SCCP.cpp

//===- SCCP.cpp - Sparse Conditional Constant Propagation -----------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements sparse conditional constant propagation and merging:
//
// Specifically, this:
//   * Assumes values are constant unless proven otherwise
//   * Assumes BasicBlocks are dead unless proven otherwise
//   * Proves values to be constant, and replaces them with constants
//   * Proves conditional branches to be unconditional
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "sccp"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/IPO.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Instructions.h"
#include "llvm/Pass.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Support/CallSite.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/InstVisitor.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/PointerIntPair.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include <algorithm>
#include <map>
using namespace llvm;

STATISTIC(NumInstRemoved, "Number of instructions removed");
STATISTIC(NumDeadBlocks , "Number of basic blocks unreachable");

STATISTIC(IPNumInstRemoved, "Number of instructions removed by IPSCCP");
STATISTIC(IPNumArgsElimed ,"Number of arguments constant propagated by IPSCCP");
STATISTIC(IPNumGlobalConst, "Number of globals found to be constant by IPSCCP");

namespace {
/// LatticeVal class - This class represents the different lattice values that
/// an LLVM value may occupy.  It is a simple class with value semantics.
///
class LatticeVal {
  enum LatticeValueTy {
    /// undefined - This LLVM Value has no known value yet.
    undefined,
    
    /// constant - This LLVM Value has a specific constant value.
    constant,

    /// forcedconstant - This LLVM Value was thought to be undef until
    /// ResolvedUndefsIn.  This is treated just like 'constant', but if merged
    /// with another (different) constant, it goes to overdefined, instead of
    /// asserting.
    forcedconstant,
    
    /// overdefined - This instruction is not known to be constant, and we know
    /// it has a value.
    overdefined
  };

  /// Val: This stores the current lattice value along with the Constant* for
  /// the constant if this is a 'constant' or 'forcedconstant' value.
  PointerIntPair<Constant *, 2, LatticeValueTy> Val;
  
  LatticeValueTy getLatticeValue() const {
    return Val.getInt();
  }
  
public:
  LatticeVal() : Val(0, undefined) {}
  
  bool isUndefined() const { return getLatticeValue() == undefined; }
  bool isConstant() const {
    return getLatticeValue() == constant || getLatticeValue() == forcedconstant;
  }
  bool isOverdefined() const { return getLatticeValue() == overdefined; }
  
  Constant *getConstant() const {
    assert(isConstant() && "Cannot get the constant of a non-constant!");
    return Val.getPointer();
  }
  
  /// markOverdefined - Return true if this is a change in status.
  bool markOverdefined() {
    if (isOverdefined())
      return false;
    
    Val.setInt(overdefined);
    return true;
  }

  /// markConstant - Return true if this is a change in status.
  bool markConstant(Constant *V) {
    if (getLatticeValue() == constant) { // Constant but not forcedconstant.
      assert(getConstant() == V && "Marking constant with different value");
      return false;
    }
    
    if (isUndefined()) {
      Val.setInt(constant);
      assert(V && "Marking constant with NULL");
      Val.setPointer(V);
    } else {
      assert(getLatticeValue() == forcedconstant && 
             "Cannot move from overdefined to constant!");
      // Stay at forcedconstant if the constant is the same.
      if (V == getConstant()) return false;
      
      // Otherwise, we go to overdefined.  Assumptions made based on the
      // forced value are possibly wrong.  Assuming this is another constant
      // could expose a contradiction.
      Val.setInt(overdefined);
    }
    return true;
  }

  /// getConstantInt - If this is a constant with a ConstantInt value, return it
  /// otherwise return null.
  ConstantInt *getConstantInt() const {
    if (isConstant())
      return dyn_cast<ConstantInt>(getConstant());
    return 0;
  }
  
  void markForcedConstant(Constant *V) {
    assert(isUndefined() && "Can't force a defined value!");
    Val.setInt(forcedconstant);
    Val.setPointer(V);
  }
};
} // end anonymous namespace.


namespace {

//===----------------------------------------------------------------------===//
//
/// SCCPSolver - This class is a general purpose solver for Sparse Conditional
/// Constant Propagation.
///
class SCCPSolver : public InstVisitor<SCCPSolver> {
  const TargetData *TD;
  SmallPtrSet<BasicBlock*, 8> BBExecutable;// The BBs that are executable.
  DenseMap<Value*, LatticeVal> ValueState;  // The state each value is in.

