LLVM API Documentation

Lint.cpp
Go to the documentation of this file.
00001 //===-- Lint.cpp - Check for common errors in LLVM IR ---------------------===//
00002 //
00003 //                     The LLVM Compiler Infrastructure
00004 //
00005 // This file is distributed under the University of Illinois Open Source
00006 // License. See LICENSE.TXT for details.
00007 //
00008 //===----------------------------------------------------------------------===//
00009 //
00010 // This pass statically checks for common and easily-identified constructs
00011 // which produce undefined or likely unintended behavior in LLVM IR.
00012 //
00013 // It is not a guarantee of correctness, in two ways. First, it isn't
00014 // comprehensive. There are checks which could be done statically which are
00015 // not yet implemented. Some of these are indicated by TODO comments, but
00016 // those aren't comprehensive either. Second, many conditions cannot be
00017 // checked statically. This pass does no dynamic instrumentation, so it
00018 // can't check for all possible problems.
00019 //
00020 // Another limitation is that it assumes all code will be executed. A store
00021 // through a null pointer in a basic block which is never reached is harmless,
00022 // but this pass will warn about it anyway. This is the main reason why most
00023 // of these checks live here instead of in the Verifier pass.
00024 //
00025 // Optimization passes may make conditions that this pass checks for more or
00026 // less obvious. If an optimization pass appears to be introducing a warning,
00027 // it may be that the optimization pass is merely exposing an existing
00028 // condition in the code.
00029 //
00030 // This code may be run before instcombine. In many cases, instcombine checks
00031 // for the same kinds of things and turns instructions with undefined behavior
00032 // into unreachable (or equivalent). Because of this, this pass makes some
00033 // effort to look through bitcasts and so on.
00034 //
00035 //===----------------------------------------------------------------------===//
00036 
00037 #include "llvm/Analysis/Lint.h"
00038 #include "llvm/ADT/STLExtras.h"
00039 #include "llvm/Analysis/AliasAnalysis.h"
00040 #include "llvm/Analysis/AssumptionTracker.h"
00041 #include "llvm/Analysis/ConstantFolding.h"
00042 #include "llvm/Analysis/InstructionSimplify.h"
00043 #include "llvm/Analysis/Loads.h"
00044 #include "llvm/Analysis/Passes.h"
00045 #include "llvm/Analysis/ValueTracking.h"
00046 #include "llvm/IR/CallSite.h"
00047 #include "llvm/IR/DataLayout.h"
00048 #include "llvm/IR/Dominators.h"
00049 #include "llvm/IR/Function.h"
00050 #include "llvm/IR/InstVisitor.h"
00051 #include "llvm/IR/IntrinsicInst.h"
00052 #include "llvm/Pass.h"
00053 #include "llvm/PassManager.h"
00054 #include "llvm/Support/Debug.h"
00055 #include "llvm/Support/raw_ostream.h"
00056 #include "llvm/Target/TargetLibraryInfo.h"
00057 using namespace llvm;
00058 
00059 namespace {
00060   namespace MemRef {
00061     static unsigned Read     = 1;
00062     static unsigned Write    = 2;
00063     static unsigned Callee   = 4;
00064     static unsigned Branchee = 8;
00065   }
00066 
00067   class Lint : public FunctionPass, public InstVisitor<Lint> {
00068     friend class InstVisitor<Lint>;
00069 
00070     void visitFunction(Function &F);
00071 
00072     void visitCallSite(CallSite CS);
00073     void visitMemoryReference(Instruction &I, Value *Ptr,
00074                               uint64_t Size, unsigned Align,
00075                               Type *Ty, unsigned Flags);
00076 
00077     void visitCallInst(CallInst &I);
00078     void visitInvokeInst(InvokeInst &I);
00079     void visitReturnInst(ReturnInst &I);
00080     void visitLoadInst(LoadInst &I);
00081     void visitStoreInst(StoreInst &I);
00082     void visitXor(BinaryOperator &I);
00083     void visitSub(BinaryOperator &I);
00084     void visitLShr(BinaryOperator &I);
00085     void visitAShr(BinaryOperator &I);
00086     void visitShl(BinaryOperator &I);
00087     void visitSDiv(BinaryOperator &I);
00088     void visitUDiv(BinaryOperator &I);
00089     void visitSRem(BinaryOperator &I);
00090     void visitURem(BinaryOperator &I);
00091     void visitAllocaInst(AllocaInst &I);
