lib/Transforms/Instrumentation/CFGMST.h

   1 //===-- CFGMST.h - Minimum Spanning Tree for CFG ----------------*- C++ -*-===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements a Union-find algorithm to compute Minimum Spanning Tree
  10 // for a given CFG.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #ifndef LLVM_LIB_TRANSFORMS_INSTRUMENTATION_CFGMST_H
  15 #define LLVM_LIB_TRANSFORMS_INSTRUMENTATION_CFGMST_H
  16
  17 #include "llvm/ADT/DenseMap.h"
  18 #include "llvm/ADT/STLExtras.h"
  19 #include "llvm/Analysis/BlockFrequencyInfo.h"
  20 #include "llvm/Analysis/BranchProbabilityInfo.h"
  21 #include "llvm/Analysis/CFG.h"
  22 #include "llvm/Support/BranchProbability.h"
  23 #include "llvm/Support/Debug.h"
  24 #include "llvm/Support/raw_ostream.h"
  25 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
  26 #include <utility>
  27 #include <vector>
  28
  29 #define DEBUG_TYPE "cfgmst"
  30
  31 namespace llvm {
  32
  33 /// An union-find based Minimum Spanning Tree for CFG
  34 ///
  35 /// Implements a Union-find algorithm to compute Minimum Spanning Tree
  36 /// for a given CFG.
  37 template <class Edge, class BBInfo> class CFGMST {
  38 public:
  39   Function &F;
  40
  41   // Store all the edges in CFG. It may contain some stale edges
  42   // when Removed is set.
  43   std::vector<std::unique_ptr<Edge>> AllEdges;
  44
  45   // This map records the auxiliary information for each BB.
  46   DenseMap<const BasicBlock *, std::unique_ptr<BBInfo>> BBInfos;
  47
  48   // Whehter the function has an exit block with no successors.
  49   // (For function with an infinite loop, this block may be absent)
  50   bool ExitBlockFound = false;
  51
  52   // Find the root group of the G and compress the path from G to the root.
  53   BBInfo *findAndCompressGroup(BBInfo *G) {
  54     if (G->Group != G)
  55       G->Group = findAndCompressGroup(static_cast<BBInfo *>(G->Group));
  56     return static_cast<BBInfo *>(G->Group);
  57   }
  58
  59   // Union BB1 and BB2 into the same group and return true.
  60   // Returns false if BB1 and BB2 are already in the same group.
  61   bool unionGroups(const BasicBlock *BB1, const BasicBlock *BB2) {
  62     BBInfo *BB1G = findAndCompressGroup(&getBBInfo(BB1));
  63     BBInfo *BB2G = findAndCompressGroup(&getBBInfo(BB2));
  64
  65     if (BB1G == BB2G)
  66       return false;
  67
  68     // Make the smaller rank tree a direct child or the root of high rank tree.
  69     if (BB1G->Rank < BB2G->Rank)
  70       BB1G->Group = BB2G;
  71     else {
  72       BB2G->Group = BB1G;
  73       // If the ranks are the same, increment root of one tree by one.
  74       if (BB1G->Rank == BB2G->Rank)
  75         BB1G->Rank++;
  76     }
  77     return true;
  78   }
  79
  80   // Give BB, return the auxiliary information.
  81   BBInfo &getBBInfo(const BasicBlock *BB) const {
  82     auto It = BBInfos.find(BB);
  83     assert(It->second.get() != nullptr);
  84     return *It->second.get();
  85   }
  86
  87   // Give BB, return the auxiliary information if it's available.
  88   BBInfo *findBBInfo(const BasicBlock *BB) const {
  89     auto It = BBInfos.find(BB);
  90     if (It == BBInfos.end())
  91       return nullptr;
  92     return It->second.get();
  93   }
  94
  95   // Traverse the CFG using a stack. Find all the edges and assign the weight.
  96   // Edges with large weight will be put into MST first so they are less likely
  97   // to be instrumented.
  98   void buildEdges() {
  99     LLVM_DEBUG(dbgs() << "Build Edge on " << F.getName() << "\n");
 100
 101     const BasicBlock *Entry = &(F.getEntryBlock());
 102     uint64_t EntryWeight = (BFI != nullptr ? BFI->getEntryFreq() : 2);
 103     Edge *EntryIncoming = nullptr, *EntryOutgoing = nullptr,
 104         *ExitOutgoing = nullptr, *ExitIncoming = nullptr;
 105     uint64_t MaxEntryOutWeight = 0, MaxExitOutWeight = 0, MaxExitInWeight = 0;
 106
 107     // Add a fake edge to the entry.
