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 = 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   AvailableValsTy::iterator AVI = getAvailableVals(AV).find(BB);
  68   return (AVI != getAvailableVals(AV).end()) ? AVI->second : nullptr;
  69 }
  70
  71 void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) {
  72   assert(ProtoType && "Need to initialize SSAUpdater");
  73   assert(ProtoType == V->getType() &&
  74          "All rewritten values must have the same type");
  75   getAvailableVals(AV)[BB] = V;
  76 }
  77
  78 static bool IsEquivalentPHI(PHINode *PHI,
  79                         SmallDenseMap<BasicBlock *, Value *, 8> &ValueMapping) {
  80   unsigned PHINumValues = PHI->getNumIncomingValues();
  81   if (PHINumValues != ValueMapping.size())
  82     return false;
  83
  84   // Scan the phi to see if it matches.
  85   for (unsigned i = 0, e = PHINumValues; i != e; ++i)
  86     if (ValueMapping[PHI->getIncomingBlock(i)] !=
  87         PHI->getIncomingValue(i)) {
  88       return false;
  89     }
  90
  91   return true;
  92 }
  93
  94 Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) {
  95   Value *Res = GetValueAtEndOfBlockInternal(BB);
  96   return Res;
  97 }
  98
  99 Value *SSAUpdater::GetValueInMiddleOfBlock(BasicBlock *BB) {
 100   // If there is no definition of the renamed variable in this block, just use
 101   // GetValueAtEndOfBlock to do our work.
 102   if (!HasValueForBlock(BB))
 103     return GetValueAtEndOfBlock(BB);
 104
 105   // Otherwise, we have the hard case.  Get the live-in values for each
 106   // predecessor.
 107   SmallVector<std::pair<BasicBlock *, Value *>, 8> PredValues;
 108   Value *SingularValue = nullptr;
 109
 110   // We can get our predecessor info by walking the pred_iterator list, but it
 111   // is relatively slow.  If we already have PHI nodes in this block, walk one
 112   // of them to get the predecessor list instead.
 113   if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
 114     for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) {
 115       BasicBlock *PredBB = SomePhi->getIncomingBlock(i);
 116       Value *PredVal = GetValueAtEndOfBlock(PredBB);
 117       PredValues.push_back(std::make_pair(PredBB, PredVal));
 118
 119       // Compute SingularValue.
 120       if (i == 0)
 121         SingularValue = PredVal;
 122       else if (PredVal != SingularValue)
 123         SingularValue = nullptr;
 124     }
 125   } else {
 126     bool isFirstPred = true;
 127     for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
 128       BasicBlock *PredBB = *PI;
 129       Value *PredVal = GetValueAtEndOfBlock(PredBB);
 130       PredValues.push_back(std::make_pair(PredBB, PredVal));
 131
 132       // Compute SingularValue.
 133       if (isFirstPred) {
 134         SingularValue = PredVal;
 135         isFirstPred = false;
 136       } else if (PredVal != SingularValue)
 137         SingularValue = nullptr;
 138     }
 139   }
 140
 141   // If there are no predecessors, just return undef.
 142   if (PredValues.empty())
 143     return UndefValue::get(ProtoType);
 144
 145   // Otherwise, if all the merged values are the same, just use it.
 146   if (SingularValue)
 147     return SingularValue;
 148
 149   // Otherwise, we do need a PHI: check to see if we already have one available
 150   // in this block that produces the right value.
 151   if (isa<PHINode>(BB->begin())) {
 152     SmallDenseMap<BasicBlock *, Value *, 8> ValueMapping(PredValues.begin(),
 153                                                          PredValues.end());
 154     for (PHINode &SomePHI : BB->phis()) {
 155       if (IsEquivalentPHI(&SomePHI, ValueMapping))
 156         return &SomePHI;
 157     }
 158   }
 159
 160   // Ok, we have no way out, insert a new one now.
 161   PHINode *InsertedPHI = PHINode::Create(ProtoType, PredValues.size(),
 162                                          ProtoName, &BB->front());
 163
 164   // Fill in all the predecessors of the PHI.
 165   for (const auto &PredValue : PredValues)
 166     InsertedPHI->addIncoming(PredValue.second, PredValue.first);
 167
 168   // See if the PHI node can be merged to a single value.  This can happen in
 169   // loop cases when we get a PHI of itself and one other value.
 170   if (Value *V =
 171           SimplifyInstruction(InsertedPHI, BB->getModule()->getDataLayout())) {
 172     InsertedPHI->eraseFromParent();
 173     return V;
 174   }
 175
 176   // Set the DebugLoc of the inserted PHI, if available.
