lib/Transforms/Instrumentation/GCOVProfiling.cpp

//===- GCOVProfiling.cpp - Insert edge counters for gcov profiling --------===//
//
//                      The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass implements GCOV-style profiling. When this pass is run it emits
// "gcno" files next to the existing source, and instruments the code that runs
// to records the edges between blocks that run and emit a complementary "gcda"
// file on exit.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "insert-gcov-profiling"

#include "ProfilingUtils.h"
#include "llvm/Transforms/Instrumentation.h"
#include "llvm/DebugInfo.h"
#include "llvm/IRBuilder.h"
#include "llvm/Instructions.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/ADT/UniqueVector.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/DebugLoc.h"
#include "llvm/Support/InstIterator.h"
#include "llvm/Support/PathV2.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/ModuleUtils.h"
#include <string>
#include <utility>
using namespace llvm;

namespace {
  class GCOVProfiler : public ModulePass {
  public:
    static char ID;
    GCOVProfiler()
        : ModulePass(ID), EmitNotes(true), EmitData(true), Use402Format(false),
          UseExtraChecksum(false) {
      initializeGCOVProfilerPass(*PassRegistry::getPassRegistry());
    }
    GCOVProfiler(bool EmitNotes, bool EmitData, bool use402Format = false,
                 bool useExtraChecksum = false)
        : ModulePass(ID), EmitNotes(EmitNotes), EmitData(EmitData),
          Use402Format(use402Format), UseExtraChecksum(useExtraChecksum) {
      assert((EmitNotes || EmitData) && "GCOVProfiler asked to do nothing?");
      initializeGCOVProfilerPass(*PassRegistry::getPassRegistry());
    }
    virtual const char *getPassName() const {
      return "GCOV Profiler";
    }
  private:
    bool runOnModule(Module &M);

    // Create the GCNO files for the Module based on DebugInfo.
    void emitGCNO();

    // Modify the program to track transitions along edges and call into the
    // profiling runtime to emit .gcda files when run.
    bool emitProfileArcs();

    // Get pointers to the functions in the runtime library.
    Constant *getStartFileFunc();
    Constant *getIncrementIndirectCounterFunc();
    Constant *getEmitFunctionFunc();
    Constant *getEmitArcsFunc();
    Constant *getEndFileFunc();

    // Create or retrieve an i32 state value that is used to represent the
    // pred block number for certain non-trivial edges.
    GlobalVariable *getEdgeStateValue();

    // Produce a table of pointers to counters, by predecessor and successor
    // block number.
    GlobalVariable *buildEdgeLookupTable(Function *F,
                                         GlobalVariable *Counter,
                                         const UniqueVector<BasicBlock *> &Preds,
                                         const UniqueVector<BasicBlock *> &Succs);

    // Add the function to write out all our counters to the global destructor
    // list.
    void insertCounterWriteout(ArrayRef<std::pair<GlobalVariable*, MDNode*> >);
    void insertIndirectCounterIncrement();

    std::string mangleName(DICompileUnit CU, std::string NewStem);

    bool EmitNotes;
    bool EmitData;
    bool Use402Format;
    bool UseExtraChecksum;

    Module *M;
    LLVMContext *Ctx;
  };
}

char GCOVProfiler::ID = 0;
INITIALIZE_PASS(GCOVProfiler, "insert-gcov-profiling",
                "Insert instrumentation for GCOV profiling", false, false)

ModulePass *llvm::createGCOVProfilerPass(bool EmitNotes, bool EmitData,
                                         bool Use402Format,
                                         bool UseExtraChecksum) {
  return new GCOVProfiler(EmitNotes, EmitData, Use402Format, UseExtraChecksum);
}

namespace {
  class GCOVRecord {
   protected:
    static const char *LinesTag;
    static const char *FunctionTag;
    static const char *BlockTag;
    static const char *EdgeTag;

    GCOVRecord() {}

    void writeBytes(const char *Bytes, int Size) {
      os->write(Bytes, Size);
    }

    void write(uint32_t i) {
      writeBytes(reinterpret_cast<char*>(&i), 4);
    }

    // Returns the length measured in 4-byte blocks that will be used to
    // represent this string in a GCOV file
    unsigned lengthOfGCOVString(StringRef s) {
      // A GCOV string is a length, followed by a NUL, then between 0 and 3 NULs
      // padding out to the next 4-byte word. The length is measured in 4-byte
      // words including padding, not bytes of actual string.
      return (s.size() / 4) + 1;
    }

    void writeGCOVString(StringRef s) {
      uint32_t Len = lengthOfGCOVString(s);
      write(Len);
      writeBytes(s.data(), s.size());

