1//===-- CFGMST.h - Minimum Spanning Tree for CFG ----------------*- C++ -*-===//
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 a Union-find algorithm to compute Minimum Spanning Tree
11// for a given CFG.
12//
13//===----------------------------------------------------------------------===//
14
15#include "llvm/ADT/DenseMap.h"
16#include "llvm/ADT/STLExtras.h"
17#include "llvm/Analysis/BlockFrequencyInfo.h"
18#include "llvm/Analysis/BranchProbabilityInfo.h"
19#include "llvm/Analysis/CFG.h"
20#include "llvm/Support/BranchProbability.h"
21#include "llvm/Support/Debug.h"
22#include "llvm/Support/raw_ostream.h"
23#include "llvm/Transforms/Utils/BasicBlockUtils.h"
24#include <utility>
25#include <vector>
26
27namespace llvm {
28
29#define DEBUG_TYPE "cfgmst"
30
31/// \brief An union-find based Minimum Spanning Tree for CFG
32///
33/// Implements a Union-find algorithm to compute Minimum Spanning Tree
34/// for a given CFG.
35template <class Edge, class BBInfo> class CFGMST {
36public:
37  Function &F;
38
39  // Store all the edges in CFG. It may contain some stale edges
40  // when Removed is set.
41  std::vector<std::unique_ptr<Edge>> AllEdges;
42
43  // This map records the auxiliary information for each BB.
44  DenseMap<const BasicBlock *, std::unique_ptr<BBInfo>> BBInfos;
45
46  // Find the root group of the G and compress the path from G to the root.
47  BBInfo *findAndCompressGroup(BBInfo *G) {
48    if (G->Group != G)
49      G->Group = findAndCompressGroup(static_cast<BBInfo *>(G->Group));
50    return static_cast<BBInfo *>(G->Group);
51  }
52
53  // Union BB1 and BB2 into the same group and return true.
54  // Returns false if BB1 and BB2 are already in the same group.
55  bool unionGroups(const BasicBlock *BB1, const BasicBlock *BB2) {
56    BBInfo *BB1G = findAndCompressGroup(&getBBInfo(BB1));
57    BBInfo *BB2G = findAndCompressGroup(&getBBInfo(BB2));
58
59    if (BB1G == BB2G)
60      return false;
61
62    // Make the smaller rank tree a direct child or the root of high rank tree.
63    if (BB1G->Rank < BB2G->Rank)
64      BB1G->Group = BB2G;
65    else {
66      BB2G->Group = BB1G;
67      // If the ranks are the same, increment root of one tree by one.
68      if (BB1G->Rank == BB2G->Rank)
69        BB1G->Rank++;
70    }
71    return true;
72  }
73
74  // Give BB, return the auxiliary information.
75  BBInfo &getBBInfo(const BasicBlock *BB) const {
76    auto It = BBInfos.find(BB);
77    assert(It->second.get() != nullptr);
78    return *It->second.get();
79  }
80
81  // Traverse the CFG using a stack. Find all the edges and assign the weight.
82  // Edges with large weight will be put into MST first so they are less likely
83  // to be instrumented.
84  void buildEdges() {
85    DEBUG(dbgs() << "Build Edge on " << F.getName() << "\n");
86
87    const BasicBlock *BB = &(F.getEntryBlock());
88    uint64_t EntryWeight = (BFI != nullptr ? BFI->getEntryFreq() : 2);
89    // Add a fake edge to the entry.
90    addEdge(nullptr, BB, EntryWeight);
91
92    // Special handling for single BB functions.
93    if (succ_empty(BB)) {
94      addEdge(BB, nullptr, EntryWeight);
95      return;
96    }
97
98    static const uint32_t CriticalEdgeMultiplier = 1000;
99
100    for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
101      TerminatorInst *TI = BB->getTerminator();
102      uint64_t BBWeight =
103          (BFI != nullptr ? BFI->getBlockFreq(&*BB).getFrequency() : 2);
104      uint64_t Weight = 2;
105      if (int successors = TI->getNumSuccessors()) {
106        for (int i = 0; i != successors; ++i) {
107          BasicBlock *TargetBB = TI->getSuccessor(i);
108          bool Critical = isCriticalEdge(TI, i);
109          uint64_t scaleFactor = BBWeight;
110          if (Critical) {
111            if (scaleFactor < UINT64_MAX / CriticalEdgeMultiplier)
112              scaleFactor *= CriticalEdgeMultiplier;
113            else
114              scaleFactor = UINT64_MAX;
115          }
116          if (BPI != nullptr)
117            Weight = BPI->getEdgeProbability(&*BB, TargetBB).scale(scaleFactor);
118          addEdge(&*BB, TargetBB, Weight).IsCritical = Critical;
119          DEBUG(dbgs() << "  Edge: from " << BB->getName() << " to "
120                       << TargetBB->getName() << "  w=" << Weight << "\n");
121        }
122      } else {
123        addEdge(&*BB, nullptr, BBWeight);
124        DEBUG(dbgs() << "  Edge: from " << BB->getName() << " to exit"
125                     << " w = " << BBWeight << "\n");
126      }
127    }
128  }
129
130  // Sort CFG edges based on its weight.