 177   DebugLoc DL;
 178   if (const Instruction *I = BB->getFirstNonPHI())
 179       DL = I->getDebugLoc();
 180   InsertedPHI->setDebugLoc(DL);
 181
 182   // If the client wants to know about all new instructions, tell it.
 183   if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
 184
 185   LLVM_DEBUG(dbgs() << "  Inserted PHI: " << *InsertedPHI << "\n");
 186   return InsertedPHI;
 187 }
 188
 189 void SSAUpdater::RewriteUse(Use &U) {
 190   Instruction *User = cast<Instruction>(U.getUser());
 191
 192   Value *V;
 193   if (PHINode *UserPN = dyn_cast<PHINode>(User))
 194     V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
 195   else
 196     V = GetValueInMiddleOfBlock(User->getParent());
 197
 198   // Notify that users of the existing value that it is being replaced.
 199   Value *OldVal = U.get();
 200   if (OldVal != V && OldVal->hasValueHandle())
 201     ValueHandleBase::ValueIsRAUWd(OldVal, V);
 202
 203   U.set(V);
 204 }
 205
 206 void SSAUpdater::RewriteUseAfterInsertions(Use &U) {
 207   Instruction *User = cast<Instruction>(U.getUser());
 208
 209   Value *V;
 210   if (PHINode *UserPN = dyn_cast<PHINode>(User))
 211     V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
 212   else
 213     V = GetValueAtEndOfBlock(User->getParent());
 214
 215   U.set(V);
 216 }
 217
 218 namespace llvm {
 219
 220 template<>
 221 class SSAUpdaterTraits<SSAUpdater> {
 222 public:
 223   using BlkT = BasicBlock;
 224   using ValT = Value *;
 225   using PhiT = PHINode;
 226   using BlkSucc_iterator = succ_iterator;
 227
 228   static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return succ_begin(BB); }
 229   static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return succ_end(BB); }
 230
 231   class PHI_iterator {
 232   private:
 233     PHINode *PHI;
 234     unsigned idx;
 235
 236   public:
 237     explicit PHI_iterator(PHINode *P) // begin iterator
 238       : PHI(P), idx(0) {}
 239     PHI_iterator(PHINode *P, bool) // end iterator
 240       : PHI(P), idx(PHI->getNumIncomingValues()) {}
 241
 242     PHI_iterator &operator++() { ++idx; return *this; }
 243     bool operator==(const PHI_iterator& x) const { return idx == x.idx; }
 244     bool operator!=(const PHI_iterator& x) const { return !operator==(x); }
 245
 246     Value *getIncomingValue() { return PHI->getIncomingValue(idx); }
 247     BasicBlock *getIncomingBlock() { return PHI->getIncomingBlock(idx); }
 248   };
 249
 250   static PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); }
 251   static PHI_iterator PHI_end(PhiT *PHI) {
 252     return PHI_iterator(PHI, true);
 253   }
 254
 255   /// FindPredecessorBlocks - Put the predecessors of Info->BB into the Preds
 256   /// vector, set Info->NumPreds, and allocate space in Info->Preds.
 257   static void FindPredecessorBlocks(BasicBlock *BB,
 258                                     SmallVectorImpl<BasicBlock *> *Preds) {
 259     // We can get our predecessor info by walking the pred_iterator list,
 260     // but it is relatively slow.  If we already have PHI nodes in this
 261     // block, walk one of them to get the predecessor list instead.
 262     if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
 263       Preds->append(SomePhi->block_begin(), SomePhi->block_end());
 264     } else {
 265       for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
 266         Preds->push_back(*PI);
 267     }
 268   }
 269
 270   /// GetUndefVal - Get an undefined value of the same type as the value
 271   /// being handled.
 272   static Value *GetUndefVal(BasicBlock *BB, SSAUpdater *Updater) {
 273     return UndefValue::get(Updater->ProtoType);
 274   }
 275
 276   /// CreateEmptyPHI - Create a new PHI instruction in the specified block.
 277   /// Reserve space for the operands but do not fill them in yet.
 278   static Value *CreateEmptyPHI(BasicBlock *BB, unsigned NumPreds,
 279                                SSAUpdater *Updater) {
 280     PHINode *PHI = PHINode::Create(Updater->ProtoType, NumPreds,
 281                                    Updater->ProtoName, &BB->front());
 282     return PHI;
 283   }
 284
 285   /// AddPHIOperand - Add the specified value as an operand of the PHI for
 286   /// the specified predecessor block.