00092     void visitVAArgInst(VAArgInst &I);
00093     void visitIndirectBrInst(IndirectBrInst &I);
00094     void visitExtractElementInst(ExtractElementInst &I);
00095     void visitInsertElementInst(InsertElementInst &I);
00096     void visitUnreachableInst(UnreachableInst &I);
00097 
00098     Value *findValue(Value *V, bool OffsetOk) const;
00099     Value *findValueImpl(Value *V, bool OffsetOk,
00100                          SmallPtrSetImpl<Value *> &Visited) const;
00101 
00102   public:
00103     Module *Mod;
00104     AliasAnalysis *AA;
00105     AssumptionTracker *AT;
00106     DominatorTree *DT;
00107     const DataLayout *DL;
00108     TargetLibraryInfo *TLI;
00109 
00110     std::string Messages;
00111     raw_string_ostream MessagesStr;
00112 
00113     static char ID; // Pass identification, replacement for typeid
00114     Lint() : FunctionPass(ID), MessagesStr(Messages) {
00115       initializeLintPass(*PassRegistry::getPassRegistry());
00116     }
00117 
00118     bool runOnFunction(Function &F) override;
00119 
00120     void getAnalysisUsage(AnalysisUsage &AU) const override {
00121       AU.setPreservesAll();
00122       AU.addRequired<AliasAnalysis>();
00123       AU.addRequired<AssumptionTracker>();
00124       AU.addRequired<TargetLibraryInfo>();
00125       AU.addRequired<DominatorTreeWrapperPass>();
00126     }
00127     void print(raw_ostream &O, const Module *M) const override {}
00128 
00129     void WriteValue(const Value *V) {
00130       if (!V) return;
00131       if (isa<Instruction>(V)) {
00132         MessagesStr << *V << '\n';
00133       } else {
00134         V->printAsOperand(MessagesStr, true, Mod);
00135         MessagesStr << '\n';
00136       }
00137     }
00138 
00139     // CheckFailed - A check failed, so print out the condition and the message
00140     // that failed.  This provides a nice place to put a breakpoint if you want
00141     // to see why something is not correct.
00142     void CheckFailed(const Twine &Message,
00143                      const Value *V1 = nullptr, const Value *V2 = nullptr,
00144                      const Value *V3 = nullptr, const Value *V4 = nullptr) {
00145       MessagesStr << Message.str() << "\n";
00146       WriteValue(V1);
00147       WriteValue(V2);
00148       WriteValue(V3);
00149       WriteValue(V4);
00150     }
00151   };
00152 }
00153 
00154 char Lint::ID = 0;
00155 INITIALIZE_PASS_BEGIN(Lint, "lint", "Statically lint-checks LLVM IR",
00156                       false, true)
00157 INITIALIZE_PASS_DEPENDENCY(AssumptionTracker)
00158 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
00159 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
00160 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
00161 INITIALIZE_PASS_END(Lint, "lint", "Statically lint-checks LLVM IR",
00162                     false, true)
00163 
00164 // Assert - We know that cond should be true, if not print an error message.
00165 #define Assert(C, M) \
00166     do { if (!(C)) { CheckFailed(M); return; } } while (0)
00167 #define Assert1(C, M, V1) \
00168     do { if (!(C)) { CheckFailed(M, V1); return; } } while (0)
00169 #define Assert2(C, M, V1, V2) \
00170     do { if (!(C)) { CheckFailed(M, V1, V2); return; } } while (0)
00171 #define Assert3(C, M, V1, V2, V3) \
00172     do { if (!(C)) { CheckFailed(M, V1, V2, V3); return; } } while (0)
00173 #define Assert4(C, M, V1, V2, V3, V4) \
00174     do { if (!(C)) { CheckFailed(M, V1, V2, V3, V4); return; } } while (0)
00175 
00176 // Lint::run - This is the main Analysis entry point for a
00177 // function.
00178 //
00179 bool Lint::runOnFunction(Function &F) {
00180   Mod = F.getParent();
00181   AA = &getAnalysis<AliasAnalysis>();
00182   AT = &getAnalysis<AssumptionTracker>();
00183   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
00184   DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
00185   DL = DLP ? &DLP->getDataLayout() : nullptr;
00186   TLI = &getAnalysis<TargetLibraryInfo>();
00187   visit(F);
00188   dbgs() << MessagesStr.str();
00189   Messages.clear();
00190   return false;
00191 }
00192 
00193 void Lint::visitFunction(Function &F) {
00194   // This isn't undefined behavior, it's just a little unusual, and it's a
00195   // fairly common mistake to neglect to name a function.
