1 //===-- Lint.cpp - Check for common errors in LLVM IR ---------------------===//
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 // This pass statically checks for common and easily-identified constructs
10 // which produce undefined or likely unintended behavior in LLVM IR.
11 //
12 // It is not a guarantee of correctness, in two ways. First, it isn't
13 // comprehensive. There are checks which could be done statically which are
14 // not yet implemented. Some of these are indicated by TODO comments, but
15 // those aren't comprehensive either. Second, many conditions cannot be
16 // checked statically. This pass does no dynamic instrumentation, so it
17 // can't check for all possible problems.
18 //
19 // Another limitation is that it assumes all code will be executed. A store
20 // through a null pointer in a basic block which is never reached is harmless,
21 // but this pass will warn about it anyway. This is the main reason why most
22 // of these checks live here instead of in the Verifier pass.
23 //
24 // Optimization passes may make conditions that this pass checks for more or
25 // less obvious. If an optimization pass appears to be introducing a warning,
26 // it may be that the optimization pass is merely exposing an existing
27 // condition in the code.
28 //
29 // This code may be run before instcombine. In many cases, instcombine checks
30 // for the same kinds of things and turns instructions with undefined behavior
31 // into unreachable (or equivalent). Because of this, this pass makes some
32 // effort to look through bitcasts and so on.
33 //
34 //===----------------------------------------------------------------------===//
35
36 #include "llvm/Analysis/Lint.h"
37 #include "llvm/ADT/APInt.h"
38 #include "llvm/ADT/ArrayRef.h"
39 #include "llvm/ADT/SmallPtrSet.h"
40 #include "llvm/ADT/Twine.h"
41 #include "llvm/Analysis/AliasAnalysis.h"
42 #include "llvm/Analysis/AssumptionCache.h"
43 #include "llvm/Analysis/BasicAliasAnalysis.h"
44 #include "llvm/Analysis/ConstantFolding.h"
45 #include "llvm/Analysis/InstructionSimplify.h"
46 #include "llvm/Analysis/Loads.h"
47 #include "llvm/Analysis/MemoryLocation.h"
48 #include "llvm/Analysis/ScopedNoAliasAA.h"
49 #include "llvm/Analysis/TargetLibraryInfo.h"
50 #include "llvm/Analysis/TypeBasedAliasAnalysis.h"
51 #include "llvm/Analysis/ValueTracking.h"
52 #include "llvm/IR/Argument.h"
53 #include "llvm/IR/BasicBlock.h"
54 #include "llvm/IR/Constant.h"
55 #include "llvm/IR/Constants.h"
56 #include "llvm/IR/DataLayout.h"
57 #include "llvm/IR/DerivedTypes.h"
58 #include "llvm/IR/Dominators.h"
59 #include "llvm/IR/Function.h"
60 #include "llvm/IR/GlobalVariable.h"
61 #include "llvm/IR/InstVisitor.h"
62 #include "llvm/IR/InstrTypes.h"
63 #include "llvm/IR/Instruction.h"
64 #include "llvm/IR/Instructions.h"
65 #include "llvm/IR/IntrinsicInst.h"
66 #include "llvm/IR/Module.h"
67 #include "llvm/IR/PassManager.h"
68 #include "llvm/IR/Type.h"
69 #include "llvm/IR/Value.h"
70 #include "llvm/Support/Casting.h"
71 #include "llvm/Support/KnownBits.h"
72 #include "llvm/Support/raw_ostream.h"
73 #include <cassert>
74 #include <cstdint>
75 #include <iterator>
76 #include <string>
77
78 using namespace llvm;
79
80 namespace {
81 namespace MemRef {
82 static const unsigned Read = 1;
83 static const unsigned Write = 2;
84 static const unsigned Callee = 4;
85 static const unsigned Branchee = 8;
86 } // end namespace MemRef
87
88 class Lint : public InstVisitor<Lint> {
89 friend class InstVisitor<Lint>;
90
91 void visitFunction(Function &F);
92
93 void visitCallBase(CallBase &CB);
94 void visitMemoryReference(Instruction &I, const MemoryLocation &Loc,
95 MaybeAlign Alignment, Type *Ty, unsigned Flags);
96
97 void visitReturnInst(ReturnInst &I);
98 void visitLoadInst(LoadInst &I);
99 void visitStoreInst(StoreInst &I);
100 void visitXor(BinaryOperator &I);
101 void visitSub(BinaryOperator &I);
