lib/Analysis/Lint.cpp

   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/ConstantFolding.h"
  44 #include "llvm/Analysis/InstructionSimplify.h"
  45 #include "llvm/Analysis/Loads.h"
  46 #include "llvm/Analysis/MemoryLocation.h"
  47 #include "llvm/Analysis/Passes.h"
  48 #include "llvm/Analysis/TargetLibraryInfo.h"
  49 #include "llvm/Analysis/ValueTracking.h"
  50 #include "llvm/IR/Argument.h"
  51 #include "llvm/IR/BasicBlock.h"
  52 #include "llvm/IR/CallSite.h"
  53 #include "llvm/IR/Constant.h"
  54 #include "llvm/IR/Constants.h"
  55 #include "llvm/IR/DataLayout.h"
  56 #include "llvm/IR/DerivedTypes.h"
  57 #include "llvm/IR/Dominators.h"
  58 #include "llvm/IR/Function.h"
  59 #include "llvm/IR/GlobalVariable.h"
  60 #include "llvm/IR/InstVisitor.h"
  61 #include "llvm/IR/InstrTypes.h"
  62 #include "llvm/IR/Instruction.h"
  63 #include "llvm/IR/Instructions.h"
  64 #include "llvm/IR/IntrinsicInst.h"
  65 #include "llvm/IR/LegacyPassManager.h"
  66 #include "llvm/IR/Module.h"
  67 #include "llvm/IR/Type.h"
  68 #include "llvm/IR/Value.h"
  69 #include "llvm/Pass.h"
  70 #include "llvm/Support/Casting.h"
  71 #include "llvm/Support/Debug.h"
  72 #include "llvm/Support/KnownBits.h"
  73 #include "llvm/Support/MathExtras.h"
  74 #include "llvm/Support/raw_ostream.h"
  75 #include <cassert>
  76 #include <cstdint>
  77 #include <iterator>
  78 #include <string>
  79
  80 using namespace llvm;
  81
  82 namespace {
  83   namespace MemRef {
  84     static const unsigned Read     = 1;
  85     static const unsigned Write    = 2;
  86     static const unsigned Callee   = 4;
  87     static const unsigned Branchee = 8;
  88   } // end namespace MemRef
  89
  90   class Lint : public FunctionPass, public InstVisitor<Lint> {
  91     friend class InstVisitor<Lint>;
  92
  93     void visitFunction(Function &F);
  94
  95     void visitCallSite(CallSite CS);
  96     void visitMemoryReference(Instruction &I, Value *Ptr,
  97                               uint64_t Size, unsigned Align,
  98                               Type *Ty, unsigned Flags);
  99     void visitEHBeginCatch(IntrinsicInst *II);
 100     void visitEHEndCatch(IntrinsicInst *II);
 101
 102     void visitCallInst(CallInst &I);
 103     void visitInvokeInst(InvokeInst &I);
 104     void visitReturnInst(ReturnInst &I);
 105     void visitLoadInst(LoadInst &I);
 106     void visitStoreInst(StoreInst &I);
 107     void visitXor(BinaryOperator &I);
 108     void visitSub(BinaryOperator &I);
 109     void visitLShr(BinaryOperator &I);
 110     void visitAShr(BinaryOperator &I);
 111     void visitShl(BinaryOperator &I);
 112     void visitSDiv(BinaryOperator &I);
 113     void visitUDiv(BinaryOperator &I);
 114     void visitSRem(BinaryOperator &I);
 115     void visitURem(BinaryOperator &I);
 116     void visitAllocaInst(AllocaInst &I);
 117     void visitVAArgInst(VAArgInst &I);
 118     void visitIndirectBrInst(IndirectBrInst &I);
 119     void visitExtractElementInst(ExtractElementInst &I);
 120     void visitInsertElementInst(InsertElementInst &I);
 121     void visitUnreachableInst(UnreachableInst &I);
 122
 123     Value *findValue(Value *V, bool OffsetOk) const;
 124     Value *findValueImpl(Value *V, bool OffsetOk,
 125                          SmallPtrSetImpl<Value *> &Visited) const;
 126
 127   public:
 128     Module *Mod;
 129     const DataLayout *DL;
 130     AliasAnalysis *AA;
 131     AssumptionCache *AC;
 132     DominatorTree *DT;
 133     TargetLibraryInfo *TLI;
 134
 135     std::string Messages;
 136     raw_string_ostream MessagesStr;
 137
 138     static char ID; // Pass identification, replacement for typeid
 139     Lint() : FunctionPass(ID), MessagesStr(Messages) {
 140       initializeLintPass(*PassRegistry::getPassRegistry());
 141     }
 142
 143     bool runOnFunction(Function &F) override;
 144
 145     void getAnalysisUsage(AnalysisUsage &AU) const override {
 146       AU.setPreservesAll();
 147       AU.addRequired<AAResultsWrapperPass>();
 148       AU.addRequired<AssumptionCacheTracker>();
 149       AU.addRequired<TargetLibraryInfoWrapperPass>();