      // Write 1 to 4 bytes of NUL padding.
      assert((unsigned)(4 - (s.size() % 4)) > 0);
      assert((unsigned)(4 - (s.size() % 4)) <= 4);
      writeBytes("\0\0\0\0", 4 - (s.size() % 4));
    }

    raw_ostream *os;
  };
  const char *GCOVRecord::LinesTag = "\0\0\x45\x01";
  const char *GCOVRecord::FunctionTag = "\0\0\0\1";
  const char *GCOVRecord::BlockTag = "\0\0\x41\x01";
  const char *GCOVRecord::EdgeTag = "\0\0\x43\x01";

  class GCOVFunction;
  class GCOVBlock;

  // Constructed only by requesting it from a GCOVBlock, this object stores a
  // list of line numbers and a single filename, representing lines that belong
  // to the block.
  class GCOVLines : public GCOVRecord {
   public:
    void addLine(uint32_t Line) {
      Lines.push_back(Line);
    }

    uint32_t length() {
      // Here 2 = 1 for string length + 1 for '0' id#.
      return lengthOfGCOVString(Filename) + 2 + Lines.size();
    }

    void writeOut() {
      write(0);
      writeGCOVString(Filename);
      for (int i = 0, e = Lines.size(); i != e; ++i)
        write(Lines[i]);
    }

    GCOVLines(StringRef F, raw_ostream *os) 
      : Filename(F) {
      this->os = os;
    }

   private:
    StringRef Filename;
    SmallVector<uint32_t, 32> Lines;
  };

  // Represent a basic block in GCOV. Each block has a unique number in the
  // function, number of lines belonging to each block, and a set of edges to
  // other blocks.
  class GCOVBlock : public GCOVRecord {
   public:
    GCOVLines &getFile(StringRef Filename) {
      GCOVLines *&Lines = LinesByFile[Filename];
      if (!Lines) {
        Lines = new GCOVLines(Filename, os);
      }
      return *Lines;
    }

    void addEdge(GCOVBlock &Successor) {
      OutEdges.push_back(&Successor);
    }

    void writeOut() {
      uint32_t Len = 3;
      for (StringMap<GCOVLines *>::iterator I = LinesByFile.begin(),
               E = LinesByFile.end(); I != E; ++I) {
        Len += I->second->length();
      }

      writeBytes(LinesTag, 4);
      write(Len);
      write(Number);
      for (StringMap<GCOVLines *>::iterator I = LinesByFile.begin(),
               E = LinesByFile.end(); I != E; ++I) 
        I->second->writeOut();
      write(0);
      write(0);
    }

    ~GCOVBlock() {
      DeleteContainerSeconds(LinesByFile);
    }

   private:
    friend class GCOVFunction;

    GCOVBlock(uint32_t Number, raw_ostream *os)
        : Number(Number) {
      this->os = os;
    }

    uint32_t Number;
    StringMap<GCOVLines *> LinesByFile;
    SmallVector<GCOVBlock *, 4> OutEdges;
  };

  // A function has a unique identifier, a checksum (we leave as zero) and a
  // set of blocks and a map of edges between blocks. This is the only GCOV
  // object users can construct, the blocks and lines will be rooted here.
  class GCOVFunction : public GCOVRecord {
   public:
    GCOVFunction(DISubprogram SP, raw_ostream *os,
                 bool Use402Format, bool UseExtraChecksum) {
      this->os = os;