102 void visitLShr(BinaryOperator &I);
103 void visitAShr(BinaryOperator &I);
104 void visitShl(BinaryOperator &I);
105 void visitSDiv(BinaryOperator &I);
106 void visitUDiv(BinaryOperator &I);
107 void visitSRem(BinaryOperator &I);
108 void visitURem(BinaryOperator &I);
109 void visitAllocaInst(AllocaInst &I);
110 void visitVAArgInst(VAArgInst &I);
111 void visitIndirectBrInst(IndirectBrInst &I);
112 void visitExtractElementInst(ExtractElementInst &I);
113 void visitInsertElementInst(InsertElementInst &I);
114 void visitUnreachableInst(UnreachableInst &I);
115
116 Value *findValue(Value *V, bool OffsetOk) const;
117 Value *findValueImpl(Value *V, bool OffsetOk,
118 SmallPtrSetImpl<Value *> &Visited) const;
119
120 public:
121 Module *Mod;
122 const DataLayout *DL;
123 AliasAnalysis *AA;
124 AssumptionCache *AC;
125 DominatorTree *DT;
126 TargetLibraryInfo *TLI;
127
128 std::string Messages;
129 raw_string_ostream MessagesStr;
130
Lint(Module * Mod,const DataLayout * DL,AliasAnalysis * AA,AssumptionCache * AC,DominatorTree * DT,TargetLibraryInfo * TLI)131 Lint(Module *Mod, const DataLayout *DL, AliasAnalysis *AA,
132 AssumptionCache *AC, DominatorTree *DT, TargetLibraryInfo *TLI)
133 : Mod(Mod), DL(DL), AA(AA), AC(AC), DT(DT), TLI(TLI),
134 MessagesStr(Messages) {}
135
WriteValues(ArrayRef<const Value * > Vs)136 void WriteValues(ArrayRef<const Value *> Vs) {
137 for (const Value *V : Vs) {
138 if (!V)
139 continue;
140 if (isa<Instruction>(V)) {
141 MessagesStr << *V << '\n';
142 } else {
143 V->printAsOperand(MessagesStr, true, Mod);
144 MessagesStr << '\n';
145 }
146 }
147 }
148
149 /// A check failed, so printout out the condition and the message.
150 ///
151 /// This provides a nice place to put a breakpoint if you want to see why
152 /// something is not correct.
CheckFailed(const Twine & Message)153 void CheckFailed(const Twine &Message) { MessagesStr << Message << '\n'; }
154
155 /// A check failed (with values to print).
156 ///
157 /// This calls the Message-only version so that the above is easier to set
158 /// a breakpoint on.
159 template <typename T1, typename... Ts>
CheckFailed(const Twine & Message,const T1 & V1,const Ts &...Vs)160 void CheckFailed(const Twine &Message, const T1 &V1, const Ts &... Vs) {
161 CheckFailed(Message);
162 WriteValues({V1, Vs...});
163 }
164 };
165 } // end anonymous namespace
166
167 // Check - We know that cond should be true, if not print an error message.
168 #define Check(C, ...) \
169 do { \
170 if (!(C)) { \
171 CheckFailed(__VA_ARGS__); \
172 return; \
173 } \
174 } while (false)
175
visitFunction(Function & F)176 void Lint::visitFunction(Function &F) {
177 // This isn't undefined behavior, it's just a little unusual, and it's a
178 // fairly common mistake to neglect to name a function.
179 Check(F.hasName() || F.hasLocalLinkage(),
180 "Unusual: Unnamed function with non-local linkage", &F);
181
182 // TODO: Check for irreducible control flow.
183 }
184
visitCallBase(CallBase & I)185 void Lint::visitCallBase(CallBase &I) {
186 Value *Callee = I.getCalledOperand();
187
188 visitMemoryReference(I, MemoryLocation::getAfter(Callee), std::nullopt,
189 nullptr, MemRef::Callee);
190
191 if (Function *F = dyn_cast<Function>(findValue(Callee,
192 /*OffsetOk=*/false))) {
193 Check(I.getCallingConv() == F->getCallingConv(),
194 "Undefined behavior: Caller and callee calling convention differ",
195 &I);
196
197 FunctionType *FT = F->getFunctionType();
198 unsigned NumActualArgs = I.arg_size();
199
200 Check(FT->isVarArg() ? FT->getNumParams() <= NumActualArgs
201 : FT->getNumParams() == NumActualArgs,
202 "Undefined behavior: Call argument count mismatches callee "
203 "argument count",
204 &I);
205
206 Check(FT->getReturnType() == I.getType(),
207 "Undefined behavior: Call return type mismatches "
208 "callee return type",
209 &I);
210
211 // Check argument types (in case the callee was casted) and attributes.
