LoopStrengthReduce.cpp

            std::swap(UsersToProcess[ScanPos], UsersToProcess.back());
            break;
          }
      }
    } while (!UsersToProcess.empty() && UsersToProcess.back().Base == Base);
    // TODO: Next, find out which base index is the most common, pull it out.
  }

  // IMPORTANT TODO: Figure out how to partition the IV's with this stride, but
  // different starting values, into different PHIs.
}

// OptimizeIndvars - Now that IVUsesByStride is set up with all of the indvar
// uses in the loop, look to see if we can eliminate some, in favor of using
// common indvars for the different uses.
void LoopStrengthReduce::OptimizeIndvars(Loop *L) {
  // TODO: implement optzns here.




  // Finally, get the terminating condition for the loop if possible.  If we
  // can, we want to change it to use a post-incremented version of its
  // induction variable, to allow coallescing the live ranges for the IV into
  // one register value.
  PHINode *SomePHI = cast<PHINode>(L->getHeader()->begin());
  BasicBlock  *Preheader = L->getLoopPreheader();
  BasicBlock *LatchBlock =
   SomePHI->getIncomingBlock(SomePHI->getIncomingBlock(0) == Preheader);
  BranchInst *TermBr = dyn_cast<BranchInst>(LatchBlock->getTerminator());
  if (!TermBr || TermBr->isUnconditional() ||
      !isa<SetCondInst>(TermBr->getCondition()))
    return;
  SetCondInst *Cond = cast<SetCondInst>(TermBr->getCondition());

  // Search IVUsesByStride to find Cond's IVUse if there is one.
  IVStrideUse *CondUse = 0;
  const SCEVHandle *CondStride = 0;

  for (unsigned Stride = 0, e = StrideOrder.size(); Stride != e && !CondUse;
       ++Stride) {
    std::map<SCEVHandle, IVUsersOfOneStride>::iterator SI = 
      IVUsesByStride.find(StrideOrder[Stride]);
    assert(SI != IVUsesByStride.end() && "Stride doesn't exist!");
    
    for (std::vector<IVStrideUse>::iterator UI = SI->second.Users.begin(),
           E = SI->second.Users.end(); UI != E; ++UI)
      if (UI->User == Cond) {
        CondUse = &*UI;
        CondStride = &SI->first;
        // NOTE: we could handle setcc instructions with multiple uses here, but
        // InstCombine does it as well for simple uses, it's not clear that it
        // occurs enough in real life to handle.
        break;
      }
  }
  if (!CondUse) return;  // setcc doesn't use the IV.

  // setcc stride is complex, don't mess with users.
  // FIXME: Evaluate whether this is a good idea or not.
  if (!isa<SCEVConstant>(*CondStride)) return;

  // It's possible for the setcc instruction to be anywhere in the loop, and
  // possible for it to have multiple users.  If it is not immediately before
  // the latch block branch, move it.
  if (&*++BasicBlock::iterator(Cond) != (Instruction*)TermBr) {
    if (Cond->hasOneUse()) {   // Condition has a single use, just move it.
      Cond->moveBefore(TermBr);
    } else {
      // Otherwise, clone the terminating condition and insert into the loopend.
      Cond = cast<SetCondInst>(Cond->clone());
      Cond->setName(L->getHeader()->getName() + ".termcond");
      LatchBlock->getInstList().insert(TermBr, Cond);
      
      // Clone the IVUse, as the old use still exists!
      IVUsesByStride[*CondStride].addUser(CondUse->Offset, Cond,
                                         CondUse->OperandValToReplace);
      CondUse = &IVUsesByStride[*CondStride].Users.back();
    }
  }

