lib/Analysis/CFG.cpp

//===--- CFG.cpp - Classes for representing and building CFGs----*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
//  This file defines the CFG and CFGBuilder classes for representing and
//  building Control-Flow Graphs (CFGs) from ASTs.
//
//===----------------------------------------------------------------------===//

#include "clang/Analysis/CFG.h"
#include "clang/AST/StmtVisitor.h"
#include "clang/AST/PrettyPrinter.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/Support/GraphWriter.h"
#include "llvm/Support/Streams.h"
#include "llvm/Support/Compiler.h"
#include <llvm/Support/Allocator.h>
#include <llvm/Support/Format.h>

using namespace clang;

namespace {

// SaveAndRestore - A utility class that uses RIIA to save and restore
//  the value of a variable.
template<typename T>
struct VISIBILITY_HIDDEN SaveAndRestore {
  SaveAndRestore(T& x) : X(x), old_value(x) {}
  ~SaveAndRestore() { X = old_value; }
  T get() { return old_value; }

  T& X;
  T old_value;
};
  
static SourceLocation GetEndLoc(Decl* D) {
  if (VarDecl* VD = dyn_cast<VarDecl>(D))
    if (Expr* Ex = VD->getInit())
      return Ex->getSourceRange().getEnd();
  
  return D->getLocation();  
}
  
/// CFGBuilder - This class implements CFG construction from an AST.
///   The builder is stateful: an instance of the builder should be used to only
///   construct a single CFG.
///
///   Example usage:
///
///     CFGBuilder builder;
///     CFG* cfg = builder.BuildAST(stmt1);
///
///  CFG construction is done via a recursive walk of an AST.
///  We actually parse the AST in reverse order so that the successor
///  of a basic block is constructed prior to its predecessor.  This
///  allows us to nicely capture implicit fall-throughs without extra
///  basic blocks.
///
class VISIBILITY_HIDDEN CFGBuilder : public StmtVisitor<CFGBuilder,CFGBlock*> {    
  CFG* cfg;
  CFGBlock* Block;
  CFGBlock* Succ;
  CFGBlock* ContinueTargetBlock;
  CFGBlock* BreakTargetBlock;
  CFGBlock* SwitchTerminatedBlock;
  CFGBlock* DefaultCaseBlock;
  
  // LabelMap records the mapping from Label expressions to their blocks.
  typedef llvm::DenseMap<LabelStmt*,CFGBlock*> LabelMapTy;
  LabelMapTy LabelMap;
  
  // A list of blocks that end with a "goto" that must be backpatched to
  // their resolved targets upon completion of CFG construction.
  typedef std::vector<CFGBlock*> BackpatchBlocksTy;
  BackpatchBlocksTy BackpatchBlocks;
  
  // A list of labels whose address has been taken (for indirect gotos).
  typedef llvm::SmallPtrSet<LabelStmt*,5> LabelSetTy;
  LabelSetTy AddressTakenLabels;
  
public:  
  explicit CFGBuilder() : cfg(NULL), Block(NULL), Succ(NULL),
                          ContinueTargetBlock(NULL), BreakTargetBlock(NULL),
                          SwitchTerminatedBlock(NULL), DefaultCaseBlock(NULL) {
    // Create an empty CFG.
    cfg = new CFG();                        
  }
  
  ~CFGBuilder() { delete cfg; }
  
  // buildCFG - Used by external clients to construct the CFG.
  CFG* buildCFG(Stmt* Statement);
  
  // Visitors to walk an AST and construct the CFG.  Called by
  // buildCFG.  Do not call directly!
  
  CFGBlock* VisitBreakStmt(BreakStmt* B);
  CFGBlock* VisitCaseStmt(CaseStmt* Terminator);
  CFGBlock* VisitCompoundStmt(CompoundStmt* C);
  CFGBlock* VisitContinueStmt(ContinueStmt* C);
  CFGBlock* VisitDefaultStmt(DefaultStmt* D);
  CFGBlock* VisitDoStmt(DoStmt* D);
  CFGBlock* VisitForStmt(ForStmt* F);
  CFGBlock* VisitGotoStmt(GotoStmt* G);
  CFGBlock* VisitIfStmt(IfStmt* I);
  CFGBlock* VisitIndirectGotoStmt(IndirectGotoStmt* I);
  CFGBlock* VisitLabelStmt(LabelStmt* L);
  CFGBlock* VisitNullStmt(NullStmt* Statement);
  CFGBlock* VisitObjCForCollectionStmt(ObjCForCollectionStmt* S);
  CFGBlock* VisitReturnStmt(ReturnStmt* R);
  CFGBlock* VisitStmt(Stmt* Statement);
  CFGBlock* VisitSwitchStmt(SwitchStmt* Terminator);
  CFGBlock* VisitWhileStmt(WhileStmt* W);

  // FIXME: Add support for ObjC-specific control-flow structures.

  // NYS == Not Yet Supported
  CFGBlock* NYS() {
    badCFG = true;
    return Block;
  }
  
  CFGBlock* VisitObjCAtTryStmt(ObjCAtTryStmt* S);
  CFGBlock* VisitObjCAtCatchStmt(ObjCAtCatchStmt* S) { 
    // FIXME: For now we pretend that @catch and the code it contains
    //  does not exit.
    return Block;
  }

  // FIXME: This is not completely supported.  We basically @throw like
  // a 'return'.
  CFGBlock* VisitObjCAtThrowStmt(ObjCAtThrowStmt* S);

  CFGBlock* VisitObjCAtSynchronizedStmt(ObjCAtSynchronizedStmt* S);
  
  // Blocks.
  CFGBlock* VisitBlockExpr(BlockExpr* E) { return NYS(); }
  CFGBlock* VisitBlockDeclRefExpr(BlockDeclRefExpr* E) { return NYS(); }  
  
private:
  CFGBlock* createBlock(bool add_successor = true);
  CFGBlock* addStmt(Stmt* Terminator);
  CFGBlock* WalkAST(Stmt* Terminator, bool AlwaysAddStmt);
  CFGBlock* WalkAST_VisitChildren(Stmt* Terminator);
  CFGBlock* WalkAST_VisitDeclSubExpr(Decl* D);
  CFGBlock* WalkAST_VisitStmtExpr(StmtExpr* Terminator);
  bool FinishBlock(CFGBlock* B);
  
  bool badCFG;
};
  
// FIXME: Add support for dependent-sized array types in C++?
// Does it even make sense to build a CFG for an uninstantiated template?
static VariableArrayType* FindVA(Type* t) {
  while (ArrayType* vt = dyn_cast<ArrayType>(t)) {
    if (VariableArrayType* vat = dyn_cast<VariableArrayType>(vt))
      if (vat->getSizeExpr())
        return vat;
    
    t = vt->getElementType().getTypePtr();
  }
  
  return 0;
}
    
/// BuildCFG - Constructs a CFG from an AST (a Stmt*).  The AST can
///  represent an arbitrary statement.  Examples include a single expression
///  or a function body (compound statement).  The ownership of the returned
///  CFG is transferred to the caller.  If CFG construction fails, this method
///  returns NULL.
CFG* CFGBuilder::buildCFG(Stmt* Statement) {
  assert (cfg);
  if (!Statement) return NULL;

  badCFG = false;
  
  // Create an empty block that will serve as the exit block for the CFG.
  // Since this is the first block added to the CFG, it will be implicitly
  // registered as the exit block.
  Succ = createBlock();
  assert (Succ == &cfg->getExit());
  Block = NULL;  // the EXIT block is empty.  Create all other blocks lazily.
  