212 // TODO: Verify that caller and callee attributes are compatible.
213 Function::arg_iterator PI = F->arg_begin(), PE = F->arg_end();
214 auto AI = I.arg_begin(), AE = I.arg_end();
215 for (; AI != AE; ++AI) {
216 Value *Actual = *AI;
217 if (PI != PE) {
218 Argument *Formal = &*PI++;
219 Check(Formal->getType() == Actual->getType(),
220 "Undefined behavior: Call argument type mismatches "
221 "callee parameter type",
222 &I);
223
224 // Check that noalias arguments don't alias other arguments. This is
225 // not fully precise because we don't know the sizes of the dereferenced
226 // memory regions.
227 if (Formal->hasNoAliasAttr() && Actual->getType()->isPointerTy()) {
228 AttributeList PAL = I.getAttributes();
229 unsigned ArgNo = 0;
230 for (auto *BI = I.arg_begin(); BI != AE; ++BI, ++ArgNo) {
231 // Skip ByVal arguments since they will be memcpy'd to the callee's
232 // stack so we're not really passing the pointer anyway.
233 if (PAL.hasParamAttr(ArgNo, Attribute::ByVal))
234 continue;
235 // If both arguments are readonly, they have no dependence.
236 if (Formal->onlyReadsMemory() && I.onlyReadsMemory(ArgNo))
237 continue;
238 // Skip readnone arguments since those are guaranteed not to be
239 // dereferenced anyway.
240 if (I.doesNotAccessMemory(ArgNo))
241 continue;
242 if (AI != BI && (*BI)->getType()->isPointerTy()) {
243 AliasResult Result = AA->alias(*AI, *BI);
244 Check(Result != AliasResult::MustAlias &&
245 Result != AliasResult::PartialAlias,
246 "Unusual: noalias argument aliases another argument", &I);
247 }
248 }
249 }
250
251 // Check that an sret argument points to valid memory.
252 if (Formal->hasStructRetAttr() && Actual->getType()->isPointerTy()) {
253 Type *Ty = Formal->getParamStructRetType();
254 MemoryLocation Loc(
255 Actual, LocationSize::precise(DL->getTypeStoreSize(Ty)));
256 visitMemoryReference(I, Loc, DL->getABITypeAlign(Ty), Ty,
257 MemRef::Read | MemRef::Write);
258 }
259 }
260 }
261 }
262
263 if (const auto *CI = dyn_cast<CallInst>(&I)) {
264 if (CI->isTailCall()) {
265 const AttributeList &PAL = CI->getAttributes();
266 unsigned ArgNo = 0;
267 for (Value *Arg : I.args()) {
268 // Skip ByVal arguments since they will be memcpy'd to the callee's
269 // stack anyway.
270 if (PAL.hasParamAttr(ArgNo++, Attribute::ByVal))
271 continue;
272 Value *Obj = findValue(Arg, /*OffsetOk=*/true);
273 Check(!isa<AllocaInst>(Obj),
274 "Undefined behavior: Call with \"tail\" keyword references "
275 "alloca",
276 &I);
277 }
278 }
279 }
280
281 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(&I))
282 switch (II->getIntrinsicID()) {
283 default:
284 break;
285
286 // TODO: Check more intrinsics
287
288 case Intrinsic::memcpy: {
289 MemCpyInst *MCI = cast<MemCpyInst>(&I);
290 visitMemoryReference(I, MemoryLocation::getForDest(MCI),
291 MCI->getDestAlign(), nullptr, MemRef::Write);
292 visitMemoryReference(I, MemoryLocation::getForSource(MCI),
293 MCI->getSourceAlign(), nullptr, MemRef::Read);
294
295 // Check that the memcpy arguments don't overlap. The AliasAnalysis API
296 // isn't expressive enough for what we really want to do. Known partial
297 // overlap is not distinguished from the case where nothing is known.