00196   Assert1(F.hasName() || F.hasLocalLinkage(),
00197           "Unusual: Unnamed function with non-local linkage", &F);
00198 
00199   // TODO: Check for irreducible control flow.
00200 }
00201 
00202 void Lint::visitCallSite(CallSite CS) {
00203   Instruction &I = *CS.getInstruction();
00204   Value *Callee = CS.getCalledValue();
00205 
00206   visitMemoryReference(I, Callee, AliasAnalysis::UnknownSize,
00207                        0, nullptr, MemRef::Callee);
00208 
00209   if (Function *F = dyn_cast<Function>(findValue(Callee, /*OffsetOk=*/false))) {
00210     Assert1(CS.getCallingConv() == F->getCallingConv(),
00211             "Undefined behavior: Caller and callee calling convention differ",
00212             &I);
00213 
00214     FunctionType *FT = F->getFunctionType();
00215     unsigned NumActualArgs = CS.arg_size();
00216 
00217     Assert1(FT->isVarArg() ?
00218               FT->getNumParams() <= NumActualArgs :
00219               FT->getNumParams() == NumActualArgs,
00220             "Undefined behavior: Call argument count mismatches callee "
00221             "argument count", &I);
00222 
00223     Assert1(FT->getReturnType() == I.getType(),
00224             "Undefined behavior: Call return type mismatches "
00225             "callee return type", &I);
00226 
00227     // Check argument types (in case the callee was casted) and attributes.
00228     // TODO: Verify that caller and callee attributes are compatible.
00229     Function::arg_iterator PI = F->arg_begin(), PE = F->arg_end();
00230     CallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
00231     for (; AI != AE; ++AI) {
00232       Value *Actual = *AI;
00233       if (PI != PE) {
00234         Argument *Formal = PI++;
00235         Assert1(Formal->getType() == Actual->getType(),
00236                 "Undefined behavior: Call argument type mismatches "
00237                 "callee parameter type", &I);
00238 
00239         // Check that noalias arguments don't alias other arguments. This is
00240         // not fully precise because we don't know the sizes of the dereferenced
00241         // memory regions.
00242         if (Formal->hasNoAliasAttr() && Actual->getType()->isPointerTy())
00243           for (CallSite::arg_iterator BI = CS.arg_begin(); BI != AE; ++BI)
00244             if (AI != BI && (*BI)->getType()->isPointerTy()) {
00245               AliasAnalysis::AliasResult Result = AA->alias(*AI, *BI);
00246               Assert1(Result != AliasAnalysis::MustAlias &&
00247                       Result != AliasAnalysis::PartialAlias,
00248                       "Unusual: noalias argument aliases another argument", &I);
00249             }
00250 
00251         // Check that an sret argument points to valid memory.
00252         if (Formal->hasStructRetAttr() && Actual->getType()->isPointerTy()) {
00253           Type *Ty =
00254             cast<PointerType>(Formal->getType())->getElementType();
00255           visitMemoryReference(I, Actual, AA->getTypeStoreSize(Ty),
00256                                DL ? DL->getABITypeAlignment(Ty) : 0,
00257                                Ty, MemRef::Read | MemRef::Write);
00258         }
00259       }
00260     }
00261   }
00262 
00263   if (CS.isCall() && cast<CallInst>(CS.getInstruction())->isTailCall())
00264     for (CallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
00265          AI != AE; ++AI) {
00266       Value *Obj = findValue(*AI, /*OffsetOk=*/true);
00267       Assert1(!isa<AllocaInst>(Obj),
00268               "Undefined behavior: Call with \"tail\" keyword references "
00269               "alloca", &I);
00270     }
00271 
00272 
00273   if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(&I))
00274     switch (II->getIntrinsicID()) {
00275     default: break;
00276 
00277     // TODO: Check more intrinsics
00278 
00279     case Intrinsic::memcpy: {
00280       MemCpyInst *MCI = cast<MemCpyInst>(&I);
00281       // TODO: If the size is known, use it.