  /// StructValueState - This maintains ValueState for values that have
  /// StructType, for example for formal arguments, calls, insertelement, etc.
  ///
  DenseMap<std::pair<Value*, unsigned>, LatticeVal> StructValueState;
  
  /// GlobalValue - If we are tracking any values for the contents of a global
  /// variable, we keep a mapping from the constant accessor to the element of
  /// the global, to the currently known value.  If the value becomes
  /// overdefined, it's entry is simply removed from this map.
  DenseMap<GlobalVariable*, LatticeVal> TrackedGlobals;

  /// TrackedRetVals - If we are tracking arguments into and the return
  /// value out of a function, it will have an entry in this map, indicating
  /// what the known return value for the function is.
  DenseMap<Function*, LatticeVal> TrackedRetVals;

  /// TrackedMultipleRetVals - Same as TrackedRetVals, but used for functions
  /// that return multiple values.
  DenseMap<std::pair<Function*, unsigned>, LatticeVal> TrackedMultipleRetVals;
  
  /// MRVFunctionsTracked - Each function in TrackedMultipleRetVals is
  /// represented here for efficient lookup.
  SmallPtrSet<Function*, 16> MRVFunctionsTracked;

  /// TrackingIncomingArguments - This is the set of functions for whose
  /// arguments we make optimistic assumptions about and try to prove as
  /// constants.
  SmallPtrSet<Function*, 16> TrackingIncomingArguments;
  
  /// The reason for two worklists is that overdefined is the lowest state
  /// on the lattice, and moving things to overdefined as fast as possible
  /// makes SCCP converge much faster.
  ///
  /// By having a separate worklist, we accomplish this because everything
  /// possibly overdefined will become overdefined at the soonest possible
  /// point.
  SmallVector<Value*, 64> OverdefinedInstWorkList;
  SmallVector<Value*, 64> InstWorkList;


  SmallVector<BasicBlock*, 64>  BBWorkList;  // The BasicBlock work list

  /// UsersOfOverdefinedPHIs - Keep track of any users of PHI nodes that are not
  /// overdefined, despite the fact that the PHI node is overdefined.
  std::multimap<PHINode*, Instruction*> UsersOfOverdefinedPHIs;

  /// KnownFeasibleEdges - Entries in this set are edges which have already had
  /// PHI nodes retriggered.
  typedef std::pair<BasicBlock*, BasicBlock*> Edge;
  DenseSet<Edge> KnownFeasibleEdges;
public:
  SCCPSolver(const TargetData *td) : TD(td) {}

  /// MarkBlockExecutable - This method can be used by clients to mark all of
  /// the blocks that are known to be intrinsically live in the processed unit.
  ///
  /// This returns true if the block was not considered live before.
  bool MarkBlockExecutable(BasicBlock *BB) {
    if (!BBExecutable.insert(BB)) return false;
    DEBUG(errs() << "Marking Block Executable: " << BB->getName() << "\n");
    BBWorkList.push_back(BB);  // Add the block to the work list!
    return true;
  }

  /// TrackValueOfGlobalVariable - Clients can use this method to
  /// inform the SCCPSolver that it should track loads and stores to the
  /// specified global variable if it can.  This is only legal to call if
  /// performing Interprocedural SCCP.
  void TrackValueOfGlobalVariable(GlobalVariable *GV) {
    // We only track the contents of scalar globals.
    if (GV->getType()->getElementType()->isSingleValueType()) {
      LatticeVal &IV = TrackedGlobals[GV];
      if (!isa<UndefValue>(GV->getInitializer()))
        IV.markConstant(GV->getInitializer());
    }
  }

  /// AddTrackedFunction - If the SCCP solver is supposed to track calls into
  /// and out of the specified function (which cannot have its address taken),
  /// this method must be called.
  void AddTrackedFunction(Function *F) {
    // Add an entry, F -> undef.
    if (const StructType *STy = dyn_cast<StructType>(F->getReturnType())) {
      MRVFunctionsTracked.insert(F);
      for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i)
        TrackedMultipleRetVals.insert(std::make_pair(std::make_pair(F, i),
                                                     LatticeVal()));
    } else
      TrackedRetVals.insert(std::make_pair(F, LatticeVal()));
  }

  void AddArgumentTrackedFunction(Function *F) {
    TrackingIncomingArguments.insert(F);
  }
  
  /// Solve - Solve for constants and executable blocks.
  ///
  void Solve();

  /// ResolvedUndefsIn - While solving the dataflow for a function, we assume
  /// that branches on undef values cannot reach any of their successors.
  /// However, this is not a safe assumption.  After we solve dataflow, this
  /// method should be use to handle this.  If this returns true, the solver
  /// should be rerun.
  bool ResolvedUndefsIn(Function &F);

  bool isBlockExecutable(BasicBlock *BB) const {
    return BBExecutable.count(BB);
  }

  LatticeVal getLatticeValueFor(Value *V) const {
    DenseMap<Value*, LatticeVal>::const_iterator I = ValueState.find(V);
    assert(I != ValueState.end() && "V is not in valuemap!");
    return I->second;
  }
  