298 auto Size = LocationSize::afterPointer();
299 if (const ConstantInt *Len =
300 dyn_cast<ConstantInt>(findValue(MCI->getLength(),
301 /*OffsetOk=*/false)))
302 if (Len->getValue().isIntN(32))
303 Size = LocationSize::precise(Len->getValue().getZExtValue());
304 Check(AA->alias(MCI->getSource(), Size, MCI->getDest(), Size) !=
305 AliasResult::MustAlias,
306 "Undefined behavior: memcpy source and destination overlap", &I);
307 break;
308 }
309 case Intrinsic::memcpy_inline: {
310 MemCpyInlineInst *MCII = cast<MemCpyInlineInst>(&I);
311 const uint64_t Size = MCII->getLength()->getValue().getLimitedValue();
312 visitMemoryReference(I, MemoryLocation::getForDest(MCII),
313 MCII->getDestAlign(), nullptr, MemRef::Write);
314 visitMemoryReference(I, MemoryLocation::getForSource(MCII),
315 MCII->getSourceAlign(), nullptr, MemRef::Read);
316
317 // Check that the memcpy arguments don't overlap. The AliasAnalysis API
318 // isn't expressive enough for what we really want to do. Known partial
319 // overlap is not distinguished from the case where nothing is known.
320 const LocationSize LS = LocationSize::precise(Size);
321 Check(AA->alias(MCII->getSource(), LS, MCII->getDest(), LS) !=
322 AliasResult::MustAlias,
323 "Undefined behavior: memcpy source and destination overlap", &I);
324 break;
325 }
326 case Intrinsic::memmove: {
327 MemMoveInst *MMI = cast<MemMoveInst>(&I);
328 visitMemoryReference(I, MemoryLocation::getForDest(MMI),
329 MMI->getDestAlign(), nullptr, MemRef::Write);
330 visitMemoryReference(I, MemoryLocation::getForSource(MMI),
331 MMI->getSourceAlign(), nullptr, MemRef::Read);
332 break;
333 }
334 case Intrinsic::memset: {
335 MemSetInst *MSI = cast<MemSetInst>(&I);
336 visitMemoryReference(I, MemoryLocation::getForDest(MSI),
337 MSI->getDestAlign(), nullptr, MemRef::Write);
338 break;
339 }
340 case Intrinsic::memset_inline: {
341 MemSetInlineInst *MSII = cast<MemSetInlineInst>(&I);
342 visitMemoryReference(I, MemoryLocation::getForDest(MSII),
343 MSII->getDestAlign(), nullptr, MemRef::Write);
344 break;
345 }
346
347 case Intrinsic::vastart:
348 Check(I.getParent()->getParent()->isVarArg(),
349 "Undefined behavior: va_start called in a non-varargs function",
350 &I);
351
352 visitMemoryReference(I, MemoryLocation::getForArgument(&I, 0, TLI),
353 std::nullopt, nullptr, MemRef::Read | MemRef::Write);
354 break;
355 case Intrinsic::vacopy:
356 visitMemoryReference(I, MemoryLocation::getForArgument(&I, 0, TLI),
357 std::nullopt, nullptr, MemRef::Write);
358 visitMemoryReference(I, MemoryLocation::getForArgument(&I, 1, TLI),
359 std::nullopt, nullptr, MemRef::Read);
360 break;
361 case Intrinsic::vaend:
362 visitMemoryReference(I, MemoryLocation::getForArgument(&I, 0, TLI),
363 std::nullopt, nullptr, MemRef::Read | MemRef::Write);
364 break;
365
366 case Intrinsic::stackrestore:
367 // Stackrestore doesn't read or write memory, but it sets the
368 // stack pointer, which the compiler may read from or write to
369 // at any time, so check it for both readability and writeability.
370 visitMemoryReference(I, MemoryLocation::getForArgument(&I, 0, TLI),
371 std::nullopt, nullptr, MemRef::Read | MemRef::Write);
372 break;
373 case Intrinsic::get_active_lane_mask:
374 if (auto *TripCount = dyn_cast<ConstantInt>(I.getArgOperand(1)))
375 Check(!TripCount->isZero(),
376 "get_active_lane_mask: operand #2 "
377 "must be greater than 0",
378 &I);
379 break;
380 }
381 }
382
visitReturnInst(ReturnInst & I)383 void Lint::visitReturnInst(ReturnInst &I) {
384 Function *F = I.getParent()->getParent();
385 Check(!F->doesNotReturn(),
386 "Unusual: Return statement in function with noreturn attribute", &I);
387
388 if (Value *V = I.getReturnValue()) {
389 Value *Obj = findValue(V, /*OffsetOk=*/true);
390 Check(!isa<AllocaInst>(Obj), "Unusual: Returning alloca value", &I);
391 }
392 }
393
394 // TODO: Check that the reference is in bounds.