00282       visitMemoryReference(I, MCI->getDest(), AliasAnalysis::UnknownSize,
00283                            MCI->getAlignment(), nullptr,
00284                            MemRef::Write);
00285       visitMemoryReference(I, MCI->getSource(), AliasAnalysis::UnknownSize,
00286                            MCI->getAlignment(), nullptr,
00287                            MemRef::Read);
00288 
00289       // Check that the memcpy arguments don't overlap. The AliasAnalysis API
00290       // isn't expressive enough for what we really want to do. Known partial
00291       // overlap is not distinguished from the case where nothing is known.
00292       uint64_t Size = 0;
00293       if (const ConstantInt *Len =
00294             dyn_cast<ConstantInt>(findValue(MCI->getLength(),
00295                                             /*OffsetOk=*/false)))
00296         if (Len->getValue().isIntN(32))
00297           Size = Len->getValue().getZExtValue();
00298       Assert1(AA->alias(MCI->getSource(), Size, MCI->getDest(), Size) !=
00299               AliasAnalysis::MustAlias,
00300               "Undefined behavior: memcpy source and destination overlap", &I);
00301       break;
00302     }
00303     case Intrinsic::memmove: {
00304       MemMoveInst *MMI = cast<MemMoveInst>(&I);
00305       // TODO: If the size is known, use it.
00306       visitMemoryReference(I, MMI->getDest(), AliasAnalysis::UnknownSize,
00307                            MMI->getAlignment(), nullptr,
00308                            MemRef::Write);
00309       visitMemoryReference(I, MMI->getSource(), AliasAnalysis::UnknownSize,
00310                            MMI->getAlignment(), nullptr,
00311                            MemRef::Read);
00312       break;
00313     }
00314     case Intrinsic::memset: {
00315       MemSetInst *MSI = cast<MemSetInst>(&I);
00316       // TODO: If the size is known, use it.
00317       visitMemoryReference(I, MSI->getDest(), AliasAnalysis::UnknownSize,
00318                            MSI->getAlignment(), nullptr,
00319                            MemRef::Write);
00320       break;
00321     }
00322 
00323     case Intrinsic::vastart:
00324       Assert1(I.getParent()->getParent()->isVarArg(),
00325               "Undefined behavior: va_start called in a non-varargs function",
00326               &I);
00327 
00328       visitMemoryReference(I, CS.getArgument(0), AliasAnalysis::UnknownSize,
00329                            0, nullptr, MemRef::Read | MemRef::Write);
00330       break;
00331     case Intrinsic::vacopy:
00332       visitMemoryReference(I, CS.getArgument(0), AliasAnalysis::UnknownSize,
00333                            0, nullptr, MemRef::Write);
00334       visitMemoryReference(I, CS.getArgument(1), AliasAnalysis::UnknownSize,
00335                            0, nullptr, MemRef::Read);
00336       break;
00337     case Intrinsic::vaend:
00338       visitMemoryReference(I, CS.getArgument(0), AliasAnalysis::UnknownSize,
00339                            0, nullptr, MemRef::Read | MemRef::Write);
00340       break;
00341 
00342     case Intrinsic::stackrestore:
00343       // Stackrestore doesn't read or write memory, but it sets the
00344       // stack pointer, which the compiler may read from or write to
00345       // at any time, so check it for both readability and writeability.
00346       visitMemoryReference(I, CS.getArgument(0), AliasAnalysis::UnknownSize,
00347                            0, nullptr, MemRef::Read | MemRef::Write);
00348       break;
00349     }
00350 }
00351 
00352 void Lint::visitCallInst(CallInst &I) {
00353   return visitCallSite(&I);
00354 }
00355 
00356 void Lint::visitInvokeInst(InvokeInst &I) {
00357   return visitCallSite(&I);
00358 }
00359 
00360 void Lint::visitReturnInst(ReturnInst &I) {
00361   Function *F = I.getParent()->getParent();
00362   Assert1(!F->doesNotReturn(),
00363           "Unusual: Return statement in function with noreturn attribute",
00364           &I);
00365 
00366   if (Value *V = I.getReturnValue()) {
00367     Value *Obj = findValue(V, /*OffsetOk=*/true);
00368     Assert1(!isa<AllocaInst>(Obj),
00369             "Unusual: Returning alloca value", &I);
00370   }
00371 }
00372 
00373 // TODO: Check that the reference is in bounds.
00374 // TODO: Check readnone/readonly function attributes.
