IndVarSimplify.cpp

//===- IndVarSimplify.cpp - Induction Variable Elimination ----------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This transformation analyzes and transforms the induction variables (and
// computations derived from them) into simpler forms suitable for subsequent
// analysis and transformation.
//
// This transformation makes the following changes to each loop with an
// identifiable induction variable:
//   1. All loops are transformed to have a SINGLE canonical induction variable
//      which starts at zero and steps by one.
//   2. The canonical induction variable is guaranteed to be the first PHI node
//      in the loop header block.
//   3. Any pointer arithmetic recurrences are raised to use array subscripts.
//
// If the trip count of a loop is computable, this pass also makes the following
// changes:
//   1. The exit condition for the loop is canonicalized to compare the
//      induction value against the exit value.  This turns loops like:
//        'for (i = 7; i*i < 1000; ++i)' into 'for (i = 0; i != 25; ++i)'
//   2. Any use outside of the loop of an expression derived from the indvar
//      is changed to compute the derived value outside of the loop, eliminating
//      the dependence on the exit value of the induction variable.  If the only
//      purpose of the loop is to compute the exit value of some derived
//      expression, this transformation will make the loop dead.
//
// This transformation should be followed by strength reduction after all of the
// desired loop transformations have been performed.  Additionally, on targets
// where it is profitable, the loop could be transformed to count down to zero
// (the "do loop" optimization).
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "indvars"
#include "llvm/Transforms/Scalar.h"
#include "llvm/BasicBlock.h"
#include "llvm/Constants.h"
#include "llvm/Instructions.h"
#include "llvm/Type.h"
#include "llvm/Analysis/ScalarEvolutionExpander.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;

STATISTIC(NumRemoved , "Number of aux indvars removed");
STATISTIC(NumPointer , "Number of pointer indvars promoted");
STATISTIC(NumInserted, "Number of canonical indvars added");
STATISTIC(NumReplaced, "Number of exit values replaced");
STATISTIC(NumLFTR    , "Number of loop exit tests replaced");

namespace {
  class VISIBILITY_HIDDEN IndVarSimplify : public LoopPass {
    LoopInfo        *LI;
    ScalarEvolution *SE;
    bool Changed;
  public:

   static char ID; // Pass identification, replacement for typeid
   IndVarSimplify() : LoopPass(&ID) {}

   virtual bool runOnLoop(Loop *L, LPPassManager &LPM);

   virtual void getAnalysisUsage(AnalysisUsage &AU) const {
     AU.addRequired<ScalarEvolution>();
     AU.addRequiredID(LCSSAID);
     AU.addRequiredID(LoopSimplifyID);
     AU.addRequired<LoopInfo>();
     AU.addPreserved<ScalarEvolution>();
     AU.addPreservedID(LoopSimplifyID);
     AU.addPreservedID(LCSSAID);
     AU.setPreservesCFG();
   }

  private:

    void RewriteNonIntegerIVs(Loop *L);

    void EliminatePointerRecurrence(PHINode *PN, BasicBlock *Preheader,
                                    SmallPtrSet<Instruction*, 16> &DeadInsts);
    void LinearFunctionTestReplace(Loop *L, SCEVHandle BackedgeTakenCount,
                                   Value *IndVar,
                                   BasicBlock *ExitingBlock,
                                   BranchInst *BI,
                                   SCEVExpander &Rewriter);
    void RewriteLoopExitValues(Loop *L, SCEV *BackedgeTakenCount);

    void DeleteTriviallyDeadInstructions(SmallPtrSet<Instruction*, 16> &Insts);

    void HandleFloatingPointIV(Loop *L, PHINode *PH,
                               SmallPtrSet<Instruction*, 16> &DeadInsts);
  };
}

char IndVarSimplify::ID = 0;
static RegisterPass<IndVarSimplify>
X("indvars", "Canonicalize Induction Variables");