395 // TODO: Check readnone/readonly function attributes.
visitMemoryReference(Instruction & I,const MemoryLocation & Loc,MaybeAlign Align,Type * Ty,unsigned Flags)396 void Lint::visitMemoryReference(Instruction &I, const MemoryLocation &Loc,
397 MaybeAlign Align, Type *Ty, unsigned Flags) {
398 // If no memory is being referenced, it doesn't matter if the pointer
399 // is valid.
400 if (Loc.Size.isZero())
401 return;
402
403 Value *Ptr = const_cast<Value *>(Loc.Ptr);
404 Value *UnderlyingObject = findValue(Ptr, /*OffsetOk=*/true);
405 Check(!isa<ConstantPointerNull>(UnderlyingObject),
406 "Undefined behavior: Null pointer dereference", &I);
407 Check(!isa<UndefValue>(UnderlyingObject),
408 "Undefined behavior: Undef pointer dereference", &I);
409 Check(!isa<ConstantInt>(UnderlyingObject) ||
410 !cast<ConstantInt>(UnderlyingObject)->isMinusOne(),
411 "Unusual: All-ones pointer dereference", &I);
412 Check(!isa<ConstantInt>(UnderlyingObject) ||
413 !cast<ConstantInt>(UnderlyingObject)->isOne(),
414 "Unusual: Address one pointer dereference", &I);
415
416 if (Flags & MemRef::Write) {
417 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(UnderlyingObject))
418 Check(!GV->isConstant(), "Undefined behavior: Write to read-only memory",
419 &I);
420 Check(!isa<Function>(UnderlyingObject) &&
421 !isa<BlockAddress>(UnderlyingObject),
422 "Undefined behavior: Write to text section", &I);
423 }
424 if (Flags & MemRef::Read) {
425 Check(!isa<Function>(UnderlyingObject), "Unusual: Load from function body",
426 &I);
427 Check(!isa<BlockAddress>(UnderlyingObject),
428 "Undefined behavior: Load from block address", &I);
429 }
430 if (Flags & MemRef::Callee) {
431 Check(!isa<BlockAddress>(UnderlyingObject),
432 "Undefined behavior: Call to block address", &I);
433 }
434 if (Flags & MemRef::Branchee) {
435 Check(!isa<Constant>(UnderlyingObject) ||
436 isa<BlockAddress>(UnderlyingObject),
437 "Undefined behavior: Branch to non-blockaddress", &I);
438 }
439
440 // Check for buffer overflows and misalignment.
441 // Only handles memory references that read/write something simple like an
442 // alloca instruction or a global variable.
443 int64_t Offset = 0;
444 if (Value *Base = GetPointerBaseWithConstantOffset(Ptr, Offset, *DL)) {
445 // OK, so the access is to a constant offset from Ptr. Check that Ptr is
446 // something we can handle and if so extract the size of this base object
447 // along with its alignment.
448 uint64_t BaseSize = MemoryLocation::UnknownSize;
449 MaybeAlign BaseAlign;
450
451 if (AllocaInst *AI = dyn_cast<AllocaInst>(Base)) {
452 Type *ATy = AI->getAllocatedType();
453 if (!AI->isArrayAllocation() && ATy->isSized())
454 BaseSize = DL->getTypeAllocSize(ATy);
455 BaseAlign = AI->getAlign();
456 } else if (GlobalVariable *GV = dyn_cast<GlobalVariable>(Base)) {
457 // If the global may be defined differently in another compilation unit
458 // then don't warn about funky memory accesses.
459 if (GV->hasDefinitiveInitializer()) {
460 Type *GTy = GV->getValueType();
461 if (GTy->isSized())
462 BaseSize = DL->getTypeAllocSize(GTy);
463 BaseAlign = GV->getAlign();
464 if (!BaseAlign && GTy->isSized())
465 BaseAlign = DL->getABITypeAlign(GTy);
466 }
467 }
468
469 // Accesses from before the start or after the end of the object are not
470 // defined.
471 Check(!Loc.Size.hasValue() || BaseSize == MemoryLocation::UnknownSize ||
472 (Offset >= 0 && Offset + Loc.Size.getValue() <= BaseSize),
473 "Undefined behavior: Buffer overflow", &I);
474
475 // Accesses that say that the memory is more aligned than it is are not
476 // defined.
