//==- IdempotentOperationChecker.cpp - Idempotent Operations ----*- C++ -*-==//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines a set of path-sensitive checks for idempotent and/or
// tautological operations. Each potential operation is checked along all paths
// to see if every path results in a pointless operation.
// +-------------------------------------------+
// |Table of idempotent/tautological operations|
// +-------------------------------------------+
//+--------------------------------------------------------------------------+
//|Operator | x op x | x op 1 | 1 op x | x op 0 | 0 op x | x op ~0 | ~0 op x |
//+--------------------------------------------------------------------------+
// +, += | | | | x | x | |
// -, -= | | | | x | -x | |
// *, *= | | x | x | 0 | 0 | |
// /, /= | 1 | x | | N/A | 0 | |
// &, &= | x | | | 0 | 0 | x | x
// |, |= | x | | | x | x | ~0 | ~0
// ^, ^= | 0 | | | x | x | |
// <<, <<= | | | | x | 0 | |
// >>, >>= | | | | x | 0 | |
// || | 1 | 1 | 1 | x | x | 1 | 1
// && | 1 | x | x | 0 | 0 | x | x
// = | x | | | | | |
// == | 1 | | | | | |
// >= | 1 | | | | | |
// <= | 1 | | | | | |
// > | 0 | | | | | |
// < | 0 | | | | | |
// != | 0 | | | | | |
//===----------------------------------------------------------------------===//
//
// Things TODO:
// - Improved error messages
// - Handle mixed assumptions (which assumptions can belong together?)
// - Finer grained false positive control (levels)
// - Handling ~0 values
#include "ClangSACheckers.h"
#include "clang/AST/Stmt.h"
#include "clang/Analysis/Analyses/CFGReachabilityAnalysis.h"
#include "clang/Analysis/Analyses/PseudoConstantAnalysis.h"
#include "clang/Analysis/CFGStmtMap.h"
#include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h"
#include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
#include "clang/StaticAnalyzer/Core/Checker.h"
#include "clang/StaticAnalyzer/Core/CheckerManager.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerHelpers.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/CoreEngine.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/SVals.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
using namespace clang;
using namespace ento;
namespace {
class IdempotentOperationChecker
: public Checker<check::PreStmt<BinaryOperator>,
check::PostStmt<BinaryOperator>,
check::EndAnalysis> {
public:
void checkPreStmt(const BinaryOperator *B, CheckerContext &C) const;
void checkPostStmt(const BinaryOperator *B, CheckerContext &C) const;
void checkEndAnalysis(ExplodedGraph &G, BugReporter &B,ExprEngine &Eng) const;
private:
// Our assumption about a particular operation.
enum Assumption { Possible = 0, Impossible, Equal, LHSis1, RHSis1, LHSis0,
RHSis0 };
static void UpdateAssumption(Assumption &A, const Assumption &New);
// False positive reduction methods
static bool isSelfAssign(const Expr *LHS, const Expr *RHS);
static bool isUnused(const Expr *E, AnalysisDeclContext *AC);
static bool isTruncationExtensionAssignment(const Expr *LHS,
const Expr *RHS);
static bool pathWasCompletelyAnalyzed(AnalysisDeclContext *AC,
const CFGBlock *CB,
const CoreEngine &CE);
static bool CanVary(const Expr *Ex,
AnalysisDeclContext *AC);
static bool isConstantOrPseudoConstant(const DeclRefExpr *DR,
AnalysisDeclContext *AC);
static bool containsNonLocalVarDecl(const Stmt *S);
// Hash table and related data structures
struct BinaryOperatorData {
BinaryOperatorData() : assumption(Possible) {}
Assumption assumption;
ExplodedNodeSet explodedNodes; // Set of ExplodedNodes that refer to a
// BinaryOperator
};
typedef llvm::DenseMap<const BinaryOperator *, BinaryOperatorData>
AssumptionMap;
mutable AssumptionMap hash;
mutable OwningPtr<BugType> BT;
};
}
void IdempotentOperationChecker::checkPreStmt(const BinaryOperator *B,
CheckerContext &C) const {
// Find or create an entry in the hash for this BinaryOperator instance.
