Reassociate.cpp

//===- Reassociate.cpp - Reassociate binary expressions -------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass reassociates commutative expressions in an order that is designed
// to promote better constant propagation, GCSE, LICM, PRE...
//
// For example: 4 + (x + 5) -> x + (4 + 5)
//
// In the implementation of this algorithm, constants are assigned rank = 0,
// function arguments are rank = 1, and other values are assigned ranks
// corresponding to the reverse post order traversal of current function
// (starting at 2), which effectively gives values in deep loops higher rank
// than values not in loops.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "reassociate"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Constants.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Pass.h"
#include "llvm/Type.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;

namespace {
  Statistic<> NumLinear ("reassociate","Number of insts linearized");
  Statistic<> NumChanged("reassociate","Number of insts reassociated");
  Statistic<> NumSwapped("reassociate","Number of insts with operands swapped");

  class Reassociate : public FunctionPass {
    std::map<BasicBlock*, unsigned> RankMap;
    std::map<Value*, unsigned> ValueRankMap;
  public:
    bool runOnFunction(Function &F);

    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.setPreservesCFG();
    }
  private:
    void BuildRankMap(Function &F);
    unsigned getRank(Value *V);
    bool ReassociateExpr(BinaryOperator *I);
    bool ReassociateBB(BasicBlock *BB);
  };

  RegisterOpt<Reassociate> X("reassociate", "Reassociate expressions");
}

// Public interface to the Reassociate pass
FunctionPass *llvm::createReassociatePass() { return new Reassociate(); }

void Reassociate::BuildRankMap(Function &F) {
  unsigned i = 2;

  // Assign distinct ranks to function arguments
  for (Function::arg_iterator I = F.arg_begin(), E = F.arg_end(); I != E; ++I)
    ValueRankMap[I] = ++i;

  ReversePostOrderTraversal<Function*> RPOT(&F);
  for (ReversePostOrderTraversal<Function*>::rpo_iterator I = RPOT.begin(),
         E = RPOT.end(); I != E; ++I)
    RankMap[*I] = ++i << 16;
}

unsigned Reassociate::getRank(Value *V) {
  if (isa<Argument>(V)) return ValueRankMap[V];   // Function argument...

  Instruction *I = dyn_cast<Instruction>(V);
  if (I == 0) return 0;  // Otherwise it's a global or constant, rank 0.

  unsigned &CachedRank = ValueRankMap[I];
  if (CachedRank) return CachedRank;    // Rank already known?
  
  // If this is an expression, return the 1+MAX(rank(LHS), rank(RHS)) so that
  // we can reassociate expressions for code motion!  Since we do not recurse
  // for PHI nodes, we cannot have infinite recursion here, because there
  // cannot be loops in the value graph that do not go through PHI nodes.
  //
  if (I->getOpcode() == Instruction::PHI ||
      I->getOpcode() == Instruction::Alloca ||
      I->getOpcode() == Instruction::Malloc || isa<TerminatorInst>(I) ||
      I->mayWriteToMemory())  // Cannot move inst if it writes to memory!
    return RankMap[I->getParent()];
  
  // If not, compute it!
  unsigned Rank = 0, MaxRank = RankMap[I->getParent()];
  for (unsigned i = 0, e = I->getNumOperands();
       i != e && Rank != MaxRank; ++i)
    Rank = std::max(Rank, getRank(I->getOperand(i)));
  
  DEBUG(std::cerr << "Calculated Rank[" << V->getName() << "] = "
        << Rank+1 << "\n");
  
  return CachedRank = Rank+1;
}


bool Reassociate::ReassociateExpr(BinaryOperator *I) {
  Value *LHS = I->getOperand(0);
  Value *RHS = I->getOperand(1);
  unsigned LHSRank = getRank(LHS);
  unsigned RHSRank = getRank(RHS);

  bool Changed = false;

  // Make sure the LHS of the operand always has the greater rank...
  if (LHSRank < RHSRank) {
    bool Success = !I->swapOperands();
    assert(Success && "swapOperands failed");

    std::swap(LHS, RHS);
    std::swap(LHSRank, RHSRank);
    Changed = true;
    ++NumSwapped;
    DEBUG(std::cerr << "Transposed: " << *I
          /* << " Result BB: " << I->getParent()*/);
  }

