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
isSimpleEnoughValueToCommitHelper(Constant * C,SmallPtrSetImpl<Constant * > & SimpleConstants,const DataLayout & DL)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
isSimpleEnoughValueToCommit(Constant * C,SmallPtrSetImpl<Constant * > & SimpleConstants,const DataLayout & DL)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
clear()123 void Evaluator::MutableValue::clear() {
124 if (auto *Agg = Val.dyn_cast<MutableAggregate *>())
125 delete Agg;
126 Val = nullptr;
127 }
128
read(Type * Ty,APInt Offset,const DataLayout & DL) const129 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 std::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
makeMutable()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
write(Constant * V,APInt Offset,const DataLayout & DL)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 std::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
toConstant() const200 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.
ComputeLoadResult(Constant * P,Type * Ty)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
ComputeLoadResult(GlobalVariable * GV,Type * Ty,const APInt & Offset)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
getFunction(Constant * C)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 *
getCalleeWithFormalArgs(CallBase & CB,SmallVectorImpl<Constant * > & Formals)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
getFormalParams(CallBase & CB,Function * F,SmallVectorImpl<Constant * > & Formals)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.
castCallResultIfNeeded(Type * ReturnType,Constant * RV)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.
EvaluateBlock(BasicBlock::iterator CurInst,BasicBlock * & NextBB,bool & StrippedPointerCastsForAliasAnalysis)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->isVolatile()) {
305 LLVM_DEBUG(dbgs() << "Store is volatile! Can not evaluate.\n");
306 return false; // no volatile 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->isVolatile()) {
341 LLVM_DEBUG(
342 dbgs() << "Found a Load! Volatile load, can not evaluate.\n");
343 return false; // no volatile 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.
EvaluateFunction(Function * F,Constant * & RetVal,const SmallVectorImpl<Constant * > & ActualArgs)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 for (const auto &[ArgNo, Arg] : llvm::enumerate(F->args()))
630 setVal(&Arg, ActualArgs[ArgNo]);
631
632 // ExecutedBlocks - We only handle non-looping, non-recursive code. As such,
633 // we can only evaluate any one basic block at most once. This set keeps
634 // track of what we have executed so we can detect recursive cases etc.
635 SmallPtrSet<BasicBlock*, 32> ExecutedBlocks;
636
637 // CurBB - The current basic block we're evaluating.
638 BasicBlock *CurBB = &F->front();
639
640 BasicBlock::iterator CurInst = CurBB->begin();
641
642 while (true) {
643 BasicBlock *NextBB = nullptr; // Initialized to avoid compiler warnings.
644 LLVM_DEBUG(dbgs() << "Trying to evaluate BB: " << *CurBB << "\n");
645
646 bool StrippedPointerCastsForAliasAnalysis = false;
647
648 if (!EvaluateBlock(CurInst, NextBB, StrippedPointerCastsForAliasAnalysis))
649 return false;
650
651 if (!NextBB) {
652 // Successfully running until there's no next block means that we found
653 // the return. Fill it the return value and pop the call stack.
654 ReturnInst *RI = cast<ReturnInst>(CurBB->getTerminator());
655 if (RI->getNumOperands()) {
656 // The Evaluator can look through pointer casts as long as alias
657 // analysis holds because it's just a simple interpreter and doesn't
658 // skip memory accesses due to invariant group metadata, but we can't
659 // let users of Evaluator use a value that's been gleaned looking
660 // through stripping pointer casts.
661 if (StrippedPointerCastsForAliasAnalysis &&
662 !RI->getReturnValue()->getType()->isVoidTy()) {
663 return false;
664 }
665 RetVal = getVal(RI->getOperand(0));
666 }
667 CallStack.pop_back();
668 return true;
669 }
670
671 // Okay, we succeeded in evaluating this control flow. See if we have
672 // executed the new block before. If so, we have a looping function,
673 // which we cannot evaluate in reasonable time.
674 if (!ExecutedBlocks.insert(NextBB).second)
675 return false; // looped!
676
677 // Okay, we have never been in this block before. Check to see if there
678 // are any PHI nodes. If so, evaluate them with information about where
679 // we came from.
680 PHINode *PN = nullptr;
681 for (CurInst = NextBB->begin();
682 (PN = dyn_cast<PHINode>(CurInst)); ++CurInst)
683 setVal(PN, getVal(PN->getIncomingValueForBlock(CurBB)));
684
685 // Advance to the next block.
686 CurBB = NextBB;
687 }
688 }
689