 287   static void AddPHIOperand(PHINode *PHI, Value *Val, BasicBlock *Pred) {
 288     PHI->addIncoming(Val, Pred);
 289   }
 290
 291   /// InstrIsPHI - Check if an instruction is a PHI.
 292   ///
 293   static PHINode *InstrIsPHI(Instruction *I) {
 294     return dyn_cast<PHINode>(I);
 295   }
 296
 297   /// ValueIsPHI - Check if a value is a PHI.
 298   static PHINode *ValueIsPHI(Value *Val, SSAUpdater *Updater) {
 299     return dyn_cast<PHINode>(Val);
 300   }
 301
 302   /// ValueIsNewPHI - Like ValueIsPHI but also check if the PHI has no source
 303   /// operands, i.e., it was just added.
 304   static PHINode *ValueIsNewPHI(Value *Val, SSAUpdater *Updater) {
 305     PHINode *PHI = ValueIsPHI(Val, Updater);
 306     if (PHI && PHI->getNumIncomingValues() == 0)
 307       return PHI;
 308     return nullptr;
 309   }
 310
 311   /// GetPHIValue - For the specified PHI instruction, return the value
 312   /// that it defines.
 313   static Value *GetPHIValue(PHINode *PHI) {
 314     return PHI;
 315   }
 316 };
 317
 318 } // end namespace llvm
 319
 320 /// Check to see if AvailableVals has an entry for the specified BB and if so,
 321 /// return it.  If not, construct SSA form by first calculating the required
 322 /// placement of PHIs and then inserting new PHIs where needed.
 323 Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *BB) {
 324   AvailableValsTy &AvailableVals = getAvailableVals(AV);
 325   if (Value *V = AvailableVals[BB])
 326     return V;
 327
 328   SSAUpdaterImpl<SSAUpdater> Impl(this, &AvailableVals, InsertedPHIs);
 329   return Impl.GetValue(BB);
 330 }
 331
 332 //===----------------------------------------------------------------------===//
 333 // LoadAndStorePromoter Implementation
 334 //===----------------------------------------------------------------------===//
 335
 336 LoadAndStorePromoter::
 337 LoadAndStorePromoter(ArrayRef<const Instruction *> Insts,
 338                      SSAUpdater &S, StringRef BaseName) : SSA(S) {
 339   if (Insts.empty()) return;
 340
 341   const Value *SomeVal;
 342   if (const LoadInst *LI = dyn_cast<LoadInst>(Insts[0]))
 343     SomeVal = LI;
 344   else
 345     SomeVal = cast<StoreInst>(Insts[0])->getOperand(0);
 346
 347   if (BaseName.empty())
 348     BaseName = SomeVal->getName();
 349   SSA.Initialize(SomeVal->getType(), BaseName);
 350 }
 351
 352 void LoadAndStorePromoter::run(const SmallVectorImpl<Instruction *> &Insts) {
 353   // First step: bucket up uses of the alloca by the block they occur in.
 354   // This is important because we have to handle multiple defs/uses in a block
 355   // ourselves: SSAUpdater is purely for cross-block references.
 356   DenseMap<BasicBlock *, TinyPtrVector<Instruction *>> UsesByBlock;
 357
 358   for (Instruction *User : Insts)
 359     UsesByBlock[User->getParent()].push_back(User);
 360
 361   // Okay, now we can iterate over all the blocks in the function with uses,
 362   // processing them.  Keep track of which loads are loading a live-in value.
 363   // Walk the uses in the use-list order to be determinstic.
 364   SmallVector<LoadInst *, 32> LiveInLoads;
 365   DenseMap<Value *, Value *> ReplacedLoads;
 366
 367   for (Instruction *User : Insts) {
 368     BasicBlock *BB = User->getParent();
 369     TinyPtrVector<Instruction *> &BlockUses = UsesByBlock[BB];
 370
 371     // If this block has already been processed, ignore this repeat use.
 372     if (BlockUses.empty()) continue;
 373
 374     // Okay, this is the first use in the block.  If this block just has a
 375     // single user in it, we can rewrite it trivially.
 376     if (BlockUses.size() == 1) {
 377       // If it is a store, it is a trivial def of the value in the block.
 378       if (StoreInst *SI = dyn_cast<StoreInst>(User)) {
 379         updateDebugInfo(SI);
 380         SSA.AddAvailableValue(BB, SI->getOperand(0));
 381       } else
 382         // Otherwise it is a load, queue it to rewrite as a live-in load.
 383         LiveInLoads.push_back(cast<LoadInst>(User));
 384       BlockUses.clear();
 385       continue;
 386     }
 387
 388     // Otherwise, check to see if this block is all loads.