  // Visit the statements and create the CFG.
  CFGBlock* B = Visit(Statement);
  if (!B) B = Succ;
  
  if (B) {
    // Finalize the last constructed block.  This usually involves
    // reversing the order of the statements in the block.
    if (Block) FinishBlock(B);
    
    // Backpatch the gotos whose label -> block mappings we didn't know
    // when we encountered them.
    for (BackpatchBlocksTy::iterator I = BackpatchBlocks.begin(), 
         E = BackpatchBlocks.end(); I != E; ++I ) {
     
      CFGBlock* B = *I;
      GotoStmt* G = cast<GotoStmt>(B->getTerminator());
      LabelMapTy::iterator LI = LabelMap.find(G->getLabel());

      // If there is no target for the goto, then we are looking at an
      // incomplete AST.  Handle this by not registering a successor.
      if (LI == LabelMap.end()) continue;
      
      B->addSuccessor(LI->second);                   
    }
    
    // Add successors to the Indirect Goto Dispatch block (if we have one).
    if (CFGBlock* B = cfg->getIndirectGotoBlock())
      for (LabelSetTy::iterator I = AddressTakenLabels.begin(),
           E = AddressTakenLabels.end(); I != E; ++I ) {

        // Lookup the target block.
        LabelMapTy::iterator LI = LabelMap.find(*I);

        // If there is no target block that contains label, then we are looking
        // at an incomplete AST.  Handle this by not registering a successor.
        if (LI == LabelMap.end()) continue;
        
        B->addSuccessor(LI->second);           
      }
                                                          
    Succ = B;
  }
  
  // Create an empty entry block that has no predecessors.    
  cfg->setEntry(createBlock());
    
  if (badCFG) {
    delete cfg;
    cfg = NULL;
    return NULL;
  }
    
  // NULL out cfg so that repeated calls to the builder will fail and that
  // the ownership of the constructed CFG is passed to the caller.
  CFG* t = cfg;
  cfg = NULL;
  return t;
}
  
/// createBlock - Used to lazily create blocks that are connected
///  to the current (global) succcessor.
CFGBlock* CFGBuilder::createBlock(bool add_successor) { 
  CFGBlock* B = cfg->createBlock();
  if (add_successor && Succ) B->addSuccessor(Succ);
  return B;
}
  
/// FinishBlock - When the last statement has been added to the block,
///  we must reverse the statements because they have been inserted
///  in reverse order.
bool CFGBuilder::FinishBlock(CFGBlock* B) {
  if (badCFG)
    return false;

  assert (B);
  B->reverseStmts();
  return true;
}

/// addStmt - Used to add statements/expressions to the current CFGBlock 
///  "Block".  This method calls WalkAST on the passed statement to see if it
///  contains any short-circuit expressions.  If so, it recursively creates
///  the necessary blocks for such expressions.  It returns the "topmost" block
///  of the created blocks, or the original value of "Block" when this method
///  was called if no additional blocks are created.
CFGBlock* CFGBuilder::addStmt(Stmt* Terminator) {
  if (!Block) Block = createBlock();
  return WalkAST(Terminator,true);
}

/// WalkAST - Used by addStmt to walk the subtree of a statement and
///   add extra blocks for ternary operators, &&, and ||.  We also
///   process "," and DeclStmts (which may contain nested control-flow).
CFGBlock* CFGBuilder::WalkAST(Stmt* Terminator, bool AlwaysAddStmt = false) {
  switch (Terminator->getStmtClass()) {
    case Stmt::ConditionalOperatorClass: {
      ConditionalOperator* C = cast<ConditionalOperator>(Terminator);

      // Create the confluence block that will "merge" the results
      // of the ternary expression.
      CFGBlock* ConfluenceBlock = (Block) ? Block : createBlock();  
      ConfluenceBlock->appendStmt(C);
      if (!FinishBlock(ConfluenceBlock))
        return 0;

      // Create a block for the LHS expression if there is an LHS expression.
      // A GCC extension allows LHS to be NULL, causing the condition to
      // be the value that is returned instead.
      //  e.g: x ?: y is shorthand for: x ? x : y;
      Succ = ConfluenceBlock;
      Block = NULL;
      CFGBlock* LHSBlock = NULL;
      if (C->getLHS()) {
        LHSBlock = Visit(C->getLHS());
        if (!FinishBlock(LHSBlock))
          return 0;
        Block = NULL;        
      }
      
      // Create the block for the RHS expression.
      Succ = ConfluenceBlock;
      CFGBlock* RHSBlock = Visit(C->getRHS());
      if (!FinishBlock(RHSBlock))
        return 0;

      // Create the block that will contain the condition.
      Block = createBlock(false);
      
      if (LHSBlock)
        Block->addSuccessor(LHSBlock);
      else {
        // If we have no LHS expression, add the ConfluenceBlock as a direct
        // successor for the block containing the condition.  Moreover,
        // we need to reverse the order of the predecessors in the
        // ConfluenceBlock because the RHSBlock will have been added to
        // the succcessors already, and we want the first predecessor to the
        // the block containing the expression for the case when the ternary
        // expression evaluates to true.
        Block->addSuccessor(ConfluenceBlock);
        assert (ConfluenceBlock->pred_size() == 2);
        std::reverse(ConfluenceBlock->pred_begin(), 
                     ConfluenceBlock->pred_end());
      }
      
      Block->addSuccessor(RHSBlock);
      
      Block->setTerminator(C);
      return addStmt(C->getCond());
    }
    
    case Stmt::ChooseExprClass: {
      ChooseExpr* C = cast<ChooseExpr>(Terminator);      
      
      CFGBlock* ConfluenceBlock = Block ? Block : createBlock();  
      ConfluenceBlock->appendStmt(C);
      if (!FinishBlock(ConfluenceBlock))
        return 0;
      
      Succ = ConfluenceBlock;
      Block = NULL;
      CFGBlock* LHSBlock = Visit(C->getLHS());
      if (!FinishBlock(LHSBlock))
        return 0;

      Succ = ConfluenceBlock;
      Block = NULL;
      CFGBlock* RHSBlock = Visit(C->getRHS());
      if (!FinishBlock(RHSBlock))
        return 0;
      
      Block = createBlock(false);
      Block->addSuccessor(LHSBlock);
      Block->addSuccessor(RHSBlock);
      Block->setTerminator(C);
      return addStmt(C->getCond());
    }

    case Stmt::DeclStmtClass: {
      DeclStmt *DS = cast<DeclStmt>(Terminator);      
      if (DS->isSingleDecl()) {      
        Block->appendStmt(Terminator);
        return WalkAST_VisitDeclSubExpr(DS->getSingleDecl());
      }
      
      CFGBlock* B = 0;

      // FIXME: Add a reverse iterator for DeclStmt to avoid this
      // extra copy.
      typedef llvm::SmallVector<Decl*,10> BufTy;
      BufTy Buf(DS->decl_begin(), DS->decl_end());
      
      for (BufTy::reverse_iterator I=Buf.rbegin(), E=Buf.rend(); I!=E; ++I) {
        // Get the alignment of the new DeclStmt, padding out to >=8 bytes.
        unsigned A = llvm::AlignOf<DeclStmt>::Alignment < 8
                     ? 8 : llvm::AlignOf<DeclStmt>::Alignment;
        
        // Allocate the DeclStmt using the BumpPtrAllocator.  It will
        // get automatically freed with the CFG. 
        DeclGroupRef DG(*I);
        Decl* D = *I;
        void* Mem = cfg->getAllocator().Allocate(sizeof(DeclStmt), A);
        
        DeclStmt* DS = new (Mem) DeclStmt(DG, D->getLocation(), GetEndLoc(D));
        
        // Append the fake DeclStmt to block.
        Block->appendStmt(DS);
        B = WalkAST_VisitDeclSubExpr(D);
      }
      return B;
    }

    case Stmt::AddrLabelExprClass: {
      AddrLabelExpr* A = cast<AddrLabelExpr>(Terminator);
      AddressTakenLabels.insert(A->getLabel());
      
      if (AlwaysAddStmt) Block->appendStmt(Terminator);
      return Block;
    }
    
    case Stmt::StmtExprClass:
      return WalkAST_VisitStmtExpr(cast<StmtExpr>(Terminator));

    case Stmt::SizeOfAlignOfExprClass: {
      SizeOfAlignOfExpr* E = cast<SizeOfAlignOfExpr>(Terminator);

      // VLA types have expressions that must be evaluated.
      if (E->isArgumentType()) {
        for (VariableArrayType* VA = FindVA(E->getArgumentType().getTypePtr());
             VA != 0; VA = FindVA(VA->getElementType().getTypePtr()))
          addStmt(VA->getSizeExpr());
      }
      // Expressions in sizeof/alignof are not evaluated and thus have no
      // control flow.
      else
        Block->appendStmt(Terminator);

      return Block;
    }
      
    case Stmt::BinaryOperatorClass: {
      BinaryOperator* B = cast<BinaryOperator>(Terminator);

      if (B->isLogicalOp()) { // && or ||
        CFGBlock* ConfluenceBlock = (Block) ? Block : createBlock();  
        ConfluenceBlock->appendStmt(B);
        if (!FinishBlock(ConfluenceBlock))
          return 0;

