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/DebugInfo.h"
  23 #include "llvm/IR/DebugLoc.h"
  24 #include "llvm/IR/Instruction.h"
  25 #include "llvm/IR/Instructions.h"
  26 #include "llvm/IR/Module.h"
  27 #include "llvm/IR/Use.h"
  28 #include "llvm/IR/Value.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 =
 160       PHINode::Create(ProtoType, PredValues.size(), ProtoName);
 161   InsertedPHI->insertBefore(BB->begin());
 162
 163   // Fill in all the predecessors of the PHI.
 164   for (const auto &PredValue : PredValues)
 165     InsertedPHI->addIncoming(PredValue.second, PredValue.first);
 166
 167   // See if the PHI node can be merged to a single value.  This can happen in
 168   // loop cases when we get a PHI of itself and one other value.
 169   if (Value *V =
 170           simplifyInstruction(InsertedPHI, BB->getModule()->getDataLayout())) {
 171     InsertedPHI->eraseFromParent();
 172     return V;
 173   }
 174
 175   // Set the DebugLoc of the inserted PHI, if available.
 176   DebugLoc DL;
 177   if (const Instruction *I = BB->getFirstNonPHI())
 178       DL = I->getDebugLoc();
 179   InsertedPHI->setDebugLoc(DL);
 180
 181   // If the client wants to know about all new instructions, tell it.
 182   if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
 183
 184   LLVM_DEBUG(dbgs() << "  Inserted PHI: " << *InsertedPHI << "\n");
 185   return InsertedPHI;
 186 }
 187
 188 void SSAUpdater::RewriteUse(Use &U) {
 189   Instruction *User = cast<Instruction>(U.getUser());
 190
 191   Value *V;
 192   if (PHINode *UserPN = dyn_cast<PHINode>(User))
 193     V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
 194   else
 195     V = GetValueInMiddleOfBlock(User->getParent());
 196
 197   U.set(V);
 198 }
 199
 200 void SSAUpdater::UpdateDebugValues(Instruction *I) {
 201   SmallVector<DbgValueInst *, 4> DbgValues;
 202   SmallVector<DPValue *, 4> DPValues;
 203   llvm::findDbgValues(DbgValues, I, &DPValues);
 204   for (auto &DbgValue : DbgValues) {
 205     if (DbgValue->getParent() == I->getParent())
 206       continue;
 207     UpdateDebugValue(I, DbgValue);
 208   }
 209   for (auto &DPV : DPValues) {
 210     if (DPV->getParent() == I->getParent())
 211       continue;
 212     UpdateDebugValue(I, DPV);
 213   }
 214 }
 215
 216 void SSAUpdater::UpdateDebugValues(Instruction *I,
 217                                    SmallVectorImpl<DbgValueInst *> &DbgValues) {
 218   for (auto &DbgValue : DbgValues) {
 219     UpdateDebugValue(I, DbgValue);
 220   }
 221 }
 222
 223 void SSAUpdater::UpdateDebugValues(Instruction *I,
 224                                    SmallVectorImpl<DPValue *> &DPValues) {
 225   for (auto &DPV : DPValues) {
 226     UpdateDebugValue(I, DPV);
 227   }
 228 }
 229
 230 void SSAUpdater::UpdateDebugValue(Instruction *I, DbgValueInst *DbgValue) {
 231   BasicBlock *UserBB = DbgValue->getParent();
 232   if (HasValueForBlock(UserBB)) {
 233     Value *NewVal = GetValueAtEndOfBlock(UserBB);
 234     DbgValue->replaceVariableLocationOp(I, NewVal);
 235   } else
 236     DbgValue->setKillLocation();
 237 }
 238
 239 void SSAUpdater::UpdateDebugValue(Instruction *I, DPValue *DPV) {
 240   BasicBlock *UserBB = DPV->getParent();
 241   if (HasValueForBlock(UserBB)) {
 242     Value *NewVal = GetValueAtEndOfBlock(UserBB);
 243     DPV->replaceVariableLocationOp(I, NewVal);
 244   } else
 245     DPV->setKillLocation();
 246 }
 247
 248 void SSAUpdater::RewriteUseAfterInsertions(Use &U) {
 249   Instruction *User = cast<Instruction>(U.getUser());
 250
 251   Value *V;
 252   if (PHINode *UserPN = dyn_cast<PHINode>(User))
 253     V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
 254   else
