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/Support/Casting.h"
  29 #include "llvm/Support/Debug.h"
  30 #include "llvm/Support/raw_ostream.h"
  31 #include "llvm/Transforms/Utils/SSAUpdaterImpl.h"
  32 #include <cassert>
  33 #include <utility>
  34
  35 using namespace llvm;
  36
  37 #define DEBUG_TYPE "ssaupdater"
  38
  39 using AvailableValsTy = DenseMap<BasicBlock *, Value *>;
  40
  41 static AvailableValsTy &getAvailableVals(void *AV) {
  42   return *static_cast<AvailableValsTy*>(AV);
  43 }
  44
  45 SSAUpdater::SSAUpdater(SmallVectorImpl<PHINode *> *NewPHI)
  46   : InsertedPHIs(NewPHI) {}
  47
  48 SSAUpdater::~SSAUpdater() {
  49   delete static_cast<AvailableValsTy*>(AV);
  50 }
  51
  52 void SSAUpdater::Initialize(Type *Ty, StringRef Name) {
  53   if (!AV)
  54     AV = new AvailableValsTy();
  55   else
  56     getAvailableVals(AV).clear();
  57   ProtoType = Ty;
  58   ProtoName = std::string(Name);
  59 }
  60
  61 bool SSAUpdater::HasValueForBlock(BasicBlock *BB) const {
  62   return getAvailableVals(AV).count(BB);
  63 }
  64
  65 Value *SSAUpdater::FindValueForBlock(BasicBlock *BB) const {
  66   return getAvailableVals(AV).lookup(BB);
  67 }
  68
  69 void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) {
  70   assert(ProtoType && "Need to initialize SSAUpdater");
  71   assert(ProtoType == V->getType() &&
  72          "All rewritten values must have the same type");
  73   getAvailableVals(AV)[BB] = V;
  74 }
  75
  76 static bool IsEquivalentPHI(PHINode *PHI,
  77                         SmallDenseMap<BasicBlock *, Value *, 8> &ValueMapping) {
  78   unsigned PHINumValues = PHI->getNumIncomingValues();
  79   if (PHINumValues != ValueMapping.size())
  80     return false;
  81
  82   // Scan the phi to see if it matches.
  83   for (unsigned i = 0, e = PHINumValues; i != e; ++i)
  84     if (ValueMapping[PHI->getIncomingBlock(i)] !=
  85         PHI->getIncomingValue(i)) {
  86       return false;
  87     }
  88
  89   return true;
  90 }
  91
  92 Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) {
  93   Value *Res = GetValueAtEndOfBlockInternal(BB);
  94   return Res;
  95 }
  96
  97 Value *SSAUpdater::GetValueInMiddleOfBlock(BasicBlock *BB) {
  98   // If there is no definition of the renamed variable in this block, just use
  99   // GetValueAtEndOfBlock to do our work.
 100   if (!HasValueForBlock(BB))
 101     return GetValueAtEndOfBlock(BB);
 102
 103   // Otherwise, we have the hard case.  Get the live-in values for each
 104   // predecessor.
 105   SmallVector<std::pair<BasicBlock *, Value *>, 8> PredValues;
 106   Value *SingularValue = nullptr;
 107
 108   // We can get our predecessor info by walking the pred_iterator list, but it
 109   // is relatively slow.  If we already have PHI nodes in this block, walk one
 110   // of them to get the predecessor list instead.
 111   if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
 112     for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) {
 113       BasicBlock *PredBB = SomePhi->getIncomingBlock(i);
 114       Value *PredVal = GetValueAtEndOfBlock(PredBB);
 115       PredValues.push_back(std::make_pair(PredBB, PredVal));
 116
 117       // Compute SingularValue.
 118       if (i == 0)
 119         SingularValue = PredVal;
 120       else if (PredVal != SingularValue)
 121         SingularValue = nullptr;
 122     }
 123   } else {
 124     bool isFirstPred = true;
 125     for (BasicBlock *PredBB : predecessors(BB)) {
 126       Value *PredVal = GetValueAtEndOfBlock(PredBB);
 127       PredValues.push_back(std::make_pair(PredBB, PredVal));
 128
 129       // Compute SingularValue.
 130       if (isFirstPred) {
 131         SingularValue = PredVal;
 132         isFirstPred = false;
 133       } else if (PredVal != SingularValue)
 134         SingularValue = nullptr;
 135     }
 136   }
 137
 138   // If there are no predecessors, just return undef.
 139   if (PredValues.empty())
 140     return UndefValue::get(ProtoType);
 141
 142   // Otherwise, if all the merged values are the same, just use it.
 143   if (SingularValue)
 144     return SingularValue;
 145
 146   // Otherwise, we do need a PHI: check to see if we already have one available
 147   // in this block that produces the right value.
