xref: /freebsd/contrib/llvm-project/llvm/lib/Transforms/Utils/Evaluator.cpp (revision ec0ea6efa1ad229d75c394c1a9b9cac33af2b1d3)
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 }
771