llvm/lib/Transforms/Utils/SSAUpdater.cpp

   1 //===- SSAUpdater.cpp - Unstructured SSA Update Tool ----------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements the SSAUpdater class.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "llvm/Transforms/Utils/SSAUpdater.h"
  14 #include "llvm/ADT/DenseMap.h"
  15 #include "llvm/ADT/STLExtras.h"
  16 #include "llvm/ADT/SmallVector.h"
  17 #include "llvm/ADT/TinyPtrVector.h"
  18 #include "llvm/Analysis/InstructionSimplify.h"
  19 #include "llvm/IR/BasicBlock.h"
  20 #include "llvm/IR/CFG.h"
  21 #include "llvm/IR/Constants.h"
  22 #include "llvm/IR/DebugLoc.h"
  23 #include "llvm/IR/Instruction.h"
  24 #include "llvm/IR/Instructions.h"
  25 #include "llvm/IR/Module.h"
  26 #include "llvm/IR/Use.h"
  27 #include "llvm/IR/Value.h"
  28 #include "llvm/IR/ValueHandle.h"
  29 #include "llvm/Support/Casting.h"
  30 #include "llvm/Support/Debug.h"
  31 #include "llvm/Support/raw_ostream.h"
  32 #include "llvm/Transforms/Utils/SSAUpdaterImpl.h"
  33 #include <cassert>
  34 #include <utility>
  35
  36 using namespace llvm;
  37
  38 #define DEBUG_TYPE "ssaupdater"
  39
  40 using AvailableValsTy = DenseMap<BasicBlock *, Value *>;
  41
  42 static AvailableValsTy &getAvailableVals(void *AV) {
  43   return *static_cast<AvailableValsTy*>(AV);
  44 }
  45
  46 SSAUpdater::SSAUpdater(SmallVectorImpl<PHINode *> *NewPHI)
  47   : InsertedPHIs(NewPHI) {}
  48
  49 SSAUpdater::~SSAUpdater() {
  50   delete static_cast<AvailableValsTy*>(AV);
  51 }
  52
  53 void SSAUpdater::Initialize(Type *Ty, StringRef Name) {
  54   if (!AV)
  55     AV = new AvailableValsTy();
  56   else
  57     getAvailableVals(AV).clear();
  58   ProtoType = Ty;
  59   ProtoName = std::string(Name);
  60 }
  61
  62 bool SSAUpdater::HasValueForBlock(BasicBlock *BB) const {
  63   return getAvailableVals(AV).count(BB);
  64 }
  65
  66 Value *SSAUpdater::FindValueForBlock(BasicBlock *BB) const {
  67   return getAvailableVals(AV).lookup(BB);
  68 }
  69
  70 void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) {
  71   assert(ProtoType && "Need to initialize SSAUpdater");
  72   assert(ProtoType == V->getType() &&
  73          "All rewritten values must have the same type");
  74   getAvailableVals(AV)[BB] = V;
  75 }
  76
  77 static bool IsEquivalentPHI(PHINode *PHI,
  78                         SmallDenseMap<BasicBlock *, Value *, 8> &ValueMapping) {
  79   unsigned PHINumValues = PHI->getNumIncomingValues();
  80   if (PHINumValues != ValueMapping.size())
  81     return false;
  82
  83   // Scan the phi to see if it matches.
  84   for (unsigned i = 0, e = PHINumValues; i != e; ++i)
  85     if (ValueMapping[PHI->getIncomingBlock(i)] !=
  86         PHI->getIncomingValue(i)) {
  87       return false;
  88     }
  89
  90   return true;
  91 }
  92
  93 Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) {
  94   Value *Res = GetValueAtEndOfBlockInternal(BB);
  95   return Res;
  96 }
  97
  98 Value *SSAUpdater::GetValueInMiddleOfBlock(BasicBlock *BB) {
  99   // If there is no definition of the renamed variable in this block, just use
 100   // GetValueAtEndOfBlock to do our work.
 101   if (!HasValueForBlock(BB))
 102     return GetValueAtEndOfBlock(BB);
 103
 104   // Otherwise, we have the hard case.  Get the live-in values for each
 105   // predecessor.