// If we haven't done a lookup before, it will get default initialized to
// 'Possible'. At this stage we do not store the ExplodedNode, as it has not
// been created yet.
BinaryOperatorData &Data = hash[B];
Assumption &A = Data.assumption;
AnalysisDeclContext *AC = C.getCurrentAnalysisDeclContext();
// If we already have visited this node on a path that does not contain an
// idempotent operation, return immediately.
if (A == Impossible)
return;
// Retrieve both sides of the operator and determine if they can vary (which
// may mean this is a false positive.
const Expr *LHS = B->getLHS();
const Expr *RHS = B->getRHS();
// At this stage we can calculate whether each side contains a false positive
// that applies to all operators. We only need to calculate this the first
// time.
bool LHSContainsFalsePositive = false, RHSContainsFalsePositive = false;
if (A == Possible) {
// An expression contains a false positive if it can't vary, or if it
// contains a known false positive VarDecl.
LHSContainsFalsePositive = !CanVary(LHS, AC)
|| containsNonLocalVarDecl(LHS);
RHSContainsFalsePositive = !CanVary(RHS, AC)
|| containsNonLocalVarDecl(RHS);
}
ProgramStateRef state = C.getState();
const LocationContext *LCtx = C.getLocationContext();
SVal LHSVal = state->getSVal(LHS, LCtx);
SVal RHSVal = state->getSVal(RHS, LCtx);
// If either value is unknown, we can't be 100% sure of all paths.
if (LHSVal.isUnknownOrUndef() || RHSVal.isUnknownOrUndef()) {
A = Impossible;
return;
}
BinaryOperator::Opcode Op = B->getOpcode();
// Dereference the LHS SVal if this is an assign operation
switch (Op) {
default:
break;
// Fall through intentional
case BO_AddAssign:
case BO_SubAssign:
case BO_MulAssign:
case BO_DivAssign:
case BO_AndAssign:
case BO_OrAssign:
case BO_XorAssign:
case BO_ShlAssign:
case BO_ShrAssign:
case BO_Assign:
// Assign statements have one extra level of indirection
if (!LHSVal.getAs<Loc>()) {
A = Impossible;
return;
}
LHSVal = state->getSVal(LHSVal.castAs<Loc>(), LHS->getType());
}
// We now check for various cases which result in an idempotent operation.
// x op x
switch (Op) {
default:
break; // We don't care about any other operators.
// Fall through intentional
case BO_Assign:
// x Assign x can be used to silence unused variable warnings intentionally.
// If this is a self assignment and the variable is referenced elsewhere,
// and the assignment is not a truncation or extension, then it is a false
// positive.
if (isSelfAssign(LHS, RHS)) {
if (!isUnused(LHS, AC) && !isTruncationExtensionAssignment(LHS, RHS)) {
UpdateAssumption(A, Equal);
return;
}
else {
A = Impossible;
return;
}
}
case BO_SubAssign:
case BO_DivAssign:
case BO_AndAssign:
case BO_OrAssign:
case BO_XorAssign:
case BO_Sub:
case BO_Div:
case BO_And:
case BO_Or:
case BO_Xor:
case BO_LOr:
case BO_LAnd:
case BO_EQ:
case BO_NE:
if (LHSVal != RHSVal || LHSContainsFalsePositive
|| RHSContainsFalsePositive)
break;
UpdateAssumption(A, Equal);
return;
}
// x op 1
switch (Op) {
default:
break; // We don't care about any other operators.
// Fall through intentional
case BO_MulAssign:
case BO_DivAssign:
case BO_Mul:
case BO_Div:
case BO_LOr:
case BO_LAnd:
if (!RHSVal.isConstant(1) || RHSContainsFalsePositive)
break;
UpdateAssumption(A, RHSis1);
return;
}
// 1 op x
switch (Op) {
default:
break; // We don't care about any other operators.
