lib/Transforms/Utils/Evaluator.cpp

   1 //===- Evaluator.cpp - LLVM IR evaluator ----------------------------------===//
   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 // Function evaluator for LLVM IR.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "llvm/Transforms/Utils/Evaluator.h"
  14 #include "llvm/ADT/DenseMap.h"
  15 #include "llvm/ADT/STLExtras.h"
  16 #include "llvm/ADT/SmallPtrSet.h"
  17 #include "llvm/ADT/SmallVector.h"
  18 #include "llvm/Analysis/ConstantFolding.h"
  19 #include "llvm/IR/BasicBlock.h"
  20 #include "llvm/IR/CallSite.h"
  21 #include "llvm/IR/Constant.h"
  22 #include "llvm/IR/Constants.h"
  23 #include "llvm/IR/DataLayout.h"
  24 #include "llvm/IR/DerivedTypes.h"
  25 #include "llvm/IR/Function.h"
  26 #include "llvm/IR/GlobalAlias.h"
  27 #include "llvm/IR/GlobalValue.h"
  28 #include "llvm/IR/GlobalVariable.h"
  29 #include "llvm/IR/InstrTypes.h"
  30 #include "llvm/IR/Instruction.h"
  31 #include "llvm/IR/Instructions.h"
  32 #include "llvm/IR/IntrinsicInst.h"
  33 #include "llvm/IR/Intrinsics.h"
  34 #include "llvm/IR/Operator.h"
  35 #include "llvm/IR/Type.h"
  36 #include "llvm/IR/User.h"
  37 #include "llvm/IR/Value.h"
  38 #include "llvm/Support/Casting.h"
  39 #include "llvm/Support/Debug.h"
  40 #include "llvm/Support/raw_ostream.h"
  41 #include <iterator>
  42
  43 #define DEBUG_TYPE "evaluator"
  44
  45 using namespace llvm;
  46
  47 static inline bool
  48 isSimpleEnoughValueToCommit(Constant *C,
  49                             SmallPtrSetImpl<Constant *> &SimpleConstants,
  50                             const DataLayout &DL);
  51
  52 /// Return true if the specified constant can be handled by the code generator.
  53 /// We don't want to generate something like:
  54 ///   void *X = &X/42;
  55 /// because the code generator doesn't have a relocation that can handle that.
  56 ///
  57 /// This function should be called if C was not found (but just got inserted)
  58 /// in SimpleConstants to avoid having to rescan the same constants all the
  59 /// time.
  60 static bool
  61 isSimpleEnoughValueToCommitHelper(Constant *C,
  62                                   SmallPtrSetImpl<Constant *> &SimpleConstants,
  63                                   const DataLayout &DL) {
  64   // Simple global addresses are supported, do not allow dllimport or
  65   // thread-local globals.
  66   if (auto *GV = dyn_cast<GlobalValue>(C))
  67     return !GV->hasDLLImportStorageClass() && !GV->isThreadLocal();
  68
  69   // Simple integer, undef, constant aggregate zero, etc are all supported.
  70   if (C->getNumOperands() == 0 || isa<BlockAddress>(C))
  71     return true;
  72
  73   // Aggregate values are safe if all their elements are.
  74   if (isa<ConstantAggregate>(C)) {
  75     for (Value *Op : C->operands())
  76       if (!isSimpleEnoughValueToCommit(cast<Constant>(Op), SimpleConstants, DL))
  77         return false;
  78     return true;
  79   }
  80
  81   // We don't know exactly what relocations are allowed in constant expressions,
  82   // so we allow &global+constantoffset, which is safe and uniformly supported
  83   // across targets.
  84   ConstantExpr *CE = cast<ConstantExpr>(C);
  85   switch (CE->getOpcode()) {
  86   case Instruction::BitCast:
  87     // Bitcast is fine if the casted value is fine.
  88     return isSimpleEnoughValueToCommit(CE->getOperand(0), SimpleConstants, DL);
  89
  90   case Instruction::IntToPtr:
  91   case Instruction::PtrToInt:
  92     // int <=> ptr is fine if the int type is the same size as the
  93     // pointer type.
