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      1 //===------ PPCLoopPreIncPrep.cpp - Loop Pre-Inc. AM Prep. Pass -----------===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 //
     10 // This file implements a pass to prepare loops for pre-increment addressing
     11 // modes. Additional PHIs are created for loop induction variables used by
     12 // load/store instructions so that the pre-increment forms can be used.
     13 // Generically, this means transforming loops like this:
     14 //   for (int i = 0; i < n; ++i)
     15 //     array[i] = c;
     16 // to look like this:
     17 //   T *p = array[-1];
     18 //   for (int i = 0; i < n; ++i)
     19 //     *++p = c;
     20 //===----------------------------------------------------------------------===//
     21 
     22 #define DEBUG_TYPE "ppc-loop-preinc-prep"
     23 #include "PPC.h"
     24 #include "PPCTargetMachine.h"
     25 #include "llvm/ADT/DepthFirstIterator.h"
     26 #include "llvm/ADT/STLExtras.h"
     27 #include "llvm/ADT/SmallSet.h"
     28 #include "llvm/ADT/Statistic.h"
     29 #include "llvm/Analysis/CodeMetrics.h"
     30 #include "llvm/Analysis/InstructionSimplify.h"
     31 #include "llvm/Analysis/LoopInfo.h"
     32 #include "llvm/Analysis/ScalarEvolution.h"
     33 #include "llvm/Analysis/ScalarEvolutionExpander.h"
     34 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
     35 #include "llvm/Analysis/ValueTracking.h"
     36 #include "llvm/IR/CFG.h"
     37 #include "llvm/IR/Dominators.h"
     38 #include "llvm/IR/Function.h"
     39 #include "llvm/IR/IntrinsicInst.h"
     40 #include "llvm/IR/Module.h"
     41 #include "llvm/Support/CommandLine.h"
     42 #include "llvm/Support/Debug.h"
     43 #include "llvm/Transforms/Scalar.h"
     44 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
     45 #include "llvm/Transforms/Utils/Local.h"
     46 #include "llvm/Transforms/Utils/LoopUtils.h"
     47 #include "llvm/Transforms/Utils/ValueMapper.h"
     48 using namespace llvm;
     49 
     50 // By default, we limit this to creating 16 PHIs (which is a little over half
     51 // of the allocatable register set).
     52 static cl::opt<unsigned> MaxVars("ppc-preinc-prep-max-vars",
     53                                  cl::Hidden, cl::init(16),
     54   cl::desc("Potential PHI threshold for PPC preinc loop prep"));
     55 
     56 namespace llvm {
     57   void initializePPCLoopPreIncPrepPass(PassRegistry&);
     58 }
     59 
     60 namespace {
     61 
     62   class PPCLoopPreIncPrep : public FunctionPass {
     63   public:
     64     static char ID; // Pass ID, replacement for typeid
     65     PPCLoopPreIncPrep() : FunctionPass(ID), TM(nullptr) {
     66       initializePPCLoopPreIncPrepPass(*PassRegistry::getPassRegistry());
     67     }
     68     PPCLoopPreIncPrep(PPCTargetMachine &TM) : FunctionPass(ID), TM(&TM) {
     69       initializePPCLoopPreIncPrepPass(*PassRegistry::getPassRegistry());
     70     }
     71 
     72     void getAnalysisUsage(AnalysisUsage &AU) const override {
     73       AU.addPreserved<DominatorTreeWrapperPass>();
     74       AU.addRequired<LoopInfoWrapperPass>();
     75       AU.addPreserved<LoopInfoWrapperPass>();
     76       AU.addRequired<ScalarEvolutionWrapperPass>();
     77     }
     78 
     79     bool runOnFunction(Function &F) override;
     80 
     81     bool runOnLoop(Loop *L);
     82     void simplifyLoopLatch(Loop *L);
     83     bool rotateLoop(Loop *L);
     84 
     85   private:
     86     PPCTargetMachine *TM;
     87     DominatorTree *DT;