 255     V = GetValueAtEndOfBlock(User->getParent());
 256
 257   U.set(V);
 258 }
 259
 260 namespace llvm {
 261
 262 template<>
 263 class SSAUpdaterTraits<SSAUpdater> {
 264 public:
 265   using BlkT = BasicBlock;
 266   using ValT = Value *;
 267   using PhiT = PHINode;
 268   using BlkSucc_iterator = succ_iterator;
 269
 270   static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return succ_begin(BB); }
 271   static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return succ_end(BB); }
 272
 273   class PHI_iterator {
 274   private:
 275     PHINode *PHI;
 276     unsigned idx;
 277
 278   public:
 279     explicit PHI_iterator(PHINode *P) // begin iterator
 280       : PHI(P), idx(0) {}
 281     PHI_iterator(PHINode *P, bool) // end iterator
 282       : PHI(P), idx(PHI->getNumIncomingValues()) {}
 283
 284     PHI_iterator &operator++() { ++idx; return *this; }
 285     bool operator==(const PHI_iterator& x) const { return idx == x.idx; }
 286     bool operator!=(const PHI_iterator& x) const { return !operator==(x); }
 287
 288     Value *getIncomingValue() { return PHI->getIncomingValue(idx); }
 289     BasicBlock *getIncomingBlock() { return PHI->getIncomingBlock(idx); }
 290   };
 291
 292   static PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); }
 293   static PHI_iterator PHI_end(PhiT *PHI) {
 294     return PHI_iterator(PHI, true);
 295   }
 296
 297   /// FindPredecessorBlocks - Put the predecessors of Info->BB into the Preds
 298   /// vector, set Info->NumPreds, and allocate space in Info->Preds.
 299   static void FindPredecessorBlocks(BasicBlock *BB,
 300                                     SmallVectorImpl<BasicBlock *> *Preds) {
 301     // We can get our predecessor info by walking the pred_iterator list,
 302     // but it is relatively slow.  If we already have PHI nodes in this
 303     // block, walk one of them to get the predecessor list instead.
 304     if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin()))
 305       append_range(*Preds, SomePhi->blocks());
 306     else
 307       append_range(*Preds, predecessors(BB));
 308   }
 309
 310   /// GetUndefVal - Get an undefined value of the same type as the value
 311   /// being handled.
 312   static Value *GetUndefVal(BasicBlock *BB, SSAUpdater *Updater) {
 313     return UndefValue::get(Updater->ProtoType);
 314   }
 315
 316   /// CreateEmptyPHI - Create a new PHI instruction in the specified block.
 317   /// Reserve space for the operands but do not fill them in yet.
 318   static Value *CreateEmptyPHI(BasicBlock *BB, unsigned NumPreds,
 319                                SSAUpdater *Updater) {
 320     PHINode *PHI =
 321         PHINode::Create(Updater->ProtoType, NumPreds, Updater->ProtoName);
 322     PHI->insertBefore(BB->begin());
 323     return PHI;
 324   }
 325
 326   /// AddPHIOperand - Add the specified value as an operand of the PHI for
 327   /// the specified predecessor block.
 328   static void AddPHIOperand(PHINode *PHI, Value *Val, BasicBlock *Pred) {
 329     PHI->addIncoming(Val, Pred);
 330   }
 331
 332   /// ValueIsPHI - Check if a value is a PHI.
 333   static PHINode *ValueIsPHI(Value *Val, SSAUpdater *Updater) {
 334     return dyn_cast<PHINode>(Val);
 335   }
 336
 337   /// ValueIsNewPHI - Like ValueIsPHI but also check if the PHI has no source
 338   /// operands, i.e., it was just added.
 339   static PHINode *ValueIsNewPHI(Value *Val, SSAUpdater *Updater) {
 340     PHINode *PHI = ValueIsPHI(Val, Updater);
 341     if (PHI && PHI->getNumIncomingValues() == 0)
 342       return PHI;
 343     return nullptr;
 344   }
 345
 346   /// GetPHIValue - For the specified PHI instruction, return the value
 347   /// that it defines.