 150       AU.addRequired<DominatorTreeWrapperPass>();
 151     }
 152     void print(raw_ostream &O, const Module *M) const override {}
 153
 154     void WriteValues(ArrayRef<const Value *> Vs) {
 155       for (const Value *V : Vs) {
 156         if (!V)
 157           continue;
 158         if (isa<Instruction>(V)) {
 159           MessagesStr << *V << '\n';
 160         } else {
 161           V->printAsOperand(MessagesStr, true, Mod);
 162           MessagesStr << '\n';
 163         }
 164       }
 165     }
 166
 167     /// A check failed, so printout out the condition and the message.
 168     ///
 169     /// This provides a nice place to put a breakpoint if you want to see why
 170     /// something is not correct.
 171     void CheckFailed(const Twine &Message) { MessagesStr << Message << '\n'; }
 172
 173     /// A check failed (with values to print).
 174     ///
 175     /// This calls the Message-only version so that the above is easier to set
 176     /// a breakpoint on.
 177     template <typename T1, typename... Ts>
 178     void CheckFailed(const Twine &Message, const T1 &V1, const Ts &...Vs) {
 179       CheckFailed(Message);
 180       WriteValues({V1, Vs...});
 181     }
 182   };
 183 } // end anonymous namespace
 184
 185 char Lint::ID = 0;
 186 INITIALIZE_PASS_BEGIN(Lint, "lint", "Statically lint-checks LLVM IR",
 187                       false, true)
 188 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
 189 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
 190 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
 191 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
 192 INITIALIZE_PASS_END(Lint, "lint", "Statically lint-checks LLVM IR",
 193                     false, true)
 194
 195 // Assert - We know that cond should be true, if not print an error message.
 196 #define Assert(C, ...) \
 197     do { if (!(C)) { CheckFailed(__VA_ARGS__); return; } } while (false)
 198
 199 // Lint::run - This is the main Analysis entry point for a
 200 // function.
 201 //
 202 bool Lint::runOnFunction(Function &F) {
 203   Mod = F.getParent();
 204   DL = &F.getParent()->getDataLayout();
 205   AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
 206   AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
 207   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
 208   TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
 209   visit(F);
 210   dbgs() << MessagesStr.str();
 211   Messages.clear();
 212   return false;
 213 }
 214
 215 void Lint::visitFunction(Function &F) {
 216   // This isn't undefined behavior, it's just a little unusual, and it's a
 217   // fairly common mistake to neglect to name a function.
 218   Assert(F.hasName() || F.hasLocalLinkage(),
 219          "Unusual: Unnamed function with non-local linkage", &F);
 220
 221   // TODO: Check for irreducible control flow.
 222 }
 223
 224 void Lint::visitCallSite(CallSite CS) {
 225   Instruction &I = *CS.getInstruction();
 226   Value *Callee = CS.getCalledValue();
 227
 228   visitMemoryReference(I, Callee, MemoryLocation::UnknownSize, 0, nullptr,
 229                        MemRef::Callee);
 230
 231   if (Function *F = dyn_cast<Function>(findValue(Callee,
 232                                                  /*OffsetOk=*/false))) {
 233     Assert(CS.getCallingConv() == F->getCallingConv(),
 234            "Undefined behavior: Caller and callee calling convention differ",
 235            &I);
 236
 237     FunctionType *FT = F->getFunctionType();
 238     unsigned NumActualArgs = CS.arg_size();
 239
 240     Assert(FT->isVarArg() ? FT->getNumParams() <= NumActualArgs
 241                           : FT->getNumParams() == NumActualArgs,
 242            "Undefined behavior: Call argument count mismatches callee "
 243            "argument count",
 244            &I);
 245
 246     Assert(FT->getReturnType() == I.getType(),
 247            "Undefined behavior: Call return type mismatches "
 248            "callee return type",
 249            &I);
 250
 251     // Check argument types (in case the callee was casted) and attributes.