477 if (!Align && Ty && Ty->isSized())
478 Align = DL->getABITypeAlign(Ty);
479 if (BaseAlign && Align)
480 Check(*Align <= commonAlignment(*BaseAlign, Offset),
481 "Undefined behavior: Memory reference address is misaligned", &I);
482 }
483 }
484
visitLoadInst(LoadInst & I)485 void Lint::visitLoadInst(LoadInst &I) {
486 visitMemoryReference(I, MemoryLocation::get(&I), I.getAlign(), I.getType(),
487 MemRef::Read);
488 }
489
visitStoreInst(StoreInst & I)490 void Lint::visitStoreInst(StoreInst &I) {
491 visitMemoryReference(I, MemoryLocation::get(&I), I.getAlign(),
492 I.getOperand(0)->getType(), MemRef::Write);
493 }
494
visitXor(BinaryOperator & I)495 void Lint::visitXor(BinaryOperator &I) {
496 Check(!isa<UndefValue>(I.getOperand(0)) || !isa<UndefValue>(I.getOperand(1)),
497 "Undefined result: xor(undef, undef)", &I);
498 }
499
visitSub(BinaryOperator & I)500 void Lint::visitSub(BinaryOperator &I) {
501 Check(!isa<UndefValue>(I.getOperand(0)) || !isa<UndefValue>(I.getOperand(1)),
502 "Undefined result: sub(undef, undef)", &I);
503 }
504
visitLShr(BinaryOperator & I)505 void Lint::visitLShr(BinaryOperator &I) {
506 if (ConstantInt *CI = dyn_cast<ConstantInt>(findValue(I.getOperand(1),
507 /*OffsetOk=*/false)))
508 Check(CI->getValue().ult(cast<IntegerType>(I.getType())->getBitWidth()),
509 "Undefined result: Shift count out of range", &I);
510 }
511
visitAShr(BinaryOperator & I)512 void Lint::visitAShr(BinaryOperator &I) {
513 if (ConstantInt *CI =
514 dyn_cast<ConstantInt>(findValue(I.getOperand(1), /*OffsetOk=*/false)))
515 Check(CI->getValue().ult(cast<IntegerType>(I.getType())->getBitWidth()),
516 "Undefined result: Shift count out of range", &I);
517 }
518
visitShl(BinaryOperator & I)519 void Lint::visitShl(BinaryOperator &I) {
520 if (ConstantInt *CI =
521 dyn_cast<ConstantInt>(findValue(I.getOperand(1), /*OffsetOk=*/false)))
522 Check(CI->getValue().ult(cast<IntegerType>(I.getType())->getBitWidth()),
523 "Undefined result: Shift count out of range", &I);
524 }
525
isZero(Value * V,const DataLayout & DL,DominatorTree * DT,AssumptionCache * AC)526 static bool isZero(Value *V, const DataLayout &DL, DominatorTree *DT,
527 AssumptionCache *AC) {
528 // Assume undef could be zero.
529 if (isa<UndefValue>(V))
530 return true;
531
532 VectorType *VecTy = dyn_cast<VectorType>(V->getType());
533 if (!VecTy) {
534 KnownBits Known =
535 computeKnownBits(V, DL, 0, AC, dyn_cast<Instruction>(V), DT);
536 return Known.isZero();
537 }
538
539 // Per-component check doesn't work with zeroinitializer
540 Constant *C = dyn_cast<Constant>(V);
541 if (!C)
542 return false;
543
544 if (C->isZeroValue())
545 return true;
546
547 // For a vector, KnownZero will only be true if all values are zero, so check
548 // this per component
549 for (unsigned I = 0, N = cast<FixedVectorType>(VecTy)->getNumElements();
550 I != N; ++I) {
551 Constant *Elem = C->getAggregateElement(I);
552 if (isa<UndefValue>(Elem))
553 return true;
554
555 KnownBits Known = computeKnownBits(Elem, DL);
556 if (Known.isZero())
557 return true;
558 }
559
560 return false;
561 }
562
visitSDiv(BinaryOperator & I)563 void Lint::visitSDiv(BinaryOperator &I) {
564 Check(!isZero(I.getOperand(1), I.getModule()->getDataLayout(), DT, AC),
565 "Undefined behavior: Division by zero", &I);
566 }
567
visitUDiv(BinaryOperator & I)568 void Lint::visitUDiv(BinaryOperator &I) {
569 Check(!isZero(I.getOperand(1), I.getModule()->getDataLayout(), DT, AC),
570 "Undefined behavior: Division by zero", &I);
571 }
572
visitSRem(BinaryOperator & I)573 void Lint::visitSRem(BinaryOperator &I) {
574 Check(!isZero(I.getOperand(1), I.getModule()->getDataLayout(), DT, AC),
575 "Undefined behavior: Division by zero", &I);
576 }
577
visitURem(BinaryOperator & I)578 void Lint::visitURem(BinaryOperator &I) {
579 Check(!isZero(I.getOperand(1), I.getModule()->getDataLayout(), DT, AC),
580 "Undefined behavior: Division by zero", &I);
581 }
582
visitAllocaInst(AllocaInst & I)583 void Lint::visitAllocaInst(AllocaInst &I) {
584 if (isa<ConstantInt>(I.getArraySize()))
585 // This isn't undefined behavior, it's just an obvious pessimization.
