clang/lib/Analysis/UninitializedValues.cpp

   1 //===- UninitializedValues.cpp - Find Uninitialized Values ----------------===//
   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 file implements uninitialized values analysis for source-level CFGs.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "clang/Analysis/Analyses/UninitializedValues.h"
  14 #include "clang/AST/Attr.h"
  15 #include "clang/AST/Decl.h"
  16 #include "clang/AST/DeclBase.h"
  17 #include "clang/AST/Expr.h"
  18 #include "clang/AST/OperationKinds.h"
  19 #include "clang/AST/Stmt.h"
  20 #include "clang/AST/StmtObjC.h"
  21 #include "clang/AST/StmtVisitor.h"
  22 #include "clang/AST/Type.h"
  23 #include "clang/Analysis/Analyses/PostOrderCFGView.h"
  24 #include "clang/Analysis/AnalysisDeclContext.h"
  25 #include "clang/Analysis/CFG.h"
  26 #include "clang/Analysis/DomainSpecific/ObjCNoReturn.h"
  27 #include "clang/Analysis/FlowSensitive/DataflowWorklist.h"
  28 #include "clang/Basic/LLVM.h"
  29 #include "llvm/ADT/BitVector.h"
  30 #include "llvm/ADT/DenseMap.h"
  31 #include "llvm/ADT/PackedVector.h"
  32 #include "llvm/ADT/SmallBitVector.h"
  33 #include "llvm/ADT/SmallVector.h"
  34 #include "llvm/Support/Casting.h"
  35 #include <algorithm>
  36 #include <cassert>
  37 #include <optional>
  38
  39 using namespace clang;
  40
  41 #define DEBUG_LOGGING 0
  42
  43 static bool recordIsNotEmpty(const RecordDecl *RD) {
  44   // We consider a record decl to be empty if it contains only unnamed bit-
  45   // fields, zero-width fields, and fields of empty record type.
  46   for (const auto *FD : RD->fields()) {
  47     if (FD->isUnnamedBitfield())
  48       continue;
  49     if (FD->isZeroSize(FD->getASTContext()))
  50       continue;
  51     // The only case remaining to check is for a field declaration of record
  52     // type and whether that record itself is empty.
  53     if (const auto *FieldRD = FD->getType()->getAsRecordDecl();
  54         !FieldRD || recordIsNotEmpty(FieldRD))
  55       return true;
  56   }
  57   return false;
  58 }
  59
  60 static bool isTrackedVar(const VarDecl *vd, const DeclContext *dc) {
  61   if (vd->isLocalVarDecl() && !vd->hasGlobalStorage() &&
  62       !vd->isExceptionVariable() && !vd->isInitCapture() && !vd->isImplicit() &&
  63       vd->getDeclContext() == dc) {
  64     QualType ty = vd->getType();
  65     if (const auto *RD = ty->getAsRecordDecl())
  66       return recordIsNotEmpty(RD);
  67     return ty->isScalarType() || ty->isVectorType() || ty->isRVVType();
  68   }
  69   return false;
  70 }
  71
  72 //------------------------------------------------------------------------====//
  73 // DeclToIndex: a mapping from Decls we track to value indices.
  74 //====------------------------------------------------------------------------//
  75
  76 namespace {
  77
  78 class DeclToIndex {
  79   llvm::DenseMap<const VarDecl *, unsigned> map;
  80
  81 public:
  82   DeclToIndex() = default;
  83
  84   /// Compute the actual mapping from declarations to bits.
  85   void computeMap(const DeclContext &dc);
  86
  87   /// Return the number of declarations in the map.
  88   unsigned size() const { return map.size(); }
  89
  90   /// Returns the bit vector index for a given declaration.
  91   std::optional<unsigned> getValueIndex(const VarDecl *d) const;
  92 };
  93
  94 } // namespace
  95
  96 void DeclToIndex::computeMap(const DeclContext &dc) {
  97   unsigned count = 0;
  98   DeclContext::specific_decl_iterator<VarDecl> I(dc.decls_begin()),
  99                                                E(dc.decls_end());
 100   for ( ; I != E; ++I) {
 101     const VarDecl *vd = *I;
 102     if (isTrackedVar(vd, &dc))
 103       map[vd] = count++;
 104   }
 105 }
 106
 107 std::optional<unsigned> DeclToIndex::getValueIndex(const VarDecl *d) const {
 108   llvm::DenseMap<const VarDecl *, unsigned>::const_iterator I = map.find(d);
 109   if (I == map.end())
 110     return std::nullopt;
 111   return I->second;
 112 }
 113
 114 //------------------------------------------------------------------------====//
 115 // CFGBlockValues: dataflow values for CFG blocks.
 116 //====------------------------------------------------------------------------//
 117
 118 // These values are defined in such a way that a merge can be done using
 119 // a bitwise OR.
