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