 148   if (isa<PHINode>(BB->begin())) {
 149     SmallDenseMap<BasicBlock *, Value *, 8> ValueMapping(PredValues.begin(),
 150                                                          PredValues.end());
 151     for (PHINode &SomePHI : BB->phis()) {
 152       if (IsEquivalentPHI(&SomePHI, ValueMapping))
 153         return &SomePHI;
 154     }
 155   }
 156
 157   // Ok, we have no way out, insert a new one now.
 158   PHINode *InsertedPHI = PHINode::Create(ProtoType, PredValues.size(),
 159                                          ProtoName, &BB->front());
 160
 161   // Fill in all the predecessors of the PHI.
 162   for (const auto &PredValue : PredValues)
 163     InsertedPHI->addIncoming(PredValue.second, PredValue.first);
 164
 165   // See if the PHI node can be merged to a single value.  This can happen in
 166   // loop cases when we get a PHI of itself and one other value.
 167   if (Value *V =
 168           simplifyInstruction(InsertedPHI, BB->getModule()->getDataLayout())) {
 169     InsertedPHI->eraseFromParent();
 170     return V;
 171   }
 172
 173   // Set the DebugLoc of the inserted PHI, if available.
 174   DebugLoc DL;
 175   if (const Instruction *I = BB->getFirstNonPHI())
 176       DL = I->getDebugLoc();
 177   InsertedPHI->setDebugLoc(DL);
 178
 179   // If the client wants to know about all new instructions, tell it.
 180   if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
 181
 182   LLVM_DEBUG(dbgs() << "  Inserted PHI: " << *InsertedPHI << "\n");
 183   return InsertedPHI;
 184 }
 185
 186 void SSAUpdater::RewriteUse(Use &U) {
 187   Instruction *User = cast<Instruction>(U.getUser());
 188
 189   Value *V;
 190   if (PHINode *UserPN = dyn_cast<PHINode>(User))
 191     V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
 192   else
 193     V = GetValueInMiddleOfBlock(User->getParent());
 194
 195   U.set(V);
 196 }
 197
 198 void SSAUpdater::RewriteUseAfterInsertions(Use &U) {
 199   Instruction *User = cast<Instruction>(U.getUser());
 200
 201   Value *V;
 202   if (PHINode *UserPN = dyn_cast<PHINode>(User))
 203     V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
 204   else
 205     V = GetValueAtEndOfBlock(User->getParent());
 206
 207   U.set(V);
 208 }
 209
 210 namespace llvm {
 211
 212 template<>
 213 class SSAUpdaterTraits<SSAUpdater> {
 214 public:
 215   using BlkT = BasicBlock;
 216   using ValT = Value *;
 217   using PhiT = PHINode;
 218   using BlkSucc_iterator = succ_iterator;
 219
 220   static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return succ_begin(BB); }
 221   static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return succ_end(BB); }
 222
 223   class PHI_iterator {
 224   private:
 225     PHINode *PHI;
 226     unsigned idx;
 227
 228   public:
 229     explicit PHI_iterator(PHINode *P) // begin iterator
 230       : PHI(P), idx(0) {}
 231     PHI_iterator(PHINode *P, bool) // end iterator
 232       : PHI(P), idx(PHI->getNumIncomingValues()) {}
 233
 234     PHI_iterator &operator++() { ++idx; return *this; }
 235     bool operator==(const PHI_iterator& x) const { return idx == x.idx; }
 236     bool operator!=(const PHI_iterator& x) const { return !operator==(x); }
 237
 238     Value *getIncomingValue() { return PHI->getIncomingValue(idx); }
 239     BasicBlock *getIncomingBlock() { return PHI->getIncomingBlock(idx); }
 240   };
 241
 242   static PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); }
 243   static PHI_iterator PHI_end(PhiT *PHI) {
 244     return PHI_iterator(PHI, true);
 245   }
 246
 247   /// FindPredecessorBlocks - Put the predecessors of Info->BB into the Preds
 248   /// vector, set Info->NumPreds, and allocate space in Info->Preds.
 249   static void FindPredecessorBlocks(BasicBlock *BB,
 250                                     SmallVectorImpl<BasicBlock *> *Preds) {
 251     // We can get our predecessor info by walking the pred_iterator list,
 252     // but it is relatively slow.  If we already have PHI nodes in this
 253     // block, walk one of them to get the predecessor list instead.
 254     if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin()))
 255       append_range(*Preds, SomePhi->blocks());
 256     else
 257       append_range(*Preds, predecessors(BB));
 258   }
 259
 260   /// GetUndefVal - Get an undefined value of the same type as the value
 261   /// being handled.
 262   static Value *GetUndefVal(BasicBlock *BB, SSAUpdater *Updater) {
 263     return UndefValue::get(Updater->ProtoType);
 264   }
 265
 266   /// CreateEmptyPHI - Create a new PHI instruction in the specified block.