 106   SmallVector<std::pair<BasicBlock *, Value *>, 8> PredValues;
 107   Value *SingularValue = nullptr;
 108
 109   // We can get our predecessor info by walking the pred_iterator list, but it
 110   // is relatively slow.  If we already have PHI nodes in this block, walk one
 111   // of them to get the predecessor list instead.
 112   if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
 113     for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) {
 114       BasicBlock *PredBB = SomePhi->getIncomingBlock(i);
 115       Value *PredVal = GetValueAtEndOfBlock(PredBB);
 116       PredValues.push_back(std::make_pair(PredBB, PredVal));
 117
 118       // Compute SingularValue.
 119       if (i == 0)
 120         SingularValue = PredVal;
 121       else if (PredVal != SingularValue)
 122         SingularValue = nullptr;
 123     }
 124   } else {
 125     bool isFirstPred = true;
 126     for (BasicBlock *PredBB : predecessors(BB)) {
 127       Value *PredVal = GetValueAtEndOfBlock(PredBB);
 128       PredValues.push_back(std::make_pair(PredBB, PredVal));
 129
 130       // Compute SingularValue.
 131       if (isFirstPred) {
 132         SingularValue = PredVal;
 133         isFirstPred = false;
 134       } else if (PredVal != SingularValue)
 135         SingularValue = nullptr;
 136     }
 137   }
 138
 139   // If there are no predecessors, just return undef.
 140   if (PredValues.empty())
 141     return UndefValue::get(ProtoType);
 142
 143   // Otherwise, if all the merged values are the same, just use it.
 144   if (SingularValue)
 145     return SingularValue;
 146
 147   // Otherwise, we do need a PHI: check to see if we already have one available
 148   // in this block that produces the right value.
 149   if (isa<PHINode>(BB->begin())) {
 150     SmallDenseMap<BasicBlock *, Value *, 8> ValueMapping(PredValues.begin(),
 151                                                          PredValues.end());
 152     for (PHINode &SomePHI : BB->phis()) {
 153       if (IsEquivalentPHI(&SomePHI, ValueMapping))
 154         return &SomePHI;
 155     }
 156   }
 157
 158   // Ok, we have no way out, insert a new one now.
 159   PHINode *InsertedPHI = PHINode::Create(ProtoType, PredValues.size(),
 160                                          ProtoName, &BB->front());
 161
 162   // Fill in all the predecessors of the PHI.
 163   for (const auto &PredValue : PredValues)
 164     InsertedPHI->addIncoming(PredValue.second, PredValue.first);
 165
 166   // See if the PHI node can be merged to a single value.  This can happen in
 167   // loop cases when we get a PHI of itself and one other value.
 168   if (Value *V =
 169           SimplifyInstruction(InsertedPHI, BB->getModule()->getDataLayout())) {
 170     InsertedPHI->eraseFromParent();
 171     return V;
 172   }
 173
 174   // Set the DebugLoc of the inserted PHI, if available.
 175   DebugLoc DL;
 176   if (const Instruction *I = BB->getFirstNonPHI())
 177       DL = I->getDebugLoc();
 178   InsertedPHI->setDebugLoc(DL);
 179
 180   // If the client wants to know about all new instructions, tell it.