131  void sortEdgesByWeight() {
132    std::stable_sort(AllEdges.begin(), AllEdges.end(),
133                     [](const std::unique_ptr<Edge> &Edge1,
134                        const std::unique_ptr<Edge> &Edge2) {
135                       return Edge1->Weight > Edge2->Weight;
136                     });
137  }
138
139  // Traverse all the edges and compute the Minimum Weight Spanning Tree
140  // using union-find algorithm.
141  void computeMinimumSpanningTree() {
142    // First, put all the critical edge with landing-pad as the Dest to MST.
143    // This works around the insufficient support of critical edges split
144    // when destination BB is a landing pad.
145    for (auto &Ei : AllEdges) {
146      if (Ei->Removed)
147        continue;
148      if (Ei->IsCritical) {
149        if (Ei->DestBB && Ei->DestBB->isLandingPad()) {
150          if (unionGroups(Ei->SrcBB, Ei->DestBB))
151            Ei->InMST = true;
152        }
153      }
154    }
155
156    for (auto &Ei : AllEdges) {
157      if (Ei->Removed)
158        continue;
159      if (unionGroups(Ei->SrcBB, Ei->DestBB))
160        Ei->InMST = true;
161    }
162  }
163
164  // Dump the Debug information about the instrumentation.
165  void dumpEdges(raw_ostream &OS, const Twine &Message) const {
166    if (!Message.str().empty())
167      OS << Message << "\n";
168    OS << "  Number of Basic Blocks: " << BBInfos.size() << "\n";
169    for (auto &BI : BBInfos) {
170      const BasicBlock *BB = BI.first;
171      OS << "  BB: " << (BB == nullptr ? "FakeNode" : BB->getName()) << "  "
172         << BI.second->infoString() << "\n";
173    }
174
175    OS << "  Number of Edges: " << AllEdges.size()
176       << " (*: Instrument, C: CriticalEdge, -: Removed)\n";
177    uint32_t Count = 0;
178    for (auto &EI : AllEdges)
179      OS << "  Edge " << Count++ << ": " << getBBInfo(EI->SrcBB).Index << "-->"
180         << getBBInfo(EI->DestBB).Index << EI->infoString() << "\n";
181  }
182
183  // Add an edge to AllEdges with weight W.
184  Edge &addEdge(const BasicBlock *Src, const BasicBlock *Dest, uint64_t W) {
185    uint32_t Index = BBInfos.size();
186    auto Iter = BBInfos.end();
187    bool Inserted;
188    std::tie(Iter, Inserted) = BBInfos.insert(std::make_pair(Src, nullptr));
189    if (Inserted) {
190      // Newly inserted, update the real info.
191      Iter->second = std::move(llvm::make_unique<BBInfo>(Index));
192      Index++;
193    }
194    std::tie(Iter, Inserted) = BBInfos.insert(std::make_pair(Dest, nullptr));
195    if (Inserted)
196      // Newly inserted, update the real info.
197      Iter->second = std::move(llvm::make_unique<BBInfo>(Index));
198    AllEdges.emplace_back(new Edge(Src, Dest, W));
199    return *AllEdges.back();
200  }
201
202  BranchProbabilityInfo *BPI;
203  BlockFrequencyInfo *BFI;
204
205public:
206  CFGMST(Function &Func, BranchProbabilityInfo *BPI_ = nullptr,
207         BlockFrequencyInfo *BFI_ = nullptr)
208      : F(Func), BPI(BPI_), BFI(BFI_) {
209    buildEdges();
210    sortEdgesByWeight();
211    computeMinimumSpanningTree();
212  }
213};
214
215#undef DEBUG_TYPE // "cfgmst"
216} // end namespace llvm
217