00375 void Lint::visitMemoryReference(Instruction &I,
00376                                 Value *Ptr, uint64_t Size, unsigned Align,
00377                                 Type *Ty, unsigned Flags) {
00378   // If no memory is being referenced, it doesn't matter if the pointer
00379   // is valid.
00380   if (Size == 0)
00381     return;
00382 
00383   Value *UnderlyingObject = findValue(Ptr, /*OffsetOk=*/true);
00384   Assert1(!isa<ConstantPointerNull>(UnderlyingObject),
00385           "Undefined behavior: Null pointer dereference", &I);
00386   Assert1(!isa<UndefValue>(UnderlyingObject),
00387           "Undefined behavior: Undef pointer dereference", &I);
00388   Assert1(!isa<ConstantInt>(UnderlyingObject) ||
00389           !cast<ConstantInt>(UnderlyingObject)->isAllOnesValue(),
00390           "Unusual: All-ones pointer dereference", &I);
00391   Assert1(!isa<ConstantInt>(UnderlyingObject) ||
00392           !cast<ConstantInt>(UnderlyingObject)->isOne(),
00393           "Unusual: Address one pointer dereference", &I);
00394 
00395   if (Flags & MemRef::Write) {
00396     if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(UnderlyingObject))
00397       Assert1(!GV->isConstant(),
00398               "Undefined behavior: Write to read-only memory", &I);
00399     Assert1(!isa<Function>(UnderlyingObject) &&
00400             !isa<BlockAddress>(UnderlyingObject),
00401             "Undefined behavior: Write to text section", &I);
00402   }
00403   if (Flags & MemRef::Read) {
00404     Assert1(!isa<Function>(UnderlyingObject),
00405             "Unusual: Load from function body", &I);
00406     Assert1(!isa<BlockAddress>(UnderlyingObject),
00407             "Undefined behavior: Load from block address", &I);
00408   }
00409   if (Flags & MemRef::Callee) {
00410     Assert1(!isa<BlockAddress>(UnderlyingObject),
00411             "Undefined behavior: Call to block address", &I);
00412   }
00413   if (Flags & MemRef::Branchee) {
00414     Assert1(!isa<Constant>(UnderlyingObject) ||
00415             isa<BlockAddress>(UnderlyingObject),
00416             "Undefined behavior: Branch to non-blockaddress", &I);
00417   }
00418 
00419   // Check for buffer overflows and misalignment.
00420   // Only handles memory references that read/write something simple like an
00421   // alloca instruction or a global variable.
00422   int64_t Offset = 0;
00423   if (Value *Base = GetPointerBaseWithConstantOffset(Ptr, Offset, DL)) {
00424     // OK, so the access is to a constant offset from Ptr.  Check that Ptr is
00425     // something we can handle and if so extract the size of this base object
00426     // along with its alignment.
00427     uint64_t BaseSize = AliasAnalysis::UnknownSize;
00428     unsigned BaseAlign = 0;
00429 
00430     if (AllocaInst *AI = dyn_cast<AllocaInst>(Base)) {
00431       Type *ATy = AI->getAllocatedType();
00432       if (DL && !AI->isArrayAllocation() && ATy->isSized())
00433         BaseSize = DL->getTypeAllocSize(ATy);
00434       BaseAlign = AI->getAlignment();
00435       if (DL && BaseAlign == 0 && ATy->isSized())
00436         BaseAlign = DL->getABITypeAlignment(ATy);
00437     } else if (GlobalVariable *GV = dyn_cast<GlobalVariable>(Base)) {
00438       // If the global may be defined differently in another compilation unit
00439       // then don't warn about funky memory accesses.
00440       if (GV->hasDefinitiveInitializer()) {
00441         Type *GTy = GV->getType()->getElementType();
00442         if (DL && GTy->isSized())
00443           BaseSize = DL->getTypeAllocSize(GTy);
00444         BaseAlign = GV->getAlignment();
00445         if (DL && BaseAlign == 0 && GTy->isSized())
00446           BaseAlign = DL->getABITypeAlignment(GTy);
00447       }
00448     }
00449 
00450     // Accesses from before the start or after the end of the object are not
00451     // defined.
00452     Assert1(Size == AliasAnalysis::UnknownSize ||
00453             BaseSize == AliasAnalysis::UnknownSize ||
00454             (Offset >= 0 && Offset + Size <= BaseSize),
00455             "Undefined behavior: Buffer overflow", &I);
00456 
00457     // Accesses that say that the memory is more aligned than it is are not
00458     // defined.