 108     EntryIncoming = &addEdge(nullptr, Entry, EntryWeight);
 109     LLVM_DEBUG(dbgs() << "  Edge: from fake node to " << Entry->getName()
 110                       << " w = " << EntryWeight << "\n");
 111
 112     // Special handling for single BB functions.
 113     if (succ_empty(Entry)) {
 114       addEdge(Entry, nullptr, EntryWeight);
 115       return;
 116     }
 117
 118     static const uint32_t CriticalEdgeMultiplier = 1000;
 119
 120     for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
 121       Instruction *TI = BB->getTerminator();
 122       uint64_t BBWeight =
 123           (BFI != nullptr ? BFI->getBlockFreq(&*BB).getFrequency() : 2);
 124       uint64_t Weight = 2;
 125       if (int successors = TI->getNumSuccessors()) {
 126         for (int i = 0; i != successors; ++i) {
 127           BasicBlock *TargetBB = TI->getSuccessor(i);
 128           bool Critical = isCriticalEdge(TI, i);
 129           uint64_t scaleFactor = BBWeight;
 130           if (Critical) {
 131             if (scaleFactor < UINT64_MAX / CriticalEdgeMultiplier)
 132               scaleFactor *= CriticalEdgeMultiplier;
 133             else
 134               scaleFactor = UINT64_MAX;
 135           }
 136           if (BPI != nullptr)
 137             Weight = BPI->getEdgeProbability(&*BB, TargetBB).scale(scaleFactor);
 138           auto *E = &addEdge(&*BB, TargetBB, Weight);
 139           E->IsCritical = Critical;
 140           LLVM_DEBUG(dbgs() << "  Edge: from " << BB->getName() << " to "
 141                             << TargetBB->getName() << "  w=" << Weight << "\n");
 142
 143           // Keep track of entry/exit edges:
 144           if (&*BB == Entry) {
 145             if (Weight > MaxEntryOutWeight) {
 146               MaxEntryOutWeight = Weight;
 147               EntryOutgoing = E;
 148             }
 149           }
 150
 151           auto *TargetTI = TargetBB->getTerminator();
 152           if (TargetTI && !TargetTI->getNumSuccessors()) {
 153             if (Weight > MaxExitInWeight) {
 154               MaxExitInWeight = Weight;
 155               ExitIncoming = E;
 156             }
 157           }
 158         }
 159       } else {
 160         ExitBlockFound = true;
 161         Edge *ExitO = &addEdge(&*BB, nullptr, BBWeight);
 162         if (BBWeight > MaxExitOutWeight) {
 163           MaxExitOutWeight = BBWeight;
 164           ExitOutgoing = ExitO;
 165         }
 166         LLVM_DEBUG(dbgs() << "  Edge: from " << BB->getName() << " to fake exit"
 167                           << " w = " << BBWeight << "\n");
 168       }
 169     }
 170
 171     // Entry/exit edge adjustment heurisitic:
 172     // prefer instrumenting entry edge over exit edge
 173     // if possible. Those exit edges may never have a chance to be
 174     // executed (for instance the program is an event handling loop)
 175     // before the profile is asynchronously dumped.
 176     //
 177     // If EntryIncoming and ExitOutgoing has similar weight, make sure
 178     // ExitOutging is selected as the min-edge. Similarly, if EntryOutgoing
 179     // and ExitIncoming has similar weight, make sure ExitIncoming becomes
 180     // the min-edge.
 181     uint64_t EntryInWeight = EntryWeight;
 182
 183     if (EntryInWeight >= MaxExitOutWeight &&
 184         EntryInWeight * 2 < MaxExitOutWeight * 3) {
 185       EntryIncoming->Weight = MaxExitOutWeight;
 186       ExitOutgoing->Weight = EntryInWeight + 1;
 187     }
 188
 189     if (MaxEntryOutWeight >= MaxExitInWeight &&
 190         MaxEntryOutWeight * 2 < MaxExitInWeight * 3) {
 191       EntryOutgoing->Weight = MaxExitInWeight;
 192       ExitIncoming->Weight = MaxEntryOutWeight + 1;
 193     }
 194   }
 195
 196   // Sort CFG edges based on its weight.