 181   if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
 182
 183   LLVM_DEBUG(dbgs() << "  Inserted PHI: " << *InsertedPHI << "\n");
 184   return InsertedPHI;
 185 }
 186
 187 void SSAUpdater::RewriteUse(Use &U) {
 188   Instruction *User = cast<Instruction>(U.getUser());
 189
 190   Value *V;
 191   if (PHINode *UserPN = dyn_cast<PHINode>(User))
 192     V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
 193   else
 194     V = GetValueInMiddleOfBlock(User->getParent());
 195
 196   U.set(V);
 197 }
 198
 199 void SSAUpdater::RewriteUseAfterInsertions(Use &U) {
 200   Instruction *User = cast<Instruction>(U.getUser());
 201
 202   Value *V;
 203   if (PHINode *UserPN = dyn_cast<PHINode>(User))
 204     V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
 205   else
 206     V = GetValueAtEndOfBlock(User->getParent());
 207
 208   U.set(V);
 209 }
 210
 211 namespace llvm {
 212
 213 template<>
 214 class SSAUpdaterTraits<SSAUpdater> {
 215 public:
 216   using BlkT = BasicBlock;
 217   using ValT = Value *;
 218   using PhiT = PHINode;
 219   using BlkSucc_iterator = succ_iterator;
 220
 221   static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return succ_begin(BB); }
 222   static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return succ_end(BB); }
 223
 224   class PHI_iterator {
 225   private:
 226     PHINode *PHI;
 227     unsigned idx;
 228
 229   public:
 230     explicit PHI_iterator(PHINode *P) // begin iterator
 231       : PHI(P), idx(0) {}
 232     PHI_iterator(PHINode *P, bool) // end iterator
 233       : PHI(P), idx(PHI->getNumIncomingValues()) {}
 234
 235     PHI_iterator &operator++() { ++idx; return *this; }
 236     bool operator==(const PHI_iterator& x) const { return idx == x.idx; }
 237     bool operator!=(const PHI_iterator& x) const { return !operator==(x); }
 238
 239     Value *getIncomingValue() { return PHI->getIncomingValue(idx); }
 240     BasicBlock *getIncomingBlock() { return PHI->getIncomingBlock(idx); }
 241   };
 242
 243   static PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); }
 244   static PHI_iterator PHI_end(PhiT *PHI) {
 245     return PHI_iterator(PHI, true);
 246   }
 247
 248   /// FindPredecessorBlocks - Put the predecessors of Info->BB into the Preds
 249   /// vector, set Info->NumPreds, and allocate space in Info->Preds.
 250   static void FindPredecessorBlocks(BasicBlock *BB,
 251                                     SmallVectorImpl<BasicBlock *> *Preds) {
 252     // We can get our predecessor info by walking the pred_iterator list,
 253     // but it is relatively slow.  If we already have PHI nodes in this
 254     // block, walk one of them to get the predecessor list instead.
 255     if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin()))
 256       append_range(*Preds, SomePhi->blocks());
 257     else
 258       append_range(*Preds, predecessors(BB));
 259   }
 260
 261   /// GetUndefVal - Get an undefined value of the same type as the value
 262   /// being handled.
 263   static Value *GetUndefVal(BasicBlock *BB, SSAUpdater *Updater) {
 264     return UndefValue::get(Updater->ProtoType);
 265   }
 266
 267   /// CreateEmptyPHI - Create a new PHI instruction in the specified block.
 268   /// Reserve space for the operands but do not fill them in yet.
 269   static Value *CreateEmptyPHI(BasicBlock *BB, unsigned NumPreds,
 270                                SSAUpdater *Updater) {
 271     PHINode *PHI = PHINode::Create(Updater->ProtoType, NumPreds,
 272                                    Updater->ProtoName, &BB->front());
 273     return PHI;
 274   }
 275
 276   /// AddPHIOperand - Add the specified value as an operand of the PHI for
 277   /// the specified predecessor block.
 278   static void AddPHIOperand(PHINode *PHI, Value *Val, BasicBlock *Pred) {
 279     PHI->addIncoming(Val, Pred);
 280   }
 281
 282   /// ValueIsPHI - Check if a value is a PHI.
 283   static PHINode *ValueIsPHI(Value *Val, SSAUpdater *Updater) {
 284     return dyn_cast<PHINode>(Val);
 285   }
 286
 287   /// ValueIsNewPHI - Like ValueIsPHI but also check if the PHI has no source
 288   /// operands, i.e., it was just added.
 289   static PHINode *ValueIsNewPHI(Value *Val, SSAUpdater *Updater) {
 290     PHINode *PHI = ValueIsPHI(Val, Updater);
 291     if (PHI && PHI->getNumIncomingValues() == 0)
 292       return PHI;
 293     return nullptr;
 294   }
 295
 296   /// GetPHIValue - For the specified PHI instruction, return the value
 297   /// that it defines.