  94     if (DL.getTypeSizeInBits(CE->getType()) !=
  95         DL.getTypeSizeInBits(CE->getOperand(0)->getType()))
  96       return false;
  97     return isSimpleEnoughValueToCommit(CE->getOperand(0), SimpleConstants, DL);
  98
  99   // GEP is fine if it is simple + constant offset.
 100   case Instruction::GetElementPtr:
 101     for (unsigned i = 1, e = CE->getNumOperands(); i != e; ++i)
 102       if (!isa<ConstantInt>(CE->getOperand(i)))
 103         return false;
 104     return isSimpleEnoughValueToCommit(CE->getOperand(0), SimpleConstants, DL);
 105
 106   case Instruction::Add:
 107     // We allow simple+cst.
 108     if (!isa<ConstantInt>(CE->getOperand(1)))
 109       return false;
 110     return isSimpleEnoughValueToCommit(CE->getOperand(0), SimpleConstants, DL);
 111   }
 112   return false;
 113 }
 114
 115 static inline bool
 116 isSimpleEnoughValueToCommit(Constant *C,
 117                             SmallPtrSetImpl<Constant *> &SimpleConstants,
 118                             const DataLayout &DL) {
 119   // If we already checked this constant, we win.
 120   if (!SimpleConstants.insert(C).second)
 121     return true;
 122   // Check the constant.
 123   return isSimpleEnoughValueToCommitHelper(C, SimpleConstants, DL);
 124 }
 125
 126 /// Return true if this constant is simple enough for us to understand.  In
 127 /// particular, if it is a cast to anything other than from one pointer type to
 128 /// another pointer type, we punt.  We basically just support direct accesses to
 129 /// globals and GEP's of globals.  This should be kept up to date with
 130 /// CommitValueTo.
 131 static bool isSimpleEnoughPointerToCommit(Constant *C) {
 132   // Conservatively, avoid aggregate types. This is because we don't
 133   // want to worry about them partially overlapping other stores.
 134   if (!cast<PointerType>(C->getType())->getElementType()->isSingleValueType())
 135     return false;
 136
 137   if (GlobalVariable *GV = dyn_cast<GlobalVariable>(C))
 138     // Do not allow weak/*_odr/linkonce linkage or external globals.
 139     return GV->hasUniqueInitializer();
 140
 141   if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C)) {
 142     // Handle a constantexpr gep.
 143     if (CE->getOpcode() == Instruction::GetElementPtr &&
 144         isa<GlobalVariable>(CE->getOperand(0)) &&
 145         cast<GEPOperator>(CE)->isInBounds()) {
 146       GlobalVariable *GV = cast<GlobalVariable>(CE->getOperand(0));
 147       // Do not allow weak/*_odr/linkonce/dllimport/dllexport linkage or
 148       // external globals.
 149       if (!GV->hasUniqueInitializer())
 150         return false;
 151
 152       // The first index must be zero.
 153       ConstantInt *CI = dyn_cast<ConstantInt>(*std::next(CE->op_begin()));
 154       if (!CI || !CI->isZero()) return false;
 155
 156       // The remaining indices must be compile-time known integers within the
 157       // notional bounds of the corresponding static array types.
 158       if (!CE->isGEPWithNoNotionalOverIndexing())
 159         return false;
 160
 161       return ConstantFoldLoadThroughGEPConstantExpr(GV->getInitializer(), CE);
 162
 163     // A constantexpr bitcast from a pointer to another pointer is a no-op,
 164     // and we know how to evaluate it by moving the bitcast from the pointer
 165     // operand to the value operand.
 166     } else if (CE->getOpcode() == Instruction::BitCast &&
 167                isa<GlobalVariable>(CE->getOperand(0))) {
 168       // Do not allow weak/*_odr/linkonce/dllimport/dllexport linkage or
 169       // external globals.
 170       return cast<GlobalVariable>(CE->getOperand(0))->hasUniqueInitializer();
 171     }
 172   }
 173
 174   return false;
 175 }
 176
 177 static Constant *getInitializer(Constant *C) {
 178   auto *GV = dyn_cast<GlobalVariable>(C);
 179   return GV && GV->hasDefinitiveInitializer() ? GV->getInitializer() : nullptr;
 180 }
 181
 182 /// Return the value that would be computed by a load from P after the stores
 183 /// reflected by 'memory' have been performed.  If we can't decide, return null.
