1 //==- IdempotentOperationChecker.cpp - Idempotent Operations ----*- C++ -*-==//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file defines a set of path-sensitive checks for idempotent and/or
11 // tautological operations. Each potential operation is checked along all paths
12 // to see if every path results in a pointless operation.
13 // +-------------------------------------------+
14 // |Table of idempotent/tautological operations|
15 // +-------------------------------------------+
16 //+--------------------------------------------------------------------------+
17 //|Operator | x op x | x op 1 | 1 op x | x op 0 | 0 op x | x op ~0 | ~0 op x |
18 //+--------------------------------------------------------------------------+
19 // +, += | | | | x | x | |
20 // -, -= | | | | x | -x | |
21 // *, *= | | x | x | 0 | 0 | |
22 // /, /= | 1 | x | | N/A | 0 | |
23 // &, &= | x | | | 0 | 0 | x | x
24 // |, |= | x | | | x | x | ~0 | ~0
25 // ^, ^= | 0 | | | x | x | |
26 // <<, <<= | | | | x | 0 | |
27 // >>, >>= | | | | x | 0 | |
28 // || | 1 | 1 | 1 | x | x | 1 | 1
29 // && | 1 | x | x | 0 | 0 | x | x
37 //===----------------------------------------------------------------------===//
40 // - Improved error messages
41 // - Handle mixed assumptions (which assumptions can belong together?)
42 // - Finer grained false positive control (levels)
43 // - Handling ~0 values
45 #include "ClangSACheckers.h"
46 #include "clang/Analysis/CFGStmtMap.h"
47 #include "clang/Analysis/Analyses/PseudoConstantAnalysis.h"
48 #include "clang/Analysis/Analyses/CFGReachabilityAnalysis.h"
49 #include "clang/StaticAnalyzer/Core/Checker.h"
50 #include "clang/StaticAnalyzer/Core/CheckerManager.h"
51 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
52 #include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h"
53 #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
54 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerHelpers.h"
55 #include "clang/StaticAnalyzer/Core/PathSensitive/CoreEngine.h"
56 #include "clang/StaticAnalyzer/Core/PathSensitive/SVals.h"
57 #include "clang/AST/Stmt.h"
58 #include "llvm/ADT/DenseMap.h"
59 #include "llvm/ADT/SmallSet.h"
60 #include "llvm/ADT/BitVector.h"
61 #include "llvm/Support/ErrorHandling.h"
63 using namespace clang;
67 class IdempotentOperationChecker
68 : public Checker<check::PreStmt<BinaryOperator>,
69 check::PostStmt<BinaryOperator>,
72 void checkPreStmt(const BinaryOperator *B, CheckerContext &C) const;
73 void checkPostStmt(const BinaryOperator *B, CheckerContext &C) const;
74 void checkEndAnalysis(ExplodedGraph &G, BugReporter &B,ExprEngine &Eng) const;
77 // Our assumption about a particular operation.
78 enum Assumption { Possible = 0, Impossible, Equal, LHSis1, RHSis1, LHSis0,
81 static void UpdateAssumption(Assumption &A, const Assumption &New);
83 // False positive reduction methods
84 static bool isSelfAssign(const Expr *LHS, const Expr *RHS);
85 static bool isUnused(const Expr *E, AnalysisContext *AC);
86 static bool isTruncationExtensionAssignment(const Expr *LHS,
88 static bool pathWasCompletelyAnalyzed(AnalysisContext *AC,
90 const CoreEngine &CE);
91 static bool CanVary(const Expr *Ex,
93 static bool isConstantOrPseudoConstant(const DeclRefExpr *DR,
95 static bool containsNonLocalVarDecl(const Stmt *S);
97 // Hash table and related data structures
98 struct BinaryOperatorData {
99 BinaryOperatorData() : assumption(Possible), analysisContext(0) {}
101 Assumption assumption;
102 AnalysisContext *analysisContext;
103 ExplodedNodeSet explodedNodes; // Set of ExplodedNodes that refer to a
106 typedef llvm::DenseMap<const BinaryOperator *, BinaryOperatorData>
108 mutable AssumptionMap hash;
112 void IdempotentOperationChecker::checkPreStmt(const BinaryOperator *B,
113 CheckerContext &C) const {
114 // Find or create an entry in the hash for this BinaryOperator instance.
