lib/Support/VirtualInstruction.cpp

   1 //===------ VirtualInstruction.cpp ------------------------------*- C++ -*-===//
   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 // Tools for determining which instructions are within a statement and the
  11 // nature of their operands.
  12 //
  13 //===----------------------------------------------------------------------===//
  14
  15 #include "polly/Support/VirtualInstruction.h"
  16 #include "polly/Support/SCEVValidator.h"
  17
  18 using namespace polly;
  19 using namespace llvm;
  20
  21 VirtualUse VirtualUse ::create(Scop *S, const Use &U, LoopInfo *LI,
  22                                bool Virtual) {
  23   auto *UserBB = getUseBlock(U);
  24   auto *UserStmt = S->getStmtFor(UserBB);
  25   auto *UserScope = LI->getLoopFor(UserBB);
  26   return create(S, UserStmt, UserScope, U.get(), Virtual);
  27 }
  28
  29 VirtualUse VirtualUse::create(Scop *S, ScopStmt *UserStmt, Loop *UserScope,
  30                               Value *Val, bool Virtual) {
  31   assert(!isa<StoreInst>(Val) && "a StoreInst cannot be used");
  32
  33   if (isa<BasicBlock>(Val))
  34     return VirtualUse(UserStmt, Val, Block, nullptr, nullptr);
  35
  36   if (isa<llvm::Constant>(Val))
  37     return VirtualUse(UserStmt, Val, Constant, nullptr, nullptr);
  38
  39   // Is the value synthesizable? If the user has been pruned
  40   // (UserStmt == nullptr), it is either not used anywhere or is synthesizable.
  41   // We assume synthesizable which practically should have the same effect.
  42   auto *SE = S->getSE();
  43   if (SE->isSCEVable(Val->getType())) {
  44     auto *ScevExpr = SE->getSCEVAtScope(Val, UserScope);
  45     if (!UserStmt || canSynthesize(Val, *UserStmt->getParent(), SE, UserScope))
  46       return VirtualUse(UserStmt, Val, Synthesizable, ScevExpr, nullptr);
  47   }
  48
  49   // FIXME: Inconsistency between lookupInvariantEquivClass and
  50   // getRequiredInvariantLoads. Querying one of them should be enough.
  51   auto &RIL = S->getRequiredInvariantLoads();
  52   if (S->lookupInvariantEquivClass(Val) || RIL.count(dyn_cast<LoadInst>(Val)))
  53     return VirtualUse(UserStmt, Val, Hoisted, nullptr, nullptr);
  54
  55   // ReadOnly uses may have MemoryAccesses that we want to associate with the
  56   // use. This is why we look for a MemoryAccess here already.
  57   MemoryAccess *InputMA = nullptr;
  58   if (UserStmt && Virtual)
  59     InputMA = UserStmt->lookupValueReadOf(Val);
  60
  61   // Uses are read-only if they have been defined before the SCoP, i.e., they
  62   // cannot be written to inside the SCoP. Arguments are defined before any
  63   // instructions, hence also before the SCoP. If the user has been pruned
  64   // (UserStmt == nullptr) and is not SCEVable, assume it is read-only as it is
  65   // neither an intra- nor an inter-use.
  66   if (!UserStmt || isa<Argument>(Val))
  67     return VirtualUse(UserStmt, Val, ReadOnly, nullptr, InputMA);
  68
  69   auto Inst = cast<Instruction>(Val);
  70   if (!S->contains(Inst))
  71     return VirtualUse(UserStmt, Val, ReadOnly, nullptr, InputMA);
  72
  73   // A use is inter-statement if either it is defined in another statement, or
  74   // there is a MemoryAccess that reads its value that has been written by
  75   // another statement.
