llvm/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/Constant.h"
  21 #include "llvm/IR/Constants.h"
  22 #include "llvm/IR/DataLayout.h"
  23 #include "llvm/IR/DerivedTypes.h"
  24 #include "llvm/IR/Function.h"
  25 #include "llvm/IR/GlobalAlias.h"
  26 #include "llvm/IR/GlobalValue.h"
  27 #include "llvm/IR/GlobalVariable.h"
  28 #include "llvm/IR/InstrTypes.h"
  29 #include "llvm/IR/Instruction.h"
  30 #include "llvm/IR/Instructions.h"
  31 #include "llvm/IR/IntrinsicInst.h"
  32 #include "llvm/IR/Intrinsics.h"
  33 #include "llvm/IR/Operator.h"
  34 #include "llvm/IR/Type.h"
  35 #include "llvm/IR/User.h"
  36 #include "llvm/IR/Value.h"
  37 #include "llvm/Support/Casting.h"
  38 #include "llvm/Support/Debug.h"
  39 #include "llvm/Support/raw_ostream.h"
  40 #include <iterator>
  41
  42 #define DEBUG_TYPE "evaluator"
  43
  44 using namespace llvm;
  45
  46 static inline bool
  47 isSimpleEnoughValueToCommit(Constant *C,
  48                             SmallPtrSetImpl<Constant *> &SimpleConstants,
  49                             const DataLayout &DL);
  50
  51 /// Return true if the specified constant can be handled by the code generator.
  52 /// We don't want to generate something like:
  53 ///   void *X = &X/42;
  54 /// because the code generator doesn't have a relocation that can handle that.
  55 ///
  56 /// This function should be called if C was not found (but just got inserted)
  57 /// in SimpleConstants to avoid having to rescan the same constants all the
  58 /// time.
  59 static bool
  60 isSimpleEnoughValueToCommitHelper(Constant *C,
  61                                   SmallPtrSetImpl<Constant *> &SimpleConstants,
  62                                   const DataLayout &DL) {
  63   // Simple global addresses are supported, do not allow dllimport or
  64   // thread-local globals.
  65   if (auto *GV = dyn_cast<GlobalValue>(C))
  66     return !GV->hasDLLImportStorageClass() && !GV->isThreadLocal();
  67
  68   // Simple integer, undef, constant aggregate zero, etc are all supported.
  69   if (C->getNumOperands() == 0 || isa<BlockAddress>(C))
  70     return true;
  71
  72   // Aggregate values are safe if all their elements are.
  73   if (isa<ConstantAggregate>(C)) {
  74     for (Value *Op : C->operands())
  75       if (!isSimpleEnoughValueToCommit(cast<Constant>(Op), SimpleConstants, DL))
  76         return false;
  77     return true;
  78   }
  79
  80   // We don't know exactly what relocations are allowed in constant expressions,
  81   // so we allow &global+constantoffset, which is safe and uniformly supported
  82   // across targets.
  83   ConstantExpr *CE = cast<ConstantExpr>(C);
  84   switch (CE->getOpcode()) {
  85   case Instruction::BitCast:
  86     // Bitcast is fine if the casted value is fine.
  87     return isSimpleEnoughValueToCommit(CE->getOperand(0), SimpleConstants, DL);
  88
  89   case Instruction::IntToPtr:
  90   case Instruction::PtrToInt:
  91     // int <=> ptr is fine if the int type is the same size as the
  92     // pointer type.
  93     if (DL.getTypeSizeInBits(CE->getType()) !=
  94         DL.getTypeSizeInBits(CE->getOperand(0)->getType()))
  95       return false;
  96     return isSimpleEnoughValueToCommit(CE->getOperand(0), SimpleConstants, DL);
  97
  98   // GEP is fine if it is simple + constant offset.
  99   case Instruction::GetElementPtr:
 100     for (unsigned i = 1, e = CE->getNumOperands(); i != e; ++i)
 101       if (!isa<ConstantInt>(CE->getOperand(i)))
 102         return false;
 103     return isSimpleEnoughValueToCommit(CE->getOperand(0), SimpleConstants, DL);
 104
 105   case Instruction::Add:
 106     // We allow simple+cst.
 107     if (!isa<ConstantInt>(CE->getOperand(1)))
 108       return false;
 109     return isSimpleEnoughValueToCommit(CE->getOperand(0), SimpleConstants, DL);
 110   }
 111   return false;
 112 }
 113
 114 static inline bool
 115 isSimpleEnoughValueToCommit(Constant *C,
 116                             SmallPtrSetImpl<Constant *> &SimpleConstants,
 117                             const DataLayout &DL) {
 118   // If we already checked this constant, we win.