 252     // TODO: Verify that caller and callee attributes are compatible.
 253     Function::arg_iterator PI = F->arg_begin(), PE = F->arg_end();
 254     CallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
 255     for (; AI != AE; ++AI) {
 256       Value *Actual = *AI;
 257       if (PI != PE) {
 258         Argument *Formal = &*PI++;
 259         Assert(Formal->getType() == Actual->getType(),
 260                "Undefined behavior: Call argument type mismatches "
 261                "callee parameter type",
 262                &I);
 263
 264         // Check that noalias arguments don't alias other arguments. This is
 265         // not fully precise because we don't know the sizes of the dereferenced
 266         // memory regions.
 267         if (Formal->hasNoAliasAttr() && Actual->getType()->isPointerTy()) {
 268           AttributeList PAL = CS.getAttributes();
 269           unsigned ArgNo = 0;
 270           for (CallSite::arg_iterator BI = CS.arg_begin(); BI != AE;
 271                ++BI, ++ArgNo) {
 272             // Skip ByVal arguments since they will be memcpy'd to the callee's
 273             // stack so we're not really passing the pointer anyway.
 274             if (PAL.hasParamAttribute(ArgNo, Attribute::ByVal))
 275               continue;
 276             // If both arguments are readonly, they have no dependence.
 277             if (Formal->onlyReadsMemory() && CS.onlyReadsMemory(ArgNo))
 278               continue;
 279             if (AI != BI && (*BI)->getType()->isPointerTy()) {
 280               AliasResult Result = AA->alias(*AI, *BI);
 281               Assert(Result != MustAlias && Result != PartialAlias,
 282                      "Unusual: noalias argument aliases another argument", &I);
 283             }
 284           }
 285         }
 286
 287         // Check that an sret argument points to valid memory.
 288         if (Formal->hasStructRetAttr() && Actual->getType()->isPointerTy()) {
 289           Type *Ty =
 290             cast<PointerType>(Formal->getType())->getElementType();
 291           visitMemoryReference(I, Actual, DL->getTypeStoreSize(Ty),
 292                                DL->getABITypeAlignment(Ty), Ty,
 293                                MemRef::Read | MemRef::Write);
 294         }
 295       }
 296     }
 297   }
 298
 299   if (CS.isCall()) {
 300     const CallInst *CI = cast<CallInst>(CS.getInstruction());
 301     if (CI->isTailCall()) {
 302       const AttributeList &PAL = CI->getAttributes();
 303       unsigned ArgNo = 0;
 304       for (Value *Arg : CS.args()) {
 305         // Skip ByVal arguments since they will be memcpy'd to the callee's
 306         // stack anyway.
 307         if (PAL.hasParamAttribute(ArgNo++, Attribute::ByVal))
 308           continue;
 309         Value *Obj = findValue(Arg, /*OffsetOk=*/true);
 310         Assert(!isa<AllocaInst>(Obj),
 311                "Undefined behavior: Call with \"tail\" keyword references "
 312                "alloca",
 313                &I);
 314       }
 315     }
 316   }
 317
 318
 319   if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(&I))
 320     switch (II->getIntrinsicID()) {
 321     default: break;
 322
 323     // TODO: Check more intrinsics
 324
 325     case Intrinsic::memcpy: {
 326       MemCpyInst *MCI = cast<MemCpyInst>(&I);
 327       // TODO: If the size is known, use it.
 328       visitMemoryReference(I, MCI->getDest(), MemoryLocation::UnknownSize,
 329                            MCI->getDestAlignment(), nullptr, MemRef::Write);
 330       visitMemoryReference(I, MCI->getSource(), MemoryLocation::UnknownSize,
 331                            MCI->getSourceAlignment(), nullptr, MemRef::Read);
 332
 333       // Check that the memcpy arguments don't overlap. The AliasAnalysis API
 334       // isn't expressive enough for what we really want to do. Known partial
 335       // overlap is not distinguished from the case where nothing is known.