 120 enum Value { Unknown = 0x0,         /* 00 */
 121              Initialized = 0x1,     /* 01 */
 122              Uninitialized = 0x2,   /* 10 */
 123              MayUninitialized = 0x3 /* 11 */ };
 124
 125 static bool isUninitialized(const Value v) {
 126   return v >= Uninitialized;
 127 }
 128
 129 static bool isAlwaysUninit(const Value v) {
 130   return v == Uninitialized;
 131 }
 132
 133 namespace {
 134
 135 using ValueVector = llvm::PackedVector<Value, 2, llvm::SmallBitVector>;
 136
 137 class CFGBlockValues {
 138   const CFG &cfg;
 139   SmallVector<ValueVector, 8> vals;
 140   ValueVector scratch;
 141   DeclToIndex declToIndex;
 142
 143 public:
 144   CFGBlockValues(const CFG &cfg);
 145
 146   unsigned getNumEntries() const { return declToIndex.size(); }
 147
 148   void computeSetOfDeclarations(const DeclContext &dc);
 149
 150   ValueVector &getValueVector(const CFGBlock *block) {
 151     return vals[block->getBlockID()];
 152   }
 153
 154   void setAllScratchValues(Value V);
 155   void mergeIntoScratch(ValueVector const &source, bool isFirst);
 156   bool updateValueVectorWithScratch(const CFGBlock *block);
 157
 158   bool hasNoDeclarations() const {
 159     return declToIndex.size() == 0;
 160   }
 161
 162   void resetScratch();
 163
 164   ValueVector::reference operator[](const VarDecl *vd);
 165
 166   Value getValue(const CFGBlock *block, const CFGBlock *dstBlock,
 167                  const VarDecl *vd) {
 168     std::optional<unsigned> idx = declToIndex.getValueIndex(vd);
 169     return getValueVector(block)[*idx];
 170   }
 171 };
 172
 173 } // namespace
 174
 175 CFGBlockValues::CFGBlockValues(const CFG &c) : cfg(c), vals(0) {}
 176
 177 void CFGBlockValues::computeSetOfDeclarations(const DeclContext &dc) {
 178   declToIndex.computeMap(dc);
 179   unsigned decls = declToIndex.size();
 180   scratch.resize(decls);
 181   unsigned n = cfg.getNumBlockIDs();
 182   if (!n)
 183     return;
 184   vals.resize(n);
 185   for (auto &val : vals)
 186     val.resize(decls);
 187 }
 188
 189 #if DEBUG_LOGGING
 190 static void printVector(const CFGBlock *block, ValueVector &bv,
 191                         unsigned num) {
 192   llvm::errs() << block->getBlockID() << " :";
 193   for (const auto &i : bv)
 194     llvm::errs() << ' ' << i;
 195   llvm::errs() << " : " << num << '\n';
 196 }
 197 #endif
 198
 199 void CFGBlockValues::setAllScratchValues(Value V) {
 200   for (unsigned I = 0, E = scratch.size(); I != E; ++I)
 201     scratch[I] = V;
 202 }
 203
 204 void CFGBlockValues::mergeIntoScratch(ValueVector const &source,
 205                                       bool isFirst) {
 206   if (isFirst)
 207     scratch = source;
 208   else
 209     scratch |= source;
 210 }
 211
 212 bool CFGBlockValues::updateValueVectorWithScratch(const CFGBlock *block) {
 213   ValueVector &dst = getValueVector(block);
 214   bool changed = (dst != scratch);
 215   if (changed)
 216     dst = scratch;
 217 #if DEBUG_LOGGING
 218   printVector(block, scratch, 0);
 219 #endif
 220   return changed;
 221 }
 222
 223 void CFGBlockValues::resetScratch() {
 224   scratch.reset();
 225 }
 226
 227 ValueVector::reference CFGBlockValues::operator[](const VarDecl *vd) {
 228   return scratch[*declToIndex.getValueIndex(vd)];
 229 }
 230
 231 //------------------------------------------------------------------------====//
 232 // Classification of DeclRefExprs as use or initialization.
 233 //====------------------------------------------------------------------------//
 234
 235 namespace {
 236
 237 class FindVarResult {
 238   const VarDecl *vd;
 239   const DeclRefExpr *dr;
 240
 241 public:
 242   FindVarResult(const VarDecl *vd, const DeclRefExpr *dr) : vd(vd), dr(dr) {}
 243
 244   const DeclRefExpr *getDeclRefExpr() const { return dr; }
 245   const VarDecl *getDecl() const { return vd; }
 246 };
 247
 248 } // namespace
 249
 250 static const Expr *stripCasts(ASTContext &C, const Expr *Ex) {
 251   while (Ex) {
 252     Ex = Ex->IgnoreParenNoopCasts(C);
 253     if (const auto *CE = dyn_cast<CastExpr>(Ex)) {
 254       if (CE->getCastKind() == CK_LValueBitCast) {
 255         Ex = CE->getSubExpr();
 256         continue;
 257       }
 258     }
 259     break;
 260   }
 261   return Ex;
 262 }
 263
 264 /// If E is an expression comprising a reference to a single variable, find that
 265 /// variable.