  // If we get to here, we know that we can transform the setcc instruction to
  // use the post-incremented version of the IV, allowing us to coallesce the
  // live ranges for the IV correctly.
  CondUse->Offset = SCEV::getMinusSCEV(CondUse->Offset, *CondStride);
  CondUse->isUseOfPostIncrementedValue = true;
}

void LoopStrengthReduce::runOnLoop(Loop *L) {
  // First step, transform all loops nesting inside of this loop.
  for (LoopInfo::iterator I = L->begin(), E = L->end(); I != E; ++I)
    runOnLoop(*I);

  // Next, find all uses of induction variables in this loop, and catagorize
  // them by stride.  Start by finding all of the PHI nodes in the header for
  // this loop.  If they are induction variables, inspect their uses.
  std::set<Instruction*> Processed;   // Don't reprocess instructions.
  for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)
    AddUsersIfInteresting(I, L, Processed);

  // If we have nothing to do, return.
  if (IVUsesByStride.empty()) return;

  // Optimize induction variables.  Some indvar uses can be transformed to use
  // strides that will be needed for other purposes.  A common example of this
  // is the exit test for the loop, which can often be rewritten to use the
  // computation of some other indvar to decide when to terminate the loop.
  OptimizeIndvars(L);


  // FIXME: We can widen subreg IV's here for RISC targets.  e.g. instead of
  // doing computation in byte values, promote to 32-bit values if safe.

  // FIXME: Attempt to reuse values across multiple IV's.  In particular, we
  // could have something like "for(i) { foo(i*8); bar(i*16) }", which should be
  // codegened as "for (j = 0;; j+=8) { foo(j); bar(j+j); }" on X86/PPC.  Need
  // to be careful that IV's are all the same type.  Only works for intptr_t
  // indvars.

  // If we only have one stride, we can more aggressively eliminate some things.
  bool HasOneStride = IVUsesByStride.size() == 1;
  
  // Note: this processes each stride/type pair individually.  All users passed
  // into StrengthReduceStridedIVUsers have the same type AND stride.  Also,
  // node that we iterate over IVUsesByStride indirectly by using StrideOrder.
  // This extra layer of indirection makes the ordering of strides deterministic
  // - not dependent on map order.
  for (unsigned Stride = 0, e = StrideOrder.size(); Stride != e; ++Stride) {
    std::map<SCEVHandle, IVUsersOfOneStride>::iterator SI = 
      IVUsesByStride.find(StrideOrder[Stride]);
    assert(SI != IVUsesByStride.end() && "Stride doesn't exist!");
    StrengthReduceStridedIVUsers(SI->first, SI->second, L, HasOneStride);
  }

  // Clean up after ourselves
  if (!DeadInsts.empty()) {
    DeleteTriviallyDeadInstructions(DeadInsts);

    BasicBlock::iterator I = L->getHeader()->begin();
    PHINode *PN;
    while ((PN = dyn_cast<PHINode>(I))) {
      ++I;  // Preincrement iterator to avoid invalidating it when deleting PN.
      
      // At this point, we know that we have killed one or more GEP
      // instructions.  It is worth checking to see if the cann indvar is also
      // dead, so that we can remove it as well.  The requirements for the cann
      // indvar to be considered dead are:
      // 1. the cann indvar has one use
      // 2. the use is an add instruction
      // 3. the add has one use
      // 4. the add is used by the cann indvar
      // If all four cases above are true, then we can remove both the add and
      // the cann indvar.
      // FIXME: this needs to eliminate an induction variable even if it's being
      // compared against some value to decide loop termination.
      if (PN->hasOneUse()) {
        BinaryOperator *BO = dyn_cast<BinaryOperator>(*(PN->use_begin()));
        if (BO && BO->hasOneUse()) {
          if (PN == *(BO->use_begin())) {
            DeadInsts.insert(BO);
            // Break the cycle, then delete the PHI.
            PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
            SE->deleteInstructionFromRecords(PN);
            PN->eraseFromParent();
          }
        }
      }
    }
    DeleteTriviallyDeadInstructions(DeadInsts);
  }

  CastedPointers.clear();
  IVUsesByStride.clear();
  StrideOrder.clear();
  return;
}