 119   if (!SimpleConstants.insert(C).second)
 120     return true;
 121   // Check the constant.
 122   return isSimpleEnoughValueToCommitHelper(C, SimpleConstants, DL);
 123 }
 124
 125 /// Return true if this constant is simple enough for us to understand.  In
 126 /// particular, if it is a cast to anything other than from one pointer type to
 127 /// another pointer type, we punt.  We basically just support direct accesses to
 128 /// globals and GEP's of globals.  This should be kept up to date with
 129 /// CommitValueTo.
 130 static bool isSimpleEnoughPointerToCommit(Constant *C, const DataLayout &DL) {
 131   // Conservatively, avoid aggregate types. This is because we don't
 132   // want to worry about them partially overlapping other stores.
 133   if (!cast<PointerType>(C->getType())->getElementType()->isSingleValueType())
 134     return false;
 135
 136   if (GlobalVariable *GV = dyn_cast<GlobalVariable>(C))
 137     // Do not allow weak/*_odr/linkonce linkage or external globals.
 138     return GV->hasUniqueInitializer();
 139
 140   if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C)) {
 141     // Handle a constantexpr gep.
 142     if (CE->getOpcode() == Instruction::GetElementPtr &&
 143         isa<GlobalVariable>(CE->getOperand(0)) &&
 144         cast<GEPOperator>(CE)->isInBounds()) {
 145       GlobalVariable *GV = cast<GlobalVariable>(CE->getOperand(0));
 146       // Do not allow weak/*_odr/linkonce/dllimport/dllexport linkage or
 147       // external globals.
 148       if (!GV->hasUniqueInitializer())
 149         return false;
 150
 151       // The first index must be zero.
 152       ConstantInt *CI = dyn_cast<ConstantInt>(*std::next(CE->op_begin()));
 153       if (!CI || !CI->isZero()) return false;
 154
 155       // The remaining indices must be compile-time known integers within the
 156       // notional bounds of the corresponding static array types.
 157       if (!CE->isGEPWithNoNotionalOverIndexing())
 158         return false;
 159
 160       return ConstantFoldLoadThroughGEPConstantExpr(
 161           GV->getInitializer(), CE,
 162           cast<GEPOperator>(CE)->getResultElementType(), DL);
 163     } else if (CE->getOpcode() == Instruction::BitCast &&
 164                isa<GlobalVariable>(CE->getOperand(0))) {
 165       // A constantexpr bitcast from a pointer to another pointer is a no-op,
 166       // and we know how to evaluate it by moving the bitcast from the pointer
 167       // operand to the value operand.
 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 /// Apply \p TryLoad to Ptr. If this returns \p nullptr, introspect the
 178 /// pointer's type and walk down through the initial elements to obtain
 179 /// additional pointers to try. Returns the first non-null return value from
 180 /// \p TryLoad, or \p nullptr if the type can't be introspected further.
 181 static Constant *
 182 evaluateBitcastFromPtr(Constant *Ptr, const DataLayout &DL,
 183                        const TargetLibraryInfo *TLI,
 184                        std::function<Constant *(Constant *)> TryLoad) {
 185   Constant *Val;
 186   while (!(Val = TryLoad(Ptr))) {
 187     // If Ty is a non-opaque struct, we can convert the pointer to the struct
 188     // into a pointer to its first member.
 189     // FIXME: This could be extended to support arrays as well.
 190     Type *Ty = cast<PointerType>(Ptr->getType())->getElementType();
 191     if (!isa<StructType>(Ty) || cast<StructType>(Ty)->isOpaque())
 192       break;
 193
 194     IntegerType *IdxTy = IntegerType::get(Ty->getContext(), 32);
 195     Constant *IdxZero = ConstantInt::get(IdxTy, 0, false);
 196     Constant *const IdxList[] = {IdxZero, IdxZero};
 197
 198     Ptr = ConstantExpr::getGetElementPtr(Ty, Ptr, IdxList);
 199     Ptr = ConstantFoldConstant(Ptr, DL, TLI);
 200   }
 201   return Val;
 202 }
 203
 204 static Constant *getInitializer(Constant *C) {
 205   auto *GV = dyn_cast<GlobalVariable>(C);
 206   return GV && GV->hasDefinitiveInitializer() ? GV->getInitializer() : nullptr;
 207 }
 208
 209 /// Return the value that would be computed by a load from P after the stores
 210 /// reflected by 'memory' have been performed.  If we can't decide, return null.