 348   static Value *GetPHIValue(PHINode *PHI) {
 349     return PHI;
 350   }
 351 };
 352
 353 } // end namespace llvm
 354
 355 /// Check to see if AvailableVals has an entry for the specified BB and if so,
 356 /// return it.  If not, construct SSA form by first calculating the required
 357 /// placement of PHIs and then inserting new PHIs where needed.
 358 Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *BB) {
 359   AvailableValsTy &AvailableVals = getAvailableVals(AV);
 360   if (Value *V = AvailableVals[BB])
 361     return V;
 362
 363   SSAUpdaterImpl<SSAUpdater> Impl(this, &AvailableVals, InsertedPHIs);
 364   return Impl.GetValue(BB);
 365 }
 366
 367 //===----------------------------------------------------------------------===//
 368 // LoadAndStorePromoter Implementation
 369 //===----------------------------------------------------------------------===//
 370
 371 LoadAndStorePromoter::
 372 LoadAndStorePromoter(ArrayRef<const Instruction *> Insts,
 373                      SSAUpdater &S, StringRef BaseName) : SSA(S) {
 374   if (Insts.empty()) return;
 375
 376   const Value *SomeVal;
 377   if (const LoadInst *LI = dyn_cast<LoadInst>(Insts[0]))
 378     SomeVal = LI;
 379   else
 380     SomeVal = cast<StoreInst>(Insts[0])->getOperand(0);
 381
 382   if (BaseName.empty())
 383     BaseName = SomeVal->getName();
 384   SSA.Initialize(SomeVal->getType(), BaseName);
 385 }
 386
 387 void LoadAndStorePromoter::run(const SmallVectorImpl<Instruction *> &Insts) {
 388   // First step: bucket up uses of the alloca by the block they occur in.
 389   // This is important because we have to handle multiple defs/uses in a block
 390   // ourselves: SSAUpdater is purely for cross-block references.
 391   DenseMap<BasicBlock *, TinyPtrVector<Instruction *>> UsesByBlock;
 392
 393   for (Instruction *User : Insts)
 394     UsesByBlock[User->getParent()].push_back(User);
 395
 396   // Okay, now we can iterate over all the blocks in the function with uses,
 397   // processing them.  Keep track of which loads are loading a live-in value.
 398   // Walk the uses in the use-list order to be determinstic.
 399   SmallVector<LoadInst *, 32> LiveInLoads;
 400   DenseMap<Value *, Value *> ReplacedLoads;
 401
 402   for (Instruction *User : Insts) {
 403     BasicBlock *BB = User->getParent();
 404     TinyPtrVector<Instruction *> &BlockUses = UsesByBlock[BB];
 405
 406     // If this block has already been processed, ignore this repeat use.
 407     if (BlockUses.empty()) continue;
 408
 409     // Okay, this is the first use in the block.  If this block just has a
 410     // single user in it, we can rewrite it trivially.
 411     if (BlockUses.size() == 1) {
 412       // If it is a store, it is a trivial def of the value in the block.
 413       if (StoreInst *SI = dyn_cast<StoreInst>(User)) {
 414         updateDebugInfo(SI);
 415         SSA.AddAvailableValue(BB, SI->getOperand(0));
 416       } else
 417         // Otherwise it is a load, queue it to rewrite as a live-in load.
 418         LiveInLoads.push_back(cast<LoadInst>(User));
 419       BlockUses.clear();
 420       continue;
 421     }
 422
 423     // Otherwise, check to see if this block is all loads.
 424     bool HasStore = false;
 425     for (Instruction *I : BlockUses) {
 426       if (isa<StoreInst>(I)) {
 427         HasStore = true;
 428         break;
 429       }
 430     }
 431
 432     // If so, we can queue them all as live in loads.  We don't have an
 433     // efficient way to tell which on is first in the block and don't want to
 434     // scan large blocks, so just add all loads as live ins.