  LatticeVal getStructLatticeValueFor(Value *V, unsigned i) const {
    DenseMap<std::pair<Value*, unsigned>, LatticeVal>::const_iterator I = 
      StructValueState.find(std::make_pair(V, i));
    assert(I != StructValueState.end() && "V is not in valuemap!");
    return I->second;
  }

  /// getTrackedRetVals - Get the inferred return value map.
  ///
  const DenseMap<Function*, LatticeVal> &getTrackedRetVals() {
    return TrackedRetVals;
  }

  /// getTrackedGlobals - Get and return the set of inferred initializers for
  /// global variables.
  const DenseMap<GlobalVariable*, LatticeVal> &getTrackedGlobals() {
    return TrackedGlobals;
  }

  void markOverdefined(Value *V) {
    assert(!isa<StructType>(V->getType()) && "Should use other method");
    markOverdefined(ValueState[V], V);
  }

  /// markAnythingOverdefined - Mark the specified value overdefined.  This
  /// works with both scalars and structs.
  void markAnythingOverdefined(Value *V) {
    if (const StructType *STy = dyn_cast<StructType>(V->getType()))
      for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i)
        markOverdefined(getStructValueState(V, i), V);
    else
      markOverdefined(V);
  }
  
private:
  // markConstant - Make a value be marked as "constant".  If the value
  // is not already a constant, add it to the instruction work list so that
  // the users of the instruction are updated later.
  //
  void markConstant(LatticeVal &IV, Value *V, Constant *C) {
    if (!IV.markConstant(C)) return;
    DEBUG(errs() << "markConstant: " << *C << ": " << *V << '\n');
    InstWorkList.push_back(V);
  }
  
  void markConstant(Value *V, Constant *C) {
    assert(!isa<StructType>(V->getType()) && "Should use other method");
    markConstant(ValueState[V], V, C);
  }

  void markForcedConstant(Value *V, Constant *C) {
    assert(!isa<StructType>(V->getType()) && "Should use other method");
    ValueState[V].markForcedConstant(C);
    DEBUG(errs() << "markForcedConstant: " << *C << ": " << *V << '\n');
    InstWorkList.push_back(V);
  }
  
  
  // markOverdefined - Make a value be marked as "overdefined". If the
  // value is not already overdefined, add it to the overdefined instruction
  // work list so that the users of the instruction are updated later.
  void markOverdefined(LatticeVal &IV, Value *V) {
    if (!IV.markOverdefined()) return;
    
    DEBUG(errs() << "markOverdefined: ";
          if (Function *F = dyn_cast<Function>(V))
            errs() << "Function '" << F->getName() << "'\n";
          else
            errs() << *V << '\n');
    // Only instructions go on the work list
    OverdefinedInstWorkList.push_back(V);
  }

  void mergeInValue(LatticeVal &IV, Value *V, LatticeVal MergeWithV) {
    if (IV.isOverdefined() || MergeWithV.isUndefined())
      return;  // Noop.
    if (MergeWithV.isOverdefined())
      markOverdefined(IV, V);
    else if (IV.isUndefined())
      markConstant(IV, V, MergeWithV.getConstant());
    else if (IV.getConstant() != MergeWithV.getConstant())
      markOverdefined(IV, V);
  }
  
  void mergeInValue(Value *V, LatticeVal MergeWithV) {
    assert(!isa<StructType>(V->getType()) && "Should use other method");
    mergeInValue(ValueState[V], V, MergeWithV);
  }


  /// getValueState - Return the LatticeVal object that corresponds to the
  /// value.  This function handles the case when the value hasn't been seen yet
  /// by properly seeding constants etc.
  LatticeVal &getValueState(Value *V) {
    assert(!isa<StructType>(V->getType()) && "Should use getStructValueState");

    std::pair<DenseMap<Value*, LatticeVal>::iterator, bool> I =
      ValueState.insert(std::make_pair(V, LatticeVal()));
    LatticeVal &LV = I.first->second;

    if (!I.second)
      return LV;  // Common case, already in the map.

    if (Constant *C = dyn_cast<Constant>(V)) {
      // Undef values remain undefined.
      if (!isa<UndefValue>(V))
        LV.markConstant(C);          // Constants are constant
    }
    