 211 Constant *Evaluator::ComputeLoadResult(Constant *P, Type *Ty) {
 212   // If this memory location has been recently stored, use the stored value: it
 213   // is the most up-to-date.
 214   auto TryFindMemLoc = [this](Constant *Ptr) {
 215     return MutatedMemory.lookup(Ptr);
 216   };
 217
 218   if (Constant *Val = TryFindMemLoc(P))
 219     return Val;
 220
 221   // Access it.
 222   if (GlobalVariable *GV = dyn_cast<GlobalVariable>(P)) {
 223     if (GV->hasDefinitiveInitializer())
 224       return GV->getInitializer();
 225     return nullptr;
 226   }
 227
 228   if (ConstantExpr *CE = dyn_cast<ConstantExpr>(P)) {
 229     switch (CE->getOpcode()) {
 230     // Handle a constantexpr getelementptr.
 231     case Instruction::GetElementPtr:
 232       if (auto *I = getInitializer(CE->getOperand(0)))
 233         return ConstantFoldLoadThroughGEPConstantExpr(I, CE, Ty, DL);
 234       break;
 235     // Handle a constantexpr bitcast.
 236     case Instruction::BitCast:
 237       // We're evaluating a load through a pointer that was bitcast to a
 238       // different type. See if the "from" pointer has recently been stored.
 239       // If it hasn't, we may still be able to find a stored pointer by
 240       // introspecting the type.
 241       Constant *Val =
 242           evaluateBitcastFromPtr(CE->getOperand(0), DL, TLI, TryFindMemLoc);
 243       if (!Val)
 244         Val = getInitializer(CE->getOperand(0));
 245       if (Val)
 246         return ConstantFoldLoadThroughBitcast(
 247             Val, P->getType()->getPointerElementType(), DL);
 248       break;
 249     }
 250   }
 251
 252   return nullptr;  // don't know how to evaluate.
 253 }
 254
 255 static Function *getFunction(Constant *C) {
 256   if (auto *Fn = dyn_cast<Function>(C))
 257     return Fn;
 258
 259   if (auto *Alias = dyn_cast<GlobalAlias>(C))
 260     if (auto *Fn = dyn_cast<Function>(Alias->getAliasee()))
 261       return Fn;
 262   return nullptr;
 263 }
 264
 265 Function *
 266 Evaluator::getCalleeWithFormalArgs(CallBase &CB,
 267                                    SmallVectorImpl<Constant *> &Formals) {
 268   auto *V = CB.getCalledOperand();
 269   if (auto *Fn = getFunction(getVal(V)))
 270     return getFormalParams(CB, Fn, Formals) ? Fn : nullptr;
 271
 272   auto *CE = dyn_cast<ConstantExpr>(V);
 273   if (!CE || CE->getOpcode() != Instruction::BitCast ||
 274       !getFormalParams(CB, getFunction(CE->getOperand(0)), Formals))
 275     return nullptr;
 276
 277   return dyn_cast<Function>(
 278       ConstantFoldLoadThroughBitcast(CE, CE->getOperand(0)->getType(), DL));
 279 }
 280
 281 bool Evaluator::getFormalParams(CallBase &CB, Function *F,
 282                                 SmallVectorImpl<Constant *> &Formals) {
 283   if (!F)
 284     return false;
 285
 286   auto *FTy = F->getFunctionType();
 287   if (FTy->getNumParams() > CB.getNumArgOperands()) {
 288     LLVM_DEBUG(dbgs() << "Too few arguments for function.\n");
 289     return false;
 290   }
 291
 292   auto ArgI = CB.arg_begin();
 293   for (auto ParI = FTy->param_begin(), ParE = FTy->param_end(); ParI != ParE;
 294        ++ParI) {
 295     auto *ArgC = ConstantFoldLoadThroughBitcast(getVal(*ArgI), *ParI, DL);
 296     if (!ArgC) {
 297       LLVM_DEBUG(dbgs() << "Can not convert function argument.\n");
 298       return false;
 299     }
 300     Formals.push_back(ArgC);
 301     ++ArgI;
 302   }
 303   return true;
 304 }
 305
 306 /// If call expression contains bitcast then we may need to cast
 307 /// evaluated return value to a type of the call expression.