  // If the LHS is the same operator as the current one is, and if we are the
  // only expression using it...
  //
  if (BinaryOperator *LHSI = dyn_cast<BinaryOperator>(LHS))
    if (LHSI->getOpcode() == I->getOpcode() && LHSI->hasOneUse()) {
      // If the rank of our current RHS is less than the rank of the LHS's LHS,
      // then we reassociate the two instructions...

      unsigned TakeOp = 0;
      if (BinaryOperator *IOp = dyn_cast<BinaryOperator>(LHSI->getOperand(0)))
        if (IOp->getOpcode() == LHSI->getOpcode())
          TakeOp = 1;   // Hoist out non-tree portion

      if (RHSRank < getRank(LHSI->getOperand(TakeOp))) {
        // Convert ((a + 12) + 10) into (a + (12 + 10))
        I->setOperand(0, LHSI->getOperand(TakeOp));
        LHSI->setOperand(TakeOp, RHS);
        I->setOperand(1, LHSI);

        // Move the LHS expression forward, to ensure that it is dominated by
        // its operands.
        LHSI->getParent()->getInstList().remove(LHSI);
        I->getParent()->getInstList().insert(I, LHSI);

        ++NumChanged;
        DEBUG(std::cerr << "Reassociated: " << *I/* << " Result BB: "
                                                   << I->getParent()*/);

        // Since we modified the RHS instruction, make sure that we recheck it.
        ReassociateExpr(LHSI);
        ReassociateExpr(I);
        return true;
      }
    }

  return Changed;
}


// NegateValue - Insert instructions before the instruction pointed to by BI,
// that computes the negative version of the value specified.  The negative
// version of the value is returned, and BI is left pointing at the instruction
// that should be processed next by the reassociation pass.
//
static Value *NegateValue(Value *V, Instruction *BI) {
  // We are trying to expose opportunity for reassociation.  One of the things
  // that we want to do to achieve this is to push a negation as deep into an
  // expression chain as possible, to expose the add instructions.  In practice,
  // this means that we turn this:
  //   X = -(A+12+C+D)   into    X = -A + -12 + -C + -D = -12 + -A + -C + -D
  // so that later, a: Y = 12+X could get reassociated with the -12 to eliminate
  // the constants.  We assume that instcombine will clean up the mess later if
  // we introduce tons of unnecessary negation instructions...
  //
  if (Instruction *I = dyn_cast<Instruction>(V))
    if (I->getOpcode() == Instruction::Add && I->hasOneUse()) {
      Value *RHS = NegateValue(I->getOperand(1), BI);
      Value *LHS = NegateValue(I->getOperand(0), BI);

      // We must actually insert a new add instruction here, because the neg
      // instructions do not dominate the old add instruction in general.  By
      // adding it now, we are assured that the neg instructions we just
      // inserted dominate the instruction we are about to insert after them.
      //
      return BinaryOperator::create(Instruction::Add, LHS, RHS,
                                    I->getName()+".neg", BI);
    }

  // Insert a 'neg' instruction that subtracts the value from zero to get the
  // negation.
  //
  return BinaryOperator::createNeg(V, V->getName() + ".neg", BI);
}

/// isReassociableOp - Return true if V is an instruction of the specified
/// opcode and if it only has one use.
static bool isReassociableOp(Value *V, unsigned Opcode) {
  return V->hasOneUse() && isa<Instruction>(V) &&
         cast<Instruction>(V)->getOpcode() == Opcode;
}

/// BreakUpSubtract - If we have (X-Y), and if either X is an add, or if this is
/// only used by an add, transform this into (X+(0-Y)) to promote better
/// reassociation.
static Instruction *BreakUpSubtract(Instruction *Sub) {
  // Reject cases where it is pointless to do this.
  if (Sub->getType()->isFloatingPoint())
    return 0;  // Floating point adds are not associative.