        // create the block evaluating the LHS
        CFGBlock* LHSBlock = createBlock(false);
        LHSBlock->setTerminator(B);
        
        // create the block evaluating the RHS
        Succ = ConfluenceBlock;
        Block = NULL;
        CFGBlock* RHSBlock = Visit(B->getRHS());
        if (!FinishBlock(RHSBlock))
          return 0;

        // Now link the LHSBlock with RHSBlock.
        if (B->getOpcode() == BinaryOperator::LOr) {
          LHSBlock->addSuccessor(ConfluenceBlock);
          LHSBlock->addSuccessor(RHSBlock);
        }
        else {
          assert (B->getOpcode() == BinaryOperator::LAnd);
          LHSBlock->addSuccessor(RHSBlock);
          LHSBlock->addSuccessor(ConfluenceBlock);
        }
        
        // Generate the blocks for evaluating the LHS.
        Block = LHSBlock;
        return addStmt(B->getLHS());                                    
      }
      else if (B->getOpcode() == BinaryOperator::Comma) { // ,
        Block->appendStmt(B);
        addStmt(B->getRHS());
        return addStmt(B->getLHS());
      }
      
      break;
    }
    
    // Blocks: No support for blocks ... yet
    case Stmt::BlockExprClass:
    case Stmt::BlockDeclRefExprClass:
      return NYS();
      
    case Stmt::ParenExprClass:
      return WalkAST(cast<ParenExpr>(Terminator)->getSubExpr(), AlwaysAddStmt);
    
  case Stmt::CallExprClass: {
    bool NoReturn = false;
    CallExpr *C = cast<CallExpr>(Terminator);
    Expr *CEE = C->getCallee()->IgnoreParenCasts();
    if (DeclRefExpr *DRE = dyn_cast<DeclRefExpr>(CEE)) {
      // FIXME: We can follow objective-c methods and C++ member functions...
      if (FunctionDecl *FD = dyn_cast<FunctionDecl>(DRE->getDecl())) {
        if (FD->hasAttr<NoReturnAttr>())
          NoReturn = true;
      }
    }

    if (!NoReturn)
      break;

    if (Block) {
      if (!FinishBlock(Block))
        return 0;
    }

    // Create new block with no successor for the remaining pieces.
    Block = createBlock(false);
    Block->appendStmt(Terminator);

    // Wire this to the exit block directly.
    Block->addSuccessor(&cfg->getExit());

    return WalkAST_VisitChildren(Terminator);
  }

    default:
      break;
  };
      
  if (AlwaysAddStmt) Block->appendStmt(Terminator);
  return WalkAST_VisitChildren(Terminator);
}
  
/// WalkAST_VisitDeclSubExpr - Utility method to add block-level expressions
///  for initializers in Decls.
CFGBlock* CFGBuilder::WalkAST_VisitDeclSubExpr(Decl* D) {
  VarDecl* VD = dyn_cast<VarDecl>(D);

  if (!VD)
    return Block;
  
  Expr* Init = VD->getInit();
  
  if (Init) {
    // Optimization: Don't create separate block-level statements for literals.
    switch (Init->getStmtClass()) {
      case Stmt::IntegerLiteralClass:
      case Stmt::CharacterLiteralClass:
      case Stmt::StringLiteralClass:
        break;
      default:
        Block = addStmt(Init);
    }
  }
    
  // If the type of VD is a VLA, then we must process its size expressions.
  for (VariableArrayType* VA = FindVA(VD->getType().getTypePtr()); VA != 0;
       VA = FindVA(VA->getElementType().getTypePtr()))
    Block = addStmt(VA->getSizeExpr());  
  
  return Block;
}

/// WalkAST_VisitChildren - Utility method to call WalkAST on the
///  children of a Stmt.
CFGBlock* CFGBuilder::WalkAST_VisitChildren(Stmt* Terminator) {
  CFGBlock* B = Block;
  for (Stmt::child_iterator I = Terminator->child_begin(),
         E = Terminator->child_end();
       I != E; ++I)
    if (*I) B = WalkAST(*I);
  
  return B;
}

/// WalkAST_VisitStmtExpr - Utility method to handle (nested) statement
///  expressions (a GCC extension).
CFGBlock* CFGBuilder::WalkAST_VisitStmtExpr(StmtExpr* Terminator) {
  Block->appendStmt(Terminator);
  return VisitCompoundStmt(Terminator->getSubStmt());  
}

/// VisitStmt - Handle statements with no branching control flow.
CFGBlock* CFGBuilder::VisitStmt(Stmt* Statement) {
  // We cannot assume that we are in the middle of a basic block, since
  // the CFG might only be constructed for this single statement.  If
  // we have no current basic block, just create one lazily.
  if (!Block) Block = createBlock();
  
  // Simply add the statement to the current block.  We actually
  // insert statements in reverse order; this order is reversed later
  // when processing the containing element in the AST.
  addStmt(Statement);

  return Block;
}

CFGBlock* CFGBuilder::VisitNullStmt(NullStmt* Statement) {
  return Block;
}

CFGBlock* CFGBuilder::VisitCompoundStmt(CompoundStmt* C) {
  
  CFGBlock* LastBlock = Block;

  for (CompoundStmt::reverse_body_iterator I=C->body_rbegin(), E=C->body_rend();
                                                               I != E; ++I ) {
    LastBlock = Visit(*I);
  }

  return LastBlock;
}

CFGBlock* CFGBuilder::VisitIfStmt(IfStmt* I) {
  // We may see an if statement in the middle of a basic block, or
  // it may be the first statement we are processing.  In either case,
  // we create a new basic block.  First, we create the blocks for
  // the then...else statements, and then we create the block containing
  // the if statement.  If we were in the middle of a block, we
  // stop processing that block and reverse its statements.  That block
  // is then the implicit successor for the "then" and "else" clauses.
  
  // The block we were proccessing is now finished.  Make it the
  // successor block.
  if (Block) { 
    Succ = Block;
    if (!FinishBlock(Block))
      return 0;
  }
  
  // Process the false branch.  NULL out Block so that the recursive
  // call to Visit will create a new basic block.
  // Null out Block so that all successor
  CFGBlock* ElseBlock = Succ;
  
  if (Stmt* Else = I->getElse()) {
    SaveAndRestore<CFGBlock*> sv(Succ);
    
    // NULL out Block so that the recursive call to Visit will
    // create a new basic block.          
    Block = NULL;
    ElseBlock = Visit(Else);
              
    if (!ElseBlock) // Can occur when the Else body has all NullStmts.
      ElseBlock = sv.get();
    else if (Block) {
      if (!FinishBlock(ElseBlock))
        return 0;
    }
  }
  
  // Process the true branch.  NULL out Block so that the recursive
  // call to Visit will create a new basic block.
  // Null out Block so that all successor
  CFGBlock* ThenBlock;
  {
    Stmt* Then = I->getThen();
    assert (Then);
    SaveAndRestore<CFGBlock*> sv(Succ);
    Block = NULL;        
    ThenBlock = Visit(Then);
    
    if (!ThenBlock) {
      // We can reach here if the "then" body has all NullStmts.
      // Create an empty block so we can distinguish between true and false
      // branches in path-sensitive analyses.
      ThenBlock = createBlock(false);
      ThenBlock->addSuccessor(sv.get());
    }
    else if (Block) {
      if (!FinishBlock(ThenBlock))
        return 0;
    }        
  }

  // Now create a new block containing the if statement.        
  Block = createBlock(false);
  
  // Set the terminator of the new block to the If statement.
  Block->setTerminator(I);
  
  // Now add the successors.
  Block->addSuccessor(ThenBlock);
  Block->addSuccessor(ElseBlock);
  
  // Add the condition as the last statement in the new block.  This
  // may create new blocks as the condition may contain control-flow.  Any
  // newly created blocks will be pointed to be "Block".
  return addStmt(I->getCond()->IgnoreParens());
}
  
    
CFGBlock* CFGBuilder::VisitReturnStmt(ReturnStmt* R) {
  // If we were in the middle of a block we stop processing that block
  // and reverse its statements.
  //
  // NOTE: If a "return" appears in the middle of a block, this means
  //       that the code afterwards is DEAD (unreachable).  We still
  //       keep a basic block for that code; a simple "mark-and-sweep"
  //       from the entry block will be able to report such dead
  //       blocks.
  if (Block) FinishBlock(Block);

  // Create the new block.
  Block = createBlock(false);
  