// Fall through intentional
case BO_MulAssign:
case BO_Mul:
case BO_LOr:
case BO_LAnd:
if (!LHSVal.isConstant(1) || LHSContainsFalsePositive)
break;
UpdateAssumption(A, LHSis1);
return;
}
// x op 0
switch (Op) {
default:
break; // We don't care about any other operators.
// Fall through intentional
case BO_AddAssign:
case BO_SubAssign:
case BO_MulAssign:
case BO_AndAssign:
case BO_OrAssign:
case BO_XorAssign:
case BO_Add:
case BO_Sub:
case BO_Mul:
case BO_And:
case BO_Or:
case BO_Xor:
case BO_Shl:
case BO_Shr:
case BO_LOr:
case BO_LAnd:
if (!RHSVal.isConstant(0) || RHSContainsFalsePositive)
break;
UpdateAssumption(A, RHSis0);
return;
}
// 0 op x
switch (Op) {
default:
break; // We don't care about any other operators.
// Fall through intentional
//case BO_AddAssign: // Common false positive
case BO_SubAssign: // Check only if unsigned
case BO_MulAssign:
case BO_DivAssign:
case BO_AndAssign:
//case BO_OrAssign: // Common false positive
//case BO_XorAssign: // Common false positive
case BO_ShlAssign:
case BO_ShrAssign:
case BO_Add:
case BO_Sub:
case BO_Mul:
case BO_Div:
case BO_And:
case BO_Or:
case BO_Xor:
case BO_Shl:
case BO_Shr:
case BO_LOr:
case BO_LAnd:
if (!LHSVal.isConstant(0) || LHSContainsFalsePositive)
break;
UpdateAssumption(A, LHSis0);
return;
}
// If we get to this point, there has been a valid use of this operation.
A = Impossible;
}
// At the post visit stage, the predecessor ExplodedNode will be the
// BinaryOperator that was just created. We use this hook to collect the
// ExplodedNode.
void IdempotentOperationChecker::checkPostStmt(const BinaryOperator *B,
CheckerContext &C) const {
// Add the ExplodedNode we just visited
BinaryOperatorData &Data = hash[B];
const Stmt *predStmt =
C.getPredecessor()->getLocation().castAs<StmtPoint>().getStmt();
// Ignore implicit calls to setters.
if (!isa<BinaryOperator>(predStmt))
return;
Data.explodedNodes.Add(C.getPredecessor());
}
void IdempotentOperationChecker::checkEndAnalysis(ExplodedGraph &G,
BugReporter &BR,
ExprEngine &Eng) const {
if (!BT)
BT.reset(new BugType("Idempotent operation", "Dead code"));
// Iterate over the hash to see if we have any paths with definite
// idempotent operations.
for (AssumptionMap::const_iterator i = hash.begin(); i != hash.end(); ++i) {
// Unpack the hash contents
const BinaryOperatorData &Data = i->second;
const Assumption &A = Data.assumption;
const ExplodedNodeSet &ES = Data.explodedNodes;
// If there are no nodes accosted with the expression, nothing to report.
// FIXME: This is possible because the checker does part of processing in
// checkPreStmt and part in checkPostStmt.
if (ES.begin() == ES.end())
continue;
const BinaryOperator *B = i->first;
if (A == Impossible)
continue;
// If the analyzer did not finish, check to see if we can still emit this
// warning
if (Eng.hasWorkRemaining()) {
// If we can trace back
AnalysisDeclContext *AC = (*ES.begin())->getLocationContext()
->getAnalysisDeclContext();
if (!pathWasCompletelyAnalyzed(AC,
AC->getCFGStmtMap()->getBlock(B),
Eng.getCoreEngine()))
continue;
}
// Select the error message and SourceRanges to report.