 266 static FindVarResult findVar(const Expr *E, const DeclContext *DC) {
 267   if (const auto *DRE =
 268           dyn_cast<DeclRefExpr>(stripCasts(DC->getParentASTContext(), E)))
 269     if (const auto *VD = dyn_cast<VarDecl>(DRE->getDecl()))
 270       if (isTrackedVar(VD, DC))
 271         return FindVarResult(VD, DRE);
 272   return FindVarResult(nullptr, nullptr);
 273 }
 274
 275 namespace {
 276
 277 /// Classify each DeclRefExpr as an initialization or a use. Any
 278 /// DeclRefExpr which isn't explicitly classified will be assumed to have
 279 /// escaped the analysis and will be treated as an initialization.
 280 class ClassifyRefs : public StmtVisitor<ClassifyRefs> {
 281 public:
 282   enum Class {
 283     Init,
 284     Use,
 285     SelfInit,
 286     ConstRefUse,
 287     Ignore
 288   };
 289
 290 private:
 291   const DeclContext *DC;
 292   llvm::DenseMap<const DeclRefExpr *, Class> Classification;
 293
 294   bool isTrackedVar(const VarDecl *VD) const {
 295     return ::isTrackedVar(VD, DC);
 296   }
 297
 298   void classify(const Expr *E, Class C);
 299
 300 public:
 301   ClassifyRefs(AnalysisDeclContext &AC) : DC(cast<DeclContext>(AC.getDecl())) {}
 302
 303   void VisitDeclStmt(DeclStmt *DS);
 304   void VisitUnaryOperator(UnaryOperator *UO);
 305   void VisitBinaryOperator(BinaryOperator *BO);
 306   void VisitCallExpr(CallExpr *CE);
 307   void VisitCastExpr(CastExpr *CE);
 308   void VisitOMPExecutableDirective(OMPExecutableDirective *ED);
 309
 310   void operator()(Stmt *S) { Visit(S); }
 311
 312   Class get(const DeclRefExpr *DRE) const {
 313     llvm::DenseMap<const DeclRefExpr*, Class>::const_iterator I
 314         = Classification.find(DRE);
 315     if (I != Classification.end())
 316       return I->second;
 317
 318     const auto *VD = dyn_cast<VarDecl>(DRE->getDecl());
 319     if (!VD || !isTrackedVar(VD))
 320       return Ignore;
 321
 322     return Init;
 323   }
 324 };
 325
 326 } // namespace
 327
 328 static const DeclRefExpr *getSelfInitExpr(VarDecl *VD) {
 329   if (VD->getType()->isRecordType())
 330     return nullptr;
 331   if (Expr *Init = VD->getInit()) {
 332     const auto *DRE =
 333         dyn_cast<DeclRefExpr>(stripCasts(VD->getASTContext(), Init));
 334     if (DRE && DRE->getDecl() == VD)
 335       return DRE;
 336   }
 337   return nullptr;
 338 }
 339
 340 void ClassifyRefs::classify(const Expr *E, Class C) {
 341   // The result of a ?: could also be an lvalue.
 342   E = E->IgnoreParens();
 343   if (const auto *CO = dyn_cast<ConditionalOperator>(E)) {
 344     classify(CO->getTrueExpr(), C);
 345     classify(CO->getFalseExpr(), C);
 346     return;
 347   }
 348
 349   if (const auto *BCO = dyn_cast<BinaryConditionalOperator>(E)) {
 350     classify(BCO->getFalseExpr(), C);
 351     return;
 352   }
 353
 354   if (const auto *OVE = dyn_cast<OpaqueValueExpr>(E)) {
 355     classify(OVE->getSourceExpr(), C);
 356     return;
 357   }
 358
 359   if (const auto *ME = dyn_cast<MemberExpr>(E)) {
 360     if (const auto *VD = dyn_cast<VarDecl>(ME->getMemberDecl())) {
 361       if (!VD->isStaticDataMember())
 362         classify(ME->getBase(), C);
 363     }
 364     return;
 365   }
 366
 367   if (const auto *BO = dyn_cast<BinaryOperator>(E)) {
 368     switch (BO->getOpcode()) {
 369     case BO_PtrMemD:
 370     case BO_PtrMemI:
 371       classify(BO->getLHS(), C);
 372       return;
 373     case BO_Comma:
 374       classify(BO->getRHS(), C);
 375       return;
 376     default:
 377       return;
 378     }
 379   }
 380
 381   FindVarResult Var = findVar(E, DC);
 382   if (const DeclRefExpr *DRE = Var.getDeclRefExpr())
 383     Classification[DRE] = std::max(Classification[DRE], C);
 384 }
 385
 386 void ClassifyRefs::VisitDeclStmt(DeclStmt *DS) {
 387   for (auto *DI : DS->decls()) {
 388     auto *VD = dyn_cast<VarDecl>(DI);
 389     if (VD && isTrackedVar(VD))
 390       if (const DeclRefExpr *DRE = getSelfInitExpr(VD))
 391         Classification[DRE] = SelfInit;
 392   }
 393 }
 394
 395 void ClassifyRefs::VisitBinaryOperator(BinaryOperator *BO) {
 396   // Ignore the evaluation of a DeclRefExpr on the LHS of an assignment. If this
 397   // is not a compound-assignment, we will treat it as initializing the variable
 398   // when TransferFunctions visits it. A compound-assignment does not affect
 399   // whether a variable is uninitialized, and there's no point counting it as a
 400   // use.