 336       auto Size = LocationSize::unknown();
 337       if (const ConstantInt *Len =
 338               dyn_cast<ConstantInt>(findValue(MCI->getLength(),
 339                                               /*OffsetOk=*/false)))
 340         if (Len->getValue().isIntN(32))
 341           Size = LocationSize::precise(Len->getValue().getZExtValue());
 342       Assert(AA->alias(MCI->getSource(), Size, MCI->getDest(), Size) !=
 343                  MustAlias,
 344              "Undefined behavior: memcpy source and destination overlap", &I);
 345       break;
 346     }
 347     case Intrinsic::memmove: {
 348       MemMoveInst *MMI = cast<MemMoveInst>(&I);
 349       // TODO: If the size is known, use it.
 350       visitMemoryReference(I, MMI->getDest(), MemoryLocation::UnknownSize,
 351                            MMI->getDestAlignment(), nullptr, MemRef::Write);
 352       visitMemoryReference(I, MMI->getSource(), MemoryLocation::UnknownSize,
 353                            MMI->getSourceAlignment(), nullptr, MemRef::Read);
 354       break;
 355     }
 356     case Intrinsic::memset: {
 357       MemSetInst *MSI = cast<MemSetInst>(&I);
 358       // TODO: If the size is known, use it.
 359       visitMemoryReference(I, MSI->getDest(), MemoryLocation::UnknownSize,
 360                            MSI->getDestAlignment(), nullptr, MemRef::Write);
 361       break;
 362     }
 363
 364     case Intrinsic::vastart:
 365       Assert(I.getParent()->getParent()->isVarArg(),
 366              "Undefined behavior: va_start called in a non-varargs function",
 367              &I);
 368
 369       visitMemoryReference(I, CS.getArgument(0), MemoryLocation::UnknownSize, 0,
 370                            nullptr, MemRef::Read | MemRef::Write);
 371       break;
 372     case Intrinsic::vacopy:
 373       visitMemoryReference(I, CS.getArgument(0), MemoryLocation::UnknownSize, 0,
 374                            nullptr, MemRef::Write);
 375       visitMemoryReference(I, CS.getArgument(1), MemoryLocation::UnknownSize, 0,
 376                            nullptr, MemRef::Read);
 377       break;
 378     case Intrinsic::vaend:
 379       visitMemoryReference(I, CS.getArgument(0), MemoryLocation::UnknownSize, 0,
 380                            nullptr, MemRef::Read | MemRef::Write);
 381       break;
 382
 383     case Intrinsic::stackrestore:
 384       // Stackrestore doesn't read or write memory, but it sets the
 385       // stack pointer, which the compiler may read from or write to
 386       // at any time, so check it for both readability and writeability.
 387       visitMemoryReference(I, CS.getArgument(0), MemoryLocation::UnknownSize, 0,
 388                            nullptr, MemRef::Read | MemRef::Write);
 389       break;
 390     }
 391 }
 392
 393 void Lint::visitCallInst(CallInst &I) {
 394   return visitCallSite(&I);
 395 }
 396
 397 void Lint::visitInvokeInst(InvokeInst &I) {
 398   return visitCallSite(&I);
 399 }
 400
 401 void Lint::visitReturnInst(ReturnInst &I) {
 402   Function *F = I.getParent()->getParent();
 403   Assert(!F->doesNotReturn(),
 404          "Unusual: Return statement in function with noreturn attribute", &I);
 405
 406   if (Value *V = I.getReturnValue()) {
 407     Value *Obj = findValue(V, /*OffsetOk=*/true);
 408     Assert(!isa<AllocaInst>(Obj), "Unusual: Returning alloca value", &I);
 409   }
 410 }
 411
 412 // TODO: Check that the reference is in bounds.
 413 // TODO: Check readnone/readonly function attributes.
 414 void Lint::visitMemoryReference(Instruction &I,
 415                                 Value *Ptr, uint64_t Size, unsigned Align,
 416                                 Type *Ty, unsigned Flags) {
 417   // If no memory is being referenced, it doesn't matter if the pointer
 418   // is valid.