 184 Constant *Evaluator::ComputeLoadResult(Constant *P) {
 185   // If this memory location has been recently stored, use the stored value: it
 186   // is the most up-to-date.
 187   DenseMap<Constant*, Constant*>::const_iterator I = MutatedMemory.find(P);
 188   if (I != MutatedMemory.end()) return I->second;
 189
 190   // Access it.
 191   if (GlobalVariable *GV = dyn_cast<GlobalVariable>(P)) {
 192     if (GV->hasDefinitiveInitializer())
 193       return GV->getInitializer();
 194     return nullptr;
 195   }
 196
 197   if (ConstantExpr *CE = dyn_cast<ConstantExpr>(P)) {
 198     switch (CE->getOpcode()) {
 199     // Handle a constantexpr getelementptr.
 200     case Instruction::GetElementPtr:
 201       if (auto *I = getInitializer(CE->getOperand(0)))
 202         return ConstantFoldLoadThroughGEPConstantExpr(I, CE);
 203       break;
 204     // Handle a constantexpr bitcast.
 205     case Instruction::BitCast:
 206       Constant *Val = getVal(CE->getOperand(0));
 207       auto MM = MutatedMemory.find(Val);
 208       auto *I = (MM != MutatedMemory.end()) ? MM->second
 209                                             : getInitializer(CE->getOperand(0));
 210       if (I)
 211         return ConstantFoldLoadThroughBitcast(
 212             I, P->getType()->getPointerElementType(), DL);
 213       break;
 214     }
 215   }
 216
 217   return nullptr;  // don't know how to evaluate.
 218 }
 219
 220 static Function *getFunction(Constant *C) {
 221   if (auto *Fn = dyn_cast<Function>(C))
 222     return Fn;
 223
 224   if (auto *Alias = dyn_cast<GlobalAlias>(C))
 225     if (auto *Fn = dyn_cast<Function>(Alias->getAliasee()))
 226       return Fn;
 227   return nullptr;
 228 }
 229
 230 Function *
 231 Evaluator::getCalleeWithFormalArgs(CallSite &CS,
 232                                    SmallVector<Constant *, 8> &Formals) {
 233   auto *V = CS.getCalledValue();
 234   if (auto *Fn = getFunction(getVal(V)))
 235     return getFormalParams(CS, Fn, Formals) ? Fn : nullptr;
 236
 237   auto *CE = dyn_cast<ConstantExpr>(V);
 238   if (!CE || CE->getOpcode() != Instruction::BitCast ||
 239       !getFormalParams(CS, getFunction(CE->getOperand(0)), Formals))
 240     return nullptr;
 241
 242   return dyn_cast<Function>(
 243       ConstantFoldLoadThroughBitcast(CE, CE->getOperand(0)->getType(), DL));
 244 }
 245
 246 bool Evaluator::getFormalParams(CallSite &CS, Function *F,
 247                                 SmallVector<Constant *, 8> &Formals) {
 248   if (!F)
 249     return false;
 250
 251   auto *FTy = F->getFunctionType();
 252   if (FTy->getNumParams() > CS.getNumArgOperands()) {
 253     LLVM_DEBUG(dbgs() << "Too few arguments for function.\n");
 254     return false;
 255   }
 256
 257   auto ArgI = CS.arg_begin();
 258   for (auto ParI = FTy->param_begin(), ParE = FTy->param_end(); ParI != ParE;
 259        ++ParI) {
 260     auto *ArgC = ConstantFoldLoadThroughBitcast(getVal(*ArgI), *ParI, DL);
 261     if (!ArgC) {
 262       LLVM_DEBUG(dbgs() << "Can not convert function argument.\n");
 263       return false;
 264     }
 265     Formals.push_back(ArgC);
 266     ++ArgI;
 267   }
 268   return true;
 269 }
 270
 271 /// If call expression contains bitcast then we may need to cast
 272 /// evaluated return value to a type of the call expression.