      Function *F = SP.getFunction();
      DEBUG(dbgs() << "Function: " << F->getName() << "\n");
      uint32_t i = 0;
      for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
        Blocks[BB] = new GCOVBlock(i++, os);
      }
      ReturnBlock = new GCOVBlock(i++, os);

      writeBytes(FunctionTag, 4);
      uint32_t BlockLen = 1 + 1 + 1 + lengthOfGCOVString(SP.getName()) +
          1 + lengthOfGCOVString(SP.getFilename()) + 1;
      if (UseExtraChecksum)
        ++BlockLen;
      write(BlockLen);
      uint32_t Ident = reinterpret_cast<intptr_t>((MDNode*)SP);
      write(Ident);
      write(0);  // lineno checksum
      if (UseExtraChecksum)
        write(0);  // cfg checksum
      writeGCOVString(SP.getName());
      writeGCOVString(SP.getFilename());
      write(SP.getLineNumber());
    }

    ~GCOVFunction() {
      DeleteContainerSeconds(Blocks);
      delete ReturnBlock;
    }

    GCOVBlock &getBlock(BasicBlock *BB) {
      return *Blocks[BB];
    }

    GCOVBlock &getReturnBlock() {
      return *ReturnBlock;
    }

    void writeOut() {
      // Emit count of blocks.
      writeBytes(BlockTag, 4);
      write(Blocks.size() + 1);
      for (int i = 0, e = Blocks.size() + 1; i != e; ++i) {
        write(0);  // No flags on our blocks.
      }
      DEBUG(dbgs() << Blocks.size() << " blocks.\n");

      // Emit edges between blocks.
      for (DenseMap<BasicBlock *, GCOVBlock *>::iterator I = Blocks.begin(),
               E = Blocks.end(); I != E; ++I) {
        GCOVBlock &Block = *I->second;
        if (Block.OutEdges.empty()) continue;

        writeBytes(EdgeTag, 4);
        write(Block.OutEdges.size() * 2 + 1);
        write(Block.Number);
        for (int i = 0, e = Block.OutEdges.size(); i != e; ++i) {
          DEBUG(dbgs() << Block.Number << " -> " << Block.OutEdges[i]->Number
                       << "\n");
          write(Block.OutEdges[i]->Number);
          write(0);  // no flags
        }
      }

      // Emit lines for each block.
      for (DenseMap<BasicBlock *, GCOVBlock *>::iterator I = Blocks.begin(),
               E = Blocks.end(); I != E; ++I) {
        I->second->writeOut();
      }
    }

   private:
    DenseMap<BasicBlock *, GCOVBlock *> Blocks;
    GCOVBlock *ReturnBlock;
  };
}

std::string GCOVProfiler::mangleName(DICompileUnit CU, std::string NewStem) {
  if (NamedMDNode *GCov = M->getNamedMetadata("llvm.gcov")) {
    for (int i = 0, e = GCov->getNumOperands(); i != e; ++i) {
      MDNode *N = GCov->getOperand(i);
      if (N->getNumOperands() != 2) continue;
      MDString *GCovFile = dyn_cast<MDString>(N->getOperand(0));
      MDNode *CompileUnit = dyn_cast<MDNode>(N->getOperand(1));
      if (!GCovFile || !CompileUnit) continue;
      if (CompileUnit == CU) {
        SmallString<128> Filename = GCovFile->getString();
        sys::path::replace_extension(Filename, NewStem);
        return Filename.str();
      }
    }
  }

  SmallString<128> Filename = CU.getFilename();
  sys::path::replace_extension(Filename, NewStem);
  return sys::path::filename(Filename.str());
}

bool GCOVProfiler::runOnModule(Module &M) {
  this->M = &M;
  Ctx = &M.getContext();

  if (EmitNotes) emitGCNO();
  if (EmitData) return emitProfileArcs();
  return false;
}

void GCOVProfiler::emitGCNO() {
  NamedMDNode *CU_Nodes = M->getNamedMetadata("llvm.dbg.cu");
  if (!CU_Nodes) return;

  for (unsigned i = 0, e = CU_Nodes->getNumOperands(); i != e; ++i) {
    // Each compile unit gets its own .gcno file. This means that whether we run
    // this pass over the original .o's as they're produced, or run it after
    // LTO, we'll generate the same .gcno files.