  // The Exit block is the only successor.
  Block->addSuccessor(&cfg->getExit());
    
  // Add the return statement to the block.  This may create new blocks
  // if R contains control-flow (short-circuit operations).
  return addStmt(R);
}

CFGBlock* CFGBuilder::VisitLabelStmt(LabelStmt* L) {
  // Get the block of the labeled statement.  Add it to our map.
  Visit(L->getSubStmt());
  CFGBlock* LabelBlock = Block;
  
  if (!LabelBlock)            // This can happen when the body is empty, i.e.
    LabelBlock=createBlock(); // scopes that only contains NullStmts.
  
  assert (LabelMap.find(L) == LabelMap.end() && "label already in map");
  LabelMap[ L ] = LabelBlock;
  
  // Labels partition blocks, so this is the end of the basic block
  // we were processing (L is the block's label).  Because this is
  // label (and we have already processed the substatement) there is no
  // extra control-flow to worry about.
  LabelBlock->setLabel(L);
  if (!FinishBlock(LabelBlock))
    return 0;
  
  // We set Block to NULL to allow lazy creation of a new block
  // (if necessary);
  Block = NULL;
  
  // This block is now the implicit successor of other blocks.
  Succ = LabelBlock;
  
  return LabelBlock;
}

CFGBlock* CFGBuilder::VisitGotoStmt(GotoStmt* G) {
  // Goto is a control-flow statement.  Thus we stop processing the
  // current block and create a new one.
  if (Block) FinishBlock(Block);
  Block = createBlock(false);
  Block->setTerminator(G);
  
  // If we already know the mapping to the label block add the
  // successor now.
  LabelMapTy::iterator I = LabelMap.find(G->getLabel());
  
  if (I == LabelMap.end())
    // We will need to backpatch this block later.
    BackpatchBlocks.push_back(Block);
  else
    Block->addSuccessor(I->second);

  return Block;            
}

CFGBlock* CFGBuilder::VisitForStmt(ForStmt* F) {
  // "for" is a control-flow statement.  Thus we stop processing the
  // current block.
  
  CFGBlock* LoopSuccessor = NULL;
  
  if (Block) {
    if (!FinishBlock(Block))
      return 0;
    LoopSuccessor = Block;
  }
  else LoopSuccessor = Succ;
  
  // Because of short-circuit evaluation, the condition of the loop
  // can span multiple basic blocks.  Thus we need the "Entry" and "Exit"
  // blocks that evaluate the condition.
  CFGBlock* ExitConditionBlock = createBlock(false);
  CFGBlock* EntryConditionBlock = ExitConditionBlock;
  
  // Set the terminator for the "exit" condition block.
  ExitConditionBlock->setTerminator(F);  
  
  // Now add the actual condition to the condition block.  Because the
  // condition itself may contain control-flow, new blocks may be created.
  if (Stmt* C = F->getCond()) {
    Block = ExitConditionBlock;
    EntryConditionBlock = addStmt(C);
    if (Block) {
      if (!FinishBlock(EntryConditionBlock))
        return 0;
    }
  }

  // The condition block is the implicit successor for the loop body as
  // well as any code above the loop.
  Succ = EntryConditionBlock;
  
  // Now create the loop body.
  {
    assert (F->getBody());
    
    // Save the current values for Block, Succ, and continue and break targets
    SaveAndRestore<CFGBlock*> save_Block(Block), save_Succ(Succ),
    save_continue(ContinueTargetBlock),
    save_break(BreakTargetBlock);      
 
    // Create a new block to contain the (bottom) of the loop body.
    Block = NULL;
    
    if (Stmt* I = F->getInc()) {
      // Generate increment code in its own basic block.  This is the target
      // of continue statements.
      Succ = Visit(I);
    }
    else {
      // No increment code.  Create a special, empty, block that is used as
      // the target block for "looping back" to the start of the loop.
      assert(Succ == EntryConditionBlock);
      Succ = createBlock();
    }
    
    // Finish up the increment (or empty) block if it hasn't been already.
    if (Block) {
      assert(Block == Succ);
      if (!FinishBlock(Block))
        return 0;
      Block = 0;
    }
    
    ContinueTargetBlock = Succ;
    
    // The starting block for the loop increment is the block that should
    // represent the 'loop target' for looping back to the start of the loop.
    ContinueTargetBlock->setLoopTarget(F);

    // All breaks should go to the code following the loop.
    BreakTargetBlock = LoopSuccessor;    
    
    // Now populate the body block, and in the process create new blocks
    // as we walk the body of the loop.
    CFGBlock* BodyBlock = Visit(F->getBody());      

    if (!BodyBlock)
      BodyBlock = EntryConditionBlock; // can happen for "for (...;...; ) ;"
    else if (Block) {
      if (!FinishBlock(BodyBlock))
        return 0;
    }      
    
    // This new body block is a successor to our "exit" condition block.
    ExitConditionBlock->addSuccessor(BodyBlock);
  }
  
  // Link up the condition block with the code that follows the loop.
  // (the false branch).
  ExitConditionBlock->addSuccessor(LoopSuccessor);
  
  // If the loop contains initialization, create a new block for those
  // statements.  This block can also contain statements that precede
  // the loop.
  if (Stmt* I = F->getInit()) {
    Block = createBlock();
    return addStmt(I);
  }
  else {
    // There is no loop initialization.   We are thus basically a while 
    // loop.  NULL out Block to force lazy block construction.
    Block = NULL;
    Succ = EntryConditionBlock;
    return EntryConditionBlock;
  }
}

CFGBlock* CFGBuilder::VisitObjCForCollectionStmt(ObjCForCollectionStmt* S) {
  // Objective-C fast enumeration 'for' statements:
  //  http://developer.apple.com/documentation/Cocoa/Conceptual/ObjectiveC
  //
  //  for ( Type newVariable in collection_expression ) { statements }
  //
  //  becomes:
  //
  //   prologue:
  //     1. collection_expression
  //     T. jump to loop_entry
  //   loop_entry:
  //     1. side-effects of element expression
  //     1. ObjCForCollectionStmt [performs binding to newVariable]
  //     T. ObjCForCollectionStmt  TB, FB  [jumps to TB if newVariable != nil]
  //   TB:
  //     statements
  //     T. jump to loop_entry
  //   FB:
  //     what comes after
  //
  //  and
  //
  //  Type existingItem;
  //  for ( existingItem in expression ) { statements }
  //
  //  becomes:
  //
  //   the same with newVariable replaced with existingItem; the binding
  //   works the same except that for one ObjCForCollectionStmt::getElement()
  //   returns a DeclStmt and the other returns a DeclRefExpr.
  //
  
  CFGBlock* LoopSuccessor = 0;
  
  if (Block) {
    if (!FinishBlock(Block))
      return 0;
    LoopSuccessor = Block;
    Block = 0;
  }
  else LoopSuccessor = Succ;
  
  // Build the condition blocks.
  CFGBlock* ExitConditionBlock = createBlock(false);
  CFGBlock* EntryConditionBlock = ExitConditionBlock;
  
  // Set the terminator for the "exit" condition block.
  ExitConditionBlock->setTerminator(S);  
  
  // The last statement in the block should be the ObjCForCollectionStmt,
  // which performs the actual binding to 'element' and determines if there
  // are any more items in the collection.
  ExitConditionBlock->appendStmt(S);
  Block = ExitConditionBlock;
  
  // Walk the 'element' expression to see if there are any side-effects.  We
  // generate new blocks as necesary.  We DON'T add the statement by default
  // to the CFG unless it contains control-flow.
  EntryConditionBlock = WalkAST(S->getElement(), false);
  if (Block) { 
    if (!FinishBlock(EntryConditionBlock))
      return 0;
    Block = 0;
  }
  
  // The condition block is the implicit successor for the loop body as
  // well as any code above the loop.
  Succ = EntryConditionBlock;
  
  // Now create the true branch.
  { 
    // Save the current values for Succ, continue and break targets.
    SaveAndRestore<CFGBlock*> save_Succ(Succ),
      save_continue(ContinueTargetBlock), save_break(BreakTargetBlock); 
    
    BreakTargetBlock = LoopSuccessor;
    ContinueTargetBlock = EntryConditionBlock;  
    
    CFGBlock* BodyBlock = Visit(S->getBody());
    
    if (!BodyBlock)
      BodyBlock = EntryConditionBlock; // can happen for "for (X in Y) ;"
    else if (Block) {
      if (!FinishBlock(BodyBlock))
        return 0;
    }
                  
    // This new body block is a successor to our "exit" condition block.
    ExitConditionBlock->addSuccessor(BodyBlock);
  }
  
  // Link up the condition block with the code that follows the loop.
  // (the false branch).
  ExitConditionBlock->addSuccessor(LoopSuccessor);

  // Now create a prologue block to contain the collection expression.
  Block = createBlock();
  return addStmt(S->getCollection());
}    
  
CFGBlock* CFGBuilder::VisitObjCAtSynchronizedStmt(ObjCAtSynchronizedStmt* S) {
  // FIXME: Add locking 'primitives' to CFG for @synchronized.
  
