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 
27 namespace 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.
35 template <class Edge, class BBInfo> class CFGMST {
36 public:
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   // Give BB, return the auxiliary information if it's available.
82   BBInfo *findBBInfo(const BasicBlock *BB) const {
83     auto It = BBInfos.find(BB);
84     if (It == BBInfos.end())
85       return nullptr;
86     return It->second.get();
87   }
88 
89   // Traverse the CFG using a stack. Find all the edges and assign the weight.
90   // Edges with large weight will be put into MST first so they are less likely
91   // to be instrumented.
92   void buildEdges() {
93     DEBUG(dbgs() << "Build Edge on " << F.getName() << "\n");
94 
95     const BasicBlock *BB = &(F.getEntryBlock());
96     uint64_t EntryWeight = (BFI != nullptr ? BFI->getEntryFreq() : 2);
97     // Add a fake edge to the entry.
98     addEdge(nullptr, BB, EntryWeight);
99 
100     // Special handling for single BB functions.
101     if (succ_empty(BB)) {
102       addEdge(BB, nullptr, EntryWeight);
103       return;
104     }
105 
106     static const uint32_t CriticalEdgeMultiplier = 1000;
107 
108     for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
109       TerminatorInst *TI = BB->getTerminator();
110       uint64_t BBWeight =
111           (BFI != nullptr ? BFI->getBlockFreq(&*BB).getFrequency() : 2);
112       uint64_t Weight = 2;
113       if (int successors = TI->getNumSuccessors()) {
114         for (int i = 0; i != successors; ++i) {
115           BasicBlock *TargetBB = TI->getSuccessor(i);
116           bool Critical = isCriticalEdge(TI, i);
117           uint64_t scaleFactor = BBWeight;
118           if (Critical) {
119             if (scaleFactor < UINT64_MAX / CriticalEdgeMultiplier)
120               scaleFactor *= CriticalEdgeMultiplier;
121             else
122               scaleFactor = UINT64_MAX;
123           }
124           if (BPI != nullptr)
125             Weight = BPI->getEdgeProbability(&*BB, TargetBB).scale(scaleFactor);
126           addEdge(&*BB, TargetBB, Weight).IsCritical = Critical;
127           DEBUG(dbgs() << "  Edge: from " << BB->getName() << " to "
128                        << TargetBB->getName() << "  w=" << Weight << "\n");
129         }
130       } else {
131         addEdge(&*BB, nullptr, BBWeight);
132         DEBUG(dbgs() << "  Edge: from " << BB->getName() << " to exit"
133                      << " w = " << BBWeight << "\n");
134       }
135     }
136   }
137 
138   // Sort CFG edges based on its weight.
139   void sortEdgesByWeight() {
140     std::stable_sort(AllEdges.begin(), AllEdges.end(),
141                      [](const std::unique_ptr<Edge> &Edge1,
142                         const std::unique_ptr<Edge> &Edge2) {
143                        return Edge1->Weight > Edge2->Weight;
144                      });
145   }
146 
147   // Traverse all the edges and compute the Minimum Weight Spanning Tree
148   // using union-find algorithm.
149   void computeMinimumSpanningTree() {
150     // First, put all the critical edge with landing-pad as the Dest to MST.
151     // This works around the insufficient support of critical edges split
152     // when destination BB is a landing pad.
153     for (auto &Ei : AllEdges) {
154       if (Ei->Removed)
155         continue;
156       if (Ei->IsCritical) {
157         if (Ei->DestBB && Ei->DestBB->isLandingPad()) {
158           if (unionGroups(Ei->SrcBB, Ei->DestBB))
159             Ei->InMST = true;
160         }
161       }
162     }
163 
164     for (auto &Ei : AllEdges) {
165       if (Ei->Removed)
166         continue;
167       if (unionGroups(Ei->SrcBB, Ei->DestBB))
168         Ei->InMST = true;
169     }
170   }
171 
172   // Dump the Debug information about the instrumentation.
173   void dumpEdges(raw_ostream &OS, const Twine &Message) const {
174     if (!Message.str().empty())
175       OS << Message << "\n";
176     OS << "  Number of Basic Blocks: " << BBInfos.size() << "\n";
177     for (auto &BI : BBInfos) {
178       const BasicBlock *BB = BI.first;
179       OS << "  BB: " << (BB == nullptr ? "FakeNode" : BB->getName()) << "  "
180          << BI.second->infoString() << "\n";
181     }
182 
183     OS << "  Number of Edges: " << AllEdges.size()
184        << " (*: Instrument, C: CriticalEdge, -: Removed)\n";
185     uint32_t Count = 0;
186     for (auto &EI : AllEdges)
187       OS << "  Edge " << Count++ << ": " << getBBInfo(EI->SrcBB).Index << "-->"
188          << getBBInfo(EI->DestBB).Index << EI->infoString() << "\n";
189   }
190 
191   // Add an edge to AllEdges with weight W.
192   Edge &addEdge(const BasicBlock *Src, const BasicBlock *Dest, uint64_t W) {
193     uint32_t Index = BBInfos.size();
194     auto Iter = BBInfos.end();
195     bool Inserted;
196     std::tie(Iter, Inserted) = BBInfos.insert(std::make_pair(Src, nullptr));
197     if (Inserted) {
198       // Newly inserted, update the real info.
199       Iter->second = std::move(llvm::make_unique<BBInfo>(Index));
200       Index++;
201     }
202     std::tie(Iter, Inserted) = BBInfos.insert(std::make_pair(Dest, nullptr));
203     if (Inserted)
204       // Newly inserted, update the real info.
205       Iter->second = std::move(llvm::make_unique<BBInfo>(Index));
206     AllEdges.emplace_back(new Edge(Src, Dest, W));
207     return *AllEdges.back();
208   }
209 
210   BranchProbabilityInfo *BPI;
211   BlockFrequencyInfo *BFI;
212 
213 public:
214   CFGMST(Function &Func, BranchProbabilityInfo *BPI_ = nullptr,
215          BlockFrequencyInfo *BFI_ = nullptr)
216       : F(Func), BPI(BPI_), BFI(BFI_) {
217     buildEdges();
218     sortEdgesByWeight();
219     computeMinimumSpanningTree();
220   }
221 };
222 
223 #undef DEBUG_TYPE // "cfgmst"
224 } // end namespace llvm
225