    DICompileUnit CU(CU_Nodes->getOperand(i));
    std::string ErrorInfo;
    raw_fd_ostream out(mangleName(CU, "gcno").c_str(), ErrorInfo,
                       raw_fd_ostream::F_Binary);
    if (!Use402Format)
      out.write("oncg*404MVLL", 12);
    else
      out.write("oncg*204MVLL", 12);

    DIArray SPs = CU.getSubprograms();
    for (unsigned i = 0, e = SPs.getNumElements(); i != e; ++i) {
      DISubprogram SP(SPs.getElement(i));
      if (!SP.Verify()) continue;

      Function *F = SP.getFunction();
      if (!F) continue;
      GCOVFunction Func(SP, &out, Use402Format, UseExtraChecksum);

      for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
        GCOVBlock &Block = Func.getBlock(BB);
        TerminatorInst *TI = BB->getTerminator();
        if (int successors = TI->getNumSuccessors()) {
          for (int i = 0; i != successors; ++i) {
            Block.addEdge(Func.getBlock(TI->getSuccessor(i)));
          }
        } else if (isa<ReturnInst>(TI)) {
          Block.addEdge(Func.getReturnBlock());
        }

        uint32_t Line = 0;
        for (BasicBlock::iterator I = BB->begin(), IE = BB->end();
             I != IE; ++I) {
          const DebugLoc &Loc = I->getDebugLoc();
          if (Loc.isUnknown()) continue;
          if (Line == Loc.getLine()) continue;
          Line = Loc.getLine();
          if (SP != getDISubprogram(Loc.getScope(*Ctx))) continue;

          GCOVLines &Lines = Block.getFile(SP.getFilename());
          Lines.addLine(Loc.getLine());
        }
      }
      Func.writeOut();
    }
    out.write("\0\0\0\0\0\0\0\0", 8);  // EOF
    out.close();
  }
}

bool GCOVProfiler::emitProfileArcs() {
  NamedMDNode *CU_Nodes = M->getNamedMetadata("llvm.dbg.cu");
  if (!CU_Nodes) return false;

  bool Result = false;  
  bool InsertIndCounterIncrCode = false;
  for (unsigned i = 0, e = CU_Nodes->getNumOperands(); i != e; ++i) {
    DICompileUnit CU(CU_Nodes->getOperand(i));
    DIArray SPs = CU.getSubprograms();
    SmallVector<std::pair<GlobalVariable *, MDNode *>, 8> CountersBySP;
    for (unsigned i = 0, e = SPs.getNumElements(); i != e; ++i) {
      DISubprogram SP(SPs.getElement(i));
      if (!SP.Verify()) continue;
      Function *F = SP.getFunction();
      if (!F) continue;
      if (!Result) Result = true;
      unsigned Edges = 0;
      for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
        TerminatorInst *TI = BB->getTerminator();
        if (isa<ReturnInst>(TI))
          ++Edges;
        else
          Edges += TI->getNumSuccessors();
      }
      
      ArrayType *CounterTy =
        ArrayType::get(Type::getInt64Ty(*Ctx), Edges);
      GlobalVariable *Counters =
        new GlobalVariable(*M, CounterTy, false,
                           GlobalValue::InternalLinkage,
                           Constant::getNullValue(CounterTy),
                           "__llvm_gcov_ctr");
      CountersBySP.push_back(std::make_pair(Counters, (MDNode*)SP));
      
      UniqueVector<BasicBlock *> ComplexEdgePreds;
      UniqueVector<BasicBlock *> ComplexEdgeSuccs;
      
      unsigned Edge = 0;
      for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
        TerminatorInst *TI = BB->getTerminator();
        int Successors = isa<ReturnInst>(TI) ? 1 : TI->getNumSuccessors();
        if (Successors) {
          IRBuilder<> Builder(TI);
          
          if (Successors == 1) {
            Value *Counter = Builder.CreateConstInBoundsGEP2_64(Counters, 0,
                                                                Edge);
            Value *Count = Builder.CreateLoad(Counter);
            Count = Builder.CreateAdd(Count,
                                      ConstantInt::get(Type::getInt64Ty(*Ctx),1));
            Builder.CreateStore(Count, Counter);
          } else if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
            Value *Sel = Builder.CreateSelect(
              BI->getCondition(),
              ConstantInt::get(Type::getInt64Ty(*Ctx), Edge),
              ConstantInt::get(Type::getInt64Ty(*Ctx), Edge + 1));
            SmallVector<Value *, 2> Idx;
            Idx.push_back(Constant::getNullValue(Type::getInt64Ty(*Ctx)));
            Idx.push_back(Sel);
            Value *Counter = Builder.CreateInBoundsGEP(Counters, Idx);
            Value *Count = Builder.CreateLoad(Counter);
            Count = Builder.CreateAdd(Count,
                                      ConstantInt::get(Type::getInt64Ty(*Ctx),1));
            Builder.CreateStore(Count, Counter);
          } else {
            ComplexEdgePreds.insert(BB);
            for (int i = 0; i != Successors; ++i)
              ComplexEdgeSuccs.insert(TI->getSuccessor(i));
          }
          Edge += Successors;
        }
      }
      