 401   if (BO->isCompoundAssignmentOp())
 402     classify(BO->getLHS(), Use);
 403   else if (BO->getOpcode() == BO_Assign || BO->getOpcode() == BO_Comma)
 404     classify(BO->getLHS(), Ignore);
 405 }
 406
 407 void ClassifyRefs::VisitUnaryOperator(UnaryOperator *UO) {
 408   // Increment and decrement are uses despite there being no lvalue-to-rvalue
 409   // conversion.
 410   if (UO->isIncrementDecrementOp())
 411     classify(UO->getSubExpr(), Use);
 412 }
 413
 414 void ClassifyRefs::VisitOMPExecutableDirective(OMPExecutableDirective *ED) {
 415   for (Stmt *S : OMPExecutableDirective::used_clauses_children(ED->clauses()))
 416     classify(cast<Expr>(S), Use);
 417 }
 418
 419 static bool isPointerToConst(const QualType &QT) {
 420   return QT->isAnyPointerType() && QT->getPointeeType().isConstQualified();
 421 }
 422
 423 static bool hasTrivialBody(CallExpr *CE) {
 424   if (FunctionDecl *FD = CE->getDirectCallee()) {
 425     if (FunctionTemplateDecl *FTD = FD->getPrimaryTemplate())
 426       return FTD->getTemplatedDecl()->hasTrivialBody();
 427     return FD->hasTrivialBody();
 428   }
 429   return false;
 430 }
 431
 432 void ClassifyRefs::VisitCallExpr(CallExpr *CE) {
 433   // Classify arguments to std::move as used.
 434   if (CE->isCallToStdMove()) {
 435     // RecordTypes are handled in SemaDeclCXX.cpp.
 436     if (!CE->getArg(0)->getType()->isRecordType())
 437       classify(CE->getArg(0), Use);
 438     return;
 439   }
 440   bool isTrivialBody = hasTrivialBody(CE);
 441   // If a value is passed by const pointer to a function,
 442   // we should not assume that it is initialized by the call, and we
 443   // conservatively do not assume that it is used.
 444   // If a value is passed by const reference to a function,
 445   // it should already be initialized.
 446   for (CallExpr::arg_iterator I = CE->arg_begin(), E = CE->arg_end();
 447        I != E; ++I) {
 448     if ((*I)->isGLValue()) {
 449       if ((*I)->getType().isConstQualified())
 450         classify((*I), isTrivialBody ? Ignore : ConstRefUse);
 451     } else if (isPointerToConst((*I)->getType())) {
 452       const Expr *Ex = stripCasts(DC->getParentASTContext(), *I);
 453       const auto *UO = dyn_cast<UnaryOperator>(Ex);
 454       if (UO && UO->getOpcode() == UO_AddrOf)
 455         Ex = UO->getSubExpr();
 456       classify(Ex, Ignore);
 457     }
 458   }
 459 }
 460
 461 void ClassifyRefs::VisitCastExpr(CastExpr *CE) {
 462   if (CE->getCastKind() == CK_LValueToRValue)
 463     classify(CE->getSubExpr(), Use);
 464   else if (const auto *CSE = dyn_cast<CStyleCastExpr>(CE)) {
 465     if (CSE->getType()->isVoidType()) {
 466       // Squelch any detected load of an uninitialized value if
 467       // we cast it to void.
 468       // e.g. (void) x;
 469       classify(CSE->getSubExpr(), Ignore);
 470     }
 471   }
 472 }
 473
 474 //------------------------------------------------------------------------====//
 475 // Transfer function for uninitialized values analysis.
 476 //====------------------------------------------------------------------------//
 477
 478 namespace {
 479
 480 class TransferFunctions : public StmtVisitor<TransferFunctions> {
 481   CFGBlockValues &vals;
 482   const CFG &cfg;
 483   const CFGBlock *block;
 484   AnalysisDeclContext &ac;
 485   const ClassifyRefs &classification;
 486   ObjCNoReturn objCNoRet;
 487   UninitVariablesHandler &handler;
 488
 489 public:
 490   TransferFunctions(CFGBlockValues &vals, const CFG &cfg,
 491                     const CFGBlock *block, AnalysisDeclContext &ac,
 492                     const ClassifyRefs &classification,
 493                     UninitVariablesHandler &handler)
 494       : vals(vals), cfg(cfg), block(block), ac(ac),
 495         classification(classification), objCNoRet(ac.getASTContext()),
 496         handler(handler) {}
 497
 498   void reportUse(const Expr *ex, const VarDecl *vd);
 499   void reportConstRefUse(const Expr *ex, const VarDecl *vd);
 500
 501   void VisitBinaryOperator(BinaryOperator *bo);
 502   void VisitBlockExpr(BlockExpr *be);
 503   void VisitCallExpr(CallExpr *ce);
 504   void VisitDeclRefExpr(DeclRefExpr *dr);
 505   void VisitDeclStmt(DeclStmt *ds);
 506   void VisitGCCAsmStmt(GCCAsmStmt *as);
 507   void VisitObjCForCollectionStmt(ObjCForCollectionStmt *FS);
 508   void VisitObjCMessageExpr(ObjCMessageExpr *ME);
 509   void VisitOMPExecutableDirective(OMPExecutableDirective *ED);
 510
 511   bool isTrackedVar(const VarDecl *vd) {
 512     return ::isTrackedVar(vd, cast<DeclContext>(ac.getDecl()));
 513   }
 514
 515   FindVarResult findVar(const Expr *ex) {
 516     return ::findVar(ex, cast<DeclContext>(ac.getDecl()));
 517   }
 518
 519   UninitUse getUninitUse(const Expr *ex, const VarDecl *vd, Value v) {
 520     UninitUse Use(ex, isAlwaysUninit(v));
 521
 522     assert(isUninitialized(v));
 523     if (Use.getKind() == UninitUse::Always)
 524       return Use;
 525
 526     // If an edge which leads unconditionally to this use did not initialize
 527     // the variable, we can say something stronger than 'may be uninitialized':
 528     // we can say 'either it's used uninitialized or you have dead code'.