115 // If we haven't done a lookup before, it will get default initialized to
116 // 'Possible'. At this stage we do not store the ExplodedNode, as it has not
118 BinaryOperatorData &Data = hash[B];
119 Assumption &A = Data.assumption;
120 AnalysisContext *AC = C.getCurrentAnalysisContext();
121 Data.analysisContext = AC;
123 // If we already have visited this node on a path that does not contain an
124 // idempotent operation, return immediately.
128 // Retrieve both sides of the operator and determine if they can vary (which
129 // may mean this is a false positive.
130 const Expr *LHS = B->getLHS();
131 const Expr *RHS = B->getRHS();
133 // At this stage we can calculate whether each side contains a false positive
134 // that applies to all operators. We only need to calculate this the first
136 bool LHSContainsFalsePositive = false, RHSContainsFalsePositive = false;
138 // An expression contains a false positive if it can't vary, or if it
139 // contains a known false positive VarDecl.
140 LHSContainsFalsePositive = !CanVary(LHS, AC)
141 || containsNonLocalVarDecl(LHS);
142 RHSContainsFalsePositive = !CanVary(RHS, AC)
143 || containsNonLocalVarDecl(RHS);
146 const GRState *state = C.getState();
148 SVal LHSVal = state->getSVal(LHS);
149 SVal RHSVal = state->getSVal(RHS);
151 // If either value is unknown, we can't be 100% sure of all paths.
152 if (LHSVal.isUnknownOrUndef() || RHSVal.isUnknownOrUndef()) {
156 BinaryOperator::Opcode Op = B->getOpcode();
158 // Dereference the LHS SVal if this is an assign operation
163 // Fall through intentional
174 // Assign statements have one extra level of indirection
175 if (!isa<Loc>(LHSVal)) {
179 LHSVal = state->getSVal(cast<Loc>(LHSVal), LHS->getType());
183 // We now check for various cases which result in an idempotent operation.
188 break; // We don't care about any other operators.
190 // Fall through intentional
192 // x Assign x can be used to silence unused variable warnings intentionally.
193 // If this is a self assignment and the variable is referenced elsewhere,
194 // and the assignment is not a truncation or extension, then it is a false
196 if (isSelfAssign(LHS, RHS)) {
197 if (!isUnused(LHS, AC) && !isTruncationExtensionAssignment(LHS, RHS)) {
198 UpdateAssumption(A, Equal);
221 if (LHSVal != RHSVal || LHSContainsFalsePositive
222 || RHSContainsFalsePositive)
224 UpdateAssumption(A, Equal);
231 break; // We don't care about any other operators.
233 // Fall through intentional
240 if (!RHSVal.isConstant(1) || RHSContainsFalsePositive)
242 UpdateAssumption(A, RHSis1);
249 break; // We don't care about any other operators.
251 // Fall through intentional
256 if (!LHSVal.isConstant(1) || LHSContainsFalsePositive)
258 UpdateAssumption(A, LHSis1);
265 break; // We don't care about any other operators.
267 // Fall through intentional
284 if (!RHSVal.isConstant(0) || RHSContainsFalsePositive)
286 UpdateAssumption(A, RHSis0);
293 break; // We don't care about any other operators.
295 // Fall through intentional
296 //case BO_AddAssign: // Common false positive
297 case BO_SubAssign: // Check only if unsigned
301 //case BO_OrAssign: // Common false positive
302 //case BO_XorAssign: // Common false positive
316 if (!LHSVal.isConstant(0) || LHSContainsFalsePositive)
318 UpdateAssumption(A, LHSis0);
322 // If we get to this point, there has been a valid use of this operation.
326 // At the post visit stage, the predecessor ExplodedNode will be the
327 // BinaryOperator that was just created. We use this hook to collect the
329 void IdempotentOperationChecker::checkPostStmt(const BinaryOperator *B,
330 CheckerContext &C) const {
331 // Add the ExplodedNode we just visited
332 BinaryOperatorData &Data = hash[B];
335 = cast<StmtPoint>(C.getPredecessor()->getLocation()).getStmt();
337 // Ignore implicit calls to setters.