 389     bool HasStore = false;
 390     for (Instruction *I : BlockUses) {
 391       if (isa<StoreInst>(I)) {
 392         HasStore = true;
 393         break;
 394       }
 395     }
 396
 397     // If so, we can queue them all as live in loads.  We don't have an
 398     // efficient way to tell which on is first in the block and don't want to
 399     // scan large blocks, so just add all loads as live ins.
 400     if (!HasStore) {
 401       for (Instruction *I : BlockUses)
 402         LiveInLoads.push_back(cast<LoadInst>(I));
 403       BlockUses.clear();
 404       continue;
 405     }
 406
 407     // Otherwise, we have mixed loads and stores (or just a bunch of stores).
 408     // Since SSAUpdater is purely for cross-block values, we need to determine
 409     // the order of these instructions in the block.  If the first use in the
 410     // block is a load, then it uses the live in value.  The last store defines
 411     // the live out value.  We handle this by doing a linear scan of the block.
 412     Value *StoredValue = nullptr;
 413     for (Instruction &I : *BB) {
 414       if (LoadInst *L = dyn_cast<LoadInst>(&I)) {
 415         // If this is a load from an unrelated pointer, ignore it.
 416         if (!isInstInList(L, Insts)) continue;
 417
 418         // If we haven't seen a store yet, this is a live in use, otherwise
 419         // use the stored value.
 420         if (StoredValue) {
 421           replaceLoadWithValue(L, StoredValue);
 422           L->replaceAllUsesWith(StoredValue);
 423           ReplacedLoads[L] = StoredValue;
 424         } else {
 425           LiveInLoads.push_back(L);
 426         }
 427         continue;
 428       }
 429
 430       if (StoreInst *SI = dyn_cast<StoreInst>(&I)) {
 431         // If this is a store to an unrelated pointer, ignore it.
 432         if (!isInstInList(SI, Insts)) continue;
 433         updateDebugInfo(SI);
 434
 435         // Remember that this is the active value in the block.
 436         StoredValue = SI->getOperand(0);
 437       }
 438     }
 439
 440     // The last stored value that happened is the live-out for the block.
 441     assert(StoredValue && "Already checked that there is a store in block");
 442     SSA.AddAvailableValue(BB, StoredValue);
 443     BlockUses.clear();
 444   }
 445
 446   // Okay, now we rewrite all loads that use live-in values in the loop,
 447   // inserting PHI nodes as necessary.
 448   for (LoadInst *ALoad : LiveInLoads) {
 449     Value *NewVal = SSA.GetValueInMiddleOfBlock(ALoad->getParent());
 450     replaceLoadWithValue(ALoad, NewVal);
 451
 452     // Avoid assertions in unreachable code.
 453     if (NewVal == ALoad) NewVal = UndefValue::get(NewVal->getType());
 454     ALoad->replaceAllUsesWith(NewVal);
 455     ReplacedLoads[ALoad] = NewVal;
 456   }
 457
 458   // Allow the client to do stuff before we start nuking things.
 459   doExtraRewritesBeforeFinalDeletion();
 460
 461   // Now that everything is rewritten, delete the old instructions from the
 462   // function.  They should all be dead now.
 463   for (Instruction *User : Insts) {
 464     // If this is a load that still has uses, then the load must have been added
 465     // as a live value in the SSAUpdate data structure for a block (e.g. because
 466     // the loaded value was stored later).  In this case, we need to recursively
 467     // propagate the updates until we get to the real value.
 468     if (!User->use_empty()) {
 469       Value *NewVal = ReplacedLoads[User];
 470       assert(NewVal && "not a replaced load?");
 471
 472       // Propagate down to the ultimate replacee.  The intermediately loads
 473       // could theoretically already have been deleted, so we don't want to
 474       // dereference the Value*'s.
 475       DenseMap<Value*, Value*>::iterator RLI = ReplacedLoads.find(NewVal);
 476       while (RLI != ReplacedLoads.end()) {
 477         NewVal = RLI->second;
 478         RLI = ReplacedLoads.find(NewVal);
 479       }
 480
 481       replaceLoadWithValue(cast<LoadInst>(User), NewVal);
 482       User->replaceAllUsesWith(NewVal);
 483     }
 484
 485     instructionDeleted(User);
 486     User->eraseFromParent();
 487   }
 488 }
 489
 490 bool
 491 LoadAndStorePromoter::isInstInList(Instruction *I,
 492                                    const SmallVectorImpl<Instruction *> &Insts)
 493                                    const {
 494   return is_contained(Insts, I);
 495 }