      if (!ComplexEdgePreds.empty()) {
        GlobalVariable *EdgeTable =
          buildEdgeLookupTable(F, Counters,
                               ComplexEdgePreds, ComplexEdgeSuccs);
        GlobalVariable *EdgeState = getEdgeStateValue();
        
        Type *Int32Ty = Type::getInt32Ty(*Ctx);
        for (int i = 0, e = ComplexEdgePreds.size(); i != e; ++i) {
          IRBuilder<> Builder(ComplexEdgePreds[i+1]->getTerminator());
          Builder.CreateStore(ConstantInt::get(Int32Ty, i), EdgeState);
        }
        for (int i = 0, e = ComplexEdgeSuccs.size(); i != e; ++i) {
          // call runtime to perform increment
          BasicBlock::iterator InsertPt =
            ComplexEdgeSuccs[i+1]->getFirstInsertionPt();
          IRBuilder<> Builder(InsertPt);
          Value *CounterPtrArray =
            Builder.CreateConstInBoundsGEP2_64(EdgeTable, 0,
                                               i * ComplexEdgePreds.size());

          // Build code to increment the counter.
          InsertIndCounterIncrCode = true;
          Builder.CreateCall2(getIncrementIndirectCounterFunc(),
                              EdgeState, CounterPtrArray);
        }
      }
    }

    insertCounterWriteout(CountersBySP);
  }

  if (InsertIndCounterIncrCode)
    insertIndirectCounterIncrement();

  return Result;
}

// All edges with successors that aren't branches are "complex", because it
// requires complex logic to pick which counter to update.
GlobalVariable *GCOVProfiler::buildEdgeLookupTable(
    Function *F,
    GlobalVariable *Counters,
    const UniqueVector<BasicBlock *> &Preds,
    const UniqueVector<BasicBlock *> &Succs) {
  // TODO: support invoke, threads. We rely on the fact that nothing can modify
  // the whole-Module pred edge# between the time we set it and the time we next
  // read it. Threads and invoke make this untrue.

  // emit [(succs * preds) x i64*], logically [succ x [pred x i64*]].
  Type *Int64PtrTy = Type::getInt64PtrTy(*Ctx);
  ArrayType *EdgeTableTy = ArrayType::get(
      Int64PtrTy, Succs.size() * Preds.size());

  Constant **EdgeTable = new Constant*[Succs.size() * Preds.size()];
  Constant *NullValue = Constant::getNullValue(Int64PtrTy);
  for (int i = 0, ie = Succs.size() * Preds.size(); i != ie; ++i)
    EdgeTable[i] = NullValue;

  unsigned Edge = 0;
  for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
    TerminatorInst *TI = BB->getTerminator();
    int Successors = isa<ReturnInst>(TI) ? 1 : TI->getNumSuccessors();
    if (Successors > 1 && !isa<BranchInst>(TI) && !isa<ReturnInst>(TI)) {
      for (int i = 0; i != Successors; ++i) {
        BasicBlock *Succ = TI->getSuccessor(i);
        IRBuilder<> builder(Succ);
        Value *Counter = builder.CreateConstInBoundsGEP2_64(Counters, 0,
                                                            Edge + i);
        EdgeTable[((Succs.idFor(Succ)-1) * Preds.size()) +
                  (Preds.idFor(BB)-1)] = cast<Constant>(Counter);
      }
    }
    Edge += Successors;
  }

  ArrayRef<Constant*> V(&EdgeTable[0], Succs.size() * Preds.size());
  GlobalVariable *EdgeTableGV =
      new GlobalVariable(
          *M, EdgeTableTy, true, GlobalValue::InternalLinkage,
          ConstantArray::get(EdgeTableTy, V),
          "__llvm_gcda_edge_table");
  EdgeTableGV->setUnnamedAddr(true);
  return EdgeTableGV;
}