 267   /// Reserve space for the operands but do not fill them in yet.
 268   static Value *CreateEmptyPHI(BasicBlock *BB, unsigned NumPreds,
 269                                SSAUpdater *Updater) {
 270     PHINode *PHI = PHINode::Create(Updater->ProtoType, NumPreds,
 271                                    Updater->ProtoName, &BB->front());
 272     return PHI;
 273   }
 274
 275   /// AddPHIOperand - Add the specified value as an operand of the PHI for
 276   /// the specified predecessor block.
 277   static void AddPHIOperand(PHINode *PHI, Value *Val, BasicBlock *Pred) {
 278     PHI->addIncoming(Val, Pred);
 279   }
 280
 281   /// ValueIsPHI - Check if a value is a PHI.
 282   static PHINode *ValueIsPHI(Value *Val, SSAUpdater *Updater) {
 283     return dyn_cast<PHINode>(Val);
 284   }
 285
 286   /// ValueIsNewPHI - Like ValueIsPHI but also check if the PHI has no source
 287   /// operands, i.e., it was just added.
 288   static PHINode *ValueIsNewPHI(Value *Val, SSAUpdater *Updater) {
 289     PHINode *PHI = ValueIsPHI(Val, Updater);
 290     if (PHI && PHI->getNumIncomingValues() == 0)
 291       return PHI;
 292     return nullptr;
 293   }
 294
 295   /// GetPHIValue - For the specified PHI instruction, return the value
 296   /// that it defines.
 297   static Value *GetPHIValue(PHINode *PHI) {
 298     return PHI;
 299   }
 300 };
 301
 302 } // end namespace llvm
 303
 304 /// Check to see if AvailableVals has an entry for the specified BB and if so,
 305 /// return it.  If not, construct SSA form by first calculating the required
 306 /// placement of PHIs and then inserting new PHIs where needed.
 307 Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *BB) {
 308   AvailableValsTy &AvailableVals = getAvailableVals(AV);
 309   if (Value *V = AvailableVals[BB])
 310     return V;
 311
 312   SSAUpdaterImpl<SSAUpdater> Impl(this, &AvailableVals, InsertedPHIs);
 313   return Impl.GetValue(BB);
 314 }
 315
 316 //===----------------------------------------------------------------------===//
 317 // LoadAndStorePromoter Implementation
 318 //===----------------------------------------------------------------------===//
 319
 320 LoadAndStorePromoter::
 321 LoadAndStorePromoter(ArrayRef<const Instruction *> Insts,
 322                      SSAUpdater &S, StringRef BaseName) : SSA(S) {
 323   if (Insts.empty()) return;
 324
 325   const Value *SomeVal;
 326   if (const LoadInst *LI = dyn_cast<LoadInst>(Insts[0]))
 327     SomeVal = LI;
 328   else
 329     SomeVal = cast<StoreInst>(Insts[0])->getOperand(0);
 330
 331   if (BaseName.empty())
 332     BaseName = SomeVal->getName();
 333   SSA.Initialize(SomeVal->getType(), BaseName);
 334 }
 335
 336 void LoadAndStorePromoter::run(const SmallVectorImpl<Instruction *> &Insts) {
 337   // First step: bucket up uses of the alloca by the block they occur in.
 338   // This is important because we have to handle multiple defs/uses in a block
 339   // ourselves: SSAUpdater is purely for cross-block references.
 340   DenseMap<BasicBlock *, TinyPtrVector<Instruction *>> UsesByBlock;
 341
 342   for (Instruction *User : Insts)
 343     UsesByBlock[User->getParent()].push_back(User);
 344
 345   // Okay, now we can iterate over all the blocks in the function with uses,
 346   // processing them.  Keep track of which loads are loading a live-in value.
 347   // Walk the uses in the use-list order to be determinstic.
 348   SmallVector<LoadInst *, 32> LiveInLoads;
 349   DenseMap<Value *, Value *> ReplacedLoads;
 350
 351   for (Instruction *User : Insts) {
 352     BasicBlock *BB = User->getParent();
 353     TinyPtrVector<Instruction *> &BlockUses = UsesByBlock[BB];
 354
 355     // If this block has already been processed, ignore this repeat use.
 356     if (BlockUses.empty()) continue;
 357
 358     // Okay, this is the first use in the block.  If this block just has a
 359     // single user in it, we can rewrite it trivially.
 360     if (BlockUses.size() == 1) {
 361       // If it is a store, it is a trivial def of the value in the block.
 362       if (StoreInst *SI = dyn_cast<StoreInst>(User)) {
 363         updateDebugInfo(SI);
 364         SSA.AddAvailableValue(BB, SI->getOperand(0));
 365       } else
 366         // Otherwise it is a load, queue it to rewrite as a live-in load.