 308 Constant *Evaluator::castCallResultIfNeeded(Value *CallExpr, Constant *RV) {
 309   ConstantExpr *CE = dyn_cast<ConstantExpr>(CallExpr);
 310   if (!RV || !CE || CE->getOpcode() != Instruction::BitCast)
 311     return RV;
 312
 313   if (auto *FT =
 314           dyn_cast<FunctionType>(CE->getType()->getPointerElementType())) {
 315     RV = ConstantFoldLoadThroughBitcast(RV, FT->getReturnType(), DL);
 316     if (!RV)
 317       LLVM_DEBUG(dbgs() << "Failed to fold bitcast call expr\n");
 318   }
 319   return RV;
 320 }
 321
 322 /// Evaluate all instructions in block BB, returning true if successful, false
 323 /// if we can't evaluate it.  NewBB returns the next BB that control flows into,
 324 /// or null upon return. StrippedPointerCastsForAliasAnalysis is set to true if
 325 /// we looked through pointer casts to evaluate something.
 326 bool Evaluator::EvaluateBlock(BasicBlock::iterator CurInst, BasicBlock *&NextBB,
 327                               bool &StrippedPointerCastsForAliasAnalysis) {
 328   // This is the main evaluation loop.
 329   while (true) {
 330     Constant *InstResult = nullptr;
 331
 332     LLVM_DEBUG(dbgs() << "Evaluating Instruction: " << *CurInst << "\n");
 333
 334     if (StoreInst *SI = dyn_cast<StoreInst>(CurInst)) {
 335       if (!SI->isSimple()) {
 336         LLVM_DEBUG(dbgs() << "Store is not simple! Can not evaluate.\n");
 337         return false;  // no volatile/atomic accesses.
 338       }
 339       Constant *Ptr = getVal(SI->getOperand(1));
 340       Constant *FoldedPtr = ConstantFoldConstant(Ptr, DL, TLI);
 341       if (Ptr != FoldedPtr) {
 342         LLVM_DEBUG(dbgs() << "Folding constant ptr expression: " << *Ptr);
 343         Ptr = FoldedPtr;
 344         LLVM_DEBUG(dbgs() << "; To: " << *Ptr << "\n");
 345       }
 346       if (!isSimpleEnoughPointerToCommit(Ptr, DL)) {
 347         // If this is too complex for us to commit, reject it.
 348         LLVM_DEBUG(
 349             dbgs() << "Pointer is too complex for us to evaluate store.");
 350         return false;
 351       }
 352
 353       Constant *Val = getVal(SI->getOperand(0));
 354
 355       // If this might be too difficult for the backend to handle (e.g. the addr
 356       // of one global variable divided by another) then we can't commit it.
 357       if (!isSimpleEnoughValueToCommit(Val, SimpleConstants, DL)) {
 358         LLVM_DEBUG(dbgs() << "Store value is too complex to evaluate store. "
 359                           << *Val << "\n");
 360         return false;
 361       }
 362
 363       if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ptr)) {
 364         if (CE->getOpcode() == Instruction::BitCast) {
 365           LLVM_DEBUG(dbgs()
 366                      << "Attempting to resolve bitcast on constant ptr.\n");
 367           // If we're evaluating a store through a bitcast, then we need
 368           // to pull the bitcast off the pointer type and push it onto the
 369           // stored value. In order to push the bitcast onto the stored value,
 370           // a bitcast from the pointer's element type to Val's type must be
 371           // legal. If it's not, we can try introspecting the type to find a
 372           // legal conversion.
 373
 374           auto TryCastValTy = [&](Constant *P) -> Constant * {
 375             // The conversion is illegal if the store is wider than the
 376             // pointee proposed by `evaluateBitcastFromPtr`, since that would
 377             // drop stores to other struct elements when the caller attempts to
 378             // look through a struct's 0th element.
