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//==- UnreachableCodeChecker.cpp - Generalized dead code checker -*- C++ -*-==//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This file implements a generalized unreachable code checker using a
// path-sensitive analysis. We mark any path visited, and then walk the CFG as a
// post-analysis to determine what was never visited.
// A similar flow-sensitive only check exists in Analysis/ReachableCode.cpp
#include "ClangSACheckers.h"
#include "clang/AST/ParentMap.h"
#include "clang/Basic/Builtins.h"
#include "clang/Basic/SourceManager.h"
#include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.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/ExplodedGraph.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/SVals.h"
#include "llvm/ADT/SmallSet.h"
using namespace clang;
using namespace ento;
namespace {
class UnreachableCodeChecker : public Checker<check::EndAnalysis> {
void checkEndAnalysis(ExplodedGraph &G, BugReporter &B,
ExprEngine &Eng) const;
typedef llvm::SmallSet<unsigned, 32> CFGBlocksSet;
static inline const Stmt *getUnreachableStmt(const CFGBlock *CB);
static void FindUnreachableEntryPoints(const CFGBlock *CB,
CFGBlocksSet &reachable,
CFGBlocksSet &visited);
static bool isInvalidPath(const CFGBlock *CB, const ParentMap &PM);
static inline bool isEmptyCFGBlock(const CFGBlock *CB);
void UnreachableCodeChecker::checkEndAnalysis(ExplodedGraph &G,
BugReporter &B,
ExprEngine &Eng) const {
CFGBlocksSet reachable, visited;
if (Eng.hasWorkRemaining())
const Decl *D = nullptr;
CFG *C = nullptr;
ParentMap *PM = nullptr;
const LocationContext *LC = nullptr;
// Iterate over ExplodedGraph
for (ExplodedGraph::node_iterator I = G.nodes_begin(), E = G.nodes_end();
I != E; ++I) {
const ProgramPoint &P = I->getLocation();
LC = P.getLocationContext();
if (!LC->inTopFrame())
if (!D)
D = LC->getAnalysisDeclContext()->getDecl();
// Save the CFG if we don't have it already
if (!C)
C = LC->getAnalysisDeclContext()->getUnoptimizedCFG();
if (!PM)
PM = &LC->getParentMap();
if (Optional<BlockEntrance> BE = P.getAs<BlockEntrance>()) {
const CFGBlock *CB = BE->getBlock();
// Bail out if we didn't get the CFG or the ParentMap.
if (!D || !C || !PM)
// Don't do anything for template instantiations. Proving that code
// in a template instantiation is unreachable means proving that it is
// unreachable in all instantiations.
if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D))
if (FD->isTemplateInstantiation())
// Find CFGBlocks that were not covered by any node
for (CFG::const_iterator I = C->begin(), E = C->end(); I != E; ++I) {
const CFGBlock *CB = *I;
// Check if the block is unreachable
if (reachable.count(CB->getBlockID()))
// Check if the block is empty (an artificial block)
if (isEmptyCFGBlock(CB))
// Find the entry points for this block
if (!visited.count(CB->getBlockID()))
FindUnreachableEntryPoints(CB, reachable, visited);
// This block may have been pruned; check if we still want to report it
if (reachable.count(CB->getBlockID()))
// Check for false positives
if (isInvalidPath(CB, *PM))
// It is good practice to always have a "default" label in a "switch", even
// if we should never get there. It can be used to detect errors, for
// instance. Unreachable code directly under a "default" label is therefore
// likely to be a false positive.
if (const Stmt *label = CB->getLabel())
if (label->getStmtClass() == Stmt::DefaultStmtClass)
// Special case for __builtin_unreachable.
