llvm-project/llvm/lib/Transforms/Scalar/SCCP.cpp

//===- SCCP.cpp - Sparse Conditional Constant Propogation -----------------===//
//
// This file implements sparse conditional constant propogation 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 constant, and unconditionalizes them
//   * Folds multiple identical constants in the constant pool together
//
// Notice that:
//   * This pass has a habit of making definitions be dead.  It is a good idea
//     to to run a DCE pass sometime after running this pass.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Scalar/ConstantProp.h"
#include "llvm/ConstantHandling.h"
#include "llvm/Function.h"
#include "llvm/iPHINode.h"
#include "llvm/iMemory.h"
#include "llvm/iTerminators.h"
#include "llvm/iOther.h"
#include "llvm/Pass.h"
#include "llvm/Support/InstVisitor.h"
#include "Support/STLExtras.h"
#include <algorithm>
#include <set>
#include <iostream>
using std::cerr;

// InstVal class - This class represents the different lattice values that an 
// instruction may occupy.  It is a simple class with value semantics.
//
namespace {
class InstVal {
  enum { 
    undefined,           // This instruction has no known value
    constant,            // This instruction has a constant value
    // Range,            // This instruction is known to fall within a range
    overdefined          // This instruction has an unknown value
  } LatticeValue;        // The current lattice position
  Constant *ConstantVal; // If Constant value, the current value
public:
  inline InstVal() : LatticeValue(undefined), ConstantVal(0) {}

  // markOverdefined - Return true if this is a new status to be in...
  inline bool markOverdefined() {
    if (LatticeValue != overdefined) {
      LatticeValue = overdefined;
      return true;
    }
    return false;
  }

  // markConstant - Return true if this is a new status for us...
  inline bool markConstant(Constant *V) {
    if (LatticeValue != constant) {
      LatticeValue = constant;
      ConstantVal = V;
      return true;
    } else {
      assert(ConstantVal == V && "Marking constant with different value");
    }
    return false;
  }

  inline bool isUndefined()   const { return LatticeValue == undefined; }
  inline bool isConstant()    const { return LatticeValue == constant; }
  inline bool isOverdefined() const { return LatticeValue == overdefined; }

  inline Constant *getConstant() const { return ConstantVal; }
};

} // end anonymous namespace


//===----------------------------------------------------------------------===//
// SCCP Class
//
// This class does all of the work of Sparse Conditional Constant Propogation.
//
namespace {
class SCCP : public FunctionPass, public InstVisitor<SCCP> {
  std::set<BasicBlock*>     BBExecutable;// The basic blocks that are executable
  std::map<Value*, InstVal> ValueState;  // The state each value is in...

  std::vector<Instruction*> InstWorkList;// The instruction work list
  std::vector<BasicBlock*>  BBWorkList;  // The BasicBlock work list
public:

  const char *getPassName() const {
    return "Sparse Conditional Constant Propogation";
  }

  // runOnFunction - Run the Sparse Conditional Constant Propogation algorithm,
  // and return true if the function was modified.
  //
  bool runOnFunction(Function *F);

  virtual void getAnalysisUsage(AnalysisUsage &AU) const {
    // FIXME: SCCP does not preserve the CFG because it folds terminators!
    //AU.preservesCFG();
  }


  //===--------------------------------------------------------------------===//
  // The implementation of this class
  //
private:
  friend class InstVisitor<SCCP>;        // Allow callbacks from visitor

  // markValueOverdefined - 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.
  //
  inline bool markConstant(Instruction *I, Constant *V) {
    //cerr << "markConstant: " << V << " = " << I;
    if (ValueState[I].markConstant(V)) {
      InstWorkList.push_back(I);
      return true;
    }
    return false;
  }

  // markValueOverdefined - Make a value be marked as "overdefined". If the
  // value is not already overdefined, add it to the instruction work list so
  // that the users of the instruction are updated later.
  //
  inline bool markOverdefined(Value *V) {
    if (ValueState[V].markOverdefined()) {
      if (Instruction *I = dyn_cast<Instruction>(V)) {
	//cerr << "markOverdefined: " << V;
	InstWorkList.push_back(I);  // Only instructions go on the work list
      }
      return true;
    }
    return false;
  }

  // getValueState - Return the InstVal object that corresponds to the value.
  // This function is neccesary because not all values should start out in the
  // underdefined state... Argument's should be overdefined, and
  // constants should be marked as constants.  If a value is not known to be an
  // Instruction object, then use this accessor to get its value from the map.
  //
  inline InstVal &getValueState(Value *V) {
    std::map<Value*, InstVal>::iterator I = ValueState.find(V);
    if (I != ValueState.end()) return I->second;  // Common case, in the map
      
    if (Constant *CPV = dyn_cast<Constant>(V)) {  // Constants are constant
      ValueState[CPV].markConstant(CPV);
    } else if (isa<Argument>(V)) {                // Arguments are overdefined
      ValueState[V].markOverdefined();
    } 
    // All others are underdefined by default...
    return ValueState[V];
  }

