lib/Target/PowerPC/PPCLoopPreIncPrep.cpp

//===------ PPCLoopPreIncPrep.cpp - Loop Pre-Inc. AM Prep. Pass -----------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements a pass to prepare loops for pre-increment addressing
// modes. Additional PHIs are created for loop induction variables used by
// load/store instructions so that the pre-increment forms can be used.
// Generically, this means transforming loops like this:
//   for (int i = 0; i < n; ++i)
//     array[i] = c;
// to look like this:
//   T *p = array[-1];
//   for (int i = 0; i < n; ++i)
//     *++p = c;
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "ppc-loop-preinc-prep"

#include "PPC.h"
#include "PPCSubtarget.h"
#include "PPCTargetMachine.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpander.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include <cassert>
#include <iterator>
#include <utility>

using namespace llvm;

// By default, we limit this to creating 16 PHIs (which is a little over half
// of the allocatable register set).
static cl::opt<unsigned> MaxVars("ppc-preinc-prep-max-vars",
                                 cl::Hidden, cl::init(16),
  cl::desc("Potential PHI threshold for PPC preinc loop prep"));

STATISTIC(PHINodeAlreadyExists, "PHI node already in pre-increment form");

namespace {

  class PPCLoopPreIncPrep : public FunctionPass {
  public:
    static char ID; // Pass ID, replacement for typeid

    PPCLoopPreIncPrep() : FunctionPass(ID) {
      initializePPCLoopPreIncPrepPass(*PassRegistry::getPassRegistry());
    }

    PPCLoopPreIncPrep(PPCTargetMachine &TM) : FunctionPass(ID), TM(&TM) {
      initializePPCLoopPreIncPrepPass(*PassRegistry::getPassRegistry());
    }

    void getAnalysisUsage(AnalysisUsage &AU) const override {
      AU.addPreserved<DominatorTreeWrapperPass>();
      AU.addRequired<LoopInfoWrapperPass>();
      AU.addPreserved<LoopInfoWrapperPass>();
      AU.addRequired<ScalarEvolutionWrapperPass>();
    }

    bool alreadyPrepared(Loop *L, Instruction* MemI,
                         const SCEV *BasePtrStartSCEV,
                         const SCEVConstant *BasePtrIncSCEV);
    bool runOnFunction(Function &F) override;

    bool runOnLoop(Loop *L);
    void simplifyLoopLatch(Loop *L);
    bool rotateLoop(Loop *L);

  private:
    PPCTargetMachine *TM = nullptr;
    DominatorTree *DT;
    LoopInfo *LI;
    ScalarEvolution *SE;
    bool PreserveLCSSA;
  };

} // end anonymous namespace

char PPCLoopPreIncPrep::ID = 0;
static const char *name = "Prepare loop for pre-inc. addressing modes";
INITIALIZE_PASS_BEGIN(PPCLoopPreIncPrep, DEBUG_TYPE, name, false, false)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
INITIALIZE_PASS_END(PPCLoopPreIncPrep, DEBUG_TYPE, name, false, false)

FunctionPass *llvm::createPPCLoopPreIncPrepPass(PPCTargetMachine &TM) {
  return new PPCLoopPreIncPrep(TM);
}

namespace {

  struct BucketElement {
    BucketElement(const SCEVConstant *O, Instruction *I) : Offset(O), Instr(I) {}
    BucketElement(Instruction *I) : Offset(nullptr), Instr(I) {}

    const SCEVConstant *Offset;
    Instruction *Instr;
  };

  struct Bucket {
    Bucket(const SCEV *B, Instruction *I) : BaseSCEV(B),
                                            Elements(1, BucketElement(I)) {}

    const SCEV *BaseSCEV;
    SmallVector<BucketElement, 16> Elements;
  };