 379             Type *NewTy = cast<PointerType>(P->getType())->getElementType();
 380             Type *STy = Val->getType();
 381             if (DL.getTypeSizeInBits(NewTy) < DL.getTypeSizeInBits(STy))
 382               return nullptr;
 383
 384             if (Constant *FV = ConstantFoldLoadThroughBitcast(Val, NewTy, DL)) {
 385               Ptr = P;
 386               return FV;
 387             }
 388             return nullptr;
 389           };
 390
 391           Constant *NewVal =
 392               evaluateBitcastFromPtr(CE->getOperand(0), DL, TLI, TryCastValTy);
 393           if (!NewVal) {
 394             LLVM_DEBUG(dbgs() << "Failed to bitcast constant ptr, can not "
 395                                  "evaluate.\n");
 396             return false;
 397           }
 398
 399           Val = NewVal;
 400           LLVM_DEBUG(dbgs() << "Evaluated bitcast: " << *Val << "\n");
 401         }
 402       }
 403
 404       MutatedMemory[Ptr] = Val;
 405     } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(CurInst)) {
 406       InstResult = ConstantExpr::get(BO->getOpcode(),
 407                                      getVal(BO->getOperand(0)),
 408                                      getVal(BO->getOperand(1)));
 409       LLVM_DEBUG(dbgs() << "Found a BinaryOperator! Simplifying: "
 410                         << *InstResult << "\n");
 411     } else if (CmpInst *CI = dyn_cast<CmpInst>(CurInst)) {
 412       InstResult = ConstantExpr::getCompare(CI->getPredicate(),
 413                                             getVal(CI->getOperand(0)),
 414                                             getVal(CI->getOperand(1)));
 415       LLVM_DEBUG(dbgs() << "Found a CmpInst! Simplifying: " << *InstResult
 416                         << "\n");
 417     } else if (CastInst *CI = dyn_cast<CastInst>(CurInst)) {
 418       InstResult = ConstantExpr::getCast(CI->getOpcode(),
 419                                          getVal(CI->getOperand(0)),
 420                                          CI->getType());
 421       LLVM_DEBUG(dbgs() << "Found a Cast! Simplifying: " << *InstResult
 422                         << "\n");
 423     } else if (SelectInst *SI = dyn_cast<SelectInst>(CurInst)) {
 424       InstResult = ConstantExpr::getSelect(getVal(SI->getOperand(0)),
 425                                            getVal(SI->getOperand(1)),
 426                                            getVal(SI->getOperand(2)));
 427       LLVM_DEBUG(dbgs() << "Found a Select! Simplifying: " << *InstResult
 428                         << "\n");
 429     } else if (auto *EVI = dyn_cast<ExtractValueInst>(CurInst)) {
 430       InstResult = ConstantExpr::getExtractValue(
 431           getVal(EVI->getAggregateOperand()), EVI->getIndices());
 432       LLVM_DEBUG(dbgs() << "Found an ExtractValueInst! Simplifying: "
 433                         << *InstResult << "\n");
 434     } else if (auto *IVI = dyn_cast<InsertValueInst>(CurInst)) {
 435       InstResult = ConstantExpr::getInsertValue(
 436           getVal(IVI->getAggregateOperand()),
 437           getVal(IVI->getInsertedValueOperand()), IVI->getIndices());
 438       LLVM_DEBUG(dbgs() << "Found an InsertValueInst! Simplifying: "
 439                         << *InstResult << "\n");
 440     } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(CurInst)) {
 441       Constant *P = getVal(GEP->getOperand(0));
 442       SmallVector<Constant*, 8> GEPOps;
 443       for (Use &Op : llvm::drop_begin(GEP->operands()))
 444         GEPOps.push_back(getVal(Op));
 445       InstResult =
 446           ConstantExpr::getGetElementPtr(GEP->getSourceElementType(), P, GEPOps,
 447                                          cast<GEPOperator>(GEP)->isInBounds());
 448       LLVM_DEBUG(dbgs() << "Found a GEP! Simplifying: " << *InstResult << "\n");
 449     } else if (LoadInst *LI = dyn_cast<LoadInst>(CurInst)) {
 450       if (!LI->isSimple()) {
 451         LLVM_DEBUG(
 452             dbgs() << "Found a Load! Not a simple load, can not evaluate.\n");
 453         return false;  // no volatile/atomic accesses.
 454       }
 455
 456       Constant *Ptr = getVal(LI->getOperand(0));
 457       Constant *FoldedPtr = ConstantFoldConstant(Ptr, DL, TLI);
 458       if (Ptr != FoldedPtr) {
 459         Ptr = FoldedPtr;
 460         LLVM_DEBUG(dbgs() << "Found a constant pointer expression, constant "
 461                              "folding: "
 462                           << *Ptr << "\n");
 463       }
 464       InstResult = ComputeLoadResult(Ptr, LI->getType());
 465       if (!InstResult) {
 466         LLVM_DEBUG(
 467             dbgs() << "Failed to compute load result. Can not evaluate load."
 468                       "\n");
 469         return false; // Could not evaluate load.
 470       }
 471
 472       LLVM_DEBUG(dbgs() << "Evaluated load: " << *InstResult << "\n");
 473     } else if (AllocaInst *AI = dyn_cast<AllocaInst>(CurInst)) {
 474       if (AI->isArrayAllocation()) {
 475         LLVM_DEBUG(dbgs() << "Found an array alloca. Can not evaluate.\n");
 476         return false;  // Cannot handle array allocs.