 273 Constant *Evaluator::castCallResultIfNeeded(Value *CallExpr, Constant *RV) {
 274   ConstantExpr *CE = dyn_cast<ConstantExpr>(CallExpr);
 275   if (!RV || !CE || CE->getOpcode() != Instruction::BitCast)
 276     return RV;
 277
 278   if (auto *FT =
 279           dyn_cast<FunctionType>(CE->getType()->getPointerElementType())) {
 280     RV = ConstantFoldLoadThroughBitcast(RV, FT->getReturnType(), DL);
 281     if (!RV)
 282       LLVM_DEBUG(dbgs() << "Failed to fold bitcast call expr\n");
 283   }
 284   return RV;
 285 }
 286
 287 /// Evaluate all instructions in block BB, returning true if successful, false
 288 /// if we can't evaluate it.  NewBB returns the next BB that control flows into,
 289 /// or null upon return.
 290 bool Evaluator::EvaluateBlock(BasicBlock::iterator CurInst,
 291                               BasicBlock *&NextBB) {
 292   // This is the main evaluation loop.
 293   while (true) {
 294     Constant *InstResult = nullptr;
 295
 296     LLVM_DEBUG(dbgs() << "Evaluating Instruction: " << *CurInst << "\n");
 297
 298     if (StoreInst *SI = dyn_cast<StoreInst>(CurInst)) {
 299       if (!SI->isSimple()) {
 300         LLVM_DEBUG(dbgs() << "Store is not simple! Can not evaluate.\n");
 301         return false;  // no volatile/atomic accesses.
 302       }
 303       Constant *Ptr = getVal(SI->getOperand(1));
 304       if (auto *FoldedPtr = ConstantFoldConstant(Ptr, DL, TLI)) {
 305         LLVM_DEBUG(dbgs() << "Folding constant ptr expression: " << *Ptr);
 306         Ptr = FoldedPtr;
 307         LLVM_DEBUG(dbgs() << "; To: " << *Ptr << "\n");
 308       }
 309       if (!isSimpleEnoughPointerToCommit(Ptr)) {
 310         // If this is too complex for us to commit, reject it.
 311         LLVM_DEBUG(
 312             dbgs() << "Pointer is too complex for us to evaluate store.");
 313         return false;
 314       }
 315
 316       Constant *Val = getVal(SI->getOperand(0));
 317
 318       // If this might be too difficult for the backend to handle (e.g. the addr
 319       // of one global variable divided by another) then we can't commit it.
 320       if (!isSimpleEnoughValueToCommit(Val, SimpleConstants, DL)) {
 321         LLVM_DEBUG(dbgs() << "Store value is too complex to evaluate store. "
 322                           << *Val << "\n");
 323         return false;
 324       }
 325
 326       if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ptr)) {
 327         if (CE->getOpcode() == Instruction::BitCast) {
 328           LLVM_DEBUG(dbgs()
 329                      << "Attempting to resolve bitcast on constant ptr.\n");
 330           // If we're evaluating a store through a bitcast, then we need
 331           // to pull the bitcast off the pointer type and push it onto the
 332           // stored value.
 333           Ptr = CE->getOperand(0);
 334
 335           Type *NewTy = cast<PointerType>(Ptr->getType())->getElementType();
 336
 337           // In order to push the bitcast onto the stored value, a bitcast
 338           // from NewTy to Val's type must be legal.  If it's not, we can try
 339           // introspecting NewTy to find a legal conversion.
 340           Constant *NewVal;
 341           while (!(NewVal = ConstantFoldLoadThroughBitcast(Val, NewTy, DL))) {
 342             // If NewTy is a struct, we can convert the pointer to the struct
 343             // into a pointer to its first member.
 344             // FIXME: This could be extended to support arrays as well.
 345             if (StructType *STy = dyn_cast<StructType>(NewTy)) {
 346
 347               IntegerType *IdxTy = IntegerType::get(NewTy->getContext(), 32);
 348               Constant *IdxZero = ConstantInt::get(IdxTy, 0, false);
 349               Constant * const IdxList[] = {IdxZero, IdxZero};
 350
 351               Ptr = ConstantExpr::getGetElementPtr(NewTy, Ptr, IdxList);
 352               if (auto *FoldedPtr = ConstantFoldConstant(Ptr, DL, TLI))
 353                 Ptr = FoldedPtr;
 354               NewTy = STy->getTypeAtIndex(0U);
 355
 356               // If we can't improve the situation by introspecting NewTy,
 357               // we have to give up.
