lib/Transforms/Instrumentation/CFGMST.h - llvm - Git at Google

 //===-- CFGMST.h - Minimum Spanning Tree for CFG ----------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file implements a Union-find algorithm to compute Minimum Spanning Tree
 // for a given CFG.
 //
 //===----------------------------------------------------------------------===//

 #ifndef LLVM_LIB_TRANSFORMS_INSTRUMENTATION_CFGMST_H
 #define LLVM_LIB_TRANSFORMS_INSTRUMENTATION_CFGMST_H

 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Analysis/BlockFrequencyInfo.h"
 #include "llvm/Analysis/BranchProbabilityInfo.h"
 #include "llvm/Analysis/CFG.h"
 #include "llvm/Support/BranchProbability.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include <utility>
 #include <vector>

 #define DEBUG_TYPE "cfgmst"

 namespace llvm {

 /// An union-find based Minimum Spanning Tree for CFG
 ///
 /// Implements a Union-find algorithm to compute Minimum Spanning Tree
 /// for a given CFG.
 template <class Edge, class BBInfo> class CFGMST {
 public:
   Function &F;

   // Store all the edges in CFG. It may contain some stale edges
   // when Removed is set.
   std::vector<std::unique_ptr<Edge>> AllEdges;

   // This map records the auxiliary information for each BB.
   DenseMap<const BasicBlock *, std::unique_ptr<BBInfo>> BBInfos;

   // Whehter the function has an exit block with no successors.
   // (For function with an infinite loop, this block may be absent)
   bool ExitBlockFound = false;

   // Find the root group of the G and compress the path from G to the root.
   BBInfo *findAndCompressGroup(BBInfo *G) {
     if (G->Group != G)
       G->Group = findAndCompressGroup(static_cast<BBInfo *>(G->Group));
     return static_cast<BBInfo *>(G->Group);
   }

   // Union BB1 and BB2 into the same group and return true.
   // Returns false if BB1 and BB2 are already in the same group.
   bool unionGroups(const BasicBlock *BB1, const BasicBlock *BB2) {
     BBInfo *BB1G = findAndCompressGroup(&getBBInfo(BB1));
     BBInfo *BB2G = findAndCompressGroup(&getBBInfo(BB2));

     if (BB1G == BB2G)
       return false;

     // Make the smaller rank tree a direct child or the root of high rank tree.
     if (BB1G->Rank < BB2G->Rank)
       BB1G->Group = BB2G;
     else {
       BB2G->Group = BB1G;
       // If the ranks are the same, increment root of one tree by one.
       if (BB1G->Rank == BB2G->Rank)
         BB1G->Rank++;
     }
     return true;
   }

   // Give BB, return the auxiliary information.
   BBInfo &getBBInfo(const BasicBlock *BB) const {
     auto It = BBInfos.find(BB);
     assert(It->second.get() != nullptr);
     return *It->second.get();
   }

   // Give BB, return the auxiliary information if it's available.
   BBInfo *findBBInfo(const BasicBlock *BB) const {
     auto It = BBInfos.find(BB);
     if (It == BBInfos.end())
       return nullptr;
     return It->second.get();
   }

   // Traverse the CFG using a stack. Find all the edges and assign the weight.
   // Edges with large weight will be put into MST first so they are less likely
   // to be instrumented.
   void buildEdges() {
     LLVM_DEBUG(dbgs() << "Build Edge on " << F.getName() << "\n");

     const BasicBlock *Entry = &(F.getEntryBlock());
     uint64_t EntryWeight = (BFI != nullptr ? BFI->getEntryFreq() : 2);
     Edge *EntryIncoming = nullptr, *EntryOutgoing = nullptr,
         *ExitOutgoing = nullptr, *ExitIncoming = nullptr;
     uint64_t MaxEntryOutWeight = 0, MaxExitOutWeight = 0, MaxExitInWeight = 0;

     // Add a fake edge to the entry.
     EntryIncoming = &addEdge(nullptr, Entry, EntryWeight);
     LLVM_DEBUG(dbgs() << "  Edge: from fake node to " << Entry->getName()
                       << " w = " << EntryWeight << "\n");