 197   void sortEdgesByWeight() {
 198     llvm::stable_sort(AllEdges, [](const std::unique_ptr<Edge> &Edge1,
 199                                    const std::unique_ptr<Edge> &Edge2) {
 200       return Edge1->Weight > Edge2->Weight;
 201     });
 202   }
 203
 204   // Traverse all the edges and compute the Minimum Weight Spanning Tree
 205   // using union-find algorithm.
 206   void computeMinimumSpanningTree() {
 207     // First, put all the critical edge with landing-pad as the Dest to MST.
 208     // This works around the insufficient support of critical edges split
 209     // when destination BB is a landing pad.
 210     for (auto &Ei : AllEdges) {
 211       if (Ei->Removed)
 212         continue;
 213       if (Ei->IsCritical) {
 214         if (Ei->DestBB && Ei->DestBB->isLandingPad()) {
 215           if (unionGroups(Ei->SrcBB, Ei->DestBB))
 216             Ei->InMST = true;
 217         }
 218       }
 219     }
 220
 221     for (auto &Ei : AllEdges) {
 222       if (Ei->Removed)
 223         continue;
 224       // If we detect infinite loops, force
 225       // instrumenting the entry edge:
 226       if (!ExitBlockFound && Ei->SrcBB == nullptr)
 227         continue;
 228       if (unionGroups(Ei->SrcBB, Ei->DestBB))
 229         Ei->InMST = true;
 230     }
 231   }
 232
 233   // Dump the Debug information about the instrumentation.
 234   void dumpEdges(raw_ostream &OS, const Twine &Message) const {
 235     if (!Message.str().empty())
 236       OS << Message << "\n";
 237     OS << "  Number of Basic Blocks: " << BBInfos.size() << "\n";
 238     for (auto &BI : BBInfos) {
 239       const BasicBlock *BB = BI.first;
 240       OS << "  BB: " << (BB == nullptr ? "FakeNode" : BB->getName()) << "  "
 241          << BI.second->infoString() << "\n";
 242     }
 243
 244     OS << "  Number of Edges: " << AllEdges.size()
 245        << " (*: Instrument, C: CriticalEdge, -: Removed)\n";
 246     uint32_t Count = 0;
 247     for (auto &EI : AllEdges)
 248       OS << "  Edge " << Count++ << ": " << getBBInfo(EI->SrcBB).Index << "-->"
 249          << getBBInfo(EI->DestBB).Index << EI->infoString() << "\n";
 250   }
 251
 252   // Add an edge to AllEdges with weight W.
 253   Edge &addEdge(const BasicBlock *Src, const BasicBlock *Dest, uint64_t W) {
 254     uint32_t Index = BBInfos.size();
 255     auto Iter = BBInfos.end();
 256     bool Inserted;
 257     std::tie(Iter, Inserted) = BBInfos.insert(std::make_pair(Src, nullptr));
 258     if (Inserted) {
 259       // Newly inserted, update the real info.
 260       Iter->second = std::move(std::make_unique<BBInfo>(Index));
 261       Index++;
 262     }
 263     std::tie(Iter, Inserted) = BBInfos.insert(std::make_pair(Dest, nullptr));
 264     if (Inserted)
 265       // Newly inserted, update the real info.
 266       Iter->second = std::move(std::make_unique<BBInfo>(Index));
 267     AllEdges.emplace_back(new Edge(Src, Dest, W));
 268     return *AllEdges.back();
 269   }
 270
 271   BranchProbabilityInfo *BPI;
 272   BlockFrequencyInfo *BFI;
 273
 274 public:
 275   CFGMST(Function &Func, BranchProbabilityInfo *BPI_ = nullptr,
 276          BlockFrequencyInfo *BFI_ = nullptr)
 277       : F(Func), BPI(BPI_), BFI(BFI_) {
 278     buildEdges();
 279     sortEdgesByWeight();
 280     computeMinimumSpanningTree();
 281   }
 282 };
 283
 284 } // end namespace llvm
 285
 286 #undef DEBUG_TYPE // "cfgmst"
 287
 288 #endif // LLVM_LIB_TRANSFORMS_INSTRUMENTATION_CFGMST_H