338 if (!isa<BinaryOperator>(predStmt))
341 Data.explodedNodes.Add(C.getPredecessor());
344 void IdempotentOperationChecker::checkEndAnalysis(ExplodedGraph &G,
346 ExprEngine &Eng) const {
347 BugType *BT = new BugType("Idempotent operation", "Dead code");
348 // Iterate over the hash to see if we have any paths with definite
349 // idempotent operations.
350 for (AssumptionMap::const_iterator i = hash.begin(); i != hash.end(); ++i) {
351 // Unpack the hash contents
352 const BinaryOperatorData &Data = i->second;
353 const Assumption &A = Data.assumption;
354 AnalysisContext *AC = Data.analysisContext;
355 const ExplodedNodeSet &ES = Data.explodedNodes;
357 const BinaryOperator *B = i->first;
362 // If the analyzer did not finish, check to see if we can still emit this
364 if (Eng.hasWorkRemaining()) {
365 // If we can trace back
366 if (!pathWasCompletelyAnalyzed(AC,
367 AC->getCFGStmtMap()->getBlock(B),
368 Eng.getCoreEngine()))
372 // Select the error message and SourceRanges to report.
373 llvm::SmallString<128> buf;
374 llvm::raw_svector_ostream os(buf);
375 bool LHSRelevant = false, RHSRelevant = false;
380 if (B->getOpcode() == BO_Assign)
381 os << "Assigned value is always the same as the existing value";
383 os << "Both operands to '" << B->getOpcodeStr()
384 << "' always have the same value";
388 os << "The left operand to '" << B->getOpcodeStr() << "' is always 1";
392 os << "The right operand to '" << B->getOpcodeStr() << "' is always 1";
396 os << "The left operand to '" << B->getOpcodeStr() << "' is always 0";
400 os << "The right operand to '" << B->getOpcodeStr() << "' is always 0";
403 llvm_unreachable("Operation was never marked with an assumption");
408 // Add a report for each ExplodedNode
409 for (ExplodedNodeSet::iterator I = ES.begin(), E = ES.end(); I != E; ++I) {
410 EnhancedBugReport *report = new EnhancedBugReport(*BT, os.str(), *I);
412 // Add source ranges and visitor hooks
414 const Expr *LHS = i->first->getLHS();
415 report->addRange(LHS->getSourceRange());
416 report->addVisitorCreator(bugreporter::registerVarDeclsLastStore, LHS);
419 const Expr *RHS = i->first->getRHS();
420 report->addRange(i->first->getRHS()->getSourceRange());
421 report->addVisitorCreator(bugreporter::registerVarDeclsLastStore, RHS);
424 BR.EmitReport(report);
431 // Updates the current assumption given the new assumption
432 inline void IdempotentOperationChecker::UpdateAssumption(Assumption &A,
433 const Assumption &New) {
434 // If the assumption is the same, there is nothing to do
439 // If we don't currently have an assumption, set it
444 // If we have determined that a valid state happened, ignore the new
449 // Any other case means that we had a different assumption last time. We don't
450 // currently support mixing assumptions for diagnostic reasons, so we set
451 // our assumption to be impossible.
458 // Check for a statement where a variable is self assigned to possibly avoid an
459 // unused variable warning.
460 bool IdempotentOperationChecker::isSelfAssign(const Expr *LHS, const Expr *RHS) {
461 LHS = LHS->IgnoreParenCasts();
462 RHS = RHS->IgnoreParenCasts();
464 const DeclRefExpr *LHS_DR = dyn_cast<DeclRefExpr>(LHS);
468 const VarDecl *VD = dyn_cast<VarDecl>(LHS_DR->getDecl());
472 const DeclRefExpr *RHS_DR = dyn_cast<DeclRefExpr>(RHS);
476 if (VD != RHS_DR->getDecl())
482 // Returns true if the Expr points to a VarDecl that is not read anywhere
483 // outside of self-assignments.