Pass *llvm::createIndVarSimplifyPass() {
  return new IndVarSimplify();
}

/// DeleteTriviallyDeadInstructions - If any of the instructions is the
/// specified set are trivially dead, delete them and see if this makes any of
/// their operands subsequently dead.
void IndVarSimplify::
DeleteTriviallyDeadInstructions(SmallPtrSet<Instruction*, 16> &Insts) {
  while (!Insts.empty()) {
    Instruction *I = *Insts.begin();
    Insts.erase(I);
    if (isInstructionTriviallyDead(I)) {
      for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
        if (Instruction *U = dyn_cast<Instruction>(I->getOperand(i)))
          Insts.insert(U);
      SE->deleteValueFromRecords(I);
      DOUT << "INDVARS: Deleting: " << *I;
      I->eraseFromParent();
      Changed = true;
    }
  }
}


/// EliminatePointerRecurrence - Check to see if this is a trivial GEP pointer
/// recurrence.  If so, change it into an integer recurrence, permitting
/// analysis by the SCEV routines.
void IndVarSimplify::EliminatePointerRecurrence(PHINode *PN,
                                                BasicBlock *Preheader,
                                     SmallPtrSet<Instruction*, 16> &DeadInsts) {
  assert(PN->getNumIncomingValues() == 2 && "Noncanonicalized loop!");
  unsigned PreheaderIdx = PN->getBasicBlockIndex(Preheader);
  unsigned BackedgeIdx = PreheaderIdx^1;
  if (GetElementPtrInst *GEPI =
          dyn_cast<GetElementPtrInst>(PN->getIncomingValue(BackedgeIdx)))
    if (GEPI->getOperand(0) == PN) {
      assert(GEPI->getNumOperands() == 2 && "GEP types must match!");
      DOUT << "INDVARS: Eliminating pointer recurrence: " << *GEPI;

      // Okay, we found a pointer recurrence.  Transform this pointer
      // recurrence into an integer recurrence.  Compute the value that gets
      // added to the pointer at every iteration.
      Value *AddedVal = GEPI->getOperand(1);

      // Insert a new integer PHI node into the top of the block.
      PHINode *NewPhi = PHINode::Create(AddedVal->getType(),
                                        PN->getName()+".rec", PN);
      NewPhi->addIncoming(Constant::getNullValue(NewPhi->getType()), Preheader);

      // Create the new add instruction.
      Value *NewAdd = BinaryOperator::CreateAdd(NewPhi, AddedVal,
                                                GEPI->getName()+".rec", GEPI);
      NewPhi->addIncoming(NewAdd, PN->getIncomingBlock(BackedgeIdx));

      // Update the existing GEP to use the recurrence.
      GEPI->setOperand(0, PN->getIncomingValue(PreheaderIdx));

      // Update the GEP to use the new recurrence we just inserted.
      GEPI->setOperand(1, NewAdd);

      // If the incoming value is a constant expr GEP, try peeling out the array
      // 0 index if possible to make things simpler.
      if (ConstantExpr *CE = dyn_cast<ConstantExpr>(GEPI->getOperand(0)))
        if (CE->getOpcode() == Instruction::GetElementPtr) {
          unsigned NumOps = CE->getNumOperands();
          assert(NumOps > 1 && "CE folding didn't work!");
          if (CE->getOperand(NumOps-1)->isNullValue()) {
            // Check to make sure the last index really is an array index.
            gep_type_iterator GTI = gep_type_begin(CE);
            for (unsigned i = 1, e = CE->getNumOperands()-1;
                 i != e; ++i, ++GTI)
              /*empty*/;
            if (isa<SequentialType>(*GTI)) {
              // Pull the last index out of the constant expr GEP.
              SmallVector<Value*, 8> CEIdxs(CE->op_begin()+1, CE->op_end()-1);
              Constant *NCE = ConstantExpr::getGetElementPtr(CE->getOperand(0),
                                                             &CEIdxs[0],
                                                             CEIdxs.size());
              Value *Idx[2];
              Idx[0] = Constant::getNullValue(Type::Int32Ty);
              Idx[1] = NewAdd;
              GetElementPtrInst *NGEPI = GetElementPtrInst::Create(
                  NCE, Idx, Idx + 2,
                  GEPI->getName(), GEPI);
              SE->deleteValueFromRecords(GEPI);
              GEPI->replaceAllUsesWith(NGEPI);