 419   if (Size == 0)
 420     return;
 421
 422   Value *UnderlyingObject = findValue(Ptr, /*OffsetOk=*/true);
 423   Assert(!isa<ConstantPointerNull>(UnderlyingObject),
 424          "Undefined behavior: Null pointer dereference", &I);
 425   Assert(!isa<UndefValue>(UnderlyingObject),
 426          "Undefined behavior: Undef pointer dereference", &I);
 427   Assert(!isa<ConstantInt>(UnderlyingObject) ||
 428              !cast<ConstantInt>(UnderlyingObject)->isMinusOne(),
 429          "Unusual: All-ones pointer dereference", &I);
 430   Assert(!isa<ConstantInt>(UnderlyingObject) ||
 431              !cast<ConstantInt>(UnderlyingObject)->isOne(),
 432          "Unusual: Address one pointer dereference", &I);
 433
 434   if (Flags & MemRef::Write) {
 435     if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(UnderlyingObject))
 436       Assert(!GV->isConstant(), "Undefined behavior: Write to read-only memory",
 437              &I);
 438     Assert(!isa<Function>(UnderlyingObject) &&
 439                !isa<BlockAddress>(UnderlyingObject),
 440            "Undefined behavior: Write to text section", &I);
 441   }
 442   if (Flags & MemRef::Read) {
 443     Assert(!isa<Function>(UnderlyingObject), "Unusual: Load from function body",
 444            &I);
 445     Assert(!isa<BlockAddress>(UnderlyingObject),
 446            "Undefined behavior: Load from block address", &I);
 447   }
 448   if (Flags & MemRef::Callee) {
 449     Assert(!isa<BlockAddress>(UnderlyingObject),
 450            "Undefined behavior: Call to block address", &I);
 451   }
 452   if (Flags & MemRef::Branchee) {
 453     Assert(!isa<Constant>(UnderlyingObject) ||
 454                isa<BlockAddress>(UnderlyingObject),
 455            "Undefined behavior: Branch to non-blockaddress", &I);
 456   }
 457
 458   // Check for buffer overflows and misalignment.
 459   // Only handles memory references that read/write something simple like an
 460   // alloca instruction or a global variable.
 461   int64_t Offset = 0;
 462   if (Value *Base = GetPointerBaseWithConstantOffset(Ptr, Offset, *DL)) {
 463     // OK, so the access is to a constant offset from Ptr.  Check that Ptr is
 464     // something we can handle and if so extract the size of this base object
 465     // along with its alignment.
 466     uint64_t BaseSize = MemoryLocation::UnknownSize;
 467     unsigned BaseAlign = 0;
 468
 469     if (AllocaInst *AI = dyn_cast<AllocaInst>(Base)) {
 470       Type *ATy = AI->getAllocatedType();
 471       if (!AI->isArrayAllocation() && ATy->isSized())
 472         BaseSize = DL->getTypeAllocSize(ATy);
 473       BaseAlign = AI->getAlignment();
 474       if (BaseAlign == 0 && ATy->isSized())
 475         BaseAlign = DL->getABITypeAlignment(ATy);
 476     } else if (GlobalVariable *GV = dyn_cast<GlobalVariable>(Base)) {
 477       // If the global may be defined differently in another compilation unit
 478       // then don't warn about funky memory accesses.
 479       if (GV->hasDefinitiveInitializer()) {
 480         Type *GTy = GV->getValueType();
 481         if (GTy->isSized())
 482           BaseSize = DL->getTypeAllocSize(GTy);
 483         BaseAlign = GV->getAlignment();
 484         if (BaseAlign == 0 && GTy->isSized())
 485           BaseAlign = DL->getABITypeAlignment(GTy);
 486       }
 487     }
 488
 489     // Accesses from before the start or after the end of the object are not
 490     // defined.
 491     Assert(Size == MemoryLocation::UnknownSize ||
 492                BaseSize == MemoryLocation::UnknownSize ||
 493                (Offset >= 0 && Offset + Size <= BaseSize),
 494            "Undefined behavior: Buffer overflow", &I);
 495
 496     // Accesses that say that the memory is more aligned than it is are not
 497     // defined.