484 bool IdempotentOperationChecker::isUnused(const Expr *E,
485 AnalysisContext *AC) {
489 const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(E->IgnoreParenCasts());
493 const VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl());
497 if (AC->getPseudoConstantAnalysis()->wasReferenced(VD))
503 // Check for self casts truncating/extending a variable
504 bool IdempotentOperationChecker::isTruncationExtensionAssignment(
508 const DeclRefExpr *LHS_DR = dyn_cast<DeclRefExpr>(LHS->IgnoreParenCasts());
512 const VarDecl *VD = dyn_cast<VarDecl>(LHS_DR->getDecl());
516 const DeclRefExpr *RHS_DR = dyn_cast<DeclRefExpr>(RHS->IgnoreParenCasts());
520 if (VD != RHS_DR->getDecl())
523 return dyn_cast<DeclRefExpr>(RHS->IgnoreParenLValueCasts()) == NULL;
526 // Returns false if a path to this block was not completely analyzed, or true
529 IdempotentOperationChecker::pathWasCompletelyAnalyzed(AnalysisContext *AC,
531 const CoreEngine &CE) {
533 CFGReverseBlockReachabilityAnalysis *CRA = AC->getCFGReachablityAnalysis();
535 // Test for reachability from any aborted blocks to this block
536 typedef CoreEngine::BlocksExhausted::const_iterator ExhaustedIterator;
537 for (ExhaustedIterator I = CE.blocks_exhausted_begin(),
538 E = CE.blocks_exhausted_end(); I != E; ++I) {
539 const BlockEdge &BE = I->first;
541 // The destination block on the BlockEdge is the first block that was not
542 // analyzed. If we can reach this block from the aborted block, then this
543 // block was not completely analyzed.
545 // Also explicitly check if the current block is the destination block.
546 // While technically reachable, it means we aborted the analysis on
547 // a path that included that block.
548 const CFGBlock *destBlock = BE.getDst();
549 if (destBlock == CB || CRA->isReachable(destBlock, CB))
553 // Test for reachability from blocks we just gave up on.
554 typedef CoreEngine::BlocksAborted::const_iterator AbortedIterator;
555 for (AbortedIterator I = CE.blocks_aborted_begin(),
556 E = CE.blocks_aborted_end(); I != E; ++I) {
557 const CFGBlock *destBlock = I->first;
558 if (destBlock == CB || CRA->isReachable(destBlock, CB))
562 // For the items still on the worklist, see if they are in blocks that
563 // can eventually reach 'CB'.
564 class VisitWL : public WorkList::Visitor {
565 const CFGStmtMap *CBM;
566 const CFGBlock *TargetBlock;
567 CFGReverseBlockReachabilityAnalysis &CRA;
569 VisitWL(const CFGStmtMap *cbm, const CFGBlock *targetBlock,
570 CFGReverseBlockReachabilityAnalysis &cra)
571 : CBM(cbm), TargetBlock(targetBlock), CRA(cra) {}
572 virtual bool visit(const WorkListUnit &U) {
573 ProgramPoint P = U.getNode()->getLocation();
574 const CFGBlock *B = 0;
575 if (StmtPoint *SP = dyn_cast<StmtPoint>(&P)) {
576 B = CBM->getBlock(SP->getStmt());
578 else if (BlockEdge *BE = dyn_cast<BlockEdge>(&P)) {
581 else if (BlockEntrance *BEnt = dyn_cast<BlockEntrance>(&P)) {
582 B = BEnt->getBlock();
584 else if (BlockExit *BExit = dyn_cast<BlockExit>(&P)) {
585 B = BExit->getBlock();
590 return B == TargetBlock || CRA.isReachable(B, TargetBlock);
593 VisitWL visitWL(AC->getCFGStmtMap(), CB, *CRA);
594 // Were there any items in the worklist that could potentially reach
596 if (CE.getWorkList()->visitItemsInWorkList(visitWL))
599 // Verify that this block is reachable from the entry block
600 if (!CRA->isReachable(&AC->getCFG()->getEntry(), CB))
603 // If we get to this point, there is no connection to the entry block or an
604 // aborted block. This path is unreachable and we can report the error.
608 // Recursive function that determines whether an expression contains any element
609 // that varies. This could be due to a compile-time constant like sizeof. An
610 // expression may also involve a variable that behaves like a constant. The
611 // function returns true if the expression varies, and false otherwise.