} // end anonymous namespace

static bool IsPtrInBounds(Value *BasePtr) {
  Value *StrippedBasePtr = BasePtr;
  while (BitCastInst *BC = dyn_cast<BitCastInst>(StrippedBasePtr))
    StrippedBasePtr = BC->getOperand(0);
  if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(StrippedBasePtr))
    return GEP->isInBounds();

  return false;
}

static Value *GetPointerOperand(Value *MemI) {
  if (LoadInst *LMemI = dyn_cast<LoadInst>(MemI)) {
    return LMemI->getPointerOperand();
  } else if (StoreInst *SMemI = dyn_cast<StoreInst>(MemI)) {
    return SMemI->getPointerOperand();
  } else if (IntrinsicInst *IMemI = dyn_cast<IntrinsicInst>(MemI)) {
    if (IMemI->getIntrinsicID() == Intrinsic::prefetch)
      return IMemI->getArgOperand(0);
  }

  return nullptr;
}

bool PPCLoopPreIncPrep::runOnFunction(Function &F) {
  if (skipFunction(F))
    return false;

  LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
  SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
  auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
  DT = DTWP ? &DTWP->getDomTree() : nullptr;
  PreserveLCSSA = mustPreserveAnalysisID(LCSSAID);

  bool MadeChange = false;

  for (auto I = LI->begin(), IE = LI->end(); I != IE; ++I)
    for (auto L = df_begin(*I), LE = df_end(*I); L != LE; ++L)
      MadeChange |= runOnLoop(*L);

  return MadeChange;
}

// In order to prepare for the pre-increment a PHI is added.
// This function will check to see if that PHI already exists and will return
//  true if it found an existing PHI with the same start and increment as the
//  one we wanted to create.
bool PPCLoopPreIncPrep::alreadyPrepared(Loop *L, Instruction* MemI,
                                        const SCEV *BasePtrStartSCEV,
                                        const SCEVConstant *BasePtrIncSCEV) {
  BasicBlock *BB = MemI->getParent();
  if (!BB)
    return false;

  BasicBlock *PredBB = L->getLoopPredecessor();
  BasicBlock *LatchBB = L->getLoopLatch();

  if (!PredBB || !LatchBB)
    return false;

  // Run through the PHIs and see if we have some that looks like a preparation
  iterator_range<BasicBlock::phi_iterator> PHIIter = BB->phis();
  for (auto & CurrentPHI : PHIIter) {
    PHINode *CurrentPHINode = dyn_cast<PHINode>(&CurrentPHI);
    if (!CurrentPHINode)
      continue;

    if (!SE->isSCEVable(CurrentPHINode->getType()))
      continue;

    const SCEV *PHISCEV = SE->getSCEVAtScope(CurrentPHINode, L);

    const SCEVAddRecExpr *PHIBasePtrSCEV = dyn_cast<SCEVAddRecExpr>(PHISCEV);
    if (!PHIBasePtrSCEV)
      continue;

    const SCEVConstant *PHIBasePtrIncSCEV =
      dyn_cast<SCEVConstant>(PHIBasePtrSCEV->getStepRecurrence(*SE));
    if (!PHIBasePtrIncSCEV)
      continue;

    if (CurrentPHINode->getNumIncomingValues() == 2) {
      if ( (CurrentPHINode->getIncomingBlock(0) == LatchBB &&
            CurrentPHINode->getIncomingBlock(1) == PredBB) ||
            (CurrentPHINode->getIncomingBlock(1) == LatchBB &&
            CurrentPHINode->getIncomingBlock(0) == PredBB) ) {
        if (PHIBasePtrSCEV->getStart() == BasePtrStartSCEV &&
            PHIBasePtrIncSCEV == BasePtrIncSCEV) {
          // The existing PHI (CurrentPHINode) has the same start and increment
          //  as the PHI that we wanted to create.
          ++PHINodeAlreadyExists;
          return true;
        }
      }
    }
  }
  return false;
}

bool PPCLoopPreIncPrep::runOnLoop(Loop *L) {
  bool MadeChange = false;

  // Only prep. the inner-most loop
  if (!L->empty())
    return MadeChange;

  LLVM_DEBUG(dbgs() << "PIP: Examining: " << *L << "\n");

  BasicBlock *Header = L->getHeader();

  const PPCSubtarget *ST =
    TM ? TM->getSubtargetImpl(*Header->getParent()) : nullptr;

  unsigned HeaderLoopPredCount = pred_size(Header);