    // All others are underdefined by default.
    return LV;
  }

  /// getStructValueState - Return the LatticeVal object that corresponds to the
  /// value/field pair.  This function handles the case when the value hasn't
  /// been seen yet by properly seeding constants etc.
  LatticeVal &getStructValueState(Value *V, unsigned i) {
    assert(isa<StructType>(V->getType()) && "Should use getValueState");
    assert(i < cast<StructType>(V->getType())->getNumElements() &&
           "Invalid element #");

    std::pair<DenseMap<std::pair<Value*, unsigned>, LatticeVal>::iterator,
              bool> I = StructValueState.insert(
                        std::make_pair(std::make_pair(V, i), LatticeVal()));
    LatticeVal &LV = I.first->second;

    if (!I.second)
      return LV;  // Common case, already in the map.

    if (Constant *C = dyn_cast<Constant>(V)) {
      if (isa<UndefValue>(C))
        ; // Undef values remain undefined.
      else if (ConstantStruct *CS = dyn_cast<ConstantStruct>(C))
        LV.markConstant(CS->getOperand(i));      // Constants are constant.
      else if (isa<ConstantAggregateZero>(C)) {
        const Type *FieldTy = cast<StructType>(V->getType())->getElementType(i);
        LV.markConstant(Constant::getNullValue(FieldTy));
      } else
        LV.markOverdefined();      // Unknown sort of constant.
    }
    
    // All others are underdefined by default.
    return LV;
  }
  

  /// markEdgeExecutable - Mark a basic block as executable, adding it to the BB
  /// work list if it is not already executable.
  void markEdgeExecutable(BasicBlock *Source, BasicBlock *Dest) {
    if (!KnownFeasibleEdges.insert(Edge(Source, Dest)).second)
      return;  // This edge is already known to be executable!

    if (!MarkBlockExecutable(Dest)) {
      // If the destination is already executable, we just made an *edge*
      // feasible that wasn't before.  Revisit the PHI nodes in the block
      // because they have potentially new operands.
      DEBUG(errs() << "Marking Edge Executable: " << Source->getName()
            << " -> " << Dest->getName() << "\n");

      PHINode *PN;
      for (BasicBlock::iterator I = Dest->begin();
           (PN = dyn_cast<PHINode>(I)); ++I)
        visitPHINode(*PN);
    }
  }

  // getFeasibleSuccessors - Return a vector of booleans to indicate which
  // successors are reachable from a given terminator instruction.
  //
  void getFeasibleSuccessors(TerminatorInst &TI, SmallVector<bool, 16> &Succs);

  // isEdgeFeasible - Return true if the control flow edge from the 'From' basic
  // block to the 'To' basic block is currently feasible.
  //
  bool isEdgeFeasible(BasicBlock *From, BasicBlock *To);

  // OperandChangedState - This method is invoked on all of the users of an
  // instruction that was just changed state somehow.  Based on this
  // information, we need to update the specified user of this instruction.
  //
  void OperandChangedState(Instruction *I) {
    if (BBExecutable.count(I->getParent()))   // Inst is executable?
      visit(*I);
  }
  
  /// RemoveFromOverdefinedPHIs - If I has any entries in the
  /// UsersOfOverdefinedPHIs map for PN, remove them now.
  void RemoveFromOverdefinedPHIs(Instruction *I, PHINode *PN) {
    if (UsersOfOverdefinedPHIs.empty()) return;
    std::multimap<PHINode*, Instruction*>::iterator It, E;
    tie(It, E) = UsersOfOverdefinedPHIs.equal_range(PN);
    while (It != E) {
      if (It->second == I)
        UsersOfOverdefinedPHIs.erase(It++);
      else
        ++It;
    }
  }

private:
  friend class InstVisitor<SCCPSolver>;

  // visit implementations - Something changed in this instruction.  Either an
  // operand made a transition, or the instruction is newly executable.  Change
  // the value type of I to reflect these changes if appropriate.
  void visitPHINode(PHINode &I);

  // Terminators
  void visitReturnInst(ReturnInst &I);
  void visitTerminatorInst(TerminatorInst &TI);

  void visitCastInst(CastInst &I);
  void visitSelectInst(SelectInst &I);
  void visitBinaryOperator(Instruction &I);
  void visitCmpInst(CmpInst &I);
  void visitExtractElementInst(ExtractElementInst &I);
  void visitInsertElementInst(InsertElementInst &I);
  void visitShuffleVectorInst(ShuffleVectorInst &I);
  void visitExtractValueInst(ExtractValueInst &EVI);
  void visitInsertValueInst(InsertValueInst &IVI);