 498     if (Align == 0 && Ty && Ty->isSized())
 499       Align = DL->getABITypeAlignment(Ty);
 500     Assert(!BaseAlign || Align <= MinAlign(BaseAlign, Offset),
 501            "Undefined behavior: Memory reference address is misaligned", &I);
 502   }
 503 }
 504
 505 void Lint::visitLoadInst(LoadInst &I) {
 506   visitMemoryReference(I, I.getPointerOperand(),
 507                        DL->getTypeStoreSize(I.getType()), I.getAlignment(),
 508                        I.getType(), MemRef::Read);
 509 }
 510
 511 void Lint::visitStoreInst(StoreInst &I) {
 512   visitMemoryReference(I, I.getPointerOperand(),
 513                        DL->getTypeStoreSize(I.getOperand(0)->getType()),
 514                        I.getAlignment(),
 515                        I.getOperand(0)->getType(), MemRef::Write);
 516 }
 517
 518 void Lint::visitXor(BinaryOperator &I) {
 519   Assert(!isa<UndefValue>(I.getOperand(0)) || !isa<UndefValue>(I.getOperand(1)),
 520          "Undefined result: xor(undef, undef)", &I);
 521 }
 522
 523 void Lint::visitSub(BinaryOperator &I) {
 524   Assert(!isa<UndefValue>(I.getOperand(0)) || !isa<UndefValue>(I.getOperand(1)),
 525          "Undefined result: sub(undef, undef)", &I);
 526 }
 527
 528 void Lint::visitLShr(BinaryOperator &I) {
 529   if (ConstantInt *CI = dyn_cast<ConstantInt>(findValue(I.getOperand(1),
 530                                                         /*OffsetOk=*/false)))
 531     Assert(CI->getValue().ult(cast<IntegerType>(I.getType())->getBitWidth()),
 532            "Undefined result: Shift count out of range", &I);
 533 }
 534
 535 void Lint::visitAShr(BinaryOperator &I) {
 536   if (ConstantInt *CI =
 537           dyn_cast<ConstantInt>(findValue(I.getOperand(1), /*OffsetOk=*/false)))
 538     Assert(CI->getValue().ult(cast<IntegerType>(I.getType())->getBitWidth()),
 539            "Undefined result: Shift count out of range", &I);
 540 }
 541
 542 void Lint::visitShl(BinaryOperator &I) {
 543   if (ConstantInt *CI =
 544           dyn_cast<ConstantInt>(findValue(I.getOperand(1), /*OffsetOk=*/false)))
 545     Assert(CI->getValue().ult(cast<IntegerType>(I.getType())->getBitWidth()),
 546            "Undefined result: Shift count out of range", &I);
 547 }
 548
 549 static bool isZero(Value *V, const DataLayout &DL, DominatorTree *DT,
 550                    AssumptionCache *AC) {
 551   // Assume undef could be zero.
 552   if (isa<UndefValue>(V))
 553     return true;
 554
 555   VectorType *VecTy = dyn_cast<VectorType>(V->getType());
 556   if (!VecTy) {
 557     KnownBits Known = computeKnownBits(V, DL, 0, AC, dyn_cast<Instruction>(V), DT);
 558     return Known.isZero();
 559   }
 560
 561   // Per-component check doesn't work with zeroinitializer
 562   Constant *C = dyn_cast<Constant>(V);
 563   if (!C)
 564     return false;
 565
 566   if (C->isZeroValue())
 567     return true;
 568
 569   // For a vector, KnownZero will only be true if all values are zero, so check
 570   // this per component
 571   for (unsigned I = 0, N = VecTy->getNumElements(); I != N; ++I) {
 572     Constant *Elem = C->getAggregateElement(I);
 573     if (isa<UndefValue>(Elem))
 574       return true;
 575
 576     KnownBits Known = computeKnownBits(Elem, DL);
 577     if (Known.isZero())
 578       return true;
 579   }
 580
 581   return false;
 582 }
 583
 584 void Lint::visitSDiv(BinaryOperator &I) {
 585   Assert(!isZero(I.getOperand(1), I.getModule()->getDataLayout(), DT, AC),
 586          "Undefined behavior: Division by zero", &I);
 587 }
 588
 589 void Lint::visitUDiv(BinaryOperator &I) {
 590   Assert(!isZero(I.getOperand(1), I.getModule()->getDataLayout(), DT, AC),
 591          "Undefined behavior: Division by zero", &I);
 592 }
 593
 594 void Lint::visitSRem(BinaryOperator &I) {
 595   Assert(!isZero(I.getOperand(1), I.getModule()->getDataLayout(), DT, AC),
 596          "Undefined behavior: Division by zero", &I);
 597 }
 598
 599 void Lint::visitURem(BinaryOperator &I) {
 600   Assert(!isZero(I.getOperand(1), I.getModule()->getDataLayout(), DT, AC),
 601          "Undefined behavior: Division by zero", &I);
 602 }
 603
 604 void Lint::visitAllocaInst(AllocaInst &I) {
 605   if (isa<ConstantInt>(I.getArraySize()))
 606     // This isn't undefined behavior, it's just an obvious pessimization.