 367         LiveInLoads.push_back(cast<LoadInst>(User));
 368       BlockUses.clear();
 369       continue;
 370     }
 371
 372     // Otherwise, check to see if this block is all loads.
 373     bool HasStore = false;
 374     for (Instruction *I : BlockUses) {
 375       if (isa<StoreInst>(I)) {
 376         HasStore = true;
 377         break;
 378       }
 379     }
 380
 381     // If so, we can queue them all as live in loads.  We don't have an
 382     // efficient way to tell which on is first in the block and don't want to
 383     // scan large blocks, so just add all loads as live ins.
 384     if (!HasStore) {
 385       for (Instruction *I : BlockUses)
 386         LiveInLoads.push_back(cast<LoadInst>(I));
 387       BlockUses.clear();
 388       continue;
 389     }
 390
 391     // Otherwise, we have mixed loads and stores (or just a bunch of stores).
 392     // Since SSAUpdater is purely for cross-block values, we need to determine
 393     // the order of these instructions in the block.  If the first use in the
 394     // block is a load, then it uses the live in value.  The last store defines
 395     // the live out value.  We handle this by doing a linear scan of the block.
 396     Value *StoredValue = nullptr;
 397     for (Instruction &I : *BB) {
 398       if (LoadInst *L = dyn_cast<LoadInst>(&I)) {
 399         // If this is a load from an unrelated pointer, ignore it.
 400         if (!isInstInList(L, Insts)) continue;
 401
 402         // If we haven't seen a store yet, this is a live in use, otherwise
 403         // use the stored value.
 404         if (StoredValue) {
 405           replaceLoadWithValue(L, StoredValue);
 406           L->replaceAllUsesWith(StoredValue);
 407           ReplacedLoads[L] = StoredValue;
 408         } else {
 409           LiveInLoads.push_back(L);
 410         }
 411         continue;
 412       }
 413
 414       if (StoreInst *SI = dyn_cast<StoreInst>(&I)) {
 415         // If this is a store to an unrelated pointer, ignore it.
 416         if (!isInstInList(SI, Insts)) continue;
 417         updateDebugInfo(SI);
 418
 419         // Remember that this is the active value in the block.
 420         StoredValue = SI->getOperand(0);
 421       }
 422     }
 423
 424     // The last stored value that happened is the live-out for the block.
 425     assert(StoredValue && "Already checked that there is a store in block");
 426     SSA.AddAvailableValue(BB, StoredValue);
 427     BlockUses.clear();
 428   }
 429
 430   // Okay, now we rewrite all loads that use live-in values in the loop,
 431   // inserting PHI nodes as necessary.
 432   for (LoadInst *ALoad : LiveInLoads) {
 433     Value *NewVal = SSA.GetValueInMiddleOfBlock(ALoad->getParent());
 434     replaceLoadWithValue(ALoad, NewVal);
 435
 436     // Avoid assertions in unreachable code.
 437     if (NewVal == ALoad) NewVal = PoisonValue::get(NewVal->getType());
 438     ALoad->replaceAllUsesWith(NewVal);
 439     ReplacedLoads[ALoad] = NewVal;
 440   }
 441
 442   // Allow the client to do stuff before we start nuking things.
 443   doExtraRewritesBeforeFinalDeletion();
 444
 445   // Now that everything is rewritten, delete the old instructions from the
 446   // function.  They should all be dead now.
 447   for (Instruction *User : Insts) {
 448     if (!shouldDelete(User))
 449       continue;
 450
 451     // If this is a load that still has uses, then the load must have been added
 452     // as a live value in the SSAUpdate data structure for a block (e.g. because
 453     // the loaded value was stored later).  In this case, we need to recursively
 454     // propagate the updates until we get to the real value.
 455     if (!User->use_empty()) {
 456       Value *NewVal = ReplacedLoads[User];
 457       assert(NewVal && "not a replaced load?");
 458
 459       // Propagate down to the ultimate replacee.  The intermediately loads
 460       // could theoretically already have been deleted, so we don't want to
 461       // dereference the Value*'s.
 462       DenseMap<Value*, Value*>::iterator RLI = ReplacedLoads.find(NewVal);
 463       while (RLI != ReplacedLoads.end()) {
 464         NewVal = RLI->second;
 465         RLI = ReplacedLoads.find(NewVal);
 466       }
 467
 468       replaceLoadWithValue(cast<LoadInst>(User), NewVal);
 469       User->replaceAllUsesWith(NewVal);
 470     }
 471
 472     instructionDeleted(User);
 473     User->eraseFromParent();
 474   }
 475 }
 476
 477 bool
 478 LoadAndStorePromoter::isInstInList(Instruction *I,
 479                                    const SmallVectorImpl<Instruction *> &Insts)
 480                                    const {
 481   return is_contained(Insts, I);
 482 }