  // Instructions that cannot be folded away.
  void visitStoreInst     (StoreInst &I);
  void visitLoadInst      (LoadInst &I);
  void visitGetElementPtrInst(GetElementPtrInst &I);
  void visitCallInst      (CallInst &I) {
    visitCallSite(CallSite::get(&I));
  }
  void visitInvokeInst    (InvokeInst &II) {
    visitCallSite(CallSite::get(&II));
    visitTerminatorInst(II);
  }
  void visitCallSite      (CallSite CS);
  void visitUnwindInst    (TerminatorInst &I) { /*returns void*/ }
  void visitUnreachableInst(TerminatorInst &I) { /*returns void*/ }
  void visitAllocaInst    (Instruction &I) { markOverdefined(&I); }
  void visitVANextInst    (Instruction &I) { markOverdefined(&I); }
  void visitVAArgInst     (Instruction &I) { markAnythingOverdefined(&I); }

  void visitInstruction(Instruction &I) {
    // If a new instruction is added to LLVM that we don't handle.
    errs() << "SCCP: Don't know how to handle: " << I;
    markAnythingOverdefined(&I);   // Just in case
  }
};

} // end anonymous namespace


// getFeasibleSuccessors - Return a vector of booleans to indicate which
// successors are reachable from a given terminator instruction.
//
void SCCPSolver::getFeasibleSuccessors(TerminatorInst &TI,
                                       SmallVector<bool, 16> &Succs) {
  Succs.resize(TI.getNumSuccessors());
  if (BranchInst *BI = dyn_cast<BranchInst>(&TI)) {
    if (BI->isUnconditional()) {
      Succs[0] = true;
      return;
    }
    
    LatticeVal BCValue = getValueState(BI->getCondition());
    ConstantInt *CI = BCValue.getConstantInt();
    if (CI == 0) {
      // Overdefined condition variables, and branches on unfoldable constant
      // conditions, mean the branch could go either way.
      if (!BCValue.isUndefined())
        Succs[0] = Succs[1] = true;
      return;
    }
    
    // Constant condition variables mean the branch can only go a single way.
    Succs[CI->isZero()] = true;
    return;
  }
  
  if (isa<InvokeInst>(TI)) {
    // Invoke instructions successors are always executable.
    Succs[0] = Succs[1] = true;
    return;
  }
  
  if (SwitchInst *SI = dyn_cast<SwitchInst>(&TI)) {
    LatticeVal SCValue = getValueState(SI->getCondition());
    ConstantInt *CI = SCValue.getConstantInt();
    
    if (CI == 0) {   // Overdefined or undefined condition?
      // All destinations are executable!
      if (!SCValue.isUndefined())
        Succs.assign(TI.getNumSuccessors(), true);
      return;
    }
      
    Succs[SI->findCaseValue(CI)] = true;
    return;
  }
  
  // TODO: This could be improved if the operand is a [cast of a] BlockAddress.
  if (isa<IndirectBrInst>(&TI)) {
    // Just mark all destinations executable!
    Succs.assign(TI.getNumSuccessors(), true);
    return;
  }
  
#ifndef NDEBUG
  errs() << "Unknown terminator instruction: " << TI << '\n';
#endif
  llvm_unreachable("SCCP: Don't know how to handle this terminator!");
}


// isEdgeFeasible - Return true if the control flow edge from the 'From' basic
// block to the 'To' basic block is currently feasible.
//
bool SCCPSolver::isEdgeFeasible(BasicBlock *From, BasicBlock *To) {
  assert(BBExecutable.count(To) && "Dest should always be alive!");

  // Make sure the source basic block is executable!!
  if (!BBExecutable.count(From)) return false;

  // Check to make sure this edge itself is actually feasible now.
  TerminatorInst *TI = From->getTerminator();
  if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
    if (BI->isUnconditional())
      return true;
    
    LatticeVal BCValue = getValueState(BI->getCondition());

    // Overdefined condition variables mean the branch could go either way,
    // undef conditions mean that neither edge is feasible yet.
    ConstantInt *CI = BCValue.getConstantInt();
    if (CI == 0)
      return !BCValue.isUndefined();
    
    // Constant condition variables mean the branch can only go a single way.
    return BI->getSuccessor(CI->isZero()) == To;
  }
  
  // Invoke instructions successors are always executable.
  if (isa<InvokeInst>(TI))
    return true;
  
  if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
    LatticeVal SCValue = getValueState(SI->getCondition());
    ConstantInt *CI = SCValue.getConstantInt();
    
    if (CI == 0)
      return !SCValue.isUndefined();

    // Make sure to skip the "default value" which isn't a value
    for (unsigned i = 1, E = SI->getNumSuccessors(); i != E; ++i)
      if (SI->getSuccessorValue(i) == CI) // Found the taken branch.
        return SI->getSuccessor(i) == To;

    // If the constant value is not equal to any of the branches, we must
    // execute default branch.
    return SI->getDefaultDest() == To;
  }
  
  // Just mark all destinations executable!
  // TODO: This could be improved if the operand is a [cast of a] BlockAddress.
  if (isa<IndirectBrInst>(&TI))
    return true;
  