612 bool IdempotentOperationChecker::CanVary(const Expr *Ex,
613 AnalysisContext *AC) {
614 // Parentheses and casts are irrelevant here
615 Ex = Ex->IgnoreParenCasts();
617 if (Ex->getLocStart().isMacroID())
620 switch (Ex->getStmtClass()) {
621 // Trivially true cases
622 case Stmt::ArraySubscriptExprClass:
623 case Stmt::MemberExprClass:
624 case Stmt::StmtExprClass:
625 case Stmt::CallExprClass:
626 case Stmt::VAArgExprClass:
627 case Stmt::ShuffleVectorExprClass:
632 // Trivially false cases
633 case Stmt::IntegerLiteralClass:
634 case Stmt::CharacterLiteralClass:
635 case Stmt::FloatingLiteralClass:
636 case Stmt::PredefinedExprClass:
637 case Stmt::ImaginaryLiteralClass:
638 case Stmt::StringLiteralClass:
639 case Stmt::OffsetOfExprClass:
640 case Stmt::CompoundLiteralExprClass:
641 case Stmt::AddrLabelExprClass:
642 case Stmt::BinaryTypeTraitExprClass:
643 case Stmt::GNUNullExprClass:
644 case Stmt::InitListExprClass:
645 case Stmt::DesignatedInitExprClass:
646 case Stmt::BlockExprClass:
647 case Stmt::BlockDeclRefExprClass:
650 // Cases requiring custom logic
651 case Stmt::UnaryExprOrTypeTraitExprClass: {
652 const UnaryExprOrTypeTraitExpr *SE =
653 cast<const UnaryExprOrTypeTraitExpr>(Ex);
654 if (SE->getKind() != UETT_SizeOf)
656 return SE->getTypeOfArgument()->isVariableArrayType();
658 case Stmt::DeclRefExprClass:
659 // Check for constants/pseudoconstants
660 return !isConstantOrPseudoConstant(cast<DeclRefExpr>(Ex), AC);
662 // The next cases require recursion for subexpressions
663 case Stmt::BinaryOperatorClass: {
664 const BinaryOperator *B = cast<const BinaryOperator>(Ex);
666 // Exclude cases involving pointer arithmetic. These are usually
668 if (B->getOpcode() == BO_Sub || B->getOpcode() == BO_Add)
669 if (B->getLHS()->getType()->getAs<PointerType>())
672 return CanVary(B->getRHS(), AC)
673 || CanVary(B->getLHS(), AC);
675 case Stmt::UnaryOperatorClass: {
676 const UnaryOperator *U = cast<const UnaryOperator>(Ex);
677 // Handle trivial case first
678 switch (U->getOpcode()) {
682 return CanVary(U->getSubExpr(), AC);
685 case Stmt::ChooseExprClass:
686 return CanVary(cast<const ChooseExpr>(Ex)->getChosenSubExpr(
687 AC->getASTContext()), AC);
688 case Stmt::ConditionalOperatorClass:
689 case Stmt::BinaryConditionalOperatorClass:
690 return CanVary(cast<AbstractConditionalOperator>(Ex)->getCond(), AC);
694 // Returns true if a DeclRefExpr is or behaves like a constant.
695 bool IdempotentOperationChecker::isConstantOrPseudoConstant(
696 const DeclRefExpr *DR,
697 AnalysisContext *AC) {
698 // Check if the type of the Decl is const-qualified
699 if (DR->getType().isConstQualified())
703 if (isa<EnumConstantDecl>(DR->getDecl()))
706 const VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl());
710 // Check if the Decl behaves like a constant. This check also takes care of
711 // static variables, which can only change between function calls if they are
712 // modified in the AST.
713 PseudoConstantAnalysis *PCA = AC->getPseudoConstantAnalysis();
714 if (PCA->isPseudoConstant(VD))
720 // Recursively find any substatements containing VarDecl's with storage other
722 bool IdempotentOperationChecker::containsNonLocalVarDecl(const Stmt *S) {
723 const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(S);
726 if (const VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl()))
727 if (!VD->hasLocalStorage())
730 for (Stmt::const_child_iterator I = S->child_begin(); I != S->child_end();
732 if (const Stmt *child = *I)
733 if (containsNonLocalVarDecl(child))
740 void ento::registerIdempotentOperationChecker(CheckerManager &mgr) {
741 mgr.registerChecker<IdempotentOperationChecker>();