 358             } else {
 359               LLVM_DEBUG(dbgs() << "Failed to bitcast constant ptr, can not "
 360                                    "evaluate.\n");
 361               return false;
 362             }
 363           }
 364
 365           Val = NewVal;
 366           LLVM_DEBUG(dbgs() << "Evaluated bitcast: " << *Val << "\n");
 367         }
 368       }
 369
 370       MutatedMemory[Ptr] = Val;
 371     } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(CurInst)) {
 372       InstResult = ConstantExpr::get(BO->getOpcode(),
 373                                      getVal(BO->getOperand(0)),
 374                                      getVal(BO->getOperand(1)));
 375       LLVM_DEBUG(dbgs() << "Found a BinaryOperator! Simplifying: "
 376                         << *InstResult << "\n");
 377     } else if (CmpInst *CI = dyn_cast<CmpInst>(CurInst)) {
 378       InstResult = ConstantExpr::getCompare(CI->getPredicate(),
 379                                             getVal(CI->getOperand(0)),
 380                                             getVal(CI->getOperand(1)));
 381       LLVM_DEBUG(dbgs() << "Found a CmpInst! Simplifying: " << *InstResult
 382                         << "\n");
 383     } else if (CastInst *CI = dyn_cast<CastInst>(CurInst)) {
 384       InstResult = ConstantExpr::getCast(CI->getOpcode(),
 385                                          getVal(CI->getOperand(0)),
 386                                          CI->getType());
 387       LLVM_DEBUG(dbgs() << "Found a Cast! Simplifying: " << *InstResult
 388                         << "\n");
 389     } else if (SelectInst *SI = dyn_cast<SelectInst>(CurInst)) {
 390       InstResult = ConstantExpr::getSelect(getVal(SI->getOperand(0)),
 391                                            getVal(SI->getOperand(1)),
 392                                            getVal(SI->getOperand(2)));
 393       LLVM_DEBUG(dbgs() << "Found a Select! Simplifying: " << *InstResult
 394                         << "\n");
 395     } else if (auto *EVI = dyn_cast<ExtractValueInst>(CurInst)) {
 396       InstResult = ConstantExpr::getExtractValue(
 397           getVal(EVI->getAggregateOperand()), EVI->getIndices());
 398       LLVM_DEBUG(dbgs() << "Found an ExtractValueInst! Simplifying: "
 399                         << *InstResult << "\n");
 400     } else if (auto *IVI = dyn_cast<InsertValueInst>(CurInst)) {
 401       InstResult = ConstantExpr::getInsertValue(
 402           getVal(IVI->getAggregateOperand()),
 403           getVal(IVI->getInsertedValueOperand()), IVI->getIndices());
 404       LLVM_DEBUG(dbgs() << "Found an InsertValueInst! Simplifying: "
 405                         << *InstResult << "\n");
 406     } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(CurInst)) {
 407       Constant *P = getVal(GEP->getOperand(0));
 408       SmallVector<Constant*, 8> GEPOps;
 409       for (User::op_iterator i = GEP->op_begin() + 1, e = GEP->op_end();
 410            i != e; ++i)
 411         GEPOps.push_back(getVal(*i));
 412       InstResult =
 413           ConstantExpr::getGetElementPtr(GEP->getSourceElementType(), P, GEPOps,
 414                                          cast<GEPOperator>(GEP)->isInBounds());
 415       LLVM_DEBUG(dbgs() << "Found a GEP! Simplifying: " << *InstResult << "\n");
 416     } else if (LoadInst *LI = dyn_cast<LoadInst>(CurInst)) {
 417       if (!LI->isSimple()) {
 418         LLVM_DEBUG(
 419             dbgs() << "Found a Load! Not a simple load, can not evaluate.\n");
 420         return false;  // no volatile/atomic accesses.
 421       }
 422
 423       Constant *Ptr = getVal(LI->getOperand(0));
 424       if (auto *FoldedPtr = ConstantFoldConstant(Ptr, DL, TLI)) {
 425         Ptr = FoldedPtr;
 426         LLVM_DEBUG(dbgs() << "Found a constant pointer expression, constant "
 427                              "folding: "
 428                           << *Ptr << "\n");
 429       }
 430       InstResult = ComputeLoadResult(Ptr);
 431       if (!InstResult) {
 432         LLVM_DEBUG(
 433             dbgs() << "Failed to compute load result. Can not evaluate load."