#ifndef NDEBUG
  errs() << "Unknown terminator instruction: " << *TI << '\n';
#endif
  llvm_unreachable(0);
}

// visit Implementations - Something changed in this instruction, either an
// operand made a transition, or the instruction is newly executable.  Change
// the value type of I to reflect these changes if appropriate.  This method
// makes sure to do the following actions:
//
// 1. If a phi node merges two constants in, and has conflicting value coming
//    from different branches, or if the PHI node merges in an overdefined
//    value, then the PHI node becomes overdefined.
// 2. If a phi node merges only constants in, and they all agree on value, the
//    PHI node becomes a constant value equal to that.
// 3. If V <- x (op) y && isConstant(x) && isConstant(y) V = Constant
// 4. If V <- x (op) y && (isOverdefined(x) || isOverdefined(y)) V = Overdefined
// 5. If V <- MEM or V <- CALL or V <- (unknown) then V = Overdefined
// 6. If a conditional branch has a value that is constant, make the selected
//    destination executable
// 7. If a conditional branch has a value that is overdefined, make all
//    successors executable.
//
void SCCPSolver::visitPHINode(PHINode &PN) {
  // If this PN returns a struct, just mark the result overdefined.
  // TODO: We could do a lot better than this if code actually uses this.
  if (isa<StructType>(PN.getType()))
    return markAnythingOverdefined(&PN);
  
  if (getValueState(&PN).isOverdefined()) {
    // There may be instructions using this PHI node that are not overdefined
    // themselves.  If so, make sure that they know that the PHI node operand
    // changed.
    std::multimap<PHINode*, Instruction*>::iterator I, E;
    tie(I, E) = UsersOfOverdefinedPHIs.equal_range(&PN);
    if (I == E)
      return;
    
    SmallVector<Instruction*, 16> Users;
    for (; I != E; ++I)
      Users.push_back(I->second);
    while (!Users.empty())
      visit(Users.pop_back_val());
    return;  // Quick exit
  }

  // Super-extra-high-degree PHI nodes are unlikely to ever be marked constant,
  // and slow us down a lot.  Just mark them overdefined.
  if (PN.getNumIncomingValues() > 64)
    return markOverdefined(&PN);
  
  // Look at all of the executable operands of the PHI node.  If any of them
  // are overdefined, the PHI becomes overdefined as well.  If they are all
  // constant, and they agree with each other, the PHI becomes the identical
  // constant.  If they are constant and don't agree, the PHI is overdefined.
  // If there are no executable operands, the PHI remains undefined.
  //
  Constant *OperandVal = 0;
  for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
    LatticeVal IV = getValueState(PN.getIncomingValue(i));
    if (IV.isUndefined()) continue;  // Doesn't influence PHI node.

    if (!isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent()))
      continue;
    
    if (IV.isOverdefined())    // PHI node becomes overdefined!
      return markOverdefined(&PN);

    if (OperandVal == 0) {   // Grab the first value.
      OperandVal = IV.getConstant();
      continue;
    }
    
    // There is already a reachable operand.  If we conflict with it,
    // then the PHI node becomes overdefined.  If we agree with it, we
    // can continue on.
    
    // Check to see if there are two different constants merging, if so, the PHI
    // node is overdefined.
    if (IV.getConstant() != OperandVal)
      return markOverdefined(&PN);
  }

  // If we exited the loop, this means that the PHI node only has constant
  // arguments that agree with each other(and OperandVal is the constant) or
  // OperandVal is null because there are no defined incoming arguments.  If
  // this is the case, the PHI remains undefined.
  //
  if (OperandVal)
    markConstant(&PN, OperandVal);      // Acquire operand value
}




void SCCPSolver::visitReturnInst(ReturnInst &I) {
  if (I.getNumOperands() == 0) return;  // ret void

  Function *F = I.getParent()->getParent();
  Value *ResultOp = I.getOperand(0);
  
  // If we are tracking the return value of this function, merge it in.
  if (!TrackedRetVals.empty() && !isa<StructType>(ResultOp->getType())) {
    DenseMap<Function*, LatticeVal>::iterator TFRVI =
      TrackedRetVals.find(F);
    if (TFRVI != TrackedRetVals.end()) {
      mergeInValue(TFRVI->second, F, getValueState(ResultOp));
      return;
    }
  }
  
  // Handle functions that return multiple values.
  if (!TrackedMultipleRetVals.empty()) {
    if (const StructType *STy = dyn_cast<StructType>(ResultOp->getType()))
      if (MRVFunctionsTracked.count(F))
        for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i)
          mergeInValue(TrackedMultipleRetVals[std::make_pair(F, i)], F,
                       getStructValueState(ResultOp, i));
    