Constant *GCOVProfiler::getStartFileFunc() {
  FunctionType *FTy = FunctionType::get(Type::getVoidTy(*Ctx),
                                              Type::getInt8PtrTy(*Ctx), false);
  return M->getOrInsertFunction("llvm_gcda_start_file", FTy);
}

Constant *GCOVProfiler::getIncrementIndirectCounterFunc() {
  Type *Int32Ty = Type::getInt32Ty(*Ctx);
  Type *Int64Ty = Type::getInt64Ty(*Ctx);
  Type *Args[] = {
    Int32Ty->getPointerTo(),                // uint32_t *predecessor
    Int64Ty->getPointerTo()->getPointerTo() // uint64_t **counters
  };
  FunctionType *FTy = FunctionType::get(Type::getVoidTy(*Ctx), Args, false);
  return M->getOrInsertFunction("__llvm_gcov_indirect_counter_increment", FTy);
}

Constant *GCOVProfiler::getEmitFunctionFunc() {
  Type *Args[2] = {
    Type::getInt32Ty(*Ctx),    // uint32_t ident
    Type::getInt8PtrTy(*Ctx),  // const char *function_name
  };
  FunctionType *FTy = FunctionType::get(Type::getVoidTy(*Ctx), Args, false);
  return M->getOrInsertFunction("llvm_gcda_emit_function", FTy);
}

Constant *GCOVProfiler::getEmitArcsFunc() {
  Type *Args[] = {
    Type::getInt32Ty(*Ctx),     // uint32_t num_counters
    Type::getInt64PtrTy(*Ctx),  // uint64_t *counters
  };
  FunctionType *FTy = FunctionType::get(Type::getVoidTy(*Ctx),
                                              Args, false);
  return M->getOrInsertFunction("llvm_gcda_emit_arcs", FTy);
}

Constant *GCOVProfiler::getEndFileFunc() {
  FunctionType *FTy = FunctionType::get(Type::getVoidTy(*Ctx), false);
  return M->getOrInsertFunction("llvm_gcda_end_file", FTy);
}

GlobalVariable *GCOVProfiler::getEdgeStateValue() {
  GlobalVariable *GV = M->getGlobalVariable("__llvm_gcov_global_state_pred");
  if (!GV) {
    GV = new GlobalVariable(*M, Type::getInt32Ty(*Ctx), false,
                            GlobalValue::InternalLinkage,
                            ConstantInt::get(Type::getInt32Ty(*Ctx),
                                             0xffffffff),
                            "__llvm_gcov_global_state_pred");
    GV->setUnnamedAddr(true);
  }
  return GV;
}

void GCOVProfiler::insertCounterWriteout(
    ArrayRef<std::pair<GlobalVariable *, MDNode *> > CountersBySP) {
  FunctionType *WriteoutFTy =
      FunctionType::get(Type::getVoidTy(*Ctx), false);
  Function *WriteoutF = Function::Create(WriteoutFTy,
                                         GlobalValue::InternalLinkage,
                                         "__llvm_gcov_writeout", M);
  WriteoutF->setUnnamedAddr(true);
  BasicBlock *BB = BasicBlock::Create(*Ctx, "", WriteoutF);
  IRBuilder<> Builder(BB);

  Constant *StartFile = getStartFileFunc();
  Constant *EmitFunction = getEmitFunctionFunc();
  Constant *EmitArcs = getEmitArcsFunc();
  Constant *EndFile = getEndFileFunc();

  NamedMDNode *CU_Nodes = M->getNamedMetadata("llvm.dbg.cu");
  if (CU_Nodes) {
    for (unsigned i = 0, e = CU_Nodes->getNumOperands(); i != e; ++i) {
      DICompileUnit compile_unit(CU_Nodes->getOperand(i));
      std::string FilenameGcda = mangleName(compile_unit, "gcda");
      Builder.CreateCall(StartFile,
                         Builder.CreateGlobalStringPtr(FilenameGcda));
      for (ArrayRef<std::pair<GlobalVariable *, MDNode *> >::iterator
             I = CountersBySP.begin(), E = CountersBySP.end();
           I != E; ++I) {
        DISubprogram SP(I->second);
        intptr_t ident = reinterpret_cast<intptr_t>(I->second);
        Builder.CreateCall2(EmitFunction,
                            ConstantInt::get(Type::getInt32Ty(*Ctx), ident),
                            Builder.CreateGlobalStringPtr(SP.getName()));
        