 434                       "\n");
 435         return false; // Could not evaluate load.
 436       }
 437
 438       LLVM_DEBUG(dbgs() << "Evaluated load: " << *InstResult << "\n");
 439     } else if (AllocaInst *AI = dyn_cast<AllocaInst>(CurInst)) {
 440       if (AI->isArrayAllocation()) {
 441         LLVM_DEBUG(dbgs() << "Found an array alloca. Can not evaluate.\n");
 442         return false;  // Cannot handle array allocs.
 443       }
 444       Type *Ty = AI->getAllocatedType();
 445       AllocaTmps.push_back(llvm::make_unique<GlobalVariable>(
 446           Ty, false, GlobalValue::InternalLinkage, UndefValue::get(Ty),
 447           AI->getName(), /*TLMode=*/GlobalValue::NotThreadLocal,
 448           AI->getType()->getPointerAddressSpace()));
 449       InstResult = AllocaTmps.back().get();
 450       LLVM_DEBUG(dbgs() << "Found an alloca. Result: " << *InstResult << "\n");
 451     } else if (isa<CallInst>(CurInst) || isa<InvokeInst>(CurInst)) {
 452       CallSite CS(&*CurInst);
 453
 454       // Debug info can safely be ignored here.
 455       if (isa<DbgInfoIntrinsic>(CS.getInstruction())) {
 456         LLVM_DEBUG(dbgs() << "Ignoring debug info.\n");
 457         ++CurInst;
 458         continue;
 459       }
 460
 461       // Cannot handle inline asm.
 462       if (isa<InlineAsm>(CS.getCalledValue())) {
 463         LLVM_DEBUG(dbgs() << "Found inline asm, can not evaluate.\n");
 464         return false;
 465       }
 466
 467       if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction())) {
 468         if (MemSetInst *MSI = dyn_cast<MemSetInst>(II)) {
 469           if (MSI->isVolatile()) {
 470             LLVM_DEBUG(dbgs() << "Can not optimize a volatile memset "
 471                               << "intrinsic.\n");
 472             return false;
 473           }
 474           Constant *Ptr = getVal(MSI->getDest());
 475           Constant *Val = getVal(MSI->getValue());
 476           Constant *DestVal = ComputeLoadResult(getVal(Ptr));
 477           if (Val->isNullValue() && DestVal && DestVal->isNullValue()) {
 478             // This memset is a no-op.
 479             LLVM_DEBUG(dbgs() << "Ignoring no-op memset.\n");
 480             ++CurInst;
 481             continue;
 482           }
 483         }
 484
 485         if (II->isLifetimeStartOrEnd()) {
 486           LLVM_DEBUG(dbgs() << "Ignoring lifetime intrinsic.\n");
 487           ++CurInst;
 488           continue;
 489         }
 490
 491         if (II->getIntrinsicID() == Intrinsic::invariant_start) {
 492           // We don't insert an entry into Values, as it doesn't have a
 493           // meaningful return value.
 494           if (!II->use_empty()) {
 495             LLVM_DEBUG(dbgs()
 496                        << "Found unused invariant_start. Can't evaluate.\n");
 497             return false;
 498           }
 499           ConstantInt *Size = cast<ConstantInt>(II->getArgOperand(0));
 500           Value *PtrArg = getVal(II->getArgOperand(1));
 501           Value *Ptr = PtrArg->stripPointerCasts();
 502           if (GlobalVariable *GV = dyn_cast<GlobalVariable>(Ptr)) {
 503             Type *ElemTy = GV->getValueType();
 504             if (!Size->isMinusOne() &&
 505                 Size->getValue().getLimitedValue() >=
 506                     DL.getTypeStoreSize(ElemTy)) {
 507               Invariants.insert(GV);
 508               LLVM_DEBUG(dbgs() << "Found a global var that is an invariant: "
 509                                 << *GV << "\n");
 510             } else {
 511               LLVM_DEBUG(dbgs()
 512                          << "Found a global var, but can not treat it as an "
 513                             "invariant.\n");
 514             }
 515           }
 516           // Continue even if we do nothing.