00459     if (DL && Align == 0 && Ty && Ty->isSized())
00460       Align = DL->getABITypeAlignment(Ty);
00461     Assert1(!BaseAlign || Align <= MinAlign(BaseAlign, Offset),
00462             "Undefined behavior: Memory reference address is misaligned", &I);
00463   }
00464 }
00465 
00466 void Lint::visitLoadInst(LoadInst &I) {
00467   visitMemoryReference(I, I.getPointerOperand(),
00468                        AA->getTypeStoreSize(I.getType()), I.getAlignment(),
00469                        I.getType(), MemRef::Read);
00470 }
00471 
00472 void Lint::visitStoreInst(StoreInst &I) {
00473   visitMemoryReference(I, I.getPointerOperand(),
00474                        AA->getTypeStoreSize(I.getOperand(0)->getType()),
00475                        I.getAlignment(),
00476                        I.getOperand(0)->getType(), MemRef::Write);
00477 }
00478 
00479 void Lint::visitXor(BinaryOperator &I) {
00480   Assert1(!isa<UndefValue>(I.getOperand(0)) ||
00481           !isa<UndefValue>(I.getOperand(1)),
00482           "Undefined result: xor(undef, undef)", &I);
00483 }
00484 
00485 void Lint::visitSub(BinaryOperator &I) {
00486   Assert1(!isa<UndefValue>(I.getOperand(0)) ||
00487           !isa<UndefValue>(I.getOperand(1)),
00488           "Undefined result: sub(undef, undef)", &I);
00489 }
00490 
00491 void Lint::visitLShr(BinaryOperator &I) {
00492   if (ConstantInt *CI =
00493         dyn_cast<ConstantInt>(findValue(I.getOperand(1), /*OffsetOk=*/false)))
00494     Assert1(CI->getValue().ult(cast<IntegerType>(I.getType())->getBitWidth()),
00495             "Undefined result: Shift count out of range", &I);
00496 }
00497 
00498 void Lint::visitAShr(BinaryOperator &I) {
00499   if (ConstantInt *CI =
00500         dyn_cast<ConstantInt>(findValue(I.getOperand(1), /*OffsetOk=*/false)))
00501     Assert1(CI->getValue().ult(cast<IntegerType>(I.getType())->getBitWidth()),
00502             "Undefined result: Shift count out of range", &I);
00503 }
00504 
00505 void Lint::visitShl(BinaryOperator &I) {
00506   if (ConstantInt *CI =
00507         dyn_cast<ConstantInt>(findValue(I.getOperand(1), /*OffsetOk=*/false)))
00508     Assert1(CI->getValue().ult(cast<IntegerType>(I.getType())->getBitWidth()),
00509             "Undefined result: Shift count out of range", &I);
00510 }
00511 
00512 static bool isZero(Value *V, const DataLayout *DL, DominatorTree *DT,
00513                    AssumptionTracker *AT) {
00514   // Assume undef could be zero.
00515   if (isa<UndefValue>(V))
00516     return true;
00517 
00518   VectorType *VecTy = dyn_cast<VectorType>(V->getType());
00519   if (!VecTy) {
00520     unsigned BitWidth = V->getType()->getIntegerBitWidth();
00521     APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
00522     computeKnownBits(V, KnownZero, KnownOne, DL,
00523                      0, AT, dyn_cast<Instruction>(V), DT);
00524     return KnownZero.isAllOnesValue();
00525   }
00526 
00527   // Per-component check doesn't work with zeroinitializer
00528   Constant *C = dyn_cast<Constant>(V);
00529   if (!C)
00530     return false;
00531 
00532   if (C->isZeroValue())
00533     return true;
00534 
00535   // For a vector, KnownZero will only be true if all values are zero, so check
00536   // this per component
00537   unsigned BitWidth = VecTy->getElementType()->getIntegerBitWidth();
00538   for (unsigned I = 0, N = VecTy->getNumElements(); I != N; ++I) {
00539     Constant *Elem = C->getAggregateElement(I);
00540     if (isa<UndefValue>(Elem))
00541       return true;
00542 
00543     APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
00544     computeKnownBits(Elem, KnownZero, KnownOne, DL);
00545     if (KnownZero.isAllOnesValue())
00546       return true;
00547   }
00548 
00549   return false;
00550 }
00551 
00552 void Lint::visitSDiv(BinaryOperator &I) {
00553   Assert1(!isZero(I.getOperand(1), DL, DT, AT),
00554           "Undefined behavior: Division by zero", &I);
00555 }
00556 
00557 void Lint::visitUDiv(BinaryOperator &I) {
00558   Assert1(!isZero(I.getOperand(1), DL, DT, AT),
00559           "Undefined behavior: Division by zero", &I);
00560 }
00561 
00562 void Lint::visitSRem(BinaryOperator &I) {
00563   Assert1(!isZero(I.getOperand(1), DL, DT, AT),
00564           "Undefined behavior: Division by zero", &I);
00565 }
00566 
00567 void Lint::visitURem(BinaryOperator &I) {
00568   Assert1(!isZero(I.getOperand(1), DL, DT, AT),
00569           "Undefined behavior: Division by zero", &I);
00570 }
00571 
00572 void Lint::visitAllocaInst(AllocaInst &I) {
00573   if (isa<ConstantInt>(I.getArraySize()))
00574     // This isn't undefined behavior, it's just an obvious pessimization.