     88     LoopInfo *LI;
     89     ScalarEvolution *SE;
     90     bool PreserveLCSSA;
     91   };
     92 }
     93 
     94 char PPCLoopPreIncPrep::ID = 0;
     95 static const char *name = "Prepare loop for pre-inc. addressing modes";
     96 INITIALIZE_PASS_BEGIN(PPCLoopPreIncPrep, DEBUG_TYPE, name, false, false)
     97 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
     98 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
     99 INITIALIZE_PASS_END(PPCLoopPreIncPrep, DEBUG_TYPE, name, false, false)
    100 
    101 FunctionPass *llvm::createPPCLoopPreIncPrepPass(PPCTargetMachine &TM) {
    102   return new PPCLoopPreIncPrep(TM);
    103 }
    104 
    105 namespace {
    106   struct BucketElement {
    107     BucketElement(const SCEVConstant *O, Instruction *I) : Offset(O), Instr(I) {}
    108     BucketElement(Instruction *I) : Offset(nullptr), Instr(I) {}
    109 
    110     const SCEVConstant *Offset;
    111     Instruction *Instr;
    112   };
    113 
    114   struct Bucket {
    115     Bucket(const SCEV *B, Instruction *I) : BaseSCEV(B),
    116                                             Elements(1, BucketElement(I)) {}
    117 
    118     const SCEV *BaseSCEV;
    119     SmallVector<BucketElement, 16> Elements;
    120   };
    121 }
    122 
    123 static bool IsPtrInBounds(Value *BasePtr) {
    124   Value *StrippedBasePtr = BasePtr;
    125   while (BitCastInst *BC = dyn_cast<BitCastInst>(StrippedBasePtr))
    126     StrippedBasePtr = BC->getOperand(0);
    127   if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(StrippedBasePtr))
    128     return GEP->isInBounds();
    129 
    130   return false;
    131 }
    132 
    133 static Value *GetPointerOperand(Value *MemI) {
    134   if (LoadInst *LMemI = dyn_cast<LoadInst>(MemI)) {
    135     return LMemI->getPointerOperand();
    136   } else if (StoreInst *SMemI = dyn_cast<StoreInst>(MemI)) {
    137     return SMemI->getPointerOperand();
    138   } else if (IntrinsicInst *IMemI = dyn_cast<IntrinsicInst>(MemI)) {
    139     if (IMemI->getIntrinsicID() == Intrinsic::prefetch)
    140       return IMemI->getArgOperand(0);
    141   }
    142 
    143   return 0;
    144 }
    145 
    146 bool PPCLoopPreIncPrep::runOnFunction(Function &F) {
    147   if (skipFunction(F))
    148     return false;
    149 
    150   LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
    151   SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
    152   auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
    153   DT = DTWP ? &DTWP->getDomTree() : nullptr;
    154   PreserveLCSSA = mustPreserveAnalysisID(LCSSAID);
    155 
    156   bool MadeChange = false;
    157 
    158   for (auto I = LI->begin(), IE = LI->end(); I != IE; ++I)
    159     for (auto L = df_begin(*I), LE = df_end(*I); L != LE; ++L)
    160       MadeChange |= runOnLoop(*L);
    161 
    162   return MadeChange;
    163 }
    164 
    165 bool PPCLoopPreIncPrep::runOnLoop(Loop *L) {
    166   bool MadeChange = false;
    167 
    168   // Only prep. the inner-most loop
    169   if (!L->empty())
    170     return MadeChange;
    171 
    172   DEBUG(dbgs() << "PIP: Examining: " << *L << "\n");
    173 
    174   BasicBlock *Header = L->getHeader();
    175 
    176   const PPCSubtarget *ST =
    177     TM ? TM->getSubtargetImpl(*Header->getParent()) : nullptr;
    178 
    179   unsigned HeaderLoopPredCount =
    180     std::distance(pred_begin(Header), pred_end(Header));
    181 
    182   // Collect buckets of comparable addresses used by loads and stores.