 517           ++CurInst;
 518           continue;
 519         } else if (II->getIntrinsicID() == Intrinsic::assume) {
 520           LLVM_DEBUG(dbgs() << "Skipping assume intrinsic.\n");
 521           ++CurInst;
 522           continue;
 523         } else if (II->getIntrinsicID() == Intrinsic::sideeffect) {
 524           LLVM_DEBUG(dbgs() << "Skipping sideeffect intrinsic.\n");
 525           ++CurInst;
 526           continue;
 527         }
 528
 529         LLVM_DEBUG(dbgs() << "Unknown intrinsic. Can not evaluate.\n");
 530         return false;
 531       }
 532
 533       // Resolve function pointers.
 534       SmallVector<Constant *, 8> Formals;
 535       Function *Callee = getCalleeWithFormalArgs(CS, Formals);
 536       if (!Callee || Callee->isInterposable()) {
 537         LLVM_DEBUG(dbgs() << "Can not resolve function pointer.\n");
 538         return false;  // Cannot resolve.
 539       }
 540
 541       if (Callee->isDeclaration()) {
 542         // If this is a function we can constant fold, do it.
 543         if (Constant *C = ConstantFoldCall(cast<CallBase>(CS.getInstruction()),
 544                                            Callee, Formals, TLI)) {
 545           InstResult = castCallResultIfNeeded(CS.getCalledValue(), C);
 546           if (!InstResult)
 547             return false;
 548           LLVM_DEBUG(dbgs() << "Constant folded function call. Result: "
 549                             << *InstResult << "\n");
 550         } else {
 551           LLVM_DEBUG(dbgs() << "Can not constant fold function call.\n");
 552           return false;
 553         }
 554       } else {
 555         if (Callee->getFunctionType()->isVarArg()) {
 556           LLVM_DEBUG(dbgs() << "Can not constant fold vararg function call.\n");
 557           return false;
 558         }
 559
 560         Constant *RetVal = nullptr;
 561         // Execute the call, if successful, use the return value.
 562         ValueStack.emplace_back();
 563         if (!EvaluateFunction(Callee, RetVal, Formals)) {
 564           LLVM_DEBUG(dbgs() << "Failed to evaluate function.\n");
 565           return false;
 566         }
 567         ValueStack.pop_back();
 568         InstResult = castCallResultIfNeeded(CS.getCalledValue(), RetVal);
 569         if (RetVal && !InstResult)
 570           return false;
 571
 572         if (InstResult) {
 573           LLVM_DEBUG(dbgs() << "Successfully evaluated function. Result: "
 574                             << *InstResult << "\n\n");
 575         } else {
 576           LLVM_DEBUG(dbgs()
 577                      << "Successfully evaluated function. Result: 0\n\n");
 578         }
 579       }
 580     } else if (CurInst->isTerminator()) {
 581       LLVM_DEBUG(dbgs() << "Found a terminator instruction.\n");
 582
 583       if (BranchInst *BI = dyn_cast<BranchInst>(CurInst)) {
 584         if (BI->isUnconditional()) {
 585           NextBB = BI->getSuccessor(0);
 586         } else {
 587           ConstantInt *Cond =
 588             dyn_cast<ConstantInt>(getVal(BI->getCondition()));
 589           if (!Cond) return false;  // Cannot determine.
 590
 591           NextBB = BI->getSuccessor(!Cond->getZExtValue());
 592         }
 593       } else if (SwitchInst *SI = dyn_cast<SwitchInst>(CurInst)) {
 594         ConstantInt *Val =
 595           dyn_cast<ConstantInt>(getVal(SI->getCondition()));
 596         if (!Val) return false;  // Cannot determine.
 597         NextBB = SI->findCaseValue(Val)->getCaseSuccessor();
 598       } else if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(CurInst)) {
 599         Value *Val = getVal(IBI->getAddress())->stripPointerCasts();
 600         if (BlockAddress *BA = dyn_cast<BlockAddress>(Val))
 601           NextBB = BA->getBasicBlock();
 602         else
 603           return false;  // Cannot determine.
 604       } else if (isa<ReturnInst>(CurInst)) {
 605         NextBB = nullptr;
 606       } else {
 607         // invoke, unwind, resume, unreachable.