00575     Assert1(&I.getParent()->getParent()->getEntryBlock() == I.getParent(),
00576             "Pessimization: Static alloca outside of entry block", &I);
00577 
00578   // TODO: Check for an unusual size (MSB set?)
00579 }
00580 
00581 void Lint::visitVAArgInst(VAArgInst &I) {
00582   visitMemoryReference(I, I.getOperand(0), AliasAnalysis::UnknownSize, 0,
00583                        nullptr, MemRef::Read | MemRef::Write);
00584 }
00585 
00586 void Lint::visitIndirectBrInst(IndirectBrInst &I) {
00587   visitMemoryReference(I, I.getAddress(), AliasAnalysis::UnknownSize, 0,
00588                        nullptr, MemRef::Branchee);
00589 
00590   Assert1(I.getNumDestinations() != 0,
00591           "Undefined behavior: indirectbr with no destinations", &I);
00592 }
00593 
00594 void Lint::visitExtractElementInst(ExtractElementInst &I) {
00595   if (ConstantInt *CI =
00596         dyn_cast<ConstantInt>(findValue(I.getIndexOperand(),
00597                                         /*OffsetOk=*/false)))
00598     Assert1(CI->getValue().ult(I.getVectorOperandType()->getNumElements()),
00599             "Undefined result: extractelement index out of range", &I);
00600 }
00601 
00602 void Lint::visitInsertElementInst(InsertElementInst &I) {
00603   if (ConstantInt *CI =
00604         dyn_cast<ConstantInt>(findValue(I.getOperand(2),
00605                                         /*OffsetOk=*/false)))
00606     Assert1(CI->getValue().ult(I.getType()->getNumElements()),
00607             "Undefined result: insertelement index out of range", &I);
00608 }
00609 
00610 void Lint::visitUnreachableInst(UnreachableInst &I) {
00611   // This isn't undefined behavior, it's merely suspicious.
00612   Assert1(&I == I.getParent()->begin() ||
00613           std::prev(BasicBlock::iterator(&I))->mayHaveSideEffects(),
00614           "Unusual: unreachable immediately preceded by instruction without "
00615           "side effects", &I);
00616 }
00617 
00618 /// findValue - Look through bitcasts and simple memory reference patterns
00619 /// to identify an equivalent, but more informative, value.  If OffsetOk
00620 /// is true, look through getelementptrs with non-zero offsets too.
00621 ///
00622 /// Most analysis passes don't require this logic, because instcombine
00623 /// will simplify most of these kinds of things away. But it's a goal of
00624 /// this Lint pass to be useful even on non-optimized IR.
00625 Value *Lint::findValue(Value *V, bool OffsetOk) const {
00626   SmallPtrSet<Value *, 4> Visited;
00627   return findValueImpl(V, OffsetOk, Visited);
00628 }
00629 
00630 /// findValueImpl - Implementation helper for findValue.
00631 Value *Lint::findValueImpl(Value *V, bool OffsetOk,
00632                            SmallPtrSetImpl<Value *> &Visited) const {
00633   // Detect self-referential values.