 529     //
 530     // We track the number of successors of a node which have been visited, and
 531     // visit a node once we have visited all of its successors. Only edges where
 532     // the variable might still be uninitialized are followed. Since a variable
 533     // can't transfer from being initialized to being uninitialized, this will
 534     // trace out the subgraph which inevitably leads to the use and does not
 535     // initialize the variable. We do not want to skip past loops, since their
 536     // non-termination might be correlated with the initialization condition.
 537     //
 538     // For example:
 539     //
 540     //         void f(bool a, bool b) {
 541     // block1:   int n;
 542     //           if (a) {
 543     // block2:     if (b)
 544     // block3:       n = 1;
 545     // block4:   } else if (b) {
 546     // block5:     while (!a) {
 547     // block6:       do_work(&a);
 548     //               n = 2;
 549     //             }
 550     //           }
 551     // block7:   if (a)
 552     // block8:     g();
 553     // block9:   return n;
 554     //         }
 555     //
 556     // Starting from the maybe-uninitialized use in block 9:
 557     //  * Block 7 is not visited because we have only visited one of its two
 558     //    successors.
 559     //  * Block 8 is visited because we've visited its only successor.
 560     // From block 8:
 561     //  * Block 7 is visited because we've now visited both of its successors.
 562     // From block 7:
 563     //  * Blocks 1, 2, 4, 5, and 6 are not visited because we didn't visit all
 564     //    of their successors (we didn't visit 4, 3, 5, 6, and 5, respectively).
 565     //  * Block 3 is not visited because it initializes 'n'.
 566     // Now the algorithm terminates, having visited blocks 7 and 8, and having
 567     // found the frontier is blocks 2, 4, and 5.
 568     //
 569     // 'n' is definitely uninitialized for two edges into block 7 (from blocks 2
 570     // and 4), so we report that any time either of those edges is taken (in
 571     // each case when 'b == false'), 'n' is used uninitialized.
 572     SmallVector<const CFGBlock*, 32> Queue;
 573     SmallVector<unsigned, 32> SuccsVisited(cfg.getNumBlockIDs(), 0);
 574     Queue.push_back(block);
 575     // Specify that we've already visited all successors of the starting block.
 576     // This has the dual purpose of ensuring we never add it to the queue, and
 577     // of marking it as not being a candidate element of the frontier.
 578     SuccsVisited[block->getBlockID()] = block->succ_size();
 579     while (!Queue.empty()) {
 580       const CFGBlock *B = Queue.pop_back_val();
 581
 582       // If the use is always reached from the entry block, make a note of that.
 583       if (B == &cfg.getEntry())
 584         Use.setUninitAfterCall();
 585
 586       for (CFGBlock::const_pred_iterator I = B->pred_begin(), E = B->pred_end();
 587            I != E; ++I) {
 588         const CFGBlock *Pred = *I;
 589         if (!Pred)
 590           continue;
 591
 592         Value AtPredExit = vals.getValue(Pred, B, vd);
 593         if (AtPredExit == Initialized)
 594           // This block initializes the variable.
 595           continue;
 596         if (AtPredExit == MayUninitialized &&
 597             vals.getValue(B, nullptr, vd) == Uninitialized) {
 598           // This block declares the variable (uninitialized), and is reachable
 599           // from a block that initializes the variable. We can't guarantee to
 600           // give an earlier location for the diagnostic (and it appears that
 601           // this code is intended to be reachable) so give a diagnostic here
 602           // and go no further down this path.
 603           Use.setUninitAfterDecl();
 604           continue;
 605         }
 606
 607         unsigned &SV = SuccsVisited[Pred->getBlockID()];
 608         if (!SV) {
 609           // When visiting the first successor of a block, mark all NULL
 610           // successors as having been visited.