  // Don't bother to break this up unless either the LHS is an associable add or
  // if this is only used by one.
  if (!isReassociableOp(Sub->getOperand(0), Instruction::Add) &&
      !isReassociableOp(Sub->getOperand(1), Instruction::Add) &&
      !(Sub->hasOneUse() &&isReassociableOp(Sub->use_back(), Instruction::Add)))
    return 0;

  // Convert a subtract into an add and a neg instruction... so that sub
  // instructions can be commuted with other add instructions...
  //
  // Calculate the negative value of Operand 1 of the sub instruction...
  // and set it as the RHS of the add instruction we just made...
  //
  std::string Name = Sub->getName();
  Sub->setName("");
  Value *NegVal = NegateValue(Sub->getOperand(1), Sub);
  Instruction *New =
    BinaryOperator::createAdd(Sub->getOperand(0), NegVal, Name, Sub);

  // Everyone now refers to the add instruction.
  Sub->replaceAllUsesWith(New);
  Sub->eraseFromParent();
  
  DEBUG(std::cerr << "Negated: " << *New);
  return New;
}

/// ConvertShiftToMul - If this is a shift of a reassociable multiply or is used
/// by one, change this into a multiply by a constant to assist with further
/// reassociation.
static Instruction *ConvertShiftToMul(Instruction *Shl) {
  if (!isReassociableOp(Shl->getOperand(0), Instruction::Mul) &&
      !(Shl->hasOneUse() && isReassociableOp(Shl->use_back(),Instruction::Mul)))
    return 0;

  Constant *MulCst = ConstantInt::get(Shl->getType(), 1);
  MulCst = ConstantExpr::getShl(MulCst, cast<Constant>(Shl->getOperand(1)));

  std::string Name = Shl->getName();  Shl->setName("");
  Instruction *Mul = BinaryOperator::createMul(Shl->getOperand(0), MulCst,
                                               Name, Shl);
  Shl->replaceAllUsesWith(Mul);
  Shl->eraseFromParent();
  return Mul;
}


/// ReassociateBB - Inspect all of the instructions in this basic block,
/// reassociating them as we go.
bool Reassociate::ReassociateBB(BasicBlock *BB) {
  bool Changed = false;
  for (BasicBlock::iterator BI = BB->begin(); BI != BB->end(); ++BI) {
    // If this is a subtract instruction which is not already in negate form,
    // see if we can convert it to X+-Y.
    if (BI->getOpcode() == Instruction::Sub && !BinaryOperator::isNeg(BI))
      if (Instruction *NI = BreakUpSubtract(BI)) {
        Changed = true;
        BI = NI;
      }
    if (BI->getOpcode() == Instruction::Shl &&
        isa<ConstantInt>(BI->getOperand(1)))
      if (Instruction *NI = ConvertShiftToMul(BI)) {
        Changed = true;
        BI = NI;
      }

    // If this instruction is a commutative binary operator, and the ranks of
    // the two operands are sorted incorrectly, fix it now.
    //
    if (BI->isAssociative()) {
      DEBUG(std::cerr << "Reassociating: " << *BI);
      BinaryOperator *I = cast<BinaryOperator>(BI);
      if (!I->use_empty()) {
        // Make sure that we don't have a tree-shaped computation.  If we do,
        // linearize it.  Convert (A+B)+(C+D) into ((A+B)+C)+D
        //
        Instruction *LHSI = dyn_cast<Instruction>(I->getOperand(0));
        Instruction *RHSI = dyn_cast<Instruction>(I->getOperand(1));
        if (LHSI && (int)LHSI->getOpcode() == I->getOpcode() &&
            RHSI && (int)RHSI->getOpcode() == I->getOpcode() &&
            RHSI->hasOneUse()) {
          // Insert a new temporary instruction... (A+B)+C
          BinaryOperator *Tmp = BinaryOperator::create(I->getOpcode(), LHSI,
                                                       RHSI->getOperand(0),
                                                       RHSI->getName()+".ra",
                                                       BI);
          BI = Tmp;
          I->setOperand(0, Tmp);
          I->setOperand(1, RHSI->getOperand(1));

          // Process the temporary instruction for reassociation now.
          I = Tmp;
          ++NumLinear;
          Changed = true;
          DEBUG(std::cerr << "Linearized: " << *I/* << " Result BB: " << BB*/);
        }

        // Make sure that this expression is correctly reassociated with respect
        // to it's used values...
        //
        Changed |= ReassociateExpr(I);
      }
    }
  }

  return Changed;
}


bool Reassociate::runOnFunction(Function &F) {
  // Recalculate the rank map for F
  BuildRankMap(F);

  bool Changed = false;
  for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
    Changed |= ReassociateBB(FI);

  // We are done with the rank map...
  RankMap.clear();
  ValueRankMap.clear();
  return Changed;
}