SmallString<128> buf;
llvm::raw_svector_ostream os(buf);
bool LHSRelevant = false, RHSRelevant = false;
switch (A) {
case Equal:
LHSRelevant = true;
RHSRelevant = true;
if (B->getOpcode() == BO_Assign)
os << "Assigned value is always the same as the existing value";
else
os << "Both operands to '" << B->getOpcodeStr()
<< "' always have the same value";
break;
case LHSis1:
LHSRelevant = true;
os << "The left operand to '" << B->getOpcodeStr() << "' is always 1";
break;
case RHSis1:
RHSRelevant = true;
os << "The right operand to '" << B->getOpcodeStr() << "' is always 1";
break;
case LHSis0:
LHSRelevant = true;
os << "The left operand to '" << B->getOpcodeStr() << "' is always 0";
break;
case RHSis0:
RHSRelevant = true;
os << "The right operand to '" << B->getOpcodeStr() << "' is always 0";
break;
case Possible:
llvm_unreachable("Operation was never marked with an assumption");
case Impossible:
llvm_unreachable(0);
}
// Add a report for each ExplodedNode
for (ExplodedNodeSet::iterator I = ES.begin(), E = ES.end(); I != E; ++I) {
BugReport *report = new BugReport(*BT, os.str(), *I);
// Add source ranges and visitor hooks
if (LHSRelevant) {
const Expr *LHS = i->first->getLHS();
report->addRange(LHS->getSourceRange());
FindLastStoreBRVisitor::registerStatementVarDecls(*report, LHS);
}
if (RHSRelevant) {
const Expr *RHS = i->first->getRHS();
report->addRange(i->first->getRHS()->getSourceRange());
FindLastStoreBRVisitor::registerStatementVarDecls(*report, RHS);
}
BR.emitReport(report);
}
}
hash.clear();
}
// Updates the current assumption given the new assumption
inline void IdempotentOperationChecker::UpdateAssumption(Assumption &A,
const Assumption &New) {
// If the assumption is the same, there is nothing to do
if (A == New)
return;
switch (A) {
// If we don't currently have an assumption, set it
case Possible:
A = New;
return;
// If we have determined that a valid state happened, ignore the new
// assumption.
case Impossible:
return;
// Any other case means that we had a different assumption last time. We don't
// currently support mixing assumptions for diagnostic reasons, so we set
// our assumption to be impossible.
default:
A = Impossible;
return;
}
}
// Check for a statement where a variable is self assigned to possibly avoid an
// unused variable warning.
bool IdempotentOperationChecker::isSelfAssign(const Expr *LHS, const Expr *RHS) {
LHS = LHS->IgnoreParenCasts();
RHS = RHS->IgnoreParenCasts();
const DeclRefExpr *LHS_DR = dyn_cast<DeclRefExpr>(LHS);
if (!LHS_DR)
return false;
const VarDecl *VD = dyn_cast<VarDecl>(LHS_DR->getDecl());
if (!VD)
return false;
const DeclRefExpr *RHS_DR = dyn_cast<DeclRefExpr>(RHS);
if (!RHS_DR)
return false;
if (VD != RHS_DR->getDecl())
return false;
return true;
}
// Returns true if the Expr points to a VarDecl that is not read anywhere
// outside of self-assignments.
bool IdempotentOperationChecker::isUnused(const Expr *E,
AnalysisDeclContext *AC) {
if (!E)
return false;
const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(E->IgnoreParenCasts());
if (!DR)
return false;
const VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl());
if (!VD)
return false;
if (AC->getPseudoConstantAnalysis()->wasReferenced(VD))
return false;
return true;
}
// Check for self casts truncating/extending a variable
bool IdempotentOperationChecker::isTruncationExtensionAssignment(
const Expr *LHS,
const Expr *RHS) {
const DeclRefExpr *LHS_DR = dyn_cast<DeclRefExpr>(LHS->IgnoreParenCasts());
if (!LHS_DR)
return false;
const VarDecl *VD = dyn_cast<VarDecl>(LHS_DR->getDecl());
if (!VD)
return false;
const DeclRefExpr *RHS_DR = dyn_cast<DeclRefExpr>(RHS->IgnoreParenCasts());
if (!RHS_DR)
return false;
if (VD != RHS_DR->getDecl())
return false;
return dyn_cast<DeclRefExpr>(RHS->IgnoreParenLValueCasts()) == NULL;
}
// Returns false if a path to this block was not completely analyzed, or true
// otherwise.