  76   if (InputMA || (!Virtual && !UserStmt->represents(Inst->getParent())))
  77     return VirtualUse(UserStmt, Val, Inter, nullptr, InputMA);
  78
  79   return VirtualUse(UserStmt, Val, Intra, nullptr, nullptr);
  80 }
  81
  82 void VirtualUse::print(raw_ostream &OS, bool Reproducible) const {
  83   OS << "User: [" << User->getBaseName() << "] ";
  84   switch (Kind) {
  85   case VirtualUse::Constant:
  86     OS << "Constant Op:";
  87     break;
  88   case VirtualUse::Block:
  89     OS << "BasicBlock Op:";
  90     break;
  91   case VirtualUse::Synthesizable:
  92     OS << "Synthesizable Op:";
  93     break;
  94   case VirtualUse::Hoisted:
  95     OS << "Hoisted load Op:";
  96     break;
  97   case VirtualUse::ReadOnly:
  98     OS << "Read-Only Op:";
  99     break;
 100   case VirtualUse::Intra:
 101     OS << "Intra Op:";
 102     break;
 103   case VirtualUse::Inter:
 104     OS << "Inter Op:";
 105     break;
 106   }
 107
 108   if (Val) {
 109     OS << ' ';
 110     if (Reproducible)
 111       OS << '"' << Val->getName() << '"';
 112     else
 113       Val->print(OS, true);
 114   }
 115   if (ScevExpr) {
 116     OS << ' ';
 117     ScevExpr->print(OS);
 118   }
 119   if (InputMA && !Reproducible)
 120     OS << ' ' << InputMA;
 121 }
 122
 123 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
 124 LLVM_DUMP_METHOD void VirtualUse::dump() const {
 125   print(errs(), false);
 126   errs() << '\n';
 127 }
 128 #endif
 129
 130 void VirtualInstruction::print(raw_ostream &OS, bool Reproducible) const {
 131   if (!Stmt || !Inst) {
 132     OS << "[null VirtualInstruction]";
 133     return;
 134   }
 135
 136   OS << "[" << Stmt->getBaseName() << "]";
 137   Inst->print(OS, !Reproducible);
 138 }
 139
 140 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
 141 LLVM_DUMP_METHOD void VirtualInstruction::dump() const {
 142   print(errs(), false);
 143   errs() << '\n';
 144 }
 145 #endif
 146
 147 /// Return true if @p Inst cannot be removed, even if it is nowhere referenced.
 148 static bool isRoot(const Instruction *Inst) {
 149   // The store is handled by its MemoryAccess. The load must be reached from the
 150   // roots in order to be marked as used.
 151   if (isa<LoadInst>(Inst) || isa<StoreInst>(Inst))
 152     return false;
 153
 154   // Terminator instructions (in region statements) are required for control
 155   // flow.
 156   if (isa<TerminatorInst>(Inst))
 157     return true;
 158
 159   // Writes to memory must be honored.
 160   if (Inst->mayWriteToMemory())
 161     return true;
 162
 163   return false;
 164 }
 165
 166 /// Return true for MemoryAccesses that cannot be removed because it represents
 167 /// an llvm::Value that is used after the SCoP.
 168 static bool isEscaping(MemoryAccess *MA) {
 169   assert(MA->isOriginalValueKind());
 170   Scop *S = MA->getStatement()->getParent();
 171   return S->isEscaping(cast<Instruction>(MA->getAccessValue()));
 172 }
 173
 174 /// Add non-removable virtual instructions in @p Stmt to @p RootInsts.
 175 static void
 176 addInstructionRoots(ScopStmt *Stmt,
 177                     SmallVectorImpl<VirtualInstruction> &RootInsts) {
 178   // For region statements we must keep all instructions because we do not
 179   // support removing instructions from region statements.
 180   if (!Stmt->isBlockStmt()) {
 181     for (auto *BB : Stmt->getRegion()->blocks())
 182       for (Instruction &Inst : *BB)
 183         RootInsts.emplace_back(Stmt, &Inst);
 184     return;
 185   }
 186
 187   for (Instruction *Inst : Stmt->getInstructions())
 188     if (isRoot(Inst))
 189       RootInsts.emplace_back(Stmt, Inst);
 190 }
 191
 192 /// Add non-removable memory accesses in @p Stmt to @p RootInsts.
 193 ///
 194 /// @param Local If true, all writes are assumed to escape. markAndSweep
 195 /// algorithms can use this to be applicable to a single ScopStmt only without
 196 /// the risk of removing definitions required by other statements.
 197 ///              If false, only writes for SCoP-escaping values are roots.  This
 198 ///              is global mode, where such writes must be marked by theirs uses
 199 ///              in order to be reachable.