00634   if (!Visited.insert(V).second)
00635     return UndefValue::get(V->getType());
00636 
00637   // TODO: Look through sext or zext cast, when the result is known to
00638   // be interpreted as signed or unsigned, respectively.
00639   // TODO: Look through eliminable cast pairs.
00640   // TODO: Look through calls with unique return values.
00641   // TODO: Look through vector insert/extract/shuffle.
00642   V = OffsetOk ? GetUnderlyingObject(V, DL) : V->stripPointerCasts();
00643   if (LoadInst *L = dyn_cast<LoadInst>(V)) {
00644     BasicBlock::iterator BBI = L;
00645     BasicBlock *BB = L->getParent();
00646     SmallPtrSet<BasicBlock *, 4> VisitedBlocks;
00647     for (;;) {
00648       if (!VisitedBlocks.insert(BB).second)
00649         break;
00650       if (Value *U = FindAvailableLoadedValue(L->getPointerOperand(),
00651                                               BB, BBI, 6, AA))
00652         return findValueImpl(U, OffsetOk, Visited);
00653       if (BBI != BB->begin()) break;
00654       BB = BB->getUniquePredecessor();
00655       if (!BB) break;
00656       BBI = BB->end();
00657     }
00658   } else if (PHINode *PN = dyn_cast<PHINode>(V)) {
00659     if (Value *W = PN->hasConstantValue())
00660       if (W != V)
00661         return findValueImpl(W, OffsetOk, Visited);
00662   } else if (CastInst *CI = dyn_cast<CastInst>(V)) {
00663     if (CI->isNoopCast(DL))
00664       return findValueImpl(CI->getOperand(0), OffsetOk, Visited);
00665   } else if (ExtractValueInst *Ex = dyn_cast<ExtractValueInst>(V)) {
00666     if (Value *W = FindInsertedValue(Ex->getAggregateOperand(),
00667                                      Ex->getIndices()))
00668       if (W != V)
00669         return findValueImpl(W, OffsetOk, Visited);
00670   } else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
00671     // Same as above, but for ConstantExpr instead of Instruction.
00672     if (Instruction::isCast(CE->getOpcode())) {
00673       if (CastInst::isNoopCast(Instruction::CastOps(CE->getOpcode()),
00674                                CE->getOperand(0)->getType(),
00675                                CE->getType(),
00676                                DL ? DL->getIntPtrType(V->getType()) :
00677                                     Type::getInt64Ty(V->getContext())))
00678         return findValueImpl(CE->getOperand(0), OffsetOk, Visited);
00679     } else if (CE->getOpcode() == Instruction::ExtractValue) {
00680       ArrayRef<unsigned> Indices = CE->getIndices();
00681       if (Value *W = FindInsertedValue(CE->getOperand(0), Indices))
00682         if (W != V)
00683           return findValueImpl(W, OffsetOk, Visited);
00684     }
00685   }
00686 
00687   // As a last resort, try SimplifyInstruction or constant folding.
00688   if (Instruction *Inst = dyn_cast<Instruction>(V)) {
00689     if (Value *W = SimplifyInstruction(Inst, DL, TLI, DT, AT))
00690       return findValueImpl(W, OffsetOk, Visited);
00691   } else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
00692     if (Value *W = ConstantFoldConstantExpression(CE, DL, TLI))
00693       if (W != V)
00694         return findValueImpl(W, OffsetOk, Visited);
00695   }
00696 
00697   return V;
00698 }
00699 
00700 //===----------------------------------------------------------------------===//
00701 //  Implement the public interfaces to this file...
00702 //===----------------------------------------------------------------------===//
00703 
00704 FunctionPass *llvm::createLintPass() {
00705   return new Lint();
00706 }
00707 
00708 /// lintFunction - Check a function for errors, printing messages on stderr.
00709 ///
00710 void llvm::lintFunction(const Function &f) {
00711   Function &F = const_cast<Function&>(f);
00712   assert(!F.isDeclaration() && "Cannot lint external functions");
00713 
00714   FunctionPassManager FPM(F.getParent());
00715   Lint *V = new Lint();
00716   FPM.add(V);
00717   FPM.run(F);
00718 }
00719 
00720 /// lintModule - Check a module for errors, printing messages on stderr.
00721 ///
00722 void llvm::lintModule(const Module &M) {
00723   PassManager PM;
00724   Lint *V = new Lint();
00725   PM.add(V);
00726   PM.run(const_cast<Module&>(M));
00727 }