 477       }
 478       Type *Ty = AI->getAllocatedType();
 479       AllocaTmps.push_back(std::make_unique<GlobalVariable>(
 480           Ty, false, GlobalValue::InternalLinkage, UndefValue::get(Ty),
 481           AI->getName(), /*TLMode=*/GlobalValue::NotThreadLocal,
 482           AI->getType()->getPointerAddressSpace()));
 483       InstResult = AllocaTmps.back().get();
 484       LLVM_DEBUG(dbgs() << "Found an alloca. Result: " << *InstResult << "\n");
 485     } else if (isa<CallInst>(CurInst) || isa<InvokeInst>(CurInst)) {
 486       CallBase &CB = *cast<CallBase>(&*CurInst);
 487
 488       // Debug info can safely be ignored here.
 489       if (isa<DbgInfoIntrinsic>(CB)) {
 490         LLVM_DEBUG(dbgs() << "Ignoring debug info.\n");
 491         ++CurInst;
 492         continue;
 493       }
 494
 495       // Cannot handle inline asm.
 496       if (CB.isInlineAsm()) {
 497         LLVM_DEBUG(dbgs() << "Found inline asm, can not evaluate.\n");
 498         return false;
 499       }
 500
 501       if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(&CB)) {
 502         if (MemSetInst *MSI = dyn_cast<MemSetInst>(II)) {
 503           if (MSI->isVolatile()) {
 504             LLVM_DEBUG(dbgs() << "Can not optimize a volatile memset "
 505                               << "intrinsic.\n");
 506             return false;
 507           }
 508           Constant *Ptr = getVal(MSI->getDest());
 509           Constant *Val = getVal(MSI->getValue());
 510           Constant *DestVal =
 511               ComputeLoadResult(getVal(Ptr), MSI->getValue()->getType());
 512           if (Val->isNullValue() && DestVal && DestVal->isNullValue()) {
 513             // This memset is a no-op.
 514             LLVM_DEBUG(dbgs() << "Ignoring no-op memset.\n");
 515             ++CurInst;
 516             continue;
 517           }
 518         }
 519
 520         if (II->isLifetimeStartOrEnd()) {
 521           LLVM_DEBUG(dbgs() << "Ignoring lifetime intrinsic.\n");
 522           ++CurInst;
 523           continue;
 524         }
 525
 526         if (II->getIntrinsicID() == Intrinsic::invariant_start) {
 527           // We don't insert an entry into Values, as it doesn't have a
 528           // meaningful return value.
 529           if (!II->use_empty()) {
 530             LLVM_DEBUG(dbgs()
 531                        << "Found unused invariant_start. Can't evaluate.\n");
 532             return false;
 533           }
 534           ConstantInt *Size = cast<ConstantInt>(II->getArgOperand(0));
 535           Value *PtrArg = getVal(II->getArgOperand(1));
 536           Value *Ptr = PtrArg->stripPointerCasts();
 537           if (GlobalVariable *GV = dyn_cast<GlobalVariable>(Ptr)) {
 538             Type *ElemTy = GV->getValueType();
 539             if (!Size->isMinusOne() &&
 540                 Size->getValue().getLimitedValue() >=
 541                     DL.getTypeStoreSize(ElemTy)) {
 542               Invariants.insert(GV);
 543               LLVM_DEBUG(dbgs() << "Found a global var that is an invariant: "
 544                                 << *GV << "\n");
 545             } else {
 546               LLVM_DEBUG(dbgs()
 547                          << "Found a global var, but can not treat it as an "
 548                             "invariant.\n");
 549             }
 550           }
 551           // Continue even if we do nothing.
 552           ++CurInst;
 553           continue;
 554         } else if (II->getIntrinsicID() == Intrinsic::assume) {
 555           LLVM_DEBUG(dbgs() << "Skipping assume intrinsic.\n");
 556           ++CurInst;
 557           continue;
 558         } else if (II->getIntrinsicID() == Intrinsic::sideeffect) {
 559           LLVM_DEBUG(dbgs() << "Skipping sideeffect intrinsic.\n");
 560           ++CurInst;
 561           continue;
 562         } else if (II->getIntrinsicID() == Intrinsic::pseudoprobe) {
 563           LLVM_DEBUG(dbgs() << "Skipping pseudoprobe intrinsic.\n");
 564           ++CurInst;
 565           continue;
 566         } else {
 567           Value *Stripped = CurInst->stripPointerCastsForAliasAnalysis();
 568           // Only attempt to getVal() if we've actually managed to strip
 569           // anything away, or else we'll call getVal() on the current
 570           // instruction.