  // Collect buckets of comparable addresses used by loads and stores.
  SmallVector<Bucket, 16> Buckets;
  for (Loop::block_iterator I = L->block_begin(), IE = L->block_end();
       I != IE; ++I) {
    for (BasicBlock::iterator J = (*I)->begin(), JE = (*I)->end();
        J != JE; ++J) {
      Value *PtrValue;
      Instruction *MemI;

      if (LoadInst *LMemI = dyn_cast<LoadInst>(J)) {
        MemI = LMemI;
        PtrValue = LMemI->getPointerOperand();
      } else if (StoreInst *SMemI = dyn_cast<StoreInst>(J)) {
        MemI = SMemI;
        PtrValue = SMemI->getPointerOperand();
      } else if (IntrinsicInst *IMemI = dyn_cast<IntrinsicInst>(J)) {
        if (IMemI->getIntrinsicID() == Intrinsic::prefetch) {
          MemI = IMemI;
          PtrValue = IMemI->getArgOperand(0);
        } else continue;
      } else continue;

      unsigned PtrAddrSpace = PtrValue->getType()->getPointerAddressSpace();
      if (PtrAddrSpace)
        continue;

      // There are no update forms for Altivec vector load/stores.
      if (ST && ST->hasAltivec() &&
          PtrValue->getType()->getPointerElementType()->isVectorTy())
        continue;

      if (L->isLoopInvariant(PtrValue))
        continue;

      const SCEV *LSCEV = SE->getSCEVAtScope(PtrValue, L);
      if (const SCEVAddRecExpr *LARSCEV = dyn_cast<SCEVAddRecExpr>(LSCEV)) {
        if (LARSCEV->getLoop() != L)
          continue;
        // See getPreIndexedAddressParts, the displacement for LDU/STDU has to
        // be 4's multiple (DS-form). For i64 loads/stores when the displacement
        // fits in a 16-bit signed field but isn't a multiple of 4, it will be
        // useless and possible to break some original well-form addressing mode
        // to make this pre-inc prep for it.
        if (PtrValue->getType()->getPointerElementType()->isIntegerTy(64)) {
          if (const SCEVConstant *StepConst =
                  dyn_cast<SCEVConstant>(LARSCEV->getStepRecurrence(*SE))) {
            const APInt &ConstInt = StepConst->getValue()->getValue();
            if (ConstInt.isSignedIntN(16) && ConstInt.srem(4) != 0)
              continue;
          }
        }
      } else {
        continue;
      }

      bool FoundBucket = false;
      for (auto &B : Buckets) {
        const SCEV *Diff = SE->getMinusSCEV(LSCEV, B.BaseSCEV);
        if (const auto *CDiff = dyn_cast<SCEVConstant>(Diff)) {
          B.Elements.push_back(BucketElement(CDiff, MemI));
          FoundBucket = true;
          break;
        }
      }

      if (!FoundBucket) {
        if (Buckets.size() == MaxVars)
          return MadeChange;
        Buckets.push_back(Bucket(LSCEV, MemI));
      }
    }
  }

  if (Buckets.empty())
    return MadeChange;

  BasicBlock *LoopPredecessor = L->getLoopPredecessor();
  // If there is no loop predecessor, or the loop predecessor's terminator
  // returns a value (which might contribute to determining the loop's
  // iteration space), insert a new preheader for the loop.
  if (!LoopPredecessor ||
      !LoopPredecessor->getTerminator()->getType()->isVoidTy()) {
    LoopPredecessor = InsertPreheaderForLoop(L, DT, LI, PreserveLCSSA);
    if (LoopPredecessor)
      MadeChange = true;
  }
  if (!LoopPredecessor)
    return MadeChange;

  LLVM_DEBUG(dbgs() << "PIP: Found " << Buckets.size() << " buckets\n");

  SmallSet<BasicBlock *, 16> BBChanged;
  for (unsigned i = 0, e = Buckets.size(); i != e; ++i) {
    // The base address of each bucket is transformed into a phi and the others
    // are rewritten as offsets of that variable.