     // Special handling for single BB functions.
     if (succ_empty(Entry)) {
       addEdge(Entry, nullptr, EntryWeight);
       return;
     }

     static const uint32_t CriticalEdgeMultiplier = 1000;

     for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
       Instruction *TI = BB->getTerminator();
       uint64_t BBWeight =
           (BFI != nullptr ? BFI->getBlockFreq(&*BB).getFrequency() : 2);
       uint64_t Weight = 2;
       if (int successors = TI->getNumSuccessors()) {
         for (int i = 0; i != successors; ++i) {
           BasicBlock *TargetBB = TI->getSuccessor(i);
           bool Critical = isCriticalEdge(TI, i);
           uint64_t scaleFactor = BBWeight;
           if (Critical) {
             if (scaleFactor < UINT64_MAX / CriticalEdgeMultiplier)
               scaleFactor *= CriticalEdgeMultiplier;
             else
               scaleFactor = UINT64_MAX;
           }
           if (BPI != nullptr)
             Weight = BPI->getEdgeProbability(&*BB, TargetBB).scale(scaleFactor);
           auto *E = &addEdge(&*BB, TargetBB, Weight);
           E->IsCritical = Critical;
           LLVM_DEBUG(dbgs() << "  Edge: from " << BB->getName() << " to "
                             << TargetBB->getName() << "  w=" << Weight << "\n");

           // Keep track of entry/exit edges:
           if (&*BB == Entry) {
             if (Weight > MaxEntryOutWeight) {
               MaxEntryOutWeight = Weight;
               EntryOutgoing = E;
             }
           }

           auto *TargetTI = TargetBB->getTerminator();
           if (TargetTI && !TargetTI->getNumSuccessors()) {
             if (Weight > MaxExitInWeight) {
               MaxExitInWeight = Weight;
               ExitIncoming = E;
             }
           }
         }
       } else {
         ExitBlockFound = true;
         Edge *ExitO = &addEdge(&*BB, nullptr, BBWeight);
         if (BBWeight > MaxExitOutWeight) {
           MaxExitOutWeight = BBWeight;
           ExitOutgoing = ExitO;
         }
         LLVM_DEBUG(dbgs() << "  Edge: from " << BB->getName() << " to fake exit"
                           << " w = " << BBWeight << "\n");
       }
     }

     // Entry/exit edge adjustment heurisitic:
     // prefer instrumenting entry edge over exit edge
     // if possible. Those exit edges may never have a chance to be
     // executed (for instance the program is an event handling loop)
     // before the profile is asynchronously dumped.
     //
     // If EntryIncoming and ExitOutgoing has similar weight, make sure
     // ExitOutging is selected as the min-edge. Similarly, if EntryOutgoing
     // and ExitIncoming has similar weight, make sure ExitIncoming becomes
     // the min-edge.
     uint64_t EntryInWeight = EntryWeight;

     if (EntryInWeight >= MaxExitOutWeight &&
         EntryInWeight * 2 < MaxExitOutWeight * 3) {
       EntryIncoming->Weight = MaxExitOutWeight;
       ExitOutgoing->Weight = EntryInWeight + 1;
     }

     if (MaxEntryOutWeight >= MaxExitInWeight &&
         MaxEntryOutWeight * 2 < MaxExitInWeight * 3) {
       EntryOutgoing->Weight = MaxExitInWeight;
       ExitIncoming->Weight = MaxEntryOutWeight + 1;
     }
   }

   // Sort CFG edges based on its weight.
   void sortEdgesByWeight() {
     llvm::stable_sort(AllEdges, [](const std::unique_ptr<Edge> &Edge1,
                                    const std::unique_ptr<Edge> &Edge2) {
       return Edge1->Weight > Edge2->Weight;
     });
   }

   // Traverse all the edges and compute the Minimum Weight Spanning Tree
   // using union-find algorithm.
   void computeMinimumSpanningTree() {
     // First, put all the critical edge with landing-pad as the Dest to MST.
     // This works around the insufficient support of critical edges split
     // when destination BB is a landing pad.
     for (auto &Ei : AllEdges) {
       if (Ei->Removed)
         continue;
       if (Ei->IsCritical) {
         if (Ei->DestBB && Ei->DestBB->isLandingPad()) {
           if (unionGroups(Ei->SrcBB, Ei->DestBB))
             Ei->InMST = true;
         }
       }
     }

     for (auto &Ei : AllEdges) {
       if (Ei->Removed)
         continue;
       // If we detect infinite loops, force
       // instrumenting the entry edge:
       if (!ExitBlockFound && Ei->SrcBB == nullptr)
         continue;
       if (unionGroups(Ei->SrcBB, Ei->DestBB))
         Ei->InMST = true;
     }
   }

   // Dump the Debug information about the instrumentation.
   void dumpEdges(raw_ostream &OS, const Twine &Message) const {
     if (!Message.str().empty())
       OS << Message << "\n";
     OS << "  Number of Basic Blocks: " << BBInfos.size() << "\n";
     for (auto &BI : BBInfos) {
       const BasicBlock *BB = BI.first;
       OS << "  BB: " << (BB == nullptr ? "FakeNode" : BB->getName()) << "  "
          << BI.second->infoString() << "\n";
     }