 611           for (CFGBlock::const_succ_iterator SI = Pred->succ_begin(),
 612                                              SE = Pred->succ_end();
 613                SI != SE; ++SI)
 614             if (!*SI)
 615               ++SV;
 616         }
 617
 618         if (++SV == Pred->succ_size())
 619           // All paths from this block lead to the use and don't initialize the
 620           // variable.
 621           Queue.push_back(Pred);
 622       }
 623     }
 624
 625     // Scan the frontier, looking for blocks where the variable was
 626     // uninitialized.
 627     for (const auto *Block : cfg) {
 628       unsigned BlockID = Block->getBlockID();
 629       const Stmt *Term = Block->getTerminatorStmt();
 630       if (SuccsVisited[BlockID] && SuccsVisited[BlockID] < Block->succ_size() &&
 631           Term) {
 632         // This block inevitably leads to the use. If we have an edge from here
 633         // to a post-dominator block, and the variable is uninitialized on that
 634         // edge, we have found a bug.
 635         for (CFGBlock::const_succ_iterator I = Block->succ_begin(),
 636              E = Block->succ_end(); I != E; ++I) {
 637           const CFGBlock *Succ = *I;
 638           if (Succ && SuccsVisited[Succ->getBlockID()] >= Succ->succ_size() &&
 639               vals.getValue(Block, Succ, vd) == Uninitialized) {
 640             // Switch cases are a special case: report the label to the caller
 641             // as the 'terminator', not the switch statement itself. Suppress
 642             // situations where no label matched: we can't be sure that's
 643             // possible.
 644             if (isa<SwitchStmt>(Term)) {
 645               const Stmt *Label = Succ->getLabel();
 646               if (!Label || !isa<SwitchCase>(Label))
 647                 // Might not be possible.
 648                 continue;
 649               UninitUse::Branch Branch;
 650               Branch.Terminator = Label;
 651               Branch.Output = 0; // Ignored.
 652               Use.addUninitBranch(Branch);
 653             } else {
 654               UninitUse::Branch Branch;
 655               Branch.Terminator = Term;
 656               Branch.Output = I - Block->succ_begin();
 657               Use.addUninitBranch(Branch);
 658             }
 659           }
 660         }
 661       }
 662     }
 663
 664     return Use;
 665   }
 666 };
 667
 668 } // namespace
 669
 670 void TransferFunctions::reportUse(const Expr *ex, const VarDecl *vd) {
 671   Value v = vals[vd];
 672   if (isUninitialized(v))
 673     handler.handleUseOfUninitVariable(vd, getUninitUse(ex, vd, v));
 674 }
 675
 676 void TransferFunctions::reportConstRefUse(const Expr *ex, const VarDecl *vd) {
 677   Value v = vals[vd];
 678   if (isAlwaysUninit(v))
 679     handler.handleConstRefUseOfUninitVariable(vd, getUninitUse(ex, vd, v));
 680 }
 681
 682 void TransferFunctions::VisitObjCForCollectionStmt(ObjCForCollectionStmt *FS) {
 683   // This represents an initialization of the 'element' value.
 684   if (const auto *DS = dyn_cast<DeclStmt>(FS->getElement())) {
 685     const auto *VD = cast<VarDecl>(DS->getSingleDecl());
 686     if (isTrackedVar(VD))
 687       vals[VD] = Initialized;
 688   }
 689 }
 690
 691 void TransferFunctions::VisitOMPExecutableDirective(
 692     OMPExecutableDirective *ED) {
 693   for (Stmt *S : OMPExecutableDirective::used_clauses_children(ED->clauses())) {
 694     assert(S && "Expected non-null used-in-clause child.");
 695     Visit(S);
 696   }
 697   if (!ED->isStandaloneDirective())
 698     Visit(ED->getStructuredBlock());
 699 }
 700
 701 void TransferFunctions::VisitBlockExpr(BlockExpr *be) {
 702   const BlockDecl *bd = be->getBlockDecl();
 703   for (const auto &I : bd->captures()) {
 704     const VarDecl *vd = I.getVariable();
 705     if (!isTrackedVar(vd))
 706       continue;
 707     if (I.isByRef()) {
 708       vals[vd] = Initialized;
 709       continue;
 710     }
 711     reportUse(be, vd);
 712   }
 713 }
 714
 715 void TransferFunctions::VisitCallExpr(CallExpr *ce) {
 716   if (Decl *Callee = ce->getCalleeDecl()) {
 717     if (Callee->hasAttr<ReturnsTwiceAttr>()) {
 718       // After a call to a function like setjmp or vfork, any variable which is
 719       // initialized anywhere within this function may now be initialized. For
 720       // now, just assume such a call initializes all variables.  FIXME: Only
 721       // mark variables as initialized if they have an initializer which is
 722       // reachable from here.
 723       vals.setAllScratchValues(Initialized);
 724     }
 725     else if (Callee->hasAttr<AnalyzerNoReturnAttr>()) {
 726       // Functions labeled like "analyzer_noreturn" are often used to denote
 727       // "panic" functions that in special debug situations can still return,
 728       // but for the most part should not be treated as returning.  This is a
 729       // useful annotation borrowed from the static analyzer that is useful for
 730       // suppressing branch-specific false positives when we call one of these
 731       // functions but keep pretending the path continues (when in reality the
 732       // user doesn't care).