 435     if (!HasStore) {
 436       for (Instruction *I : BlockUses)
 437         LiveInLoads.push_back(cast<LoadInst>(I));
 438       BlockUses.clear();
 439       continue;
 440     }
 441
 442     // Otherwise, we have mixed loads and stores (or just a bunch of stores).
 443     // Since SSAUpdater is purely for cross-block values, we need to determine
 444     // the order of these instructions in the block.  If the first use in the
 445     // block is a load, then it uses the live in value.  The last store defines
 446     // the live out value.  We handle this by doing a linear scan of the block.
 447     Value *StoredValue = nullptr;
 448     for (Instruction &I : *BB) {
 449       if (LoadInst *L = dyn_cast<LoadInst>(&I)) {
 450         // If this is a load from an unrelated pointer, ignore it.
 451         if (!isInstInList(L, Insts)) continue;
 452
 453         // If we haven't seen a store yet, this is a live in use, otherwise
 454         // use the stored value.
 455         if (StoredValue) {
 456           replaceLoadWithValue(L, StoredValue);
 457           L->replaceAllUsesWith(StoredValue);
 458           ReplacedLoads[L] = StoredValue;
 459         } else {
 460           LiveInLoads.push_back(L);
 461         }
 462         continue;
 463       }
 464
 465       if (StoreInst *SI = dyn_cast<StoreInst>(&I)) {
 466         // If this is a store to an unrelated pointer, ignore it.
 467         if (!isInstInList(SI, Insts)) continue;
 468         updateDebugInfo(SI);
 469
 470         // Remember that this is the active value in the block.
 471         StoredValue = SI->getOperand(0);
 472       }
 473     }
 474
 475     // The last stored value that happened is the live-out for the block.
 476     assert(StoredValue && "Already checked that there is a store in block");
 477     SSA.AddAvailableValue(BB, StoredValue);
 478     BlockUses.clear();
 479   }
 480
 481   // Okay, now we rewrite all loads that use live-in values in the loop,
 482   // inserting PHI nodes as necessary.
 483   for (LoadInst *ALoad : LiveInLoads) {
 484     Value *NewVal = SSA.GetValueInMiddleOfBlock(ALoad->getParent());
 485     replaceLoadWithValue(ALoad, NewVal);
 486
 487     // Avoid assertions in unreachable code.
 488     if (NewVal == ALoad) NewVal = PoisonValue::get(NewVal->getType());
 489     ALoad->replaceAllUsesWith(NewVal);
 490     ReplacedLoads[ALoad] = NewVal;
 491   }
 492
 493   // Allow the client to do stuff before we start nuking things.
 494   doExtraRewritesBeforeFinalDeletion();
 495
 496   // Now that everything is rewritten, delete the old instructions from the
 497   // function.  They should all be dead now.
 498   for (Instruction *User : Insts) {
 499     if (!shouldDelete(User))
 500       continue;
 501
 502     // If this is a load that still has uses, then the load must have been added
 503     // as a live value in the SSAUpdate data structure for a block (e.g. because
 504     // the loaded value was stored later).  In this case, we need to recursively
 505     // propagate the updates until we get to the real value.
 506     if (!User->use_empty()) {
 507       Value *NewVal = ReplacedLoads[User];
 508       assert(NewVal && "not a replaced load?");
 509
 510       // Propagate down to the ultimate replacee.  The intermediately loads
 511       // could theoretically already have been deleted, so we don't want to
 512       // dereference the Value*'s.
 513       DenseMap<Value*, Value*>::iterator RLI = ReplacedLoads.find(NewVal);
 514       while (RLI != ReplacedLoads.end()) {
 515         NewVal = RLI->second;
 516         RLI = ReplacedLoads.find(NewVal);
 517       }
 518
 519       replaceLoadWithValue(cast<LoadInst>(User), NewVal);
 520       User->replaceAllUsesWith(NewVal);
 521     }
 522
 523     instructionDeleted(User);
 524     User->eraseFromParent();
 525   }
 526 }
 527
 528 bool
 529 LoadAndStorePromoter::isInstInList(Instruction *I,
 530                                    const SmallVectorImpl<Instruction *> &Insts)
 531                                    const {
 532   return is_contained(Insts, I);
 533 }