  }
}

void SCCPSolver::visitTerminatorInst(TerminatorInst &TI) {
  SmallVector<bool, 16> SuccFeasible;
  getFeasibleSuccessors(TI, SuccFeasible);

  BasicBlock *BB = TI.getParent();

  // Mark all feasible successors executable.
  for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
    if (SuccFeasible[i])
      markEdgeExecutable(BB, TI.getSuccessor(i));
}

void SCCPSolver::visitCastInst(CastInst &I) {
  LatticeVal OpSt = getValueState(I.getOperand(0));
  if (OpSt.isOverdefined())          // Inherit overdefinedness of operand
    markOverdefined(&I);
  else if (OpSt.isConstant())        // Propagate constant value
    markConstant(&I, ConstantExpr::getCast(I.getOpcode(), 
                                           OpSt.getConstant(), I.getType()));
}


void SCCPSolver::visitExtractValueInst(ExtractValueInst &EVI) {
  // If this returns a struct, mark all elements over defined, we don't track
  // structs in structs.
  if (isa<StructType>(EVI.getType()))
    return markAnythingOverdefined(&EVI);
    
  // If this is extracting from more than one level of struct, we don't know.
  if (EVI.getNumIndices() != 1)
    return markOverdefined(&EVI);

  Value *AggVal = EVI.getAggregateOperand();
  unsigned i = *EVI.idx_begin();
  LatticeVal EltVal = getStructValueState(AggVal, i);
  mergeInValue(getValueState(&EVI), &EVI, EltVal);
}

void SCCPSolver::visitInsertValueInst(InsertValueInst &IVI) {
  const StructType *STy = dyn_cast<StructType>(IVI.getType());
  if (STy == 0)
    return markOverdefined(&IVI);
  
  // If this has more than one index, we can't handle it, drive all results to
  // undef.
  if (IVI.getNumIndices() != 1)
    return markAnythingOverdefined(&IVI);
  
  Value *Aggr = IVI.getAggregateOperand();
  unsigned Idx = *IVI.idx_begin();
  
  // Compute the result based on what we're inserting.
  for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
    // This passes through all values that aren't the inserted element.
    if (i != Idx) {
      LatticeVal EltVal = getStructValueState(Aggr, i);
      mergeInValue(getStructValueState(&IVI, i), &IVI, EltVal);
      continue;
    }
    
    Value *Val = IVI.getInsertedValueOperand();
    if (isa<StructType>(Val->getType()))
      // We don't track structs in structs.
      markOverdefined(getStructValueState(&IVI, i), &IVI);
    else {
      LatticeVal InVal = getValueState(Val);
      mergeInValue(getStructValueState(&IVI, i), &IVI, InVal);
    }
  }
}

void SCCPSolver::visitSelectInst(SelectInst &I) {
  // If this select returns a struct, just mark the result overdefined.
  // TODO: We could do a lot better than this if code actually uses this.
  if (isa<StructType>(I.getType()))
    return markAnythingOverdefined(&I);
  
  LatticeVal CondValue = getValueState(I.getCondition());
  if (CondValue.isUndefined())
    return;
  
  if (ConstantInt *CondCB = CondValue.getConstantInt()) {
    Value *OpVal = CondCB->isZero() ? I.getFalseValue() : I.getTrueValue();
    mergeInValue(&I, getValueState(OpVal));
    return;
  }
  
  // Otherwise, the condition is overdefined or a constant we can't evaluate.
  // See if we can produce something better than overdefined based on the T/F
  // value.
  LatticeVal TVal = getValueState(I.getTrueValue());
  LatticeVal FVal = getValueState(I.getFalseValue());
  
  // select ?, C, C -> C.
  if (TVal.isConstant() && FVal.isConstant() && 
      TVal.getConstant() == FVal.getConstant())
    return markConstant(&I, FVal.getConstant());

  if (TVal.isUndefined())   // select ?, undef, X -> X.
    return mergeInValue(&I, FVal);
  if (FVal.isUndefined())   // select ?, X, undef -> X.
    return mergeInValue(&I, TVal);
  markOverdefined(&I);
}

// Handle Binary Operators.
void SCCPSolver::visitBinaryOperator(Instruction &I) {
  LatticeVal V1State = getValueState(I.getOperand(0));
  LatticeVal V2State = getValueState(I.getOperand(1));
  
  LatticeVal &IV = ValueState[&I];
  if (IV.isOverdefined()) return;

  if (V1State.isConstant() && V2State.isConstant())
    return markConstant(IV, &I,
                        ConstantExpr::get(I.getOpcode(), V1State.getConstant(),
                                          V2State.getConstant()));
  
  // If something is undef, wait for it to resolve.
  if (!V1State.isOverdefined() && !V2State.isOverdefined())
    return;
  
  // Otherwise, one of our operands is overdefined.  Try to produce something
  // better than overdefined with some tricks.
  