  // Inline the body.
  CFGBlock *SyncBlock = Visit(S->getSynchBody());
  
  // The sync body starts its own basic block.  This makes it a little easier
  // for diagnostic clients.
  if (SyncBlock) {
    if (!FinishBlock(SyncBlock))
      return 0;
    
    Block = 0;
  }
    
  Succ = SyncBlock;
  
  // Inline the sync expression.
  return Visit(S->getSynchExpr());
}
  
CFGBlock* CFGBuilder::VisitObjCAtTryStmt(ObjCAtTryStmt* S) {
  return NYS();
}

CFGBlock* CFGBuilder::VisitWhileStmt(WhileStmt* W) {
  // "while" is a control-flow statement.  Thus we stop processing the
  // current block.
  
  CFGBlock* LoopSuccessor = NULL;
  
  if (Block) {
    if (!FinishBlock(Block))
      return 0;
    LoopSuccessor = Block;
  }
  else LoopSuccessor = Succ;
            
  // Because of short-circuit evaluation, the condition of the loop
  // can span multiple basic blocks.  Thus we need the "Entry" and "Exit"
  // blocks that evaluate the condition.
  CFGBlock* ExitConditionBlock = createBlock(false);
  CFGBlock* EntryConditionBlock = ExitConditionBlock;
  
  // Set the terminator for the "exit" condition block.
  ExitConditionBlock->setTerminator(W);
  
  // Now add the actual condition to the condition block.  Because the
  // condition itself may contain control-flow, new blocks may be created.
  // Thus we update "Succ" after adding the condition.
  if (Stmt* C = W->getCond()) {
    Block = ExitConditionBlock;
    EntryConditionBlock = addStmt(C);
    assert(Block == EntryConditionBlock);
    if (Block) {
      if (!FinishBlock(EntryConditionBlock))
        return 0;
    }
  }
  
  // The condition block is the implicit successor for the loop body as
  // well as any code above the loop.
  Succ = EntryConditionBlock;
  
  // Process the loop body.
  {
    assert(W->getBody());

    // Save the current values for Block, Succ, and continue and break targets
    SaveAndRestore<CFGBlock*> save_Block(Block), save_Succ(Succ),
                              save_continue(ContinueTargetBlock),
                              save_break(BreakTargetBlock);

    // Create an empty block to represent the transition block for looping
    // back to the head of the loop.
    Block = 0;
    assert(Succ == EntryConditionBlock);
    Succ = createBlock();
    Succ->setLoopTarget(W);
    ContinueTargetBlock = Succ;    
    
    // All breaks should go to the code following the loop.
    BreakTargetBlock = LoopSuccessor;
    
    // NULL out Block to force lazy instantiation of blocks for the body.
    Block = NULL;
    
    // Create the body.  The returned block is the entry to the loop body.
    CFGBlock* BodyBlock = Visit(W->getBody());
    
    if (!BodyBlock)
      BodyBlock = EntryConditionBlock; // can happen for "while(...) ;"
    else if (Block) {
      if (!FinishBlock(BodyBlock))
        return 0;
    }
    
    // Add the loop body entry as a successor to the condition.
    ExitConditionBlock->addSuccessor(BodyBlock);
  }
  
  // Link up the condition block with the code that follows the loop.
  // (the false branch).
  ExitConditionBlock->addSuccessor(LoopSuccessor);
  
  // There can be no more statements in the condition block
  // since we loop back to this block.  NULL out Block to force
  // lazy creation of another block.
  Block = NULL;
  
  // Return the condition block, which is the dominating block for the loop.
  Succ = EntryConditionBlock;
  return EntryConditionBlock;
}
  
CFGBlock* CFGBuilder::VisitObjCAtThrowStmt(ObjCAtThrowStmt* S) {
  // FIXME: This isn't complete.  We basically treat @throw like a return
  //  statement.
  
  // If we were in the middle of a block we stop processing that block
  // and reverse its statements.
  if (Block) {
    if (!FinishBlock(Block))
      return 0;
  }
  
  // Create the new block.
  Block = createBlock(false);
  
  // The Exit block is the only successor.
  Block->addSuccessor(&cfg->getExit());
  
  // Add the statement to the block.  This may create new blocks
  // if S contains control-flow (short-circuit operations).
  return addStmt(S);
}

CFGBlock* CFGBuilder::VisitDoStmt(DoStmt* D) {
  // "do...while" is a control-flow statement.  Thus we stop processing the
  // current block.
  
  CFGBlock* LoopSuccessor = NULL;
  
  if (Block) {
    if (!FinishBlock(Block))
      return 0;
    LoopSuccessor = Block;
  }
  else LoopSuccessor = Succ;
  
  // Because of short-circuit evaluation, the condition of the loop
  // can span multiple basic blocks.  Thus we need the "Entry" and "Exit"
  // blocks that evaluate the condition.
  CFGBlock* ExitConditionBlock = createBlock(false);
  CFGBlock* EntryConditionBlock = ExitConditionBlock;
        
  // Set the terminator for the "exit" condition block.
  ExitConditionBlock->setTerminator(D);  
  
  // Now add the actual condition to the condition block.  Because the
  // condition itself may contain control-flow, new blocks may be created.
  if (Stmt* C = D->getCond()) {
    Block = ExitConditionBlock;
    EntryConditionBlock = addStmt(C);
    if (Block) {
      if (!FinishBlock(EntryConditionBlock))
        return 0;
    }
  }
  
  // The condition block is the implicit successor for the loop body.
  Succ = EntryConditionBlock;

  // Process the loop body.
  CFGBlock* BodyBlock = NULL;
  {
    assert (D->getBody());
    
    // Save the current values for Block, Succ, and continue and break targets
    SaveAndRestore<CFGBlock*> save_Block(Block), save_Succ(Succ),
    save_continue(ContinueTargetBlock),
    save_break(BreakTargetBlock);
    
    // All continues within this loop should go to the condition block
    ContinueTargetBlock = EntryConditionBlock;
    
    // All breaks should go to the code following the loop.
    BreakTargetBlock = LoopSuccessor;
    
    // NULL out Block to force lazy instantiation of blocks for the body.
    Block = NULL;
    
    // Create the body.  The returned block is the entry to the loop body.
    BodyBlock = Visit(D->getBody());
    
    if (!BodyBlock)
      BodyBlock = EntryConditionBlock; // can happen for "do ; while(...)"
    else if (Block) {
      if (!FinishBlock(BodyBlock))
        return 0;
    }
        
    // Add an intermediate block between the BodyBlock and the
    // ExitConditionBlock to represent the "loop back" transition.
    // Create an empty block to represent the transition block for looping
    // back to the head of the loop.
    // FIXME: Can we do this more efficiently without adding another block?
    Block = NULL;
    Succ = BodyBlock;
    CFGBlock *LoopBackBlock = createBlock();
    LoopBackBlock->setLoopTarget(D);
    
    // Add the loop body entry as a successor to the condition.
    ExitConditionBlock->addSuccessor(LoopBackBlock);
  }
  
  // Link up the condition block with the code that follows the loop.
  // (the false branch).
  ExitConditionBlock->addSuccessor(LoopSuccessor);
  
  // There can be no more statements in the body block(s)
  // since we loop back to the body.  NULL out Block to force
  // lazy creation of another block.
  Block = NULL;
  
  // Return the loop body, which is the dominating block for the loop.
  Succ = BodyBlock;
  return BodyBlock;
}

CFGBlock* CFGBuilder::VisitContinueStmt(ContinueStmt* C) {
  // "continue" is a control-flow statement.  Thus we stop processing the
  // current block.
  if (Block) {
    if (!FinishBlock(Block))
      return 0;
  }
  
