llvm/lib/Transforms/Scalar/IndVarSimplify.cpp

//===- IndVarSimplify.cpp - Induction Variable Elimination ----------------===//
//
// InductionVariableSimplify - Transform induction variables in a program
//   to all use a single cannonical induction variable per loop.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Scalar/IndVarSimplify.h"
#include "llvm/Analysis/InductionVariable.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/iPHINode.h"
#include "llvm/iOther.h"
#include "llvm/Type.h"
#include "llvm/ConstantVals.h"
#include "Support/STLExtras.h"

#if 0
#define DEBUG
#include "llvm/Analysis/Writer.h"
#endif

// InsertCast - Cast Val to Ty, setting a useful name on the cast if Val has a
// name...
//
static Instruction *InsertCast(Instruction *Val, const Type *Ty,
                               BasicBlock::iterator It) {
  Instruction *Cast = new CastInst(Val, Ty);
  if (Val->hasName()) Cast->setName(Val->getName()+"-casted");
  Val->getParent()->getInstList().insert(It, Cast);
  return Cast;
}

static bool TransformLoop(cfg::LoopInfo *Loops, cfg::Loop *Loop) {
  // Transform all subloops before this loop...
  bool Changed = reduce_apply_bool(Loop->getSubLoops().begin(),
                                   Loop->getSubLoops().end(),
                                   std::bind1st(ptr_fun(TransformLoop), Loops));
  // Get the header node for this loop.  All of the phi nodes that could be
  // induction variables must live in this basic block.
  BasicBlock *Header = (BasicBlock*)Loop->getBlocks().front();
  
  // Loop over all of the PHI nodes in the basic block, calculating the
  // induction variables that they represent... stuffing the induction variable
  // info into a vector...
  //
  vector<InductionVariable> IndVars;    // Induction variables for block
  for (BasicBlock::iterator I = Header->begin(); 
       PHINode *PN = dyn_cast<PHINode>(*I); ++I)
    IndVars.push_back(InductionVariable(PN, Loops));

  // If there are no phi nodes in this basic block, there can't be indvars...
  if (IndVars.empty()) return Changed;
  
  // Loop over the induction variables, looking for a cannonical induction
  // variable, and checking to make sure they are not all unknown induction
  // variables.
  //
  bool FoundIndVars = false;
  InductionVariable *Cannonical = 0;
  for (unsigned i = 0; i < IndVars.size(); ++i) {
    if (IndVars[i].InductionType == InductionVariable::Cannonical)
      Cannonical = &IndVars[i];
    if (IndVars[i].InductionType != InductionVariable::Unknown)
      FoundIndVars = true;
  }

  // No induction variables, bail early... don't add a cannonnical indvar
  if (!FoundIndVars) return Changed;

  // Okay, we want to convert other induction variables to use a cannonical
  // indvar.  If we don't have one, add one now...
  if (!Cannonical) {
    // Create the PHI node for the new induction variable
    PHINode *PN = new PHINode(Type::UIntTy, "cann-indvar");

    // Insert the phi node at the end of the other phi nodes...
    Header->getInstList().insert(Header->begin()+IndVars.size(), PN);

    // Create the increment instruction to add one to the counter...
    Instruction *Add = BinaryOperator::create(Instruction::Add, PN,
                                              ConstantUInt::get(Type::UIntTy,1),
                                              "add1-indvar");

    // Insert the add instruction after all of the PHI nodes...
    Header->getInstList().insert(Header->begin()+(IndVars.size()+1), Add);

    // Figure out which block is incoming and which is the backedge for the loop
    BasicBlock *Incoming, *BackEdgeBlock;
    BasicBlock::pred_iterator PI = Header->pred_begin();
    assert(PI != Header->pred_end() && "Loop headers should have 2 preds!");
    if (Loop->contains(*PI)) {  // First pred is back edge...
      BackEdgeBlock = *PI++;
      Incoming      = *PI++;
    } else {
      Incoming      = *PI++;
      BackEdgeBlock = *PI++;
    }
    assert(PI == Header->pred_end() && "Loop headers should have 2 preds!");
    
