lib/VMCore/DominatorInternals.cpp

   1 //==- DominatorInternals.cpp - Dominator Calculation -------------*- C++ -*-==//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file was developed by Owen Anderson and is distributed under
   6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9
  10 #ifndef LLVM_ANALYSIS_DOMINATOR_INTERNALS_H
  11 #define LLVM_ANALYSIS_DOMINATOR_INTERNALS_H
  12
  13 #include "llvm/Analysis/Dominators.h"
  14 #include "llvm/ADT/DenseMap.h"
  15 #include "llvm/ADT/SmallPtrSet.h"
  16 //===----------------------------------------------------------------------===//
  17 //
  18 // DominatorTree construction - This pass constructs immediate dominator
  19 // information for a flow-graph based on the algorithm described in this
  20 // document:
  21 //
  22 //   A Fast Algorithm for Finding Dominators in a Flowgraph
  23 //   T. Lengauer & R. Tarjan, ACM TOPLAS July 1979, pgs 121-141.
  24 //
  25 // This implements both the O(n*ack(n)) and the O(n*log(n)) versions of EVAL and
  26 // LINK, but it turns out that the theoretically slower O(n*log(n))
  27 // implementation is actually faster than the "efficient" algorithm (even for
  28 // large CFGs) because the constant overheads are substantially smaller.  The
  29 // lower-complexity version can be enabled with the following #define:
  30 //
  31 #define BALANCE_IDOM_TREE 0
  32 //
  33 //===----------------------------------------------------------------------===//
  34
  35 namespace llvm {
  36
  37 void Compress(DominatorTreeBase& DT, BasicBlock *VIn) {
  38
  39   std::vector<BasicBlock *> Work;
  40   SmallPtrSet<BasicBlock *, 32> Visited;
  41   BasicBlock *VInAncestor = DT.Info[VIn].Ancestor;
  42   DominatorTreeBase::InfoRec &VInVAInfo = DT.Info[VInAncestor];
  43
  44   if (VInVAInfo.Ancestor != 0)
  45     Work.push_back(VIn);
  46
  47   while (!Work.empty()) {
  48     BasicBlock *V = Work.back();
  49     DominatorTree::InfoRec &VInfo = DT.Info[V];
  50     BasicBlock *VAncestor = VInfo.Ancestor;
  51     DominatorTreeBase::InfoRec &VAInfo = DT.Info[VAncestor];
  52
  53     // Process Ancestor first
  54     if (Visited.insert(VAncestor) &&
  55         VAInfo.Ancestor != 0) {
  56       Work.push_back(VAncestor);
  57       continue;
  58     }
  59     Work.pop_back();
  60
  61     // Update VInfo based on Ancestor info
  62     if (VAInfo.Ancestor == 0)
  63       continue;
  64     BasicBlock *VAncestorLabel = VAInfo.Label;
  65     BasicBlock *VLabel = VInfo.Label;
  66     if (DT.Info[VAncestorLabel].Semi < DT.Info[VLabel].Semi)
  67       VInfo.Label = VAncestorLabel;
  68     VInfo.Ancestor = VAInfo.Ancestor;
  69   }
  70 }
  71
  72 BasicBlock *Eval(DominatorTreeBase& DT, BasicBlock *V) {
  73                  DominatorTreeBase::InfoRec &VInfo = DT.Info[V];
  74 #if !BALANCE_IDOM_TREE
  75   // Higher-complexity but faster implementation
  76   if (VInfo.Ancestor == 0)
  77     return V;
  78   Compress(DT, V);
  79   return VInfo.Label;
  80 #else
  81   // Lower-complexity but slower implementation
  82   if (VInfo.Ancestor == 0)
  83     return VInfo.Label;
  84   Compress(DT, V);
  85   BasicBlock *VLabel = VInfo.Label;
  86
  87   BasicBlock *VAncestorLabel = DT.Info[VInfo.Ancestor].Label;
  88   if (DT.Info[VAncestorLabel].Semi >= DT.Info[VLabel].Semi)
  89     return VLabel;
  90   else
  91     return VAncestorLabel;
  92 #endif
  93 }
  94
  95 void Link(DominatorTreeBase& DT, BasicBlock *V, BasicBlock *W,
  96           DominatorTreeBase::InfoRec &WInfo) {
  97 #if !BALANCE_IDOM_TREE
  98   // Higher-complexity but faster implementation
  99   WInfo.Ancestor = V;
 100 #else
 101   // Lower-complexity but slower implementation
 102   BasicBlock *WLabel = WInfo.Label;
 103   unsigned WLabelSemi = DT.Info[WLabel].Semi;
 104   BasicBlock *S = W;
 105   InfoRec *SInfo = &DT.Info[S];
 106
 107   BasicBlock *SChild = SInfo->Child;
 108   InfoRec *SChildInfo = &DT.Info[SChild];
 109
 110   while (WLabelSemi < DT.Info[SChildInfo->Label].Semi) {
 111     BasicBlock *SChildChild = SChildInfo->Child;
 112     if (SInfo->Size+DT.Info[SChildChild].Size >= 2*SChildInfo->Size) {
 113       SChildInfo->Ancestor = S;
 114       SInfo->Child = SChild = SChildChild;
 115       SChildInfo = &DT.Info[SChild];
 116     } else {
 117       SChildInfo->Size = SInfo->Size;
 118       S = SInfo->Ancestor = SChild;
 119       SInfo = SChildInfo;
 120       SChild = SChildChild;
 121       SChildInfo = &DT.Info[SChild];
 122     }
 123   }
 124
 125   DominatorTreeBase::InfoRec &VInfo = DT.Info[V];
 126   SInfo->Label = WLabel;
 127
 128   assert(V != W && "The optimization here will not work in this case!");
 129   unsigned WSize = WInfo.Size;
 130   unsigned VSize = (VInfo.Size += WSize);
 131
 132   if (VSize < 2*WSize)
 133     std::swap(S, VInfo.Child);
 134
 135   while (S) {
 136     SInfo = &DT.Info[S];
 137     SInfo->Ancestor = V;
 138     S = SInfo->Child;
 139   }
 140 #endif
 141 }
 142
 143 }
 144
 145 #endif