  // markExecutable - Mark a basic block as executable, adding it to the BB 
  // work list if it is not already executable...
  // 
  void markExecutable(BasicBlock *BB) {
    if (BBExecutable.count(BB)) return;
    //cerr << "Marking BB Executable: " << BB;
    BBExecutable.insert(BB);   // Basic block is executable!
    BBWorkList.push_back(BB);  // Add the block to the work list!
  }


  // 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) { /*does not have an effect*/ }
  void visitBranchInst(BranchInst *I);
  void visitSwitchInst(SwitchInst *I);

  void visitUnaryOperator(Instruction *I);
  void visitCastInst(CastInst *I) { visitUnaryOperator(I); }
  void visitBinaryOperator(Instruction *I);
  void visitShiftInst(ShiftInst *I) { visitBinaryOperator(I); }

  // Instructions that cannot be folded away...
  void visitMemAccessInst (Instruction *I) { markOverdefined(I); }
  void visitCallInst      (Instruction *I) { markOverdefined(I); }
  void visitInvokeInst    (Instruction *I) { markOverdefined(I); }
  void visitAllocationInst(Instruction *I) { markOverdefined(I); }
  void visitFreeInst      (Instruction *I) { markOverdefined(I); }

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

  // 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(User *U);
};
} // end anonymous namespace


// createSCCPPass - This is the public interface to this file...
//
Pass *createSCCPPass() {
  return new SCCP();
}



//===----------------------------------------------------------------------===//
// SCCP Class Implementation


// runOnFunction() - Run the Sparse Conditional Constant Propogation algorithm,
// and return true if the function was modified.
//
bool SCCP::runOnFunction(Function *F) {
  // Mark the first block of the function as being executable...
  markExecutable(F->front());

  // Process the work lists until their are empty!
  while (!BBWorkList.empty() || !InstWorkList.empty()) {
    // Process the instruction work list...
    while (!InstWorkList.empty()) {
      Instruction *I = InstWorkList.back();
      InstWorkList.pop_back();

      //cerr << "\nPopped off I-WL: " << I;

      
      // "I" got into the work list because it either made the transition from
      // bottom to constant, or to Overdefined.
      //
      // Update all of the users of this instruction's value...
      //
      for_each(I->use_begin(), I->use_end(),
	       bind_obj(this, &SCCP::OperandChangedState));
    }

    // Process the basic block work list...
    while (!BBWorkList.empty()) {
      BasicBlock *BB = BBWorkList.back();
      BBWorkList.pop_back();

      //cerr << "\nPopped off BBWL: " << BB;

      // If this block only has a single successor, mark it as executable as
      // well... if not, terminate the do loop.
      //
      if (BB->getTerminator()->getNumSuccessors() == 1)
        markExecutable(BB->getTerminator()->getSuccessor(0));

      // Notify all instructions in this basic block that they are newly
      // executable.
      visit(BB);
    }
  }

#if 0
  for (Function::iterator BBI = F->begin(), BBEnd = F->end();
       BBI != BBEnd; ++BBI)
    if (!BBExecutable.count(*BBI))
      cerr << "BasicBlock Dead:" << *BBI;
#endif


  // Iterate over all of the instructions in a function, replacing them with
  // constants if we have found them to be of constant values.
  //
  bool MadeChanges = false;
  for (Function::iterator FI = F->begin(), FE = F->end(); FI != FE; ++FI) {
    BasicBlock *BB = *FI;
    for (BasicBlock::iterator BI = BB->begin(); BI != BB->end();) {
      Instruction *Inst = *BI;
      InstVal &IV = ValueState[Inst];
      if (IV.isConstant()) {
        Constant *Const = IV.getConstant();
        // cerr << "Constant: " << Inst << "  is: " << Const;

        // Replaces all of the uses of a variable with uses of the constant.
        Inst->replaceAllUsesWith(Const);

        // Remove the operator from the list of definitions... and delete it.
        delete BB->getInstList().remove(BI);

        // Hey, we just changed something!
        MadeChanges = true;

        // Do NOT advance the iterator, skipping the next instruction...
        continue;

      } else if (TerminatorInst *TI = dyn_cast<TerminatorInst>(Inst)) {
        MadeChanges |= ConstantFoldTerminator(BB, BI, TI);
      }

      ++BI;
    }
  }

  // Reset state so that the next invokation will have empty data structures
  BBExecutable.clear();
  ValueState.clear();