  // Now create a new block that ends with the continue statement.
  Block = createBlock(false);
  Block->setTerminator(C);
  
  // If there is no target for the continue, then we are looking at an
  // incomplete AST.  This means the CFG cannot be constructed.
  if (ContinueTargetBlock)
    Block->addSuccessor(ContinueTargetBlock);
  else
    badCFG = true;
  
  return Block;
}

CFGBlock* CFGBuilder::VisitBreakStmt(BreakStmt* B) {
  // "break" is a control-flow statement.  Thus we stop processing the
  // current block.
  if (Block) {
    if (!FinishBlock(Block))
      return 0;
  }
  
  // Now create a new block that ends with the break statement.
  Block = createBlock(false);
  Block->setTerminator(B);
  
  // If there is no target for the break, then we are looking at an
  // incomplete AST.  This means that the CFG cannot be constructed.
  if (BreakTargetBlock)
    Block->addSuccessor(BreakTargetBlock);
  else 
    badCFG = true;


  return Block;  
}

CFGBlock* CFGBuilder::VisitSwitchStmt(SwitchStmt* Terminator) {
  // "switch" is a control-flow statement.  Thus we stop processing the
  // current block.    
  CFGBlock* SwitchSuccessor = NULL;
  
  if (Block) {
    if (!FinishBlock(Block))
      return 0;
    SwitchSuccessor = Block;
  }
  else SwitchSuccessor = Succ;

  // Save the current "switch" context.
  SaveAndRestore<CFGBlock*> save_switch(SwitchTerminatedBlock),
                            save_break(BreakTargetBlock),
                            save_default(DefaultCaseBlock);

  // Set the "default" case to be the block after the switch statement.
  // If the switch statement contains a "default:", this value will
  // be overwritten with the block for that code.
  DefaultCaseBlock = SwitchSuccessor;
  
  // Create a new block that will contain the switch statement.
  SwitchTerminatedBlock = createBlock(false);
  
  // Now process the switch body.  The code after the switch is the implicit
  // successor.
  Succ = SwitchSuccessor;
  BreakTargetBlock = SwitchSuccessor;
  
  // When visiting the body, the case statements should automatically get
  // linked up to the switch.  We also don't keep a pointer to the body,
  // since all control-flow from the switch goes to case/default statements.
  assert (Terminator->getBody() && "switch must contain a non-NULL body");
  Block = NULL;
  CFGBlock *BodyBlock = Visit(Terminator->getBody());
  if (Block) {
    if (!FinishBlock(BodyBlock))
      return 0;
  }

  // If we have no "default:" case, the default transition is to the
  // code following the switch body.
  SwitchTerminatedBlock->addSuccessor(DefaultCaseBlock);
  
  // Add the terminator and condition in the switch block.
  SwitchTerminatedBlock->setTerminator(Terminator);
  assert (Terminator->getCond() && "switch condition must be non-NULL");
  Block = SwitchTerminatedBlock;
  
  return addStmt(Terminator->getCond());
}

CFGBlock* CFGBuilder::VisitCaseStmt(CaseStmt* Terminator) {
  // CaseStmts are essentially labels, so they are the
  // first statement in a block.      

  if (Terminator->getSubStmt()) Visit(Terminator->getSubStmt());
  CFGBlock* CaseBlock = Block;
  if (!CaseBlock) CaseBlock = createBlock();  
    
  // Cases statements partition blocks, so this is the top of
  // the basic block we were processing (the "case XXX:" is the label).
  CaseBlock->setLabel(Terminator);
  if (!FinishBlock(CaseBlock))
    return 0;
  
  // Add this block to the list of successors for the block with the
  // switch statement.
  assert (SwitchTerminatedBlock);
  SwitchTerminatedBlock->addSuccessor(CaseBlock);
  
  // We set Block to NULL to allow lazy creation of a new block (if necessary)
  Block = NULL;
  
  // This block is now the implicit successor of other blocks.
  Succ = CaseBlock;
  
  return CaseBlock;
}
  
CFGBlock* CFGBuilder::VisitDefaultStmt(DefaultStmt* Terminator) {
  if (Terminator->getSubStmt()) Visit(Terminator->getSubStmt());
  DefaultCaseBlock = Block;
  if (!DefaultCaseBlock) DefaultCaseBlock = createBlock();  
  
  // Default statements partition blocks, so this is the top of
  // the basic block we were processing (the "default:" is the label).
  DefaultCaseBlock->setLabel(Terminator);
  if (!FinishBlock(DefaultCaseBlock))
    return 0;

  // Unlike case statements, we don't add the default block to the
  // successors for the switch statement immediately.  This is done
  // when we finish processing the switch statement.  This allows for
  // the default case (including a fall-through to the code after the
  // switch statement) to always be the last successor of a switch-terminated
  // block.
  
  // We set Block to NULL to allow lazy creation of a new block (if necessary)
  Block = NULL;
  
  // This block is now the implicit successor of other blocks.
  Succ = DefaultCaseBlock;
  
  return DefaultCaseBlock;  
}

CFGBlock* CFGBuilder::VisitIndirectGotoStmt(IndirectGotoStmt* I) {
  // Lazily create the indirect-goto dispatch block if there isn't one
  // already.
  CFGBlock* IBlock = cfg->getIndirectGotoBlock();
  
  if (!IBlock) {
    IBlock = createBlock(false);
    cfg->setIndirectGotoBlock(IBlock);
  }
  
  // IndirectGoto is a control-flow statement.  Thus we stop processing the
  // current block and create a new one.
  if (Block) {
    if (!FinishBlock(Block))
      return 0;
  }
  Block = createBlock(false);
  Block->setTerminator(I);
  Block->addSuccessor(IBlock);
  return addStmt(I->getTarget());
}


} // end anonymous namespace

/// createBlock - Constructs and adds a new CFGBlock to the CFG.  The
///  block has no successors or predecessors.  If this is the first block
///  created in the CFG, it is automatically set to be the Entry and Exit
///  of the CFG.
CFGBlock* CFG::createBlock() {
  bool first_block = begin() == end();

  // Create the block.
  Blocks.push_front(CFGBlock(NumBlockIDs++));

  // If this is the first block, set it as the Entry and Exit.
  if (first_block) Entry = Exit = &front();

  // Return the block.
  return &front();
}

/// buildCFG - Constructs a CFG from an AST.  Ownership of the returned
///  CFG is returned to the caller.
CFG* CFG::buildCFG(Stmt* Statement) {
  CFGBuilder Builder;
  return Builder.buildCFG(Statement);
}

/// reverseStmts - Reverses the orders of statements within a CFGBlock.
void CFGBlock::reverseStmts() { std::reverse(Stmts.begin(),Stmts.end()); }

//===----------------------------------------------------------------------===//
// CFG: Queries for BlkExprs.
//===----------------------------------------------------------------------===//

namespace {
  typedef llvm::DenseMap<const Stmt*,unsigned> BlkExprMapTy;
}

static void FindSubExprAssignments(Stmt* Terminator, llvm::SmallPtrSet<Expr*,50>& Set) {
  if (!Terminator)
    return;
  
  for (Stmt::child_iterator I=Terminator->child_begin(), E=Terminator->child_end(); I!=E; ++I) {
    if (!*I) continue;
    
    if (BinaryOperator* B = dyn_cast<BinaryOperator>(*I))
      if (B->isAssignmentOp()) Set.insert(B);
    
    FindSubExprAssignments(*I, Set);
  }
}

static BlkExprMapTy* PopulateBlkExprMap(CFG& cfg) {
  BlkExprMapTy* M = new BlkExprMapTy();
  
  // Look for assignments that are used as subexpressions.  These are the
  // only assignments that we want to *possibly* register as a block-level
  // expression.  Basically, if an assignment occurs both in a subexpression
  // and at the block-level, it is a block-level expression.
  llvm::SmallPtrSet<Expr*,50> SubExprAssignments;
  
  for (CFG::iterator I=cfg.begin(), E=cfg.end(); I != E; ++I)
    for (CFGBlock::iterator BI=I->begin(), EI=I->end(); BI != EI; ++BI)
      FindSubExprAssignments(*BI, SubExprAssignments);

  for (CFG::iterator I=cfg.begin(), E=cfg.end(); I != E; ++I) {
    
    // Iterate over the statements again on identify the Expr* and Stmt* at
    // the block-level that are block-level expressions.

    for (CFGBlock::iterator BI=I->begin(), EI=I->end(); BI != EI; ++BI)
      if (Expr* Exp = dyn_cast<Expr>(*BI)) {
        
        if (BinaryOperator* B = dyn_cast<BinaryOperator>(Exp)) {
          // Assignment expressions that are not nested within another
          // expression are really "statements" whose value is never
          // used by another expression.
          if (B->isAssignmentOp() && !SubExprAssignments.count(Exp))
            continue;
        }
        else if (const StmtExpr* Terminator = dyn_cast<StmtExpr>(Exp)) {
          // Special handling for statement expressions.  The last statement
          // in the statement expression is also a block-level expr.
          const CompoundStmt* C = Terminator->getSubStmt();
          if (!C->body_empty()) {
            unsigned x = M->size();
            (*M)[C->body_back()] = x;
          }
        }

        unsigned x = M->size();
        (*M)[Exp] = x;
      }
    
    // Look at terminators.  The condition is a block-level expression.
    
