Keep it here for
  // documentation purposes.
#if 0
  VInfo.Semi = ++N;
  VInfo.Label = V;

  Vertex.push_back(V);        // Vertex[n] = V;
  //Info[V].Ancestor = 0;     // Ancestor[n] = 0
  //Info[V].Child = 0;        // Child[v] = 0
  VInfo.Size = 1;             // Size[v] = 1

  for (succ_iterator SI = succ_begin(V), E = succ_end(V); SI != E; ++SI) {
    InfoRec &SuccVInfo = Info[*SI];
    if (SuccVInfo.Semi == 0) {
      SuccVInfo.Parent = V;
      N = DFSPass(*SI, SuccVInfo, N);
    }
  }
#else
  std::vector<std::pair<BasicBlock*, unsigned> > Worklist;
  Worklist.push_back(std::make_pair(V, 0U));
  while (!Worklist.empty()) {
    BasicBlock *BB = Worklist.back().first;
    unsigned NextSucc = Worklist.back().second;
    
    // First time we visited this BB?
    if (NextSucc == 0) {
      InfoRec &BBInfo = Info[BB];
      BBInfo.Semi = ++N;
      BBInfo.Label = BB;
      
      Vertex.push_back(BB);       // Vertex[n] = V;
      //BBInfo[V].Ancestor = 0;   // Ancestor[n] = 0
      //BBInfo[V].Child = 0;      // Child[v] = 0
      BBInfo.Size = 1;            // Size[v] = 1
    }
    
    // If we are done with this block, remove it from the worklist.
    if (NextSucc == BB->getTerminator()->getNumSuccessors()) {
      Worklist.pop_back();
      continue;
    }
    
    // Otherwise, increment the successor number for the next time we get to it.
    ++Worklist.back().second;
    
    // Visit the successor next, if it isn't already visited.
    BasicBlock *Succ = BB->getTerminator()->getSuccessor(NextSucc);
    
    InfoRec &SuccVInfo = Info[Succ];
    if (SuccVInfo.Semi == 0) {
      SuccVInfo.Parent = BB;
      Worklist.push_back(std::make_pair(Succ, 0U));
    }
  }
#endif
  return N;
}

void DominatorTree::Compress(BasicBlock *VIn) {

  std::vector<BasicBlock *> Work;
  std::set<BasicBlock *> Visited;
  InfoRec &VInInfo = Info[VIn];
  BasicBlock *VInAncestor = VInInfo.Ancestor;
  InfoRec &VInVAInfo = Info[VInAncestor];

  if (VInVAInfo.Ancestor != 0)
    Work.push_back(VIn);
  
  while (!Work.empty()) {
    BasicBlock *V = Work.back();
    InfoRec &VInfo = Info[V];
    BasicBlock *VAncestor =