    // We have a choice now of which instruction's memory operand we use as the
    // base for the generated PHI. Always picking the first instruction in each
    // bucket does not work well, specifically because that instruction might
    // be a prefetch (and there are no pre-increment dcbt variants). Otherwise,
    // the choice is somewhat arbitrary, because the backend will happily
    // generate direct offsets from both the pre-incremented and
    // post-incremented pointer values. Thus, we'll pick the first non-prefetch
    // instruction in each bucket, and adjust the recurrence and other offsets
    // accordingly.
    for (int j = 0, je = Buckets[i].Elements.size(); j != je; ++j) {
      if (auto *II = dyn_cast<IntrinsicInst>(Buckets[i].Elements[j].Instr))
        if (II->getIntrinsicID() == Intrinsic::prefetch)
          continue;

      // If we'd otherwise pick the first element anyway, there's nothing to do.
      if (j == 0)
        break;

      // If our chosen element has no offset from the base pointer, there's
      // nothing to do.
      if (!Buckets[i].Elements[j].Offset ||
          Buckets[i].Elements[j].Offset->isZero())
        break;

      const SCEV *Offset = Buckets[i].Elements[j].Offset;
      Buckets[i].BaseSCEV = SE->getAddExpr(Buckets[i].BaseSCEV, Offset);
      for (auto &E : Buckets[i].Elements) {
        if (E.Offset)
          E.Offset = cast<SCEVConstant>(SE->getMinusSCEV(E.Offset, Offset));
        else
          E.Offset = cast<SCEVConstant>(SE->getNegativeSCEV(Offset));
      }

      std::swap(Buckets[i].Elements[j], Buckets[i].Elements[0]);
      break;
    }

    const SCEVAddRecExpr *BasePtrSCEV =
      cast<SCEVAddRecExpr>(Buckets[i].BaseSCEV);
    if (!BasePtrSCEV->isAffine())
      continue;

    LLVM_DEBUG(dbgs() << "PIP: Transforming: " << *BasePtrSCEV << "\n");
    assert(BasePtrSCEV->getLoop() == L &&
           "AddRec for the wrong loop?");

    // The instruction corresponding to the Bucket's BaseSCEV must be the first
    // in the vector of elements.
    Instruction *MemI = Buckets[i].Elements.begin()->Instr;
    Value *BasePtr = GetPointerOperand(MemI);
    assert(BasePtr && "No pointer operand");

    Type *I8Ty = Type::getInt8Ty(MemI->getParent()->getContext());
    Type *I8PtrTy = Type::getInt8PtrTy(MemI->getParent()->getContext(),
      BasePtr->getType()->getPointerAddressSpace());

    const SCEV *BasePtrStartSCEV = BasePtrSCEV->getStart();
    if (!SE->isLoopInvariant(BasePtrStartSCEV, L))
      continue;

    const SCEVConstant *BasePtrIncSCEV =
      dyn_cast<SCEVConstant>(BasePtrSCEV->getStepRecurrence(*SE));
    if (!BasePtrIncSCEV)
      continue;
    BasePtrStartSCEV = SE->getMinusSCEV(BasePtrStartSCEV, BasePtrIncSCEV);
    if (!isSafeToExpand(BasePtrStartSCEV, *SE))
      continue;

    LLVM_DEBUG(dbgs() << "PIP: New start is: " << *BasePtrStartSCEV << "\n");

    if (alreadyPrepared(L, MemI, BasePtrStartSCEV, BasePtrIncSCEV))
      continue;

    PHINode *NewPHI = PHINode::Create(I8PtrTy, HeaderLoopPredCount,
      MemI->hasName() ? MemI->getName() + ".phi" : "",
      Header->getFirstNonPHI());

    SCEVExpander SCEVE(*SE, Header->getModule()->getDataLayout(), "pistart");
    Value *BasePtrStart = SCEVE.expandCodeFor(BasePtrStartSCEV, I8PtrTy,
      LoopPredecessor->getTerminator());