    183   SmallVector<Bucket, 16> Buckets;
    184   for (Loop::block_iterator I = L->block_begin(), IE = L->block_end();
    185        I != IE; ++I) {
    186     for (BasicBlock::iterator J = (*I)->begin(), JE = (*I)->end();
    187         J != JE; ++J) {
    188       Value *PtrValue;
    189       Instruction *MemI;
    190 
    191       if (LoadInst *LMemI = dyn_cast<LoadInst>(J)) {
    192         MemI = LMemI;
    193         PtrValue = LMemI->getPointerOperand();
    194       } else if (StoreInst *SMemI = dyn_cast<StoreInst>(J)) {
    195         MemI = SMemI;
    196         PtrValue = SMemI->getPointerOperand();
    197       } else if (IntrinsicInst *IMemI = dyn_cast<IntrinsicInst>(J)) {
    198         if (IMemI->getIntrinsicID() == Intrinsic::prefetch) {
    199           MemI = IMemI;
    200           PtrValue = IMemI->getArgOperand(0);
    201         } else continue;
    202       } else continue;
    203 
    204       unsigned PtrAddrSpace = PtrValue->getType()->getPointerAddressSpace();
    205       if (PtrAddrSpace)
    206         continue;
    207 
    208       // There are no update forms for Altivec vector load/stores.
    209       if (ST && ST->hasAltivec() &&
    210           PtrValue->getType()->getPointerElementType()->isVectorTy())
    211         continue;
    212 
    213       if (L->isLoopInvariant(PtrValue))
    214         continue;
    215 
    216       const SCEV *LSCEV = SE->getSCEVAtScope(PtrValue, L);
    217       if (const SCEVAddRecExpr *LARSCEV = dyn_cast<SCEVAddRecExpr>(LSCEV)) {
    218         if (LARSCEV->getLoop() != L)
    219           continue;
    220       } else {
    221         continue;
    222       }
    223 
    224       bool FoundBucket = false;
    225       for (auto &B : Buckets) {
    226         const SCEV *Diff = SE->getMinusSCEV(LSCEV, B.BaseSCEV);
    227         if (const auto *CDiff = dyn_cast<SCEVConstant>(Diff)) {
    228           B.Elements.push_back(BucketElement(CDiff, MemI));
    229           FoundBucket = true;
    230           break;
    231         }
    232       }
    233 
    234       if (!FoundBucket) {
    235         if (Buckets.size() == MaxVars)
    236           return MadeChange;
    237         Buckets.push_back(Bucket(LSCEV, MemI));
    238       }
    239     }
    240   }
    241 
    242   if (Buckets.empty())
    243     return MadeChange;
    244 
    245   BasicBlock *LoopPredecessor = L->getLoopPredecessor();
    246   // If there is no loop predecessor, or the loop predecessor's terminator
    247   // returns a value (which might contribute to determining the loop's
    248   // iteration space), insert a new preheader for the loop.
    249   if (!LoopPredecessor ||
    250       !LoopPredecessor->getTerminator()->getType()->isVoidTy()) {
    251     LoopPredecessor = InsertPreheaderForLoop(L, DT, LI, PreserveLCSSA);
    252     if (LoopPredecessor)
    253       MadeChange = true;
    254   }
    255   if (!LoopPredecessor)
    256     return MadeChange;
    257 
    258   DEBUG(dbgs() << "PIP: Found " << Buckets.size() << " buckets\n");
    259 
    260   SmallSet<BasicBlock *, 16> BBChanged;
    261   for (unsigned i = 0, e = Buckets.size(); i != e; ++i) {
    262     // The base address of each bucket is transformed into a phi and the others
    263     // are rewritten as offsets of that variable.
    264 
    265     // We have a choice now of which instruction's memory operand we use as the
    266     // base for the generated PHI. Always picking the first instruction in each
    267     // bucket does not work well, specifically because that instruction might
    268     // be a prefetch (and there are no pre-increment dcbt variants). Otherwise,
    269     // the choice is somewhat arbitrary, because the backend will happily
    270     // generate direct offsets from both the pre-incremented and
    271     // post-incremented pointer values. Thus, we'll pick the first non-prefetch
    272     // instruction in each bucket, and adjust the recurrence and other offsets
    273     // accordingly.