586 Check(&I.getParent()->getParent()->getEntryBlock() == I.getParent(),
587 "Pessimization: Static alloca outside of entry block", &I);
588
589 // TODO: Check for an unusual size (MSB set?)
590 }
591
visitVAArgInst(VAArgInst & I)592 void Lint::visitVAArgInst(VAArgInst &I) {
593 visitMemoryReference(I, MemoryLocation::get(&I), std::nullopt, nullptr,
594 MemRef::Read | MemRef::Write);
595 }
596
visitIndirectBrInst(IndirectBrInst & I)597 void Lint::visitIndirectBrInst(IndirectBrInst &I) {
598 visitMemoryReference(I, MemoryLocation::getAfter(I.getAddress()),
599 std::nullopt, nullptr, MemRef::Branchee);
600
601 Check(I.getNumDestinations() != 0,
602 "Undefined behavior: indirectbr with no destinations", &I);
603 }
604
visitExtractElementInst(ExtractElementInst & I)605 void Lint::visitExtractElementInst(ExtractElementInst &I) {
606 if (ConstantInt *CI = dyn_cast<ConstantInt>(findValue(I.getIndexOperand(),
607 /*OffsetOk=*/false)))
608 Check(
609 CI->getValue().ult(
610 cast<FixedVectorType>(I.getVectorOperandType())->getNumElements()),
611 "Undefined result: extractelement index out of range", &I);
612 }
613
visitInsertElementInst(InsertElementInst & I)614 void Lint::visitInsertElementInst(InsertElementInst &I) {
615 if (ConstantInt *CI = dyn_cast<ConstantInt>(findValue(I.getOperand(2),
616 /*OffsetOk=*/false)))
617 Check(CI->getValue().ult(
618 cast<FixedVectorType>(I.getType())->getNumElements()),
619 "Undefined result: insertelement index out of range", &I);
620 }
621
visitUnreachableInst(UnreachableInst & I)622 void Lint::visitUnreachableInst(UnreachableInst &I) {
623 // This isn't undefined behavior, it's merely suspicious.
624 Check(&I == &I.getParent()->front() ||
625 std::prev(I.getIterator())->mayHaveSideEffects(),
626 "Unusual: unreachable immediately preceded by instruction without "
627 "side effects",
628 &I);
629 }
630
631 /// findValue - Look through bitcasts and simple memory reference patterns
632 /// to identify an equivalent, but more informative, value. If OffsetOk
633 /// is true, look through getelementptrs with non-zero offsets too.
634 ///
635 /// Most analysis passes don't require this logic, because instcombine
636 /// will simplify most of these kinds of things away. But it's a goal of
637 /// this Lint pass to be useful even on non-optimized IR.
findValue(Value * V,bool OffsetOk) const638 Value *Lint::findValue(Value *V, bool OffsetOk) const {
639 SmallPtrSet<Value *, 4> Visited;
640 return findValueImpl(V, OffsetOk, Visited);
641 }
642
643 /// findValueImpl - Implementation helper for findValue.
findValueImpl(Value * V,bool OffsetOk,SmallPtrSetImpl<Value * > & Visited) const644 Value *Lint::findValueImpl(Value *V, bool OffsetOk,
645 SmallPtrSetImpl<Value *> &Visited) const {
646 // Detect self-referential values.
647 if (!Visited.insert(V).second)
648 return UndefValue::get(V->getType());
649
650 // TODO: Look through sext or zext cast, when the result is known to
651 // be interpreted as signed or unsigned, respectively.
652 // TODO: Look through eliminable cast pairs.
653 // TODO: Look through calls with unique return values.
654 // TODO: Look through vector insert/extract/shuffle.