// FIXME: This should be extended to include other unreachable markers,
// such as llvm_unreachable.
if (!CB->empty()) {
bool foundUnreachable = false;
for (CFGBlock::const_iterator ci = CB->begin(), ce = CB->end();
ci != ce; ++ci) {
if (Optional<CFGStmt> S = (*ci).getAs<CFGStmt>())
if (const CallExpr *CE = dyn_cast<CallExpr>(S->getStmt())) {
if (CE->getBuiltinCallee() == Builtin::BI__builtin_unreachable ||
CE->isBuiltinAssumeFalse(Eng.getContext())) {
foundUnreachable = true;
if (foundUnreachable)
// We found a block that wasn't covered - find the statement to report
SourceRange SR;
PathDiagnosticLocation DL;
SourceLocation SL;
if (const Stmt *S = getUnreachableStmt(CB)) {
// In macros, 'do {...} while (0)' is often used. Don't warn about the
// condition 0 when it is unreachable.
if (S->getLocStart().isMacroID())
if (const auto *I = dyn_cast<IntegerLiteral>(S))
if (I->getValue() == 0ULL)
if (const Stmt *Parent = PM->getParent(S))
if (isa<DoStmt>(Parent))
SR = S->getSourceRange();
DL = PathDiagnosticLocation::createBegin(S, B.getSourceManager(), LC);
SL = DL.asLocation();
if (SR.isInvalid() || !SL.isValid())
// Check if the SourceLocation is in a system header
const SourceManager &SM = B.getSourceManager();
if (SM.isInSystemHeader(SL) || SM.isInExternCSystemHeader(SL))
B.EmitBasicReport(D, this, "Unreachable code", "Dead code",
"This statement is never executed", DL, SR);
// Recursively finds the entry point(s) for this dead CFGBlock.
void UnreachableCodeChecker::FindUnreachableEntryPoints(const CFGBlock *CB,
CFGBlocksSet &reachable,
CFGBlocksSet &visited) {
for (CFGBlock::const_pred_iterator I = CB->pred_begin(), E = CB->pred_end();
I != E; ++I) {
if (!*I)
if (!reachable.count((*I)->getBlockID())) {
// If we find an unreachable predecessor, mark this block as reachable so
// we don't report this block
if (!visited.count((*I)->getBlockID()))
// If we haven't previously visited the unreachable predecessor, recurse
FindUnreachableEntryPoints(*I, reachable, visited);
// Find the Stmt* in a CFGBlock for reporting a warning
const Stmt *UnreachableCodeChecker::getUnreachableStmt(const CFGBlock *CB) {
for (CFGBlock::const_iterator I = CB->begin(), E = CB->end(); I != E; ++I) {
if (Optional<CFGStmt> S = I->getAs<CFGStmt>()) {
if (!isa<DeclStmt>(S->getStmt()))
return S->getStmt();
if (const Stmt *S = CB->getTerminator())
return S;
return nullptr;
// Determines if the path to this CFGBlock contained an element that infers this
// block is a false positive. We assume that FindUnreachableEntryPoints has
// already marked only the entry points to any dead code, so we need only to
// find the condition that led to this block (the predecessor of this block.)
// There will never be more than one predecessor.
bool UnreachableCodeChecker::isInvalidPath(const CFGBlock *CB,
const ParentMap &PM) {
// We only expect a predecessor size of 0 or 1. If it is >1, then an external
// condition has broken our assumption (for example, a sink being placed by
// another check). In these cases, we choose not to report.
if (CB->pred_size() > 1)
return true;
// If there are no predecessors, then this block is trivially unreachable
if (CB->pred_size() == 0)
return false;
const CFGBlock *pred = *CB->pred_begin();
if (!pred)
return false;
// Get the predecessor block's terminator conditon
const Stmt *cond = pred->getTerminatorCondition();
//assert(cond && "CFGBlock's predecessor has a terminator condition");
// The previous assertion is invalid in some cases (eg do/while). Leaving
// reporting of these situations on at the moment to help triage these cases.
if (!cond)
return false;
// Run each of the checks on the conditions
return containsMacro(cond) || containsEnum(cond) ||
containsStaticLocal(cond) || containsBuiltinOffsetOf(cond) ||
// Returns true if the given CFGBlock is empty
bool UnreachableCodeChecker::isEmptyCFGBlock(const CFGBlock *CB) {
return CB->getLabel() == nullptr // No labels
&& CB->size() == 0 // No statements
&& !CB->getTerminator(); // No terminator
void ento::registerUnreachableCodeChecker(CheckerManager &mgr) {