    274     for (int j = 0, je = Buckets[i].Elements.size(); j != je; ++j) {
    275       if (auto *II = dyn_cast<IntrinsicInst>(Buckets[i].Elements[j].Instr))
    276         if (II->getIntrinsicID() == Intrinsic::prefetch)
    277           continue;
    278 
    279       // If we'd otherwise pick the first element anyway, there's nothing to do.
    280       if (j == 0)
    281         break;
    282 
    283       // If our chosen element has no offset from the base pointer, there's
    284       // nothing to do.
    285       if (!Buckets[i].Elements[j].Offset ||
    286           Buckets[i].Elements[j].Offset->isZero())
    287         break;
    288 
    289       const SCEV *Offset = Buckets[i].Elements[j].Offset;
    290       Buckets[i].BaseSCEV = SE->getAddExpr(Buckets[i].BaseSCEV, Offset);
    291       for (auto &E : Buckets[i].Elements) {
    292         if (E.Offset)
    293           E.Offset = cast<SCEVConstant>(SE->getMinusSCEV(E.Offset, Offset));
    294         else
    295           E.Offset = cast<SCEVConstant>(SE->getNegativeSCEV(Offset));
    296       }
    297 
    298       std::swap(Buckets[i].Elements[j], Buckets[i].Elements[0]);
    299       break;
    300     }
    301 
    302     const SCEVAddRecExpr *BasePtrSCEV =
    303       cast<SCEVAddRecExpr>(Buckets[i].BaseSCEV);
    304     if (!BasePtrSCEV->isAffine())
    305       continue;
    306 
    307     DEBUG(dbgs() << "PIP: Transforming: " << *BasePtrSCEV << "\n");
    308     assert(BasePtrSCEV->getLoop() == L &&
    309            "AddRec for the wrong loop?");
    310 
    311     // The instruction corresponding to the Bucket's BaseSCEV must be the first
    312     // in the vector of elements.
    313     Instruction *MemI = Buckets[i].Elements.begin()->Instr;
    314     Value *BasePtr = GetPointerOperand(MemI);
    315     assert(BasePtr && "No pointer operand");
    316 
    317     Type *I8Ty = Type::getInt8Ty(MemI->getParent()->getContext());
    318     Type *I8PtrTy = Type::getInt8PtrTy(MemI->getParent()->getContext(),
    319       BasePtr->getType()->getPointerAddressSpace());
    320 
    321     const SCEV *BasePtrStartSCEV = BasePtrSCEV->getStart();
    322     if (!SE->isLoopInvariant(BasePtrStartSCEV, L))
    323       continue;
    324 
    325     const SCEVConstant *BasePtrIncSCEV =
    326       dyn_cast<SCEVConstant>(BasePtrSCEV->getStepRecurrence(*SE));
    327     if (!BasePtrIncSCEV)
    328       continue;
    329     BasePtrStartSCEV = SE->getMinusSCEV(BasePtrStartSCEV, BasePtrIncSCEV);
    330     if (!isSafeToExpand(BasePtrStartSCEV, *SE))
    331       continue;
    332 
    333     DEBUG(dbgs() << "PIP: New start is: " << *BasePtrStartSCEV << "\n");
    334 
    335     PHINode *NewPHI = PHINode::Create(I8PtrTy, HeaderLoopPredCount,
    336       MemI->hasName() ? MemI->getName() + ".phi" : "",
    337       Header->getFirstNonPHI());
    338 
    339     SCEVExpander SCEVE(*SE, Header->getModule()->getDataLayout(), "pistart");
    340     Value *BasePtrStart = SCEVE.expandCodeFor(BasePtrStartSCEV, I8PtrTy,
    341       LoopPredecessor->getTerminator());
    342 
    343     // Note that LoopPredecessor might occur in the predecessor list multiple
    344     // times, and we need to add it the right number of times.