655 V = OffsetOk ? getUnderlyingObject(V) : V->stripPointerCasts();
656 if (LoadInst *L = dyn_cast<LoadInst>(V)) {
657 BasicBlock::iterator BBI = L->getIterator();
658 BasicBlock *BB = L->getParent();
659 SmallPtrSet<BasicBlock *, 4> VisitedBlocks;
660 BatchAAResults BatchAA(*AA);
661 for (;;) {
662 if (!VisitedBlocks.insert(BB).second)
663 break;
664 if (Value *U =
665 FindAvailableLoadedValue(L, BB, BBI, DefMaxInstsToScan, &BatchAA))
666 return findValueImpl(U, OffsetOk, Visited);
667 if (BBI != BB->begin())
668 break;
669 BB = BB->getUniquePredecessor();
670 if (!BB)
671 break;
672 BBI = BB->end();
673 }
674 } else if (PHINode *PN = dyn_cast<PHINode>(V)) {
675 if (Value *W = PN->hasConstantValue())
676 return findValueImpl(W, OffsetOk, Visited);
677 } else if (CastInst *CI = dyn_cast<CastInst>(V)) {
678 if (CI->isNoopCast(*DL))
679 return findValueImpl(CI->getOperand(0), OffsetOk, Visited);
680 } else if (ExtractValueInst *Ex = dyn_cast<ExtractValueInst>(V)) {
681 if (Value *W =
682 FindInsertedValue(Ex->getAggregateOperand(), Ex->getIndices()))
683 if (W != V)
684 return findValueImpl(W, OffsetOk, Visited);
685 } else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
686 // Same as above, but for ConstantExpr instead of Instruction.
687 if (Instruction::isCast(CE->getOpcode())) {
688 if (CastInst::isNoopCast(Instruction::CastOps(CE->getOpcode()),
689 CE->getOperand(0)->getType(), CE->getType(),
690 *DL))
691 return findValueImpl(CE->getOperand(0), OffsetOk, Visited);
692 }
693 }
694
695 // As a last resort, try SimplifyInstruction or constant folding.
696 if (Instruction *Inst = dyn_cast<Instruction>(V)) {
697 if (Value *W = simplifyInstruction(Inst, {*DL, TLI, DT, AC}))
698 return findValueImpl(W, OffsetOk, Visited);
699 } else if (auto *C = dyn_cast<Constant>(V)) {
700 Value *W = ConstantFoldConstant(C, *DL, TLI);
701 if (W != V)
702 return findValueImpl(W, OffsetOk, Visited);
703 }
704
705 return V;
706 }
707
run(Function & F,FunctionAnalysisManager & AM)708 PreservedAnalyses LintPass::run(Function &F, FunctionAnalysisManager &AM) {
709 auto *Mod = F.getParent();
710 auto *DL = &F.getParent()->getDataLayout();
711 auto *AA = &AM.getResult<AAManager>(F);
712 auto *AC = &AM.getResult<AssumptionAnalysis>(F);
713 auto *DT = &AM.getResult<DominatorTreeAnalysis>(F);
714 auto *TLI = &AM.getResult<TargetLibraryAnalysis>(F);
715 Lint L(Mod, DL, AA, AC, DT, TLI);
716 L.visit(F);
717 dbgs() << L.MessagesStr.str();
718 return PreservedAnalyses::all();
719 }
720
721 //===----------------------------------------------------------------------===//
722 // Implement the public interfaces to this file...
723 //===----------------------------------------------------------------------===//
724
725 /// lintFunction - Check a function for errors, printing messages on stderr.
726 ///
lintFunction(const Function & f)727 void llvm::lintFunction(const Function &f) {
728 Function &F = const_cast<Function &>(f);
729 assert(!F.isDeclaration() && "Cannot lint external functions");
730
731 FunctionAnalysisManager FAM;
732 FAM.registerPass([&] { return TargetLibraryAnalysis(); });
733 FAM.registerPass([&] { return DominatorTreeAnalysis(); });
734 FAM.registerPass([&] { return AssumptionAnalysis(); });
735 FAM.registerPass([&] {
736 AAManager AA;
737 AA.registerFunctionAnalysis<BasicAA>();
738 AA.registerFunctionAnalysis<ScopedNoAliasAA>();
739 AA.registerFunctionAnalysis<TypeBasedAA>();
740 return AA;
741 });
742 LintPass().run(F, FAM);
743 }
744
745 /// lintModule - Check a module for errors, printing messages on stderr.
746 ///
lintModule(const Module & M)747 void llvm::lintModule(const Module &M) {
748 for (const Function &F : M) {
749 if (!F.isDeclaration())
750 lintFunction(F);
751 }
752 }
753