 298   static Value *GetPHIValue(PHINode *PHI) {
 299     return PHI;
 300   }
 301 };
 302
 303 } // end namespace llvm
 304
 305 /// Check to see if AvailableVals has an entry for the specified BB and if so,
 306 /// return it.  If not, construct SSA form by first calculating the required
 307 /// placement of PHIs and then inserting new PHIs where needed.
 308 Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *BB) {
 309   AvailableValsTy &AvailableVals = getAvailableVals(AV);
 310   if (Value *V = AvailableVals[BB])
 311     return V;
 312
 313   SSAUpdaterImpl<SSAUpdater> Impl(this, &AvailableVals, InsertedPHIs);
 314   return Impl.GetValue(BB);
 315 }
 316
 317 //===----------------------------------------------------------------------===//
 318 // LoadAndStorePromoter Implementation
 319 //===----------------------------------------------------------------------===//
 320
 321 LoadAndStorePromoter::
 322 LoadAndStorePromoter(ArrayRef<const Instruction *> Insts,
 323                      SSAUpdater &S, StringRef BaseName) : SSA(S) {
 324   if (Insts.empty()) return;
 325
 326   const Value *SomeVal;
 327   if (const LoadInst *LI = dyn_cast<LoadInst>(Insts[0]))
 328     SomeVal = LI;
 329   else
 330     SomeVal = cast<StoreInst>(Insts[0])->getOperand(0);
 331
 332   if (BaseName.empty())
 333     BaseName = SomeVal->getName();
 334   SSA.Initialize(SomeVal->getType(), BaseName);
 335 }
 336
 337 void LoadAndStorePromoter::run(const SmallVectorImpl<Instruction *> &Insts) {
 338   // First step: bucket up uses of the alloca by the block they occur in.
 339   // This is important because we have to handle multiple defs/uses in a block
 340   // ourselves: SSAUpdater is purely for cross-block references.
 341   DenseMap<BasicBlock *, TinyPtrVector<Instruction *>> UsesByBlock;
 342
 343   for (Instruction *User : Insts)
 344     UsesByBlock[User->getParent()].push_back(User);
 345
 346   // Okay, now we can iterate over all the blocks in the function with uses,
 347   // processing them.  Keep track of which loads are loading a live-in value.
 348   // Walk the uses in the use-list order to be determinstic.
 349   SmallVector<LoadInst *, 32> LiveInLoads;
 350   DenseMap<Value *, Value *> ReplacedLoads;
 351
 352   for (Instruction *User : Insts) {
 353     BasicBlock *BB = User->getParent();
 354     TinyPtrVector<Instruction *> &BlockUses = UsesByBlock[BB];
 355
 356     // If this block has already been processed, ignore this repeat use.
 357     if (BlockUses.empty()) continue;
 358
 359     // Okay, this is the first use in the block.  If this block just has a
 360     // single user in it, we can rewrite it trivially.
 361     if (BlockUses.size() == 1) {
 362       // If it is a store, it is a trivial def of the value in the block.
 363       if (StoreInst *SI = dyn_cast<StoreInst>(User)) {
 364         updateDebugInfo(SI);
 365         SSA.AddAvailableValue(BB, SI->getOperand(0));
 366       } else
 367         // Otherwise it is a load, queue it to rewrite as a live-in load.
 368         LiveInLoads.push_back(cast<LoadInst>(User));
 369       BlockUses.clear();
 370       continue;
 371     }
 372
 373     // Otherwise, check to see if this block is all loads.
 374     bool HasStore = false;
 375     for (Instruction *I : BlockUses) {
 376       if (isa<StoreInst>(I)) {
 377         HasStore = true;
 378         break;
 379       }
 380     }
 381
 382     // If so, we can queue them all as live in loads.  We don't have an
 383     // efficient way to tell which on is first in the block and don't want to
 384     // scan large blocks, so just add all loads as live ins.