    345     for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
    346          PI != PE; ++PI) {
    347       if (*PI != LoopPredecessor)
    348         continue;
    349 
    350       NewPHI->addIncoming(BasePtrStart, LoopPredecessor);
    351     }
    352 
    353     Instruction *InsPoint = &*Header->getFirstInsertionPt();
    354     GetElementPtrInst *PtrInc = GetElementPtrInst::Create(
    355         I8Ty, NewPHI, BasePtrIncSCEV->getValue(),
    356         MemI->hasName() ? MemI->getName() + ".inc" : "", InsPoint);
    357     PtrInc->setIsInBounds(IsPtrInBounds(BasePtr));
    358     for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
    359          PI != PE; ++PI) {
    360       if (*PI == LoopPredecessor)
    361         continue;
    362 
    363       NewPHI->addIncoming(PtrInc, *PI);
    364     }
    365 
    366     Instruction *NewBasePtr;
    367     if (PtrInc->getType() != BasePtr->getType())
    368       NewBasePtr = new BitCastInst(PtrInc, BasePtr->getType(),
    369         PtrInc->hasName() ? PtrInc->getName() + ".cast" : "", InsPoint);
    370     else
    371       NewBasePtr = PtrInc;
    372 
    373     if (Instruction *IDel = dyn_cast<Instruction>(BasePtr))
    374       BBChanged.insert(IDel->getParent());
    375     BasePtr->replaceAllUsesWith(NewBasePtr);
    376     RecursivelyDeleteTriviallyDeadInstructions(BasePtr);
    377 
    378     // Keep track of the replacement pointer values we've inserted so that we
    379     // don't generate more pointer values than necessary.
    380     SmallPtrSet<Value *, 16> NewPtrs;
    381     NewPtrs.insert( NewBasePtr);
    382 
    383     for (auto I = std::next(Buckets[i].Elements.begin()),
    384          IE = Buckets[i].Elements.end(); I != IE; ++I) {
    385       Value *Ptr = GetPointerOperand(I->Instr);
    386       assert(Ptr && "No pointer operand");
    387       if (NewPtrs.count(Ptr))
    388         continue;
    389 
    390       Instruction *RealNewPtr;
    391       if (!I->Offset || I->Offset->getValue()->isZero()) {
    392         RealNewPtr = NewBasePtr;
    393       } else {
    394         Instruction *PtrIP = dyn_cast<Instruction>(Ptr);
    395         if (PtrIP && isa<Instruction>(NewBasePtr) &&
    396             cast<Instruction>(NewBasePtr)->getParent() == PtrIP->getParent())
    397           PtrIP = 0;
    398         else if (isa<PHINode>(PtrIP))
    399           PtrIP = &*PtrIP->getParent()->getFirstInsertionPt();
    400         else if (!PtrIP)
    401           PtrIP = I->Instr;
    402 
    403         GetElementPtrInst *NewPtr = GetElementPtrInst::Create(
    404             I8Ty, PtrInc, I->Offset->getValue(),
    405             I->Instr->hasName() ? I->Instr->getName() + ".off" : "", PtrIP);
    406         if (!PtrIP)
    407           NewPtr->insertAfter(cast<Instruction>(PtrInc));
    408         NewPtr->setIsInBounds(IsPtrInBounds(Ptr));
    409         RealNewPtr = NewPtr;
    410       }
    411 
    412       if (Instruction *IDel = dyn_cast<Instruction>(Ptr))
    413         BBChanged.insert(IDel->getParent());
    414 
    415       Instruction *ReplNewPtr;
    416       if (Ptr->getType() != RealNewPtr->getType()) {
    417         ReplNewPtr = new BitCastInst(RealNewPtr, Ptr->getType(),
    418           Ptr->hasName() ? Ptr->getName() + ".cast" : "");
    419         ReplNewPtr->insertAfter(RealNewPtr);
    420       } else
    421         ReplNewPtr = RealNewPtr;
    422 
    423       Ptr->replaceAllUsesWith(ReplNewPtr);
    424       RecursivelyDeleteTriviallyDeadInstructions(Ptr);
    425 
    426       NewPtrs.insert(RealNewPtr);
    427     }
    428 
    429     MadeChange = true;
    430   }
    431 
    432   for (Loop::block_iterator I = L->block_begin(), IE = L->block_end();
    433        I != IE; ++I) {
    434     if (BBChanged.count(*I))
    435       DeleteDeadPHIs(*I);
    436   }
    437 
    438   return MadeChange;
    439 }
    440 
    441