 200 static void addAccessRoots(ScopStmt *Stmt,
 201                            SmallVectorImpl<MemoryAccess *> &RootAccs,
 202                            bool Local) {
 203   for (auto *MA : *Stmt) {
 204     if (!MA->isWrite())
 205       continue;
 206
 207     // Writes to arrays are always used.
 208     if (MA->isLatestArrayKind())
 209       RootAccs.push_back(MA);
 210
 211     // Values are roots if they are escaping.
 212     else if (MA->isLatestValueKind()) {
 213       if (Local || isEscaping(MA))
 214         RootAccs.push_back(MA);
 215     }
 216
 217     // Exit phis are, by definition, escaping.
 218     else if (MA->isLatestExitPHIKind())
 219       RootAccs.push_back(MA);
 220
 221     // phi writes are only roots if we are not visiting the statement
 222     // containing the PHINode.
 223     else if (Local && MA->isLatestPHIKind())
 224       RootAccs.push_back(MA);
 225   }
 226 }
 227
 228 /// Determine all instruction and access roots.
 229 static void addRoots(ScopStmt *Stmt,
 230                      SmallVectorImpl<VirtualInstruction> &RootInsts,
 231                      SmallVectorImpl<MemoryAccess *> &RootAccs, bool Local) {
 232   addInstructionRoots(Stmt, RootInsts);
 233   addAccessRoots(Stmt, RootAccs, Local);
 234 }
 235
 236 /// Mark accesses and instructions as used if they are reachable from a root,
 237 /// walking the operand trees.
 238 ///
 239 /// @param S              The SCoP to walk.
 240 /// @param LI             The LoopInfo Analysis.
 241 /// @param RootInsts      List of root instructions.
 242 /// @param RootAccs       List of root accesses.
 243 /// @param UsesInsts[out] Receives all reachable instructions, including the
 244 /// roots.
 245 /// @param UsedAccs[out]  Receives all reachable accesses, including the roots.
 246 /// @param OnlyLocal      If non-nullptr, restricts walking to a single
 247 /// statement.
 248 static void walkReachable(Scop *S, LoopInfo *LI,
 249                           ArrayRef<VirtualInstruction> RootInsts,
 250                           ArrayRef<MemoryAccess *> RootAccs,
 251                           DenseSet<VirtualInstruction> &UsedInsts,
 252                           DenseSet<MemoryAccess *> &UsedAccs,
 253                           ScopStmt *OnlyLocal = nullptr) {
 254   UsedInsts.clear();
 255   UsedAccs.clear();
 256
 257   SmallVector<VirtualInstruction, 32> WorklistInsts;
 258   SmallVector<MemoryAccess *, 32> WorklistAccs;
 259
 260   WorklistInsts.append(RootInsts.begin(), RootInsts.end());
 261   WorklistAccs.append(RootAccs.begin(), RootAccs.end());
 262
 263   auto AddToWorklist = [&](VirtualUse VUse) {
 264     switch (VUse.getKind()) {
 265     case VirtualUse::Block:
 266     case VirtualUse::Constant:
 267     case VirtualUse::Synthesizable:
 268     case VirtualUse::Hoisted:
 269       break;
 270     case VirtualUse::ReadOnly:
 271       // Read-only scalars only have MemoryAccesses if ModelReadOnlyScalars is
 272       // enabled.
 273       if (!VUse.getMemoryAccess())
 274         break;
 275       LLVM_FALLTHROUGH;
 276     case VirtualUse::Inter:
 277       assert(VUse.getMemoryAccess());
 278       WorklistAccs.push_back(VUse.getMemoryAccess());
 279       break;
 280     case VirtualUse::Intra:
 281       WorklistInsts.emplace_back(VUse.getUser(),
 282                                  cast<Instruction>(VUse.getValue()));
 283       break;
 284     }
 285   };
 286
 287   while (true) {
 288     // We have two worklists to process: Only when the MemoryAccess worklist is
 289     // empty, we process the instruction worklist.