    Stmt* S = I->getTerminatorCondition();
    
    if (S && M->find(S) == M->end()) {
        unsigned x = M->size();
        (*M)[S] = x;
    }
  }
    
  return M;
}

CFG::BlkExprNumTy CFG::getBlkExprNum(const Stmt* S) {
  assert(S != NULL);
  if (!BlkExprMap) { BlkExprMap = (void*) PopulateBlkExprMap(*this); }
  
  BlkExprMapTy* M = reinterpret_cast<BlkExprMapTy*>(BlkExprMap);
  BlkExprMapTy::iterator I = M->find(S);
  
  if (I == M->end()) return CFG::BlkExprNumTy();
  else return CFG::BlkExprNumTy(I->second);
}

unsigned CFG::getNumBlkExprs() {
  if (const BlkExprMapTy* M = reinterpret_cast<const BlkExprMapTy*>(BlkExprMap))
    return M->size();
  else {
    // We assume callers interested in the number of BlkExprs will want
    // the map constructed if it doesn't already exist.
    BlkExprMap = (void*) PopulateBlkExprMap(*this);
    return reinterpret_cast<BlkExprMapTy*>(BlkExprMap)->size();
  }
}

//===----------------------------------------------------------------------===//
// Cleanup: CFG dstor.
//===----------------------------------------------------------------------===//

CFG::~CFG() {
  delete reinterpret_cast<const BlkExprMapTy*>(BlkExprMap);
}
  
//===----------------------------------------------------------------------===//
// CFG pretty printing
//===----------------------------------------------------------------------===//

namespace {

class VISIBILITY_HIDDEN StmtPrinterHelper : public PrinterHelper  {
                          
  typedef llvm::DenseMap<Stmt*,std::pair<unsigned,unsigned> > StmtMapTy;
  StmtMapTy StmtMap;
  signed CurrentBlock;
  unsigned CurrentStmt;
  const LangOptions &LangOpts;
public:

  StmtPrinterHelper(const CFG* cfg, const LangOptions &LO)
    : CurrentBlock(0), CurrentStmt(0), LangOpts(LO) {
    for (CFG::const_iterator I = cfg->begin(), E = cfg->end(); I != E; ++I ) {
      unsigned j = 1;
      for (CFGBlock::const_iterator BI = I->begin(), BEnd = I->end() ;
           BI != BEnd; ++BI, ++j )
        StmtMap[*BI] = std::make_pair(I->getBlockID(),j);
      }
  }
            
  virtual ~StmtPrinterHelper() {}
  
  const LangOptions &getLangOpts() const { return LangOpts; }
  void setBlockID(signed i) { CurrentBlock = i; }
  void setStmtID(unsigned i) { CurrentStmt = i; }
  
  virtual bool handledStmt(Stmt* Terminator, llvm::raw_ostream& OS) {
    
    StmtMapTy::iterator I = StmtMap.find(Terminator);

    if (I == StmtMap.end())
      return false;
    
    if (CurrentBlock >= 0 && I->second.first == (unsigned) CurrentBlock 
                          && I->second.second == CurrentStmt)
      return false;
      
      OS << "[B" << I->second.first << "." << I->second.second << "]";
    return true;
  }
};
} // end anonymous namespace


namespace {
class VISIBILITY_HIDDEN CFGBlockTerminatorPrint
  : public StmtVisitor<CFGBlockTerminatorPrint,void> {
  
  llvm::raw_ostream& OS;
  StmtPrinterHelper* Helper;
  PrintingPolicy Policy;

public:
  CFGBlockTerminatorPrint(llvm::raw_ostream& os, StmtPrinterHelper* helper,
                          const PrintingPolicy &Policy)
    : OS(os), Helper(helper), Policy(Policy) {}
  
  void VisitIfStmt(IfStmt* I) {
    OS << "if ";
    I->getCond()->printPretty(OS,Helper,Policy);
  }
  
  // Default case.
  void VisitStmt(Stmt* Terminator) { Terminator->printPretty(OS, Helper, Policy); }
  
  void VisitForStmt(ForStmt* F) {
    OS << "for (" ;
    if (F->getInit()) OS << "...";
    OS << "; ";
    if (Stmt* C = F->getCond()) C->printPretty(OS, Helper, Policy);
    OS << "; ";
    if (F->getInc()) OS << "...";
    OS << ")";
  }
  
  void VisitWhileStmt(WhileStmt* W) {
    OS << "while " ;
    if (Stmt* C = W->getCond()) C->printPretty(OS, Helper, Policy);
  }
  
  void VisitDoStmt(DoStmt* D) {
    OS << "do ... while ";
    if (Stmt* C = D->getCond()) C->printPretty(OS, Helper, Policy);
  }
  
  void VisitSwitchStmt(SwitchStmt* Terminator) {
    OS << "switch ";
    Terminator->getCond()->printPretty(OS, Helper, Policy);
  }
  
  void VisitConditionalOperator(ConditionalOperator* C) {
    C->getCond()->printPretty(OS, Helper, Policy);
    OS << " ? ... : ...";  
  }
  
  void VisitChooseExpr(ChooseExpr* C) {
    OS << "__builtin_choose_expr( ";
    C->getCond()->printPretty(OS, Helper, Policy);
    OS << " )";
  }
  
  void VisitIndirectGotoStmt(IndirectGotoStmt* I) {
    OS << "goto *";
    I->getTarget()->printPretty(OS, Helper, Policy);
  }
  
  void VisitBinaryOperator(BinaryOperator* B) {
    if (!B->isLogicalOp()) {
      VisitExpr(B);
      return;
    }
    
    B->getLHS()->printPretty(OS, Helper, Policy);
    
    switch (B->getOpcode()) {
      case BinaryOperator::LOr:
        OS << " || ...";
        return;
      case BinaryOperator::LAnd:
        OS << " && ...";
        return;
      default:
        assert(false && "Invalid logical operator.");
    }  
  }
  
  void VisitExpr(Expr* E) {
    E->printPretty(OS, Helper, Policy);
  }                                                       
};
} // end anonymous namespace

  
static void print_stmt(llvm::raw_ostream &OS, StmtPrinterHelper* Helper,
                       Stmt* Terminator) {
  if (Helper) {
    // special printing for statement-expressions.
    if (StmtExpr* SE = dyn_cast<StmtExpr>(Terminator)) {
      CompoundStmt* Sub = SE->getSubStmt();
      
      if (Sub->child_begin() != Sub->child_end()) {
        OS << "({ ... ; ";
        Helper->handledStmt(*SE->getSubStmt()->body_rbegin(),OS);
        OS << " })\n";
        return;
      }
    }
    
    // special printing for comma expressions.
    if (BinaryOperator* B = dyn_cast<BinaryOperator>(Terminator)) {
      if (B->getOpcode() == BinaryOperator::Comma) {
        OS << "... , ";
        Helper->handledStmt(B->getRHS(),OS);
        OS << '\n';
        return;
      }          
    }  
  }
  
  Terminator->printPretty(OS, Helper, PrintingPolicy(Helper->getLangOpts()));
  
  // Expressions need a newline.
  if (isa<Expr>(Terminator)) OS << '\n';
}
  
static void print_block(llvm::raw_ostream& OS, const CFG* cfg,
                        const CFGBlock& B,
                        StmtPrinterHelper* Helper, bool print_edges) {
 
  if (Helper) Helper->setBlockID(B.getBlockID());
  