     OS << "  Number of Edges: " << AllEdges.size()
        << " (*: Instrument, C: CriticalEdge, -: Removed)\n";
     uint32_t Count = 0;
     for (auto &EI : AllEdges)
       OS << "  Edge " << Count++ << ": " << getBBInfo(EI->SrcBB).Index << "-->"
          << getBBInfo(EI->DestBB).Index << EI->infoString() << "\n";
   }

   // Add an edge to AllEdges with weight W.
   Edge &addEdge(const BasicBlock *Src, const BasicBlock *Dest, uint64_t W) {
     uint32_t Index = BBInfos.size();
     auto Iter = BBInfos.end();
     bool Inserted;
     std::tie(Iter, Inserted) = BBInfos.insert(std::make_pair(Src, nullptr));
     if (Inserted) {
       // Newly inserted, update the real info.
       Iter->second = std::move(std::make_unique<BBInfo>(Index));
       Index++;
     }
     std::tie(Iter, Inserted) = BBInfos.insert(std::make_pair(Dest, nullptr));
     if (Inserted)
       // Newly inserted, update the real info.
       Iter->second = std::move(std::make_unique<BBInfo>(Index));
     AllEdges.emplace_back(new Edge(Src, Dest, W));
     return *AllEdges.back();
   }

   BranchProbabilityInfo *BPI;
   BlockFrequencyInfo *BFI;

 public:
   CFGMST(Function &Func, BranchProbabilityInfo *BPI_ = nullptr,
          BlockFrequencyInfo *BFI_ = nullptr)
       : F(Func), BPI(BPI_), BFI(BFI_) {
     buildEdges();
     sortEdgesByWeight();
     computeMinimumSpanningTree();
   }
 };

 } // end namespace llvm

 #undef DEBUG_TYPE // "cfgmst"

 #endif // LLVM_LIB_TRANSFORMS_INSTRUMENTATION_CFGMST_H
	//===-- CFGMST.h - Minimum Spanning Tree for CFG ----------------- C++ --===//
	//
	// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
	// See https://llvm.org/LICENSE.txt for license information.
	// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
	//
	//===----------------------------------------------------------------------===//
	//
	// This file implements a Union-find algorithm to compute Minimum Spanning Tree
	// for a given CFG.
	//
	//===----------------------------------------------------------------------===//

	#ifndef LLVM_LIB_TRANSFORMS_INSTRUMENTATION_CFGMST_H
	#define LLVM_LIB_TRANSFORMS_INSTRUMENTATION_CFGMST_H

	#include "llvm/ADT/DenseMap.h"
	#include "llvm/ADT/STLExtras.h"
	#include "llvm/Analysis/BlockFrequencyInfo.h"
	#include "llvm/Analysis/BranchProbabilityInfo.h"
	#include "llvm/Analysis/CFG.h"
	#include "llvm/Support/BranchProbability.h"
	#include "llvm/Support/Debug.h"
	#include "llvm/Support/raw_ostream.h"
	#include "llvm/Transforms/Utils/BasicBlockUtils.h"
	#include <utility>
	#include <vector>

	#define DEBUG_TYPE "cfgmst"

	namespace llvm {

	/// An union-find based Minimum Spanning Tree for CFG
	///
	/// Implements a Union-find algorithm to compute Minimum Spanning Tree
	/// for a given CFG.
	template <class Edge, class BBInfo> class CFGMST {
	public:
	Function &F;

	// Store all the edges in CFG. It may contain some stale edges
	// when Removed is set.
	std::vector<std::unique_ptr<Edge>> AllEdges;

	// This map records the auxiliary information for each BB.
	DenseMap<const BasicBlock *, std::unique_ptr<BBInfo>> BBInfos;

	// Whehter the function has an exit block with no successors.
	// (For function with an infinite loop, this block may be absent)
	bool ExitBlockFound = false;

	// Find the root group of the G and compress the path from G to the root.
	BBInfo findAndCompressGroup(BBInfo G) {
	if (G->Group != G)
	G->Group = findAndCompressGroup(static_cast<BBInfo *>(G->Group));
	return static_cast<BBInfo *>(G->Group);
	}

	// Union BB1 and BB2 into the same group and return true.
	// Returns false if BB1 and BB2 are already in the same group.
	bool unionGroups(const BasicBlock BB1, const BasicBlock BB2) {
	BBInfo *BB1G = findAndCompressGroup(&getBBInfo(BB1));
	BBInfo *BB2G = findAndCompressGroup(&getBBInfo(BB2));

	if (BB1G == BB2G)
	return false;