 290
 291     while (!WorklistAccs.empty()) {
 292       auto *Acc = WorklistAccs.pop_back_val();
 293
 294       ScopStmt *Stmt = Acc->getStatement();
 295       if (OnlyLocal && Stmt != OnlyLocal)
 296         continue;
 297
 298       auto Inserted = UsedAccs.insert(Acc);
 299       if (!Inserted.second)
 300         continue;
 301
 302       if (Acc->isRead()) {
 303         const ScopArrayInfo *SAI = Acc->getScopArrayInfo();
 304
 305         if (Acc->isOriginalValueKind()) {
 306           MemoryAccess *DefAcc = S->getValueDef(SAI);
 307
 308           // Accesses to read-only values do not have a definition.
 309           if (DefAcc)
 310             WorklistAccs.push_back(S->getValueDef(SAI));
 311         }
 312
 313         if (Acc->isOriginalAnyPHIKind()) {
 314           auto IncomingMAs = S->getPHIIncomings(SAI);
 315           WorklistAccs.append(IncomingMAs.begin(), IncomingMAs.end());
 316         }
 317       }
 318
 319       if (Acc->isWrite()) {
 320         if (Acc->isOriginalValueKind() ||
 321             (Acc->isOriginalArrayKind() && Acc->getAccessValue())) {
 322           Loop *Scope = Stmt->getSurroundingLoop();
 323           VirtualUse VUse =
 324               VirtualUse::create(S, Stmt, Scope, Acc->getAccessValue(), true);
 325           AddToWorklist(VUse);
 326         }
 327
 328         if (Acc->isOriginalAnyPHIKind()) {
 329           for (auto Incoming : Acc->getIncoming()) {
 330             VirtualUse VUse = VirtualUse::create(
 331                 S, Stmt, LI->getLoopFor(Incoming.first), Incoming.second, true);
 332             AddToWorklist(VUse);
 333           }
 334         }
 335
 336         if (Acc->isOriginalArrayKind())
 337           WorklistInsts.emplace_back(Stmt, Acc->getAccessInstruction());
 338       }
 339     }
 340
 341     // If both worklists are empty, stop walking.
 342     if (WorklistInsts.empty())
 343       break;
 344
 345     VirtualInstruction VInst = WorklistInsts.pop_back_val();
 346     ScopStmt *Stmt = VInst.getStmt();
 347     Instruction *Inst = VInst.getInstruction();
 348
 349     // Do not process statements other than the local.
 350     if (OnlyLocal && Stmt != OnlyLocal)
 351       continue;
 352
 353     auto InsertResult = UsedInsts.insert(VInst);
 354     if (!InsertResult.second)
 355       continue;
 356
 357     // Add all operands to the worklists.
 358     PHINode *PHI = dyn_cast<PHINode>(Inst);
 359     if (PHI && PHI->getParent() == Stmt->getEntryBlock()) {
 360       if (MemoryAccess *PHIRead = Stmt->lookupPHIReadOf(PHI))
 361         WorklistAccs.push_back(PHIRead);
 362     } else {
 363       for (VirtualUse VUse : VInst.operands())
 364         AddToWorklist(VUse);
 365     }
 366
 367     // If there is an array access, also add its MemoryAccesses to the worklist.
 368     const MemoryAccessList *Accs = Stmt->lookupArrayAccessesFor(Inst);
 369     if (!Accs)
 370       continue;
 371
 372     for (MemoryAccess *Acc : *Accs)
 373       WorklistAccs.push_back(Acc);
 374   }
 375 }
 376
 377 void polly::markReachable(Scop *S, LoopInfo *LI,
 378                           DenseSet<VirtualInstruction> &UsedInsts,
 379                           DenseSet<MemoryAccess *> &UsedAccs,
 380                           ScopStmt *OnlyLocal) {
 381   SmallVector<VirtualInstruction, 32> RootInsts;
 382   SmallVector<MemoryAccess *, 32> RootAccs;
 383
 384   if (OnlyLocal) {
 385     addRoots(OnlyLocal, RootInsts, RootAccs, true);
 386   } else {
 387     for (auto &Stmt : *S)
 388       addRoots(&Stmt, RootInsts, RootAccs, false);
 389   }
 390
 391   walkReachable(S, LI, RootInsts, RootAccs, UsedInsts, UsedAccs, OnlyLocal);
 392 }