  // Merge identical constants last: this is important because we may have just
  // introduced constants that already exist, and we don't want to pollute later
  // stages with extraneous constants.
  //
  return MadeChanges;
}


// 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 SCCP::visitPHINode(PHINode *PN) {
  unsigned NumValues = PN->getNumIncomingValues(), i;
  InstVal *OperandIV = 0;

  // 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.
  //
  for (i = 0; i < NumValues; ++i) {
    if (BBExecutable.count(PN->getIncomingBlock(i))) {
      InstVal &IV = getValueState(PN->getIncomingValue(i));
      if (IV.isUndefined()) continue;  // Doesn't influence PHI node.
      if (IV.isOverdefined()) {   // PHI node becomes overdefined!
        markOverdefined(PN);
        return;
      }

      if (OperandIV == 0) {   // Grab the first value...
        OperandIV = &IV;
      } else {                // Another value is being merged in!
        // 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 (IV.getConstant() != OperandIV->getConstant()) {
          // Yes there is.  This means the PHI node is not constant.
          // You must be overdefined poor PHI.
          //
          markOverdefined(PN);         // The PHI node now becomes overdefined
          return;    // I'm done analyzing you
        }
      }
    }
  }

  // If we exited the loop, this means that the PHI node only has constant
  // arguments that agree with each other(and OperandIV is a pointer to one
  // of their InstVal's) or OperandIV is null because there are no defined
  // incoming arguments.  If this is the case, the PHI remains undefined.
  //
  if (OperandIV) {
    assert(OperandIV->isConstant() && "Should only be here for constants!");
    markConstant(PN, OperandIV->getConstant());  // Aquire operand value
  }
}

void SCCP::visitBranchInst(BranchInst *BI) {
  if (BI->isUnconditional())
    return; // Unconditional branches are already handled!

  InstVal &BCValue = getValueState(BI->getCondition());
  if (BCValue.isOverdefined()) {
    // Overdefined condition variables mean the branch could go either way.
    markExecutable(BI->getSuccessor(0));
    markExecutable(BI->getSuccessor(1));
  } else if (BCValue.isConstant()) {
    // Constant condition variables mean the branch can only go a single way.
    if (BCValue.getConstant() == ConstantBool::True)
      markExecutable(BI->getSuccessor(0));
    else
      markExecutable(BI->getSuccessor(1));
  }
}

void SCCP::visitSwitchInst(SwitchInst *SI) {
  InstVal &SCValue = getValueState(SI->getCondition());
  if (SCValue.isOverdefined()) {  // Overdefined condition?  All dests are exe
    for(unsigned i = 0, E = SI->getNumSuccessors(); i != E; ++i)
      markExecutable(SI->getSuccessor(i));
  } else if (SCValue.isConstant()) {
    Constant *CPV = SCValue.getConstant();
    // 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) == CPV) {// Found the right branch...
        markExecutable(SI->getSuccessor(i));
        return;
      }
    }

    // Constant value not equal to any of the branches... must execute
    // default branch then...
    markExecutable(SI->getDefaultDest());
  }
}

void SCCP::visitUnaryOperator(Instruction *I) {
  Value *V = I->getOperand(0);
  InstVal &VState = getValueState(V);
  if (VState.isOverdefined()) {        // Inherit overdefinedness of operand
    markOverdefined(I);
  } else if (VState.isConstant()) {    // Propogate constant value
    Constant *Result = isa<CastInst>(I)
      ? ConstantFoldCastInstruction(VState.getConstant(), I->getType())
      : ConstantFoldUnaryInstruction(I->getOpcode(), VState.getConstant());

    if (Result) {
      // This instruction constant folds!
      markConstant(I, Result);
    } else {
      markOverdefined(I);   // Don't know how to fold this instruction.  :(
    }
  }
}

// Handle BinaryOperators and Shift Instructions...
void SCCP::visitBinaryOperator(Instruction *I) {
  InstVal &V1State = getValueState(I->getOperand(0));
  InstVal &V2State = getValueState(I->getOperand(1));
  if (V1State.isOverdefined() || V2State.isOverdefined()) {
    markOverdefined(I);
  } else if (V1State.isConstant() && V2State.isConstant()) {
    Constant *Result = ConstantFoldBinaryInstruction(I->getOpcode(),
                                                     V1State.getConstant(),
                                                     V2State.getConstant());
    if (Result)
      markConstant(I, Result);      // This instruction constant folds!
    else
      markOverdefined(I);   // Don't know how to fold this instruction.  :(
  }
}

// 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 SCCP::OperandChangedState(User *U) {
  // Only instructions use other variable values!
  Instruction *I = cast<Instruction>(U);
  if (!BBExecutable.count(I->getParent())) return;  // Inst not executable yet!

  visit(I);
}