 385     if (!HasStore) {
 386       for (Instruction *I : BlockUses)
 387         LiveInLoads.push_back(cast<LoadInst>(I));
 388       BlockUses.clear();
 389       continue;
 390     }
 391
 392     // Otherwise, we have mixed loads and stores (or just a bunch of stores).
 393     // Since SSAUpdater is purely for cross-block values, we need to determine
 394     // the order of these instructions in the block.  If the first use in the
 395     // block is a load, then it uses the live in value.  The last store defines
 396     // the live out value.  We handle this by doing a linear scan of the block.
 397     Value *StoredValue = nullptr;
 398     for (Instruction &I : *BB) {
 399       if (LoadInst *L = dyn_cast<LoadInst>(&I)) {
 400         // If this is a load from an unrelated pointer, ignore it.
 401         if (!isInstInList(L, Insts)) continue;
 402
 403         // If we haven't seen a store yet, this is a live in use, otherwise
 404         // use the stored value.
 405         if (StoredValue) {
 406           replaceLoadWithValue(L, StoredValue);
 407           L->replaceAllUsesWith(StoredValue);
 408           ReplacedLoads[L] = StoredValue;
 409         } else {
 410           LiveInLoads.push_back(L);
 411         }
 412         continue;
 413       }
 414
 415       if (StoreInst *SI = dyn_cast<StoreInst>(&I)) {
 416         // If this is a store to an unrelated pointer, ignore it.
 417         if (!isInstInList(SI, Insts)) continue;
 418         updateDebugInfo(SI);
 419
 420         // Remember that this is the active value in the block.
 421         StoredValue = SI->getOperand(0);
 422       }
 423     }
 424
 425     // The last stored value that happened is the live-out for the block.
 426     assert(StoredValue && "Already checked that there is a store in block");
 427     SSA.AddAvailableValue(BB, StoredValue);
 428     BlockUses.clear();
 429   }
 430
 431   // Okay, now we rewrite all loads that use live-in values in the loop,
 432   // inserting PHI nodes as necessary.
 433   for (LoadInst *ALoad : LiveInLoads) {
 434     Value *NewVal = SSA.GetValueInMiddleOfBlock(ALoad->getParent());
 435     replaceLoadWithValue(ALoad, NewVal);
 436
 437     // Avoid assertions in unreachable code.
 438     if (NewVal == ALoad) NewVal = UndefValue::get(NewVal->getType());
 439     ALoad->replaceAllUsesWith(NewVal);
 440     ReplacedLoads[ALoad] = NewVal;
 441   }
 442
 443   // Allow the client to do stuff before we start nuking things.
 444   doExtraRewritesBeforeFinalDeletion();
 445
 446   // Now that everything is rewritten, delete the old instructions from the
 447   // function.  They should all be dead now.
 448   for (Instruction *User : Insts) {
 449     // If this is a load that still has uses, then the load must have been added
 450     // as a live value in the SSAUpdate data structure for a block (e.g. because
 451     // the loaded value was stored later).  In this case, we need to recursively
 452     // propagate the updates until we get to the real value.
 453     if (!User->use_empty()) {
 454       Value *NewVal = ReplacedLoads[User];
 455       assert(NewVal && "not a replaced load?");
 456
 457       // Propagate down to the ultimate replacee.  The intermediately loads
 458       // could theoretically already have been deleted, so we don't want to
 459       // dereference the Value*'s.
 460       DenseMap<Value*, Value*>::iterator RLI = ReplacedLoads.find(NewVal);
 461       while (RLI != ReplacedLoads.end()) {
 462         NewVal = RLI->second;
 463         RLI = ReplacedLoads.find(NewVal);
 464       }
 465
 466       replaceLoadWithValue(cast<LoadInst>(User), NewVal);
 467       User->replaceAllUsesWith(NewVal);
 468     }
 469
 470     instructionDeleted(User);
 471     User->eraseFromParent();
 472   }
 473 }
 474
 475 bool
 476 LoadAndStorePromoter::isInstInList(Instruction *I,
 477                                    const SmallVectorImpl<Instruction *> &Insts)
 478                                    const {
 479   return is_contained(Insts, I);
 480 }