 571           if (Stripped != &*CurInst) {
 572             InstResult = getVal(Stripped);
 573           }
 574           if (InstResult) {
 575             LLVM_DEBUG(dbgs()
 576                        << "Stripped pointer casts for alias analysis for "
 577                           "intrinsic call.\n");
 578             StrippedPointerCastsForAliasAnalysis = true;
 579             InstResult = ConstantExpr::getBitCast(InstResult, II->getType());
 580           } else {
 581             LLVM_DEBUG(dbgs() << "Unknown intrinsic. Cannot evaluate.\n");
 582             return false;
 583           }
 584         }
 585       }
 586
 587       if (!InstResult) {
 588         // Resolve function pointers.
 589         SmallVector<Constant *, 8> Formals;
 590         Function *Callee = getCalleeWithFormalArgs(CB, Formals);
 591         if (!Callee || Callee->isInterposable()) {
 592           LLVM_DEBUG(dbgs() << "Can not resolve function pointer.\n");
 593           return false; // Cannot resolve.
 594         }
 595
 596         if (Callee->isDeclaration()) {
 597           // If this is a function we can constant fold, do it.
 598           if (Constant *C = ConstantFoldCall(&CB, Callee, Formals, TLI)) {
 599             InstResult = castCallResultIfNeeded(CB.getCalledOperand(), C);
 600             if (!InstResult)
 601               return false;
 602             LLVM_DEBUG(dbgs() << "Constant folded function call. Result: "
 603                               << *InstResult << "\n");
 604           } else {
 605             LLVM_DEBUG(dbgs() << "Can not constant fold function call.\n");
 606             return false;
 607           }
 608         } else {
 609           if (Callee->getFunctionType()->isVarArg()) {
 610             LLVM_DEBUG(dbgs()
 611                        << "Can not constant fold vararg function call.\n");
 612             return false;
 613           }
 614
 615           Constant *RetVal = nullptr;
 616           // Execute the call, if successful, use the return value.
 617           ValueStack.emplace_back();
 618           if (!EvaluateFunction(Callee, RetVal, Formals)) {
 619             LLVM_DEBUG(dbgs() << "Failed to evaluate function.\n");
 620             return false;
 621           }
 622           ValueStack.pop_back();
 623           InstResult = castCallResultIfNeeded(CB.getCalledOperand(), RetVal);
 624           if (RetVal && !InstResult)
 625             return false;
 626
 627           if (InstResult) {
 628             LLVM_DEBUG(dbgs() << "Successfully evaluated function. Result: "
 629                               << *InstResult << "\n\n");
 630           } else {
 631             LLVM_DEBUG(dbgs()
 632                        << "Successfully evaluated function. Result: 0\n\n");
 633           }
 634         }
 635       }
 636     } else if (CurInst->isTerminator()) {
 637       LLVM_DEBUG(dbgs() << "Found a terminator instruction.\n");
 638
 639       if (BranchInst *BI = dyn_cast<BranchInst>(CurInst)) {
 640         if (BI->isUnconditional()) {
 641           NextBB = BI->getSuccessor(0);
 642         } else {
 643           ConstantInt *Cond =
 644             dyn_cast<ConstantInt>(getVal(BI->getCondition()));
 645           if (!Cond) return false;  // Cannot determine.
 646
 647           NextBB = BI->getSuccessor(!Cond->getZExtValue());
 648         }
 649       } else if (SwitchInst *SI = dyn_cast<SwitchInst>(CurInst)) {
 650         ConstantInt *Val =
 651           dyn_cast<ConstantInt>(getVal(SI->getCondition()));
 652         if (!Val) return false;  // Cannot determine.
 653         NextBB = SI->findCaseValue(Val)->getCaseSuccessor();
 654       } else if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(CurInst)) {
 655         Value *Val = getVal(IBI->getAddress())->stripPointerCasts();
 656         if (BlockAddress *BA = dyn_cast<BlockAddress>(Val))
 657           NextBB = BA->getBasicBlock();
 658         else
 659           return false;  // Cannot determine.
 660       } else if (isa<ReturnInst>(CurInst)) {
 661         NextBB = nullptr;
 662       } else {
 663         // invoke, unwind, resume, unreachable.
 664         LLVM_DEBUG(dbgs() << "Can not handle terminator.");
 665         return false;  // Cannot handle this terminator.
 666       }
 667
 668       // We succeeded at evaluating this block!