 733       vals.setAllScratchValues(Unknown);
 734     }
 735   }
 736 }
 737
 738 void TransferFunctions::VisitDeclRefExpr(DeclRefExpr *dr) {
 739   switch (classification.get(dr)) {
 740   case ClassifyRefs::Ignore:
 741     break;
 742   case ClassifyRefs::Use:
 743     reportUse(dr, cast<VarDecl>(dr->getDecl()));
 744     break;
 745   case ClassifyRefs::Init:
 746     vals[cast<VarDecl>(dr->getDecl())] = Initialized;
 747     break;
 748   case ClassifyRefs::SelfInit:
 749     handler.handleSelfInit(cast<VarDecl>(dr->getDecl()));
 750     break;
 751   case ClassifyRefs::ConstRefUse:
 752     reportConstRefUse(dr, cast<VarDecl>(dr->getDecl()));
 753     break;
 754   }
 755 }
 756
 757 void TransferFunctions::VisitBinaryOperator(BinaryOperator *BO) {
 758   if (BO->getOpcode() == BO_Assign) {
 759     FindVarResult Var = findVar(BO->getLHS());
 760     if (const VarDecl *VD = Var.getDecl())
 761       vals[VD] = Initialized;
 762   }
 763 }
 764
 765 void TransferFunctions::VisitDeclStmt(DeclStmt *DS) {
 766   for (auto *DI : DS->decls()) {
 767     auto *VD = dyn_cast<VarDecl>(DI);
 768     if (VD && isTrackedVar(VD)) {
 769       if (getSelfInitExpr(VD)) {
 770         // If the initializer consists solely of a reference to itself, we
 771         // explicitly mark the variable as uninitialized. This allows code
 772         // like the following:
 773         //
 774         //   int x = x;
 775         //
 776         // to deliberately leave a variable uninitialized. Different analysis
 777         // clients can detect this pattern and adjust their reporting
 778         // appropriately, but we need to continue to analyze subsequent uses
 779         // of the variable.
 780         vals[VD] = Uninitialized;
 781       } else if (VD->getInit()) {
 782         // Treat the new variable as initialized.
 783         vals[VD] = Initialized;
 784       } else {
 785         // No initializer: the variable is now uninitialized. This matters
 786         // for cases like:
 787         //   while (...) {
 788         //     int n;
 789         //     use(n);
 790         //     n = 0;
 791         //   }
 792         // FIXME: Mark the variable as uninitialized whenever its scope is
 793         // left, since its scope could be re-entered by a jump over the
 794         // declaration.
 795         vals[VD] = Uninitialized;
 796       }
 797     }
 798   }
 799 }
 800
 801 void TransferFunctions::VisitGCCAsmStmt(GCCAsmStmt *as) {
 802   // An "asm goto" statement is a terminator that may initialize some variables.
 803   if (!as->isAsmGoto())
 804     return;
 805
 806   ASTContext &C = ac.getASTContext();
 807   for (const Expr *O : as->outputs()) {
 808     const Expr *Ex = stripCasts(C, O);
 809
 810     // Strip away any unary operators. Invalid l-values are reported by other
 811     // semantic analysis passes.
 812     while (const auto *UO = dyn_cast<UnaryOperator>(Ex))
 813       Ex = stripCasts(C, UO->getSubExpr());
 814
 815     // Mark the variable as potentially uninitialized for those cases where
 816     // it's used on an indirect path, where it's not guaranteed to be
 817     // defined.
 818     if (const VarDecl *VD = findVar(Ex).getDecl())
 819       if (vals[VD] != Initialized)
 820         vals[VD] = MayUninitialized;
 821   }
 822 }
 823
 824 void TransferFunctions::VisitObjCMessageExpr(ObjCMessageExpr *ME) {
 825   // If the Objective-C message expression is an implicit no-return that
 826   // is not modeled in the CFG, set the tracked dataflow values to Unknown.
 827   if (objCNoRet.isImplicitNoReturn(ME)) {
 828     vals.setAllScratchValues(Unknown);
 829   }
 830 }
 831
 832 //------------------------------------------------------------------------====//
 833 // High-level "driver" logic for uninitialized values analysis.
 834 //====------------------------------------------------------------------------//
 835
 836 static bool runOnBlock(const CFGBlock *block, const CFG &cfg,
 837                        AnalysisDeclContext &ac, CFGBlockValues &vals,
 838                        const ClassifyRefs &classification,
 839                        llvm::BitVector &wasAnalyzed,
 840                        UninitVariablesHandler &handler) {
 841   wasAnalyzed[block->getBlockID()] = true;
 842   vals.resetScratch();
 843   // Merge in values of predecessor blocks.
 844   bool isFirst = true;
 845   for (CFGBlock::const_pred_iterator I = block->pred_begin(),
 846        E = block->pred_end(); I != E; ++I) {
 847     const CFGBlock *pred = *I;
 848     if (!pred)
 849       continue;
 850     if (wasAnalyzed[pred->getBlockID()]) {
 851       vals.mergeIntoScratch(vals.getValueVector(pred), isFirst);
 852       isFirst = false;
 853     }
 854   }
 855   // Apply the transfer function.