  // Print the header.
  OS << "\n [ B" << B.getBlockID();  
    
  if (&B == &cfg->getEntry())
    OS << " (ENTRY) ]\n";
  else if (&B == &cfg->getExit())
    OS << " (EXIT) ]\n";
  else if (&B == cfg->getIndirectGotoBlock())
    OS << " (INDIRECT GOTO DISPATCH) ]\n";
  else
    OS << " ]\n";
 
  // Print the label of this block.
  if (Stmt* Terminator = const_cast<Stmt*>(B.getLabel())) {

    if (print_edges)
      OS << "    ";
  
    if (LabelStmt* L = dyn_cast<LabelStmt>(Terminator))
      OS << L->getName();
    else if (CaseStmt* C = dyn_cast<CaseStmt>(Terminator)) {
      OS << "case ";
      C->getLHS()->printPretty(OS, Helper,
                               PrintingPolicy(Helper->getLangOpts()));
      if (C->getRHS()) {
        OS << " ... ";
        C->getRHS()->printPretty(OS, Helper,
                                 PrintingPolicy(Helper->getLangOpts()));
      }
    }  
    else if (isa<DefaultStmt>(Terminator))
      OS << "default";
    else
      assert(false && "Invalid label statement in CFGBlock.");
 
    OS << ":\n";
  }
 
  // Iterate through the statements in the block and print them.
  unsigned j = 1;
  
  for (CFGBlock::const_iterator I = B.begin(), E = B.end() ;
       I != E ; ++I, ++j ) {
       
    // Print the statement # in the basic block and the statement itself.
    if (print_edges)
      OS << "    ";
      
    OS << llvm::format("%3d", j) << ": ";
    
    if (Helper)
      Helper->setStmtID(j);
     
    print_stmt(OS,Helper,*I);
  }
 
  // Print the terminator of this block.
  if (B.getTerminator()) {
    if (print_edges)
      OS << "    ";
      
    OS << "  T: ";
    
    if (Helper) Helper->setBlockID(-1);
    
    CFGBlockTerminatorPrint TPrinter(OS, Helper,
                                     PrintingPolicy(Helper->getLangOpts()));
    TPrinter.Visit(const_cast<Stmt*>(B.getTerminator()));
    OS << '\n';
  }
 
  if (print_edges) {
    // Print the predecessors of this block.
    OS << "    Predecessors (" << B.pred_size() << "):";
    unsigned i = 0;

    for (CFGBlock::const_pred_iterator I = B.pred_begin(), E = B.pred_end();
         I != E; ++I, ++i) {
                  
      if (i == 8 || (i-8) == 0)
        OS << "\n     ";
      
      OS << " B" << (*I)->getBlockID();
    }
    
    OS << '\n';
 
    // Print the successors of this block.
    OS << "    Successors (" << B.succ_size() << "):";
    i = 0;

    for (CFGBlock::const_succ_iterator I = B.succ_begin(), E = B.succ_end();
         I != E; ++I, ++i) {
         
      if (i == 8 || (i-8) % 10 == 0)
        OS << "\n    ";

      OS << " B" << (*I)->getBlockID();
    }
    
    OS << '\n';
  }
}                   


/// dump - A simple pretty printer of a CFG that outputs to stderr.
void CFG::dump(const LangOptions &LO) const { print(llvm::errs(), LO); }

/// print - A simple pretty printer of a CFG that outputs to an ostream.
void CFG::print(llvm::raw_ostream &OS, const LangOptions &LO) const {
  StmtPrinterHelper Helper(this, LO);
  
  // Print the entry block.
  print_block(OS, this, getEntry(), &Helper, true);
                    
  // Iterate through the CFGBlocks and print them one by one.
  for (const_iterator I = Blocks.begin(), E = Blocks.end() ; I != E ; ++I) {
    // Skip the entry block, because we already printed it.
    if (&(*I) == &getEntry() || &(*I) == &getExit())
      continue;
      
    print_block(OS, this, *I, &Helper, true);
  }
  
  // Print the exit block.
  print_block(OS, this, getExit(), &Helper, true);
  OS.flush();
}  

/// dump - A simply pretty printer of a CFGBlock that outputs to stderr.
void CFGBlock::dump(const CFG* cfg, const LangOptions &LO) const {
  print(llvm::errs(), cfg, LO);
}

/// print - A simple pretty printer of a CFGBlock that outputs to an ostream.
///   Generally this will only be called from CFG::print.
void CFGBlock::print(llvm::raw_ostream& OS, const CFG* cfg,
                     const LangOptions &LO) const {
  StmtPrinterHelper Helper(cfg, LO);
  print_block(OS, cfg, *this, &Helper, true);
}

/// printTerminator - A simple pretty printer of the terminator of a CFGBlock.
void CFGBlock::printTerminator(llvm::raw_ostream &OS,
                               const LangOptions &LO) const {  
  CFGBlockTerminatorPrint TPrinter(OS, NULL, PrintingPolicy(LO));
  TPrinter.Visit(const_cast<Stmt*>(getTerminator()));
}

Stmt* CFGBlock::getTerminatorCondition() {
  
  if (!Terminator)
    return NULL;
  
  Expr* E = NULL;
  
  switch (Terminator->getStmtClass()) {
    default:
      break;
      
    case Stmt::ForStmtClass:
      E = cast<ForStmt>(Terminator)->getCond();
      break;
      
    case Stmt::WhileStmtClass:
      E = cast<WhileStmt>(Terminator)->getCond();
      break;
      
    case Stmt::DoStmtClass:
      E = cast<DoStmt>(Terminator)->getCond();
      break;
      
    case Stmt::IfStmtClass:
      E = cast<IfStmt>(Terminator)->getCond();
      break;
      
    case Stmt::ChooseExprClass:
      E = cast<ChooseExpr>(Terminator)->getCond();
      break;
      
    case Stmt::IndirectGotoStmtClass:
      E = cast<IndirectGotoStmt>(Terminator)->getTarget();
      break;
      
    case Stmt::SwitchStmtClass:
      E = cast<SwitchStmt>(Terminator)->getCond();
      break;
      
    case Stmt::ConditionalOperatorClass:
      E = cast<ConditionalOperator>(Terminator)->getCond();
      break;
      
    case Stmt::BinaryOperatorClass: // '&&' and '||'
      E = cast<BinaryOperator>(Terminator)->getLHS();
      break;
      
    case Stmt::ObjCForCollectionStmtClass:
      return Terminator;      
  }
  
  return E ? E->IgnoreParens() : NULL;
}

bool CFGBlock::hasBinaryBranchTerminator() const {
  
  if (!Terminator)
    return false;
  
  Expr* E = NULL;
  
  switch (Terminator->getStmtClass()) {
    default:
      return false;
      
    case Stmt::ForStmtClass:      
    case Stmt::WhileStmtClass:
    case Stmt::DoStmtClass:
    case Stmt::IfStmtClass:
    case Stmt::ChooseExprClass:
    case Stmt::ConditionalOperatorClass:
    case Stmt::BinaryOperatorClass:
      return true;      
  }
  
  return E ? E->IgnoreParens() : NULL;
}


//===----------------------------------------------------------------------===//
// CFG Graphviz Visualization
//===----------------------------------------------------------------------===//


#ifndef NDEBUG
static StmtPrinterHelper* GraphHelper;  
#endif

void CFG::viewCFG(const LangOptions &LO) const {
#ifndef NDEBUG
  StmtPrinterHelper H(this, LO);
  GraphHelper = &H;
  llvm::ViewGraph(this,"CFG");
  GraphHelper = NULL;
#endif
}

namespace llvm {
template<>
struct DOTGraphTraits<const CFG*> : public DefaultDOTGraphTraits {
  static std::string getNodeLabel(const CFGBlock* Node, const CFG* Graph,
                                  bool ShortNames) {

#ifndef NDEBUG
    std::string OutSStr;
    llvm::raw_string_ostream Out(OutSStr);
    print_block(Out,Graph, *Node, GraphHelper, false);
    std::string& OutStr = Out.str();

    if (OutStr[0] == '\n') OutStr.erase(OutStr.begin());

    // Process string output to make it nicer...
    for (unsigned i = 0; i != OutStr.length(); ++i)
      if (OutStr[i] == '\n') {                            // Left justify
        OutStr[i] = '\\';
        OutStr.insert(OutStr.begin()+i+1, 'l');
      }
      
    return OutStr;
#else
    return "";
#endif
  }
};
} // end namespace llvm