	// Make the smaller rank tree a direct child or the root of high rank tree.
	if (BB1G->Rank < BB2G->Rank)
	BB1G->Group = BB2G;
	else {
	BB2G->Group = BB1G;
	// If the ranks are the same, increment root of one tree by one.
	if (BB1G->Rank == BB2G->Rank)
	BB1G->Rank++;
	}
	return true;
	}

	// Give BB, return the auxiliary information.
	BBInfo &getBBInfo(const BasicBlock *BB) const {
	auto It = BBInfos.find(BB);
	assert(It->second.get() != nullptr);
	return *It->second.get();
	}

	// Give BB, return the auxiliary information if it's available.
	BBInfo findBBInfo(const BasicBlock BB) const {
	auto It = BBInfos.find(BB);
	if (It == BBInfos.end())
	return nullptr;
	return It->second.get();
	}

	// Traverse the CFG using a stack. Find all the edges and assign the weight.
	// Edges with large weight will be put into MST first so they are less likely
	// to be instrumented.
	void buildEdges() {
	LLVM_DEBUG(dbgs() << "Build Edge on " << F.getName() << "\n");

	const BasicBlock *Entry = &(F.getEntryBlock());
	uint64_t EntryWeight = (BFI != nullptr ? BFI->getEntryFreq() : 2);
	Edge EntryIncoming = nullptr, EntryOutgoing = nullptr,
	ExitOutgoing = nullptr, ExitIncoming = nullptr;
	uint64_t MaxEntryOutWeight = 0, MaxExitOutWeight = 0, MaxExitInWeight = 0;

	// Add a fake edge to the entry.
	EntryIncoming = &addEdge(nullptr, Entry, EntryWeight);
	LLVM_DEBUG(dbgs() << " Edge: from fake node to " << Entry->getName()
	<< " w = " << EntryWeight << "\n");

	// Special handling for single BB functions.
	if (succ_empty(Entry)) {
	addEdge(Entry, nullptr, EntryWeight);
	return;
	}

	static const uint32_t CriticalEdgeMultiplier = 1000;

	for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
	Instruction *TI = BB->getTerminator();
	uint64_t BBWeight =
	(BFI != nullptr ? BFI->getBlockFreq(&*BB).getFrequency() : 2);
	uint64_t Weight = 2;
	if (int successors = TI->getNumSuccessors()) {
	for (int i = 0; i != successors; ++i) {
	BasicBlock *TargetBB = TI->getSuccessor(i);
	bool Critical = isCriticalEdge(TI, i);
	uint64_t scaleFactor = BBWeight;
	if (Critical) {
	if (scaleFactor < UINT64_MAX / CriticalEdgeMultiplier)
	scaleFactor *= CriticalEdgeMultiplier;
	else
	scaleFactor = UINT64_MAX;
	}
	if (BPI != nullptr)
	Weight = BPI->getEdgeProbability(&*BB, TargetBB).scale(scaleFactor);
	auto E = &addEdge(&BB, TargetBB, Weight);
	E->IsCritical = Critical;
	LLVM_DEBUG(dbgs() << " Edge: from " << BB->getName() << " to "
	<< TargetBB->getName() << " w=" << Weight << "\n");

	// Keep track of entry/exit edges:
	if (&*BB == Entry) {
	if (Weight > MaxEntryOutWeight) {
	MaxEntryOutWeight = Weight;
	EntryOutgoing = E;
	}
	}

	auto *TargetTI = TargetBB->getTerminator();
	if (TargetTI && !TargetTI->getNumSuccessors()) {
	if (Weight > MaxExitInWeight) {
	MaxExitInWeight = Weight;
	ExitIncoming = E;
	}
	}
	}
	} else {
	ExitBlockFound = true;
	Edge ExitO = &addEdge(&BB, nullptr, BBWeight);
	if (BBWeight > MaxExitOutWeight) {
	MaxExitOutWeight = BBWeight;
	ExitOutgoing = ExitO;
	}
	LLVM_DEBUG(dbgs() << " Edge: from " << BB->getName() << " to fake exit"
	<< " w = " << BBWeight << "\n");
	}
	}