    // Note that LoopPredecessor might occur in the predecessor list multiple
    // times, and we need to add it the right number of times.
    for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
         PI != PE; ++PI) {
      if (*PI != LoopPredecessor)
        continue;

      NewPHI->addIncoming(BasePtrStart, LoopPredecessor);
    }

    Instruction *InsPoint = &*Header->getFirstInsertionPt();
    GetElementPtrInst *PtrInc = GetElementPtrInst::Create(
        I8Ty, NewPHI, BasePtrIncSCEV->getValue(),
        MemI->hasName() ? MemI->getName() + ".inc" : "", InsPoint);
    PtrInc->setIsInBounds(IsPtrInBounds(BasePtr));
    for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
         PI != PE; ++PI) {
      if (*PI == LoopPredecessor)
        continue;

      NewPHI->addIncoming(PtrInc, *PI);
    }

    Instruction *NewBasePtr;
    if (PtrInc->getType() != BasePtr->getType())
      NewBasePtr = new BitCastInst(PtrInc, BasePtr->getType(),
        PtrInc->hasName() ? PtrInc->getName() + ".cast" : "", InsPoint);
    else
      NewBasePtr = PtrInc;

    if (Instruction *IDel = dyn_cast<Instruction>(BasePtr))
      BBChanged.insert(IDel->getParent());
    BasePtr->replaceAllUsesWith(NewBasePtr);
    RecursivelyDeleteTriviallyDeadInstructions(BasePtr);

    // Keep track of the replacement pointer values we've inserted so that we
    // don't generate more pointer values than necessary.
    SmallPtrSet<Value *, 16> NewPtrs;
    NewPtrs.insert( NewBasePtr);

    for (auto I = std::next(Buckets[i].Elements.begin()),
         IE = Buckets[i].Elements.end(); I != IE; ++I) {
      Value *Ptr = GetPointerOperand(I->Instr);
      assert(Ptr && "No pointer operand");
      if (NewPtrs.count(Ptr))
        continue;

      Instruction *RealNewPtr;
      if (!I->Offset || I->Offset->getValue()->isZero()) {
        RealNewPtr = NewBasePtr;
      } else {
        Instruction *PtrIP = dyn_cast<Instruction>(Ptr);
        if (PtrIP && isa<Instruction>(NewBasePtr) &&
            cast<Instruction>(NewBasePtr)->getParent() == PtrIP->getParent())
          PtrIP = nullptr;
        else if (isa<PHINode>(PtrIP))
          PtrIP = &*PtrIP->getParent()->getFirstInsertionPt();
        else if (!PtrIP)
          PtrIP = I->Instr;

        GetElementPtrInst *NewPtr = GetElementPtrInst::Create(
            I8Ty, PtrInc, I->Offset->getValue(),
            I->Instr->hasName() ? I->Instr->getName() + ".off" : "", PtrIP);
        if (!PtrIP)
          NewPtr->insertAfter(cast<Instruction>(PtrInc));
        NewPtr->setIsInBounds(IsPtrInBounds(Ptr));
        RealNewPtr = NewPtr;
      }

      if (Instruction *IDel = dyn_cast<Instruction>(Ptr))
        BBChanged.insert(IDel->getParent());

      Instruction *ReplNewPtr;
      if (Ptr->getType() != RealNewPtr->getType()) {
        ReplNewPtr = new BitCastInst(RealNewPtr, Ptr->getType(),
          Ptr->hasName() ? Ptr->getName() + ".cast" : "");
        ReplNewPtr->insertAfter(RealNewPtr);
      } else
        ReplNewPtr = RealNewPtr;

      Ptr->replaceAllUsesWith(ReplNewPtr);
      RecursivelyDeleteTriviallyDeadInstructions(Ptr);

      NewPtrs.insert(RealNewPtr);
    }

    MadeChange = true;
  }

  for (Loop::block_iterator I = L->block_begin(), IE = L->block_end();
       I != IE; ++I) {
    if (BBChanged.count(*I))
      DeleteDeadPHIs(*I);
  }

  return MadeChange;
}