 607     Assert(&I.getParent()->getParent()->getEntryBlock() == I.getParent(),
 608            "Pessimization: Static alloca outside of entry block", &I);
 609
 610   // TODO: Check for an unusual size (MSB set?)
 611 }
 612
 613 void Lint::visitVAArgInst(VAArgInst &I) {
 614   visitMemoryReference(I, I.getOperand(0), MemoryLocation::UnknownSize, 0,
 615                        nullptr, MemRef::Read | MemRef::Write);
 616 }
 617
 618 void Lint::visitIndirectBrInst(IndirectBrInst &I) {
 619   visitMemoryReference(I, I.getAddress(), MemoryLocation::UnknownSize, 0,
 620                        nullptr, MemRef::Branchee);
 621
 622   Assert(I.getNumDestinations() != 0,
 623          "Undefined behavior: indirectbr with no destinations", &I);
 624 }
 625
 626 void Lint::visitExtractElementInst(ExtractElementInst &I) {
 627   if (ConstantInt *CI = dyn_cast<ConstantInt>(findValue(I.getIndexOperand(),
 628                                                         /*OffsetOk=*/false)))
 629     Assert(CI->getValue().ult(I.getVectorOperandType()->getNumElements()),
 630            "Undefined result: extractelement index out of range", &I);
 631 }
 632
 633 void Lint::visitInsertElementInst(InsertElementInst &I) {
 634   if (ConstantInt *CI = dyn_cast<ConstantInt>(findValue(I.getOperand(2),
 635                                                         /*OffsetOk=*/false)))
 636     Assert(CI->getValue().ult(I.getType()->getNumElements()),
 637            "Undefined result: insertelement index out of range", &I);
 638 }
 639
 640 void Lint::visitUnreachableInst(UnreachableInst &I) {
 641   // This isn't undefined behavior, it's merely suspicious.
 642   Assert(&I == &I.getParent()->front() ||
 643              std::prev(I.getIterator())->mayHaveSideEffects(),
 644          "Unusual: unreachable immediately preceded by instruction without "
 645          "side effects",
 646          &I);
 647 }
 648
 649 /// findValue - Look through bitcasts and simple memory reference patterns
 650 /// to identify an equivalent, but more informative, value.  If OffsetOk
 651 /// is true, look through getelementptrs with non-zero offsets too.
 652 ///
 653 /// Most analysis passes don't require this logic, because instcombine
 654 /// will simplify most of these kinds of things away. But it's a goal of
 655 /// this Lint pass to be useful even on non-optimized IR.
 656 Value *Lint::findValue(Value *V, bool OffsetOk) const {
 657   SmallPtrSet<Value *, 4> Visited;
 658   return findValueImpl(V, OffsetOk, Visited);
 659 }
 660
 661 /// findValueImpl - Implementation helper for findValue.
 662 Value *Lint::findValueImpl(Value *V, bool OffsetOk,
 663                            SmallPtrSetImpl<Value *> &Visited) const {
 664   // Detect self-referential values.
 665   if (!Visited.insert(V).second)
 666     return UndefValue::get(V->getType());
 667
 668   // TODO: Look through sext or zext cast, when the result is known to
 669   // be interpreted as signed or unsigned, respectively.