  // If this is an AND or OR with 0 or -1, it doesn't matter that the other
  // operand is overdefined.
  if (I.getOpcode() == Instruction::And || I.getOpcode() == Instruction::Or) {
    LatticeVal *NonOverdefVal = 0;
    if (!V1State.isOverdefined())
      NonOverdefVal = &V1State;
    else if (!V2State.isOverdefined())
      NonOverdefVal = &V2State;

    if (NonOverdefVal) {
      if (NonOverdefVal->isUndefined()) {
        // Could annihilate value.
        if (I.getOpcode() == Instruction::And)
          markConstant(IV, &I, Constant::getNullValue(I.getType()));
        else if (const VectorType *PT = dyn_cast<VectorType>(I.getType()))
          markConstant(IV, &I, Constant::getAllOnesValue(PT));
        else
          markConstant(IV, &I,
                       Constant::getAllOnesValue(I.getType()));
        return;
      }
      
      if (I.getOpcode() == Instruction::And) {
        // X and 0 = 0
        if (NonOverdefVal->getConstant()->isNullValue())
          return markConstant(IV, &I, NonOverdefVal->getConstant());
      } else {
        if (ConstantInt *CI = NonOverdefVal->getConstantInt())
          if (CI->isAllOnesValue())     // X or -1 = -1
            return markConstant(IV, &I, NonOverdefVal->getConstant());
      }
    }
  }


  // If both operands are PHI nodes, it is possible that this instruction has
  // a constant value, despite the fact that the PHI node doesn't.  Check for
  // this condition now.
  if (PHINode *PN1 = dyn_cast<PHINode>(I.getOperand(0)))
    if (PHINode *PN2 = dyn_cast<PHINode>(I.getOperand(1)))
      if (PN1->getParent() == PN2->getParent()) {
        // Since the two PHI nodes are in the same basic block, they must have
        // entries for the same predecessors.  Walk the predecessor list, and
        // if all of the incoming values are constants, and the result of
        // evaluating this expression with all incoming value pairs is the
        // same, then this expression is a constant even though the PHI node
        // is not a constant!
        LatticeVal Result;
        for (unsigned i = 0, e = PN1->getNumIncomingValues(); i != e; ++i) {
          LatticeVal In1 = getValueState(PN1->getIncomingValue(i));
          BasicBlock *InBlock = PN1->getIncomingBlock(i);
          LatticeVal In2 =getValueState(PN2->getIncomingValueForBlock(InBlock));

          if (In1.isOverdefined() || In2.isOverdefined()) {
            Result.markOverdefined();
            break;  // Cannot fold this operation over the PHI nodes!
          }
          
          if (In1.isConstant() && In2.isConstant()) {
            Constant *V = ConstantExpr::get(I.getOpcode(), In1.getConstant(),
                                            In2.getConstant());
            if (Result.isUndefined())
              Result.markConstant(V);
            else if (Result.isConstant() && Result.getConstant() != V) {
              Result.markOverdefined();
              break;
            }
          }
        }

        // If we found a constant value here, then we know the instruction is
        // constant despite the fact that the PHI nodes are overdefined.
        if (Result.isConstant()) {
          markConstant(IV, &I, Result.getConstant());
          // Remember that this instruction is virtually using the PHI node
          // operands.
          UsersOfOverdefinedPHIs.insert(std::make_pair(PN1, &I));
          UsersOfOverdefinedPHIs.insert(std::make_pair(PN2, &I));
          return;
        }
        
        if (Result.isUndefined())
          return;

        // Okay, this really is overdefined now.  Since we might have
        // speculatively thought that this was not overdefined before, and
        // added ourselves to the UsersOfOverdefinedPHIs list for the PHIs,
        // make sure to clean out any entries that we put there, for
        // efficiency.
        RemoveFromOverdefinedPHIs(&I, PN1);
        RemoveFromOverdefinedPHIs(&I, PN2);
      }

  markOverdefined(&I);
}

// Handle ICmpInst instruction.
void SCCPSolver::visitCmpInst(CmpInst &I) {
  LatticeVal V1State = getValueState(I.getOperand(0));
  LatticeVal V2State = getValueState(I.getOperand(1));

  LatticeVal &IV = ValueState[&I];
  if (IV.isOverdefined()) return;

  if (V1State.isConstant() && V2State.isConstant())
    return markConstant(IV, &I, ConstantExpr::getCompare(I.getPredicate(), 
                                                         V1State.getConstant(), 
                                                        V2State.getConstant()));
  
  // If operands are still undefined, wait for it to resolve.
  if (!V1State.isOverdefined() && !V2State.isOverdefined())