 669       LLVM_DEBUG(dbgs() << "Successfully evaluated block.\n");
 670       return true;
 671     } else {
 672       // Did not know how to evaluate this!
 673       LLVM_DEBUG(
 674           dbgs() << "Failed to evaluate block due to unhandled instruction."
 675                     "\n");
 676       return false;
 677     }
 678
 679     if (!CurInst->use_empty()) {
 680       InstResult = ConstantFoldConstant(InstResult, DL, TLI);
 681       setVal(&*CurInst, InstResult);
 682     }
 683
 684     // If we just processed an invoke, we finished evaluating the block.
 685     if (InvokeInst *II = dyn_cast<InvokeInst>(CurInst)) {
 686       NextBB = II->getNormalDest();
 687       LLVM_DEBUG(dbgs() << "Found an invoke instruction. Finished Block.\n\n");
 688       return true;
 689     }
 690
 691     // Advance program counter.
 692     ++CurInst;
 693   }
 694 }
 695
 696 /// Evaluate a call to function F, returning true if successful, false if we
 697 /// can't evaluate it.  ActualArgs contains the formal arguments for the
 698 /// function.
 699 bool Evaluator::EvaluateFunction(Function *F, Constant *&RetVal,
 700                                  const SmallVectorImpl<Constant*> &ActualArgs) {
 701   // Check to see if this function is already executing (recursion).  If so,
 702   // bail out.  TODO: we might want to accept limited recursion.
 703   if (is_contained(CallStack, F))
 704     return false;
 705
 706   CallStack.push_back(F);
 707
 708   // Initialize arguments to the incoming values specified.
 709   unsigned ArgNo = 0;
 710   for (Function::arg_iterator AI = F->arg_begin(), E = F->arg_end(); AI != E;
 711        ++AI, ++ArgNo)
 712     setVal(&*AI, ActualArgs[ArgNo]);
 713
 714   // ExecutedBlocks - We only handle non-looping, non-recursive code.  As such,
 715   // we can only evaluate any one basic block at most once.  This set keeps
 716   // track of what we have executed so we can detect recursive cases etc.
 717   SmallPtrSet<BasicBlock*, 32> ExecutedBlocks;
 718
 719   // CurBB - The current basic block we're evaluating.
 720   BasicBlock *CurBB = &F->front();
 721
 722   BasicBlock::iterator CurInst = CurBB->begin();
 723
 724   while (true) {
 725     BasicBlock *NextBB = nullptr; // Initialized to avoid compiler warnings.
 726     LLVM_DEBUG(dbgs() << "Trying to evaluate BB: " << *CurBB << "\n");
 727
 728     bool StrippedPointerCastsForAliasAnalysis = false;
 729
 730     if (!EvaluateBlock(CurInst, NextBB, StrippedPointerCastsForAliasAnalysis))
 731       return false;
 732
 733     if (!NextBB) {
 734       // Successfully running until there's no next block means that we found
 735       // the return.  Fill it the return value and pop the call stack.
 736       ReturnInst *RI = cast<ReturnInst>(CurBB->getTerminator());
 737       if (RI->getNumOperands()) {
 738         // The Evaluator can look through pointer casts as long as alias
 739         // analysis holds because it's just a simple interpreter and doesn't
 740         // skip memory accesses due to invariant group metadata, but we can't
 741         // let users of Evaluator use a value that's been gleaned looking
 742         // through stripping pointer casts.
 743         if (StrippedPointerCastsForAliasAnalysis &&
 744             !RI->getReturnValue()->getType()->isVoidTy()) {
 745           return false;
 746         }
 747         RetVal = getVal(RI->getOperand(0));
 748       }
 749       CallStack.pop_back();
 750       return true;
 751     }
 752
 753     // Okay, we succeeded in evaluating this control flow.  See if we have
 754     // executed the new block before.  If so, we have a looping function,
 755     // which we cannot evaluate in reasonable time.
 756     if (!ExecutedBlocks.insert(NextBB).second)
 757       return false;  // looped!
 758
 759     // Okay, we have never been in this block before.  Check to see if there
 760     // are any PHI nodes.  If so, evaluate them with information about where
 761     // we came from.
 762     PHINode *PN = nullptr;
 763     for (CurInst = NextBB->begin();
 764          (PN = dyn_cast<PHINode>(CurInst)); ++CurInst)
 765       setVal(PN, getVal(PN->getIncomingValueForBlock(CurBB)));
 766
 767     // Advance to the next block.
 768     CurBB = NextBB;
 769   }
 770 }