bool
IdempotentOperationChecker::pathWasCompletelyAnalyzed(AnalysisDeclContext *AC,
const CFGBlock *CB,
const CoreEngine &CE) {
CFGReverseBlockReachabilityAnalysis *CRA = AC->getCFGReachablityAnalysis();
// Test for reachability from any aborted blocks to this block
typedef CoreEngine::BlocksExhausted::const_iterator ExhaustedIterator;
for (ExhaustedIterator I = CE.blocks_exhausted_begin(),
E = CE.blocks_exhausted_end(); I != E; ++I) {
const BlockEdge &BE = I->first;
// The destination block on the BlockEdge is the first block that was not
// analyzed. If we can reach this block from the aborted block, then this
// block was not completely analyzed.
//
// Also explicitly check if the current block is the destination block.
// While technically reachable, it means we aborted the analysis on
// a path that included that block.
const CFGBlock *destBlock = BE.getDst();
if (destBlock == CB || CRA->isReachable(destBlock, CB))
return false;
}
// Test for reachability from blocks we just gave up on.
typedef CoreEngine::BlocksAborted::const_iterator AbortedIterator;
for (AbortedIterator I = CE.blocks_aborted_begin(),
E = CE.blocks_aborted_end(); I != E; ++I) {
const CFGBlock *destBlock = I->first;
if (destBlock == CB || CRA->isReachable(destBlock, CB))
return false;
}
// For the items still on the worklist, see if they are in blocks that
// can eventually reach 'CB'.
class VisitWL : public WorkList::Visitor {
const CFGStmtMap *CBM;
const CFGBlock *TargetBlock;
CFGReverseBlockReachabilityAnalysis &CRA;
public:
VisitWL(const CFGStmtMap *cbm, const CFGBlock *targetBlock,
CFGReverseBlockReachabilityAnalysis &cra)
: CBM(cbm), TargetBlock(targetBlock), CRA(cra) {}
virtual bool visit(const WorkListUnit &U) {
ProgramPoint P = U.getNode()->getLocation();
const CFGBlock *B = 0;
if (Optional<StmtPoint> SP = P.getAs<StmtPoint>()) {
B = CBM->getBlock(SP->getStmt());
} else if (Optional<BlockEdge> BE = P.getAs<BlockEdge>()) {
B = BE->getDst();
} else if (Optional<BlockEntrance> BEnt = P.getAs<BlockEntrance>()) {
B = BEnt->getBlock();
} else if (Optional<BlockExit> BExit = P.getAs<BlockExit>()) {
B = BExit->getBlock();
}
if (!B)
return true;
return B == TargetBlock || CRA.isReachable(B, TargetBlock);
}
};
VisitWL visitWL(AC->getCFGStmtMap(), CB, *CRA);
// Were there any items in the worklist that could potentially reach
// this block?
if (CE.getWorkList()->visitItemsInWorkList(visitWL))
return false;
// Verify that this block is reachable from the entry block
if (!CRA->isReachable(&AC->getCFG()->getEntry(), CB))
return false;
// If we get to this point, there is no connection to the entry block or an
// aborted block. This path is unreachable and we can report the error.
return true;
}
// Recursive function that determines whether an expression contains any element
// that varies. This could be due to a compile-time constant like sizeof. An
// expression may also involve a variable that behaves like a constant. The
// function returns true if the expression varies, and false otherwise.
