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 <algorithm>
using namespace llvm;

//===----------------------------------------------------------------------===//
//  ImmediateDominators Implementation
//===----------------------------------------------------------------------===//
//
// Immediate Dominators 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
//
//===----------------------------------------------------------------------===//

static RegisterAnalysis<ImmediateDominators>
C("idom", "Immediate Dominators Construction", true);

unsigned ImmediateDominators::DFSPass(BasicBlock *V, InfoRec &VInfo,
                                      unsigned N) {
  VInfo.Semi = ++N;
  VInfo.Label = V;

  Vertex.push_back(V);        // Vertex[n] = V;
  //Info[V].Ancestor = 0;     // Ancestor[n] = 0
  //Child[V] = 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);
    }
  }
  return N;
}

void ImmediateDominators::Compress(BasicBlock *V, InfoRec &VInfo) {
  BasicBlock *VAncestor = VInfo.Ancestor;
  InfoRec &VAInfo = Info[VAncestor];
  if (VAInfo.Ancestor == 0)
    return;

  Compress(VAncestor, VAInfo);

  BasicBlock *VAncestorLabel = VAInfo.Label;
  BasicBlock *VLabel = VInfo.Label;
  if (Info[VAncestorLabel].Semi < Info[VLabel].Semi)
    VInfo.Label = VAncestorLabel;

  VInfo.Ancestor = VAInfo.Ancestor;
}

BasicBlock *ImmediateDominators::Eval(BasicBlock *V) {
  InfoRec &VInfo = Info[V];
#if !BALANCE_IDOM_TREE
  // Higher-complexity but faster implementation
  if (VInfo.Ancestor == 0)
    return V;
  Compress(V, VInfo);
  return VInfo.Label;
#else
  // Lower-complexity but slower implementation
  if (VInfo.Ancestor == 0)
    return VInfo.Label;
  Compress(V, VInfo);
  BasicBlock *VLabel = VInfo.Label;

  BasicBlock *VAncestorLabel = Info[VInfo.Ancestor].Label;
  if (Info[VAncestorLabel].Semi >= Info[VLabel].Semi)
    return VLabel;
  else
    return VAncestorLabel;
#endif
}

void ImmediateDominators::Link(BasicBlock *V, BasicBlock *W, InfoRec &WInfo){
#if !BALANCE_IDOM_TREE
  // Higher-complexity but faster implementation
  WInfo.Ancestor = V;
#else
  // Lower-complexity but slower implementation
  BasicBlock *WLabel = WInfo.Label;
  unsigned WLabelSemi = Info[WLabel].Semi;
  BasicBlock *S = W;
  InfoRec *SInfo = &Info[S];

  BasicBlock *SChild = SInfo->Child;
  InfoRec *SChildInfo = &Info[SChild];

  while (WLabelSemi < Info[SChildInfo->Label].Semi) {
    BasicBlock *SChildChild = SChildInfo->Child;
    if (SInfo->Size+Info[SChildChild].Size >= 2*SChildInfo->Size) {
      SChildInfo->Ancestor = S;
      SInfo->Child = SChild = SChildChild;
      SChildInfo = &Info[SChild];
    } else {
      SChildInfo->Size = SInfo->Size;
      S = SInfo->Ancestor = SChild;
      SInfo = SChildInfo;
      SChild = SChildChild;
      SChildInfo = &Info[SChild];
    }
  }

  InfoRec &VInfo = Info[V];
  SInfo->Label = WLabel;

  assert(V != W && "The optimization here will not work in this case!");
  unsigned WSize = WInfo.Size;
  unsigned VSize = (VInfo.Size += WSize);

  if (VSize < 2*WSize)
    std::swap(S, VInfo.Child);

  while (S) {
    SInfo = &Info[S];
    SInfo->Ancestor = V;
    S = SInfo->Child;
  }
#endif
}



bool ImmediateDominators::runOnFunction(Function &F) {
  IDoms.clear();     // Reset from the last time we were run...
  BasicBlock *Root = &F.getEntryBlock();
  Roots.clear();
  Roots.push_back(Root);

  Vertex.push_back(0);

  // Step #1: Number blocks in depth-first order and initialize variables used
  // in later stages of the algorithm.
  unsigned N = 0;
  for (unsigned i = 0, e = Roots.size(); i != e; ++i)
    N = DFSPass(Roots[i], Info[Roots[i]], 0);

  for (unsigned i = N; i >= 2; --i) {
    BasicBlock *W = Vertex[i];
    InfoRec &WInfo = Info[W];

    // Step #2: Calculate the semidominators of all vertices
    for (pred_iterator PI = pred_begin(W), E = pred_end(W); PI != E; ++PI)
      if (Info.count(*PI)) {  // Only if this predecessor is reachable!
        unsigned SemiU = Info[Eval(*PI)].Semi;
        if (SemiU < WInfo.Semi)
          WInfo.Semi = SemiU;
      }

    Info[Vertex[WInfo.Semi]].Bucket.push_back(W);

    BasicBlock *WParent = WInfo.Parent;
    Link(WParent, W, WInfo);

    // Step #3: Implicitly define the immediate dominator of vertices
    std::vector<BasicBlock*> &WParentBucket = Info[WParent].Bucket;
    while (!WParentBucket.empty()) {
      BasicBlock *V = WParentBucket.back();
      WParentBucket.pop_back();
      BasicBlock *U = Eval(V);
      IDoms[V] = Info[U].Semi < Info[V].Semi ? U : WParent;
    }
  }

  // Step #4: Explicitly define the immediate dominator of each vertex
  for (unsigned i = 2; i <= N; ++i) {
    BasicBlock *W = Vertex[i];
    BasicBlock *&WIDom = IDoms[W];
    if (WIDom != Vertex[Info[W].Semi])
      WIDom = IDoms[WIDom];
  }

  // Free temporary memory used to construct idom's
  Info.clear();
  std::vector<BasicBlock*>().swap(Vertex);

  return false;
}

void ImmediateDominatorsBase::print(std::ostream &o, const Module* ) const {
  Function *F = getRoots()[0]->