 608         LLVM_DEBUG(dbgs() << "Can not handle terminator.");
 609         return false;  // Cannot handle this terminator.
 610       }
 611
 612       // We succeeded at evaluating this block!
 613       LLVM_DEBUG(dbgs() << "Successfully evaluated block.\n");
 614       return true;
 615     } else {
 616       // Did not know how to evaluate this!
 617       LLVM_DEBUG(
 618           dbgs() << "Failed to evaluate block due to unhandled instruction."
 619                     "\n");
 620       return false;
 621     }
 622
 623     if (!CurInst->use_empty()) {
 624       if (auto *FoldedInstResult = ConstantFoldConstant(InstResult, DL, TLI))
 625         InstResult = FoldedInstResult;
 626
 627       setVal(&*CurInst, InstResult);
 628     }
 629
 630     // If we just processed an invoke, we finished evaluating the block.
 631     if (InvokeInst *II = dyn_cast<InvokeInst>(CurInst)) {
 632       NextBB = II->getNormalDest();
 633       LLVM_DEBUG(dbgs() << "Found an invoke instruction. Finished Block.\n\n");
 634       return true;
 635     }
 636
 637     // Advance program counter.
 638     ++CurInst;
 639   }
 640 }
 641
 642 /// Evaluate a call to function F, returning true if successful, false if we
 643 /// can't evaluate it.  ActualArgs contains the formal arguments for the
 644 /// function.
 645 bool Evaluator::EvaluateFunction(Function *F, Constant *&RetVal,
 646                                  const SmallVectorImpl<Constant*> &ActualArgs) {
 647   // Check to see if this function is already executing (recursion).  If so,
 648   // bail out.  TODO: we might want to accept limited recursion.
 649   if (is_contained(CallStack, F))
 650     return false;
 651
 652   CallStack.push_back(F);
 653
 654   // Initialize arguments to the incoming values specified.
 655   unsigned ArgNo = 0;
 656   for (Function::arg_iterator AI = F->arg_begin(), E = F->arg_end(); AI != E;
 657        ++AI, ++ArgNo)
 658     setVal(&*AI, ActualArgs[ArgNo]);
 659
 660   // ExecutedBlocks - We only handle non-looping, non-recursive code.  As such,
 661   // we can only evaluate any one basic block at most once.  This set keeps
 662   // track of what we have executed so we can detect recursive cases etc.
 663   SmallPtrSet<BasicBlock*, 32> ExecutedBlocks;
 664
 665   // CurBB - The current basic block we're evaluating.
 666   BasicBlock *CurBB = &F->front();
 667
 668   BasicBlock::iterator CurInst = CurBB->begin();
 669
 670   while (true) {
 671     BasicBlock *NextBB = nullptr; // Initialized to avoid compiler warnings.
 672     LLVM_DEBUG(dbgs() << "Trying to evaluate BB: " << *CurBB << "\n");
 673
 674     if (!EvaluateBlock(CurInst, NextBB))
 675       return false;
 676
 677     if (!NextBB) {
 678       // Successfully running until there's no next block means that we found
 679       // the return.  Fill it the return value and pop the call stack.
 680       ReturnInst *RI = cast<ReturnInst>(CurBB->getTerminator());
 681       if (RI->getNumOperands())
 682         RetVal = getVal(RI->getOperand(0));
 683       CallStack.pop_back();
 684       return true;
 685     }
 686
 687     // Okay, we succeeded in evaluating this control flow.  See if we have
 688     // executed the new block before.  If so, we have a looping function,
 689     // which we cannot evaluate in reasonable time.
 690     if (!ExecutedBlocks.insert(NextBB).second)
 691       return false;  // looped!
 692
 693     // Okay, we have never been in this block before.  Check to see if there
 694     // are any PHI nodes.  If so, evaluate them with information about where
 695     // we came from.
 696     PHINode *PN = nullptr;
 697     for (CurInst = NextBB->begin();
 698          (PN = dyn_cast<PHINode>(CurInst)); ++CurInst)
 699       setVal(PN, getVal(PN->getIncomingValueForBlock(CurBB)));
 700
 701     // Advance to the next block.
 702     CurBB = NextBB;
 703   }
 704 }