 670   // TODO: Look through eliminable cast pairs.
 671   // TODO: Look through calls with unique return values.
 672   // TODO: Look through vector insert/extract/shuffle.
 673   V = OffsetOk ? GetUnderlyingObject(V, *DL) : V->stripPointerCasts();
 674   if (LoadInst *L = dyn_cast<LoadInst>(V)) {
 675     BasicBlock::iterator BBI = L->getIterator();
 676     BasicBlock *BB = L->getParent();
 677     SmallPtrSet<BasicBlock *, 4> VisitedBlocks;
 678     for (;;) {
 679       if (!VisitedBlocks.insert(BB).second)
 680         break;
 681       if (Value *U =
 682           FindAvailableLoadedValue(L, BB, BBI, DefMaxInstsToScan, AA))
 683         return findValueImpl(U, OffsetOk, Visited);
 684       if (BBI != BB->begin()) break;
 685       BB = BB->getUniquePredecessor();
 686       if (!BB) break;
 687       BBI = BB->end();
 688     }
 689   } else if (PHINode *PN = dyn_cast<PHINode>(V)) {
 690     if (Value *W = PN->hasConstantValue())
 691       if (W != V)
 692         return findValueImpl(W, OffsetOk, Visited);
 693   } else if (CastInst *CI = dyn_cast<CastInst>(V)) {
 694     if (CI->isNoopCast(*DL))
 695       return findValueImpl(CI->getOperand(0), OffsetOk, Visited);
 696   } else if (ExtractValueInst *Ex = dyn_cast<ExtractValueInst>(V)) {
 697     if (Value *W = FindInsertedValue(Ex->getAggregateOperand(),
 698                                      Ex->getIndices()))
 699       if (W != V)
 700         return findValueImpl(W, OffsetOk, Visited);
 701   } else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
 702     // Same as above, but for ConstantExpr instead of Instruction.
 703     if (Instruction::isCast(CE->getOpcode())) {
 704       if (CastInst::isNoopCast(Instruction::CastOps(CE->getOpcode()),
 705                                CE->getOperand(0)->getType(), CE->getType(),
 706                                *DL))
 707         return findValueImpl(CE->getOperand(0), OffsetOk, Visited);
 708     } else if (CE->getOpcode() == Instruction::ExtractValue) {
 709       ArrayRef<unsigned> Indices = CE->getIndices();
 710       if (Value *W = FindInsertedValue(CE->getOperand(0), Indices))
 711         if (W != V)
 712           return findValueImpl(W, OffsetOk, Visited);
 713     }
 714   }
 715
 716   // As a last resort, try SimplifyInstruction or constant folding.
 717   if (Instruction *Inst = dyn_cast<Instruction>(V)) {
 718     if (Value *W = SimplifyInstruction(Inst, {*DL, TLI, DT, AC}))
 719       return findValueImpl(W, OffsetOk, Visited);
 720   } else if (auto *C = dyn_cast<Constant>(V)) {
 721     if (Value *W = ConstantFoldConstant(C, *DL, TLI))
 722       if (W && W != V)
 723         return findValueImpl(W, OffsetOk, Visited);
 724   }
 725
 726   return V;
 727 }
 728
 729 //===----------------------------------------------------------------------===//
 730 //  Implement the public interfaces to this file...
 731 //===----------------------------------------------------------------------===//
 732
 733 FunctionPass *llvm::createLintPass() {
 734   return new Lint();
 735 }
 736
 737 /// lintFunction - Check a function for errors, printing messages on stderr.
 738 ///
 739 void llvm::lintFunction(const Function &f) {
 740   Function &F = const_cast<Function&>(f);
 741   assert(!F.isDeclaration() && "Cannot lint external functions");
 742
 743   legacy::FunctionPassManager FPM(F.getParent());
 744   Lint *V = new Lint();
 745   FPM.add(V);
 746   FPM.run(F);
 747 }
 748
 749 /// lintModule - Check a module for errors, printing messages on stderr.
 750 ///
 751 void llvm::lintModule(const Module &M) {
 752   legacy::PassManager PM;
 753   Lint *V = new Lint();
 754   PM.add(V);
 755   PM.run(const_cast<Module&>(M));
 756 }