        GlobalVariable *GV = I->first;
        unsigned Arcs =
          cast<ArrayType>(GV->getType()->getElementType())->getNumElements();
        Builder.CreateCall2(EmitArcs,
                            ConstantInt::get(Type::getInt32Ty(*Ctx), Arcs),
                            Builder.CreateConstGEP2_64(GV, 0, 0));
      }
      Builder.CreateCall(EndFile);
    }
  }
  Builder.CreateRetVoid();

  // Create a small bit of code that registers the "__llvm_gcov_writeout"
  // function to be executed at exit.
  FunctionType *FTy = FunctionType::get(Type::getVoidTy(*Ctx), false);
  Function *F = Function::Create(FTy, GlobalValue::InternalLinkage,
                                 "__llvm_gcov_init", M);
  F->setUnnamedAddr(true);
  F->setLinkage(GlobalValue::InternalLinkage);
  F->addFnAttr(Attribute::NoInline);

  BB = BasicBlock::Create(*Ctx, "entry", F);
  Builder.SetInsertPoint(BB);

  FTy = FunctionType::get(Type::getInt32Ty(*Ctx),
                          PointerType::get(FTy, 0), false);
  Constant *AtExitFn = M->getOrInsertFunction("atexit", FTy);
  Builder.CreateCall(AtExitFn, WriteoutF);
  Builder.CreateRetVoid();

  appendToGlobalCtors(*M, F, 0);
}

void GCOVProfiler::insertIndirectCounterIncrement() {
  Function *Fn =
    cast<Function>(GCOVProfiler::getIncrementIndirectCounterFunc());
  Fn->setUnnamedAddr(true);
  Fn->setLinkage(GlobalValue::InternalLinkage);
  Fn->addFnAttr(Attribute::NoInline);

  Type *Int32Ty = Type::getInt32Ty(*Ctx);
  Type *Int64Ty = Type::getInt64Ty(*Ctx);
  Constant *NegOne = ConstantInt::get(Int32Ty, 0xffffffff);

  // Create basic blocks for function.
  BasicBlock *BB = BasicBlock::Create(*Ctx, "entry", Fn);
  IRBuilder<> Builder(BB);

  BasicBlock *PredNotNegOne = BasicBlock::Create(*Ctx, "", Fn);
  BasicBlock *CounterEnd = BasicBlock::Create(*Ctx, "", Fn);
  BasicBlock *Exit = BasicBlock::Create(*Ctx, "exit", Fn);

  // uint32_t pred = *predecessor;
  // if (pred == 0xffffffff) return;
  Argument *Arg = Fn->arg_begin();
  Arg->setName("predecessor");
  Value *Pred = Builder.CreateLoad(Arg, "pred");
  Value *Cond = Builder.CreateICmpEQ(Pred, NegOne);
  BranchInst::Create(Exit, PredNotNegOne, Cond, BB);

  Builder.SetInsertPoint(PredNotNegOne);

  // uint64_t *counter = counters[pred];
  // if (!counter) return;
  Value *ZExtPred = Builder.CreateZExt(Pred, Int64Ty);
  Arg = llvm::next(Fn->arg_begin());
  Arg->setName("counters");
  Value *GEP = Builder.CreateGEP(Arg, ZExtPred);
  Value *Counter = Builder.CreateLoad(GEP, "counter");
  Cond = Builder.CreateICmpEQ(Counter,
                              Constant::getNullValue(Int64Ty->getPointerTo()));
  Builder.CreateCondBr(Cond, Exit, CounterEnd);

  // ++*counter;
  Builder.SetInsertPoint(CounterEnd);
  Value *Add = Builder.CreateAdd(Builder.CreateLoad(Counter),
                                 ConstantInt::get(Int64Ty, 1));
  Builder.CreateStore(Add, Counter);
  Builder.CreateBr(Exit);

  // Fill in the exit block.
  Builder.SetInsertPoint(Exit);
  Builder.CreateRetVoid();
}