	// Entry/exit edge adjustment heurisitic:
	// prefer instrumenting entry edge over exit edge
	// if possible. Those exit edges may never have a chance to be
	// executed (for instance the program is an event handling loop)
	// before the profile is asynchronously dumped.
	//
	// If EntryIncoming and ExitOutgoing has similar weight, make sure
	// ExitOutging is selected as the min-edge. Similarly, if EntryOutgoing
	// and ExitIncoming has similar weight, make sure ExitIncoming becomes
	// the min-edge.
	uint64_t EntryInWeight = EntryWeight;

	if (EntryInWeight >= MaxExitOutWeight &&
	EntryInWeight * 2 < MaxExitOutWeight * 3) {
	EntryIncoming->Weight = MaxExitOutWeight;
	ExitOutgoing->Weight = EntryInWeight + 1;
	}

	if (MaxEntryOutWeight >= MaxExitInWeight &&
	MaxEntryOutWeight * 2 < MaxExitInWeight * 3) {
	EntryOutgoing->Weight = MaxExitInWeight;
	ExitIncoming->Weight = MaxEntryOutWeight + 1;
	}
	}

	// Sort CFG edges based on its weight.
	void sortEdgesByWeight() {
	llvm::stable_sort(AllEdges, [](const std::unique_ptr<Edge> &Edge1,
	const std::unique_ptr<Edge> &Edge2) {
	return Edge1->Weight > Edge2->Weight;
	});
	}

	// Traverse all the edges and compute the Minimum Weight Spanning Tree
	// using union-find algorithm.
	void computeMinimumSpanningTree() {
	// First, put all the critical edge with landing-pad as the Dest to MST.
	// This works around the insufficient support of critical edges split
	// when destination BB is a landing pad.
	for (auto &Ei : AllEdges) {
	if (Ei->Removed)
	continue;
	if (Ei->IsCritical) {
	if (Ei->DestBB && Ei->DestBB->isLandingPad()) {
	if (unionGroups(Ei->SrcBB, Ei->DestBB))
	Ei->InMST = true;
	}
	}
	}

	for (auto &Ei : AllEdges) {
	if (Ei->Removed)
	continue;
	// If we detect infinite loops, force
	// instrumenting the entry edge:
	if (!ExitBlockFound && Ei->SrcBB == nullptr)
	continue;
	if (unionGroups(Ei->SrcBB, Ei->DestBB))
	Ei->InMST = true;
	}
	}

	// Dump the Debug information about the instrumentation.
	void dumpEdges(raw_ostream &OS, const Twine &Message) const {
	if (!Message.str().empty())
	OS << Message << "\n";
	OS << " Number of Basic Blocks: " << BBInfos.size() << "\n";
	for (auto &BI : BBInfos) {
	const BasicBlock *BB = BI.first;
	OS << " BB: " << (BB == nullptr ? "FakeNode" : BB->getName()) << " "
	<< BI.second->infoString() << "\n";
	}

	OS << " Number of Edges: " << AllEdges.size()
	<< " (*: Instrument, C: CriticalEdge, -: Removed)\n";
	uint32_t Count = 0;
	for (auto &EI : AllEdges)
	OS << " Edge " << Count++ << ": " << getBBInfo(EI->SrcBB).Index << "-->"
	<< getBBInfo(EI->DestBB).Index << EI->infoString() << "\n";
	}

	// Add an edge to AllEdges with weight W.
	Edge &addEdge(const BasicBlock Src, const BasicBlock Dest, uint64_t W) {
	uint32_t Index = BBInfos.size();
	auto Iter = BBInfos.end();
	bool Inserted;
	std::tie(Iter, Inserted) = BBInfos.insert(std::make_pair(Src, nullptr));
	if (Inserted) {
	// Newly inserted, update the real info.
	Iter->second = std::move(std::make_unique<BBInfo>(Index));
	Index++;
	}
	std::tie(Iter, Inserted) = BBInfos.insert(std::make_pair(Dest, nullptr));
	if (Inserted)
	// Newly inserted, update the real info.
	Iter->second = std::move(std::make_unique<BBInfo>(Index));
	AllEdges.emplace_back(new Edge(Src, Dest, W));
	return *AllEdges.back();
	}

	BranchProbabilityInfo *BPI;
	BlockFrequencyInfo *BFI;

	public:
	CFGMST(Function &Func, BranchProbabilityInfo *BPI_ = nullptr,
	BlockFrequencyInfo *BFI_ = nullptr)
	: F(Func), BPI(BPI_), BFI(BFI_) {
	buildEdges();
	sortEdgesByWeight();
	computeMinimumSpanningTree();
	}
	};

	} // end namespace llvm

	#undef DEBUG_TYPE // "cfgmst"

	#endif // LLVM_LIB_TRANSFORMS_INSTRUMENTATION_CFGMST_H