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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 "clang/AST/ParentMap.h"
#include "clang/Basic/Builtins.h"
#include "clang/Basic/SourceManager.h"
#include "clang/EntoSA/PathSensitive/CheckerVisitor.h"
#include "clang/EntoSA/PathSensitive/ExplodedGraph.h"
#include "clang/EntoSA/PathSensitive/SVals.h"
#include "clang/EntoSA/PathSensitive/CheckerHelpers.h"
#include "clang/EntoSA/BugReporter/BugReporter.h"
#include "ExprEngineExperimentalChecks.h"
#include "llvm/ADT/SmallPtrSet.h"
// The number of CFGBlock pointers we want to reserve memory for. This is used
// once for each function we analyze.
#define DEFAULT_CFGBLOCKS 256
using namespace clang;
using namespace ento;
namespace {
class UnreachableCodeChecker : public