bool IdempotentOperationChecker::CanVary(const Expr *Ex,
AnalysisDeclContext *AC) {
// Parentheses and casts are irrelevant here
Ex = Ex->IgnoreParenCasts();
if (Ex->getLocStart().isMacroID())
return false;
switch (Ex->getStmtClass()) {
// Trivially true cases
case Stmt::ArraySubscriptExprClass:
case Stmt::MemberExprClass:
case Stmt::StmtExprClass:
case Stmt::CallExprClass:
case Stmt::VAArgExprClass:
case Stmt::ShuffleVectorExprClass:
return true;
default:
return true;
// Trivially false cases
case Stmt::IntegerLiteralClass:
case Stmt::CharacterLiteralClass:
case Stmt::FloatingLiteralClass:
case Stmt::PredefinedExprClass:
case Stmt::ImaginaryLiteralClass:
case Stmt::StringLiteralClass:
case Stmt::OffsetOfExprClass:
case Stmt::CompoundLiteralExprClass:
case Stmt::AddrLabelExprClass:
case Stmt::BinaryTypeTraitExprClass:
case Stmt::GNUNullExprClass:
case Stmt::InitListExprClass:
case Stmt::DesignatedInitExprClass:
case Stmt::BlockExprClass:
return false;
// Cases requiring custom logic
case Stmt::UnaryExprOrTypeTraitExprClass: {
const UnaryExprOrTypeTraitExpr *SE =
cast<const UnaryExprOrTypeTraitExpr>(Ex);
if (SE->getKind() != UETT_SizeOf)
return false;
return SE->getTypeOfArgument()->isVariableArrayType();
}
case Stmt::DeclRefExprClass:
// Check for constants/pseudoconstants
return !isConstantOrPseudoConstant(cast<DeclRefExpr>(Ex), AC);
// The next cases require recursion for subexpressions
case Stmt::BinaryOperatorClass: {
const BinaryOperator *B = cast<const BinaryOperator>(Ex);
// Exclude cases involving pointer arithmetic. These are usually
// false positives.
if (B->getOpcode() == BO_Sub || B->getOpcode() == BO_Add)
if (B->getLHS()->getType()->getAs<PointerType>())
return false;
return CanVary(B->getRHS(), AC)
|| CanVary(B->getLHS(), AC);
}
case Stmt::UnaryOperatorClass: {
const UnaryOperator *U = cast<const UnaryOperator>(Ex);
// Handle trivial case first
switch (U->getOpcode()) {
case UO_Extension:
return false;
default:
return CanVary(U->getSubExpr(), AC);
}
}
case Stmt::ChooseExprClass:
return CanVary(cast<const ChooseExpr>(Ex)->getChosenSubExpr(
AC->getASTContext()), AC);
case Stmt::ConditionalOperatorClass:
case Stmt::BinaryConditionalOperatorClass:
return CanVary(cast<AbstractConditionalOperator>(Ex)->getCond(), AC);
}
}
// Returns true if a DeclRefExpr is or behaves like a constant.
bool IdempotentOperationChecker::isConstantOrPseudoConstant(
const DeclRefExpr *DR,
AnalysisDeclContext *AC) {
// Check if the type of the Decl is const-qualified
if (DR->getType().isConstQualified())
return true;
// Check for an enum
if (isa<EnumConstantDecl>(DR->getDecl()))
return true;
const VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl());
if (!VD)
return true;
// Check if the Decl behaves like a constant. This check also takes care of
// static variables, which can only change between function calls if they are
// modified in the AST.
PseudoConstantAnalysis *PCA = AC->getPseudoConstantAnalysis();
if (PCA->isPseudoConstant(VD))
return true;
return false;
}
// Recursively find any substatements containing VarDecl's with storage other
// than local
bool IdempotentOperationChecker::containsNonLocalVarDecl(const Stmt *S) {
const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(S);
if (DR)
if (const VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl()))
if (!VD->hasLocalStorage())
return true;
for (Stmt::const_child_iterator I = S->child_begin(); I != S->child_end();
++I)
if (const Stmt *child = *I)
if (containsNonLocalVarDecl(child))
return true;
return false;
}
void ento::registerIdempotentOperationChecker(CheckerManager &mgr) {
mgr.registerChecker<IdempotentOperationChecker>();
}