 856   TransferFunctions tf(vals, cfg, block, ac, classification, handler);
 857   for (const auto &I : *block) {
 858     if (std::optional<CFGStmt> cs = I.getAs<CFGStmt>())
 859       tf.Visit(const_cast<Stmt *>(cs->getStmt()));
 860   }
 861   CFGTerminator terminator = block->getTerminator();
 862   if (auto *as = dyn_cast_or_null<GCCAsmStmt>(terminator.getStmt()))
 863     if (as->isAsmGoto())
 864       tf.Visit(as);
 865   return vals.updateValueVectorWithScratch(block);
 866 }
 867
 868 namespace {
 869
 870 /// PruneBlocksHandler is a special UninitVariablesHandler that is used
 871 /// to detect when a CFGBlock has any *potential* use of an uninitialized
 872 /// variable.  It is mainly used to prune out work during the final
 873 /// reporting pass.
 874 struct PruneBlocksHandler : public UninitVariablesHandler {
 875   /// Records if a CFGBlock had a potential use of an uninitialized variable.
 876   llvm::BitVector hadUse;
 877
 878   /// Records if any CFGBlock had a potential use of an uninitialized variable.
 879   bool hadAnyUse = false;
 880
 881   /// The current block to scribble use information.
 882   unsigned currentBlock = 0;
 883
 884   PruneBlocksHandler(unsigned numBlocks) : hadUse(numBlocks, false) {}
 885
 886   ~PruneBlocksHandler() override = default;
 887
 888   void handleUseOfUninitVariable(const VarDecl *vd,
 889                                  const UninitUse &use) override {
 890     hadUse[currentBlock] = true;
 891     hadAnyUse = true;
 892   }
 893
 894   void handleConstRefUseOfUninitVariable(const VarDecl *vd,
 895                                          const UninitUse &use) override {
 896     hadUse[currentBlock] = true;
 897     hadAnyUse = true;
 898   }
 899
 900   /// Called when the uninitialized variable analysis detects the
 901   /// idiom 'int x = x'.  All other uses of 'x' within the initializer
 902   /// are handled by handleUseOfUninitVariable.
 903   void handleSelfInit(const VarDecl *vd) override {
 904     hadUse[currentBlock] = true;
 905     hadAnyUse = true;
 906   }
 907 };
 908
 909 } // namespace
 910
 911 void clang::runUninitializedVariablesAnalysis(
 912     const DeclContext &dc,
 913     const CFG &cfg,
 914     AnalysisDeclContext &ac,
 915     UninitVariablesHandler &handler,
 916     UninitVariablesAnalysisStats &stats) {
 917   CFGBlockValues vals(cfg);
 918   vals.computeSetOfDeclarations(dc);
 919   if (vals.hasNoDeclarations())
 920     return;
 921
 922   stats.NumVariablesAnalyzed = vals.getNumEntries();
 923
 924   // Precompute which expressions are uses and which are initializations.
 925   ClassifyRefs classification(ac);
 926   cfg.VisitBlockStmts(classification);
 927
 928   // Mark all variables uninitialized at the entry.
 929   const CFGBlock &entry = cfg.getEntry();
 930   ValueVector &vec = vals.getValueVector(&entry);
 931   const unsigned n = vals.getNumEntries();
 932   for (unsigned j = 0; j < n; ++j) {
 933     vec[j] = Uninitialized;
 934   }
 935
 936   // Proceed with the workist.
 937   ForwardDataflowWorklist worklist(cfg, ac);
 938   llvm::BitVector previouslyVisited(cfg.getNumBlockIDs());
 939   worklist.enqueueSuccessors(&cfg.getEntry());
 940   llvm::BitVector wasAnalyzed(cfg.getNumBlockIDs(), false);
 941   wasAnalyzed[cfg.getEntry().getBlockID()] = true;
 942   PruneBlocksHandler PBH(cfg.getNumBlockIDs());
 943
 944   while (const CFGBlock *block = worklist.dequeue()) {
 945     PBH.currentBlock = block->getBlockID();
 946
 947     // Did the block change?
 948     bool changed = runOnBlock(block, cfg, ac, vals,
 949                               classification, wasAnalyzed, PBH);
 950     ++stats.NumBlockVisits;
 951     if (changed || !previouslyVisited[block->getBlockID()])
 952       worklist.enqueueSuccessors(block);
 953     previouslyVisited[block->getBlockID()] = true;
 954   }
 955
 956   if (!PBH.hadAnyUse)
 957     return;
 958
 959   // Run through the blocks one more time, and report uninitialized variables.
 960   for (const auto *block : cfg)
 961     if (PBH.hadUse[block->getBlockID()]) {
 962       runOnBlock(block, cfg, ac, vals, classification, wasAnalyzed, handler);
 963       ++stats.NumBlockVisits;
 964     }
 965 }
 966
 967 UninitVariablesHandler::~UninitVariablesHandler() = default;