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"
48using 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).
52static 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
56namespace llvm {
57  void initializePPCLoopPreIncPrepPass(PassRegistry&);
58}
59
60namespace {
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
94char PPCLoopPreIncPrep::ID = 0;
95static const char *name = "Prepare loop for pre-inc. addressing modes";
96INITIALIZE_PASS_BEGIN(PPCLoopPreIncPrep, DEBUG_TYPE, name, false, false)
97INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
98INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
99INITIALIZE_PASS_END(PPCLoopPreIncPrep, DEBUG_TYPE, name, false, false)
100
101FunctionPass *llvm::createPPCLoopPreIncPrepPass(PPCTargetMachine &TM) {
102  return new PPCLoopPreIncPrep(TM);
103}
104
105namespace {
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
123static 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
133static 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
146bool 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
165bool 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