    // Add incoming values for the PHI node...
    PN->addIncoming(Constant::getNullConstant(Type::UIntTy), Incoming);
    PN->addIncoming(Add, BackEdgeBlock);

    // Analyze the new induction variable...
    IndVars.push_back(InductionVariable(PN, Loops));
    assert(IndVars.back().InductionType == InductionVariable::Cannonical &&
           "Just inserted cannonical indvar that is not cannonical!");
    Cannonical = &IndVars.back();
  }

#ifdef DEBUG
  cerr << "Induction variables:\n";
#endif

  // Get the current loop iteration count, which is always the value of the
  // cannonical phi node...
  //
  PHINode *IterCount = Cannonical->Phi;

  // Loop through and replace all of the auxillary induction variables with
  // references to the primary induction variable...
  //
  unsigned InsertPos = IndVars.size();
  for (unsigned i = 0; i < IndVars.size(); ++i) {
    InductionVariable *IV = &IndVars[i];
#ifdef DEBUG
    cerr << IndVars[i];
#endif
    if (IV != Cannonical) {  // Don't modify the cannonical indvar
      Instruction *Val = IterCount;
      if (!isa<ConstantInt>(IV->Step) ||   // If the step != 1
          !cast<ConstantInt>(IV->Step)->equalsInt(1)) {
        string Name;   // Create a scale by the step value...
        if (IV->Phi->hasName()) Name = IV->Phi->getName()+"-scale";

        // If the types are not compatible, insert a cast now...
        if (Val->getType() != IV->Step->getType())
          Val = InsertCast(Val, IV->Step->getType(),
                           Header->begin()+InsertPos++);

        Val = BinaryOperator::create(Instruction::Mul, Val, IV->Step, Name);
        // Insert the phi node at the end of the other phi nodes...
        Header->getInstList().insert(Header->begin()+InsertPos++, Val);
      }

      if (!isa<Constant>(IV->Start) ||   // If the start != 0
          !cast<Constant>(IV->Start)->isNullValue()) {
        string Name;   // Create a offset by the start value...
        if (IV->Phi->hasName()) Name = IV->Phi->getName()+"-offset";

        // If the types are not compatible, insert a cast now...
        if (Val->getType() != IV->Start->getType())
          Val = InsertCast(Val, IV->Start->getType(),
                           Header->begin()+InsertPos++);

        Val = BinaryOperator::create(Instruction::Add, Val, IV->Start, Name);
        // Insert the phi node at the end of the other phi nodes...
        Header->getInstList().insert(Header->begin()+InsertPos++, Val);
      }

      // If the PHI node has a different type than val is, insert a cast now...
      if (Val->getType() != IV->Phi->getType())
          Val = InsertCast(Val, IV->Phi->getType(),
                           Header->begin()+InsertPos++);
      
      // Replace all uses of the old PHI node with the new computed value...
      IV->Phi->replaceAllUsesWith(Val);

      // Move the PHI name to it's new equivalent value...
      string OldName = IV->Phi->getName();
      IV->Phi->setName("");
      Val->setName(OldName);

      // Delete the old, now unused, phi node...
      Header->getInstList().remove(IV->Phi);
      delete IV->Phi;
      InsertPos--;            // Deleted an instr, decrement insert position
    }
  }

  return Changed;
}

bool InductionVariableSimplify::doit(Method *M) {
  // Figure out the loop structure of the method...
  cfg::LoopInfo Loops(M);

  // Induction Variables live in the header nodes of the loops of the method...
  return reduce_apply_bool(Loops.getTopLevelLoops().begin(),
                           Loops.getTopLevelLoops().end(),
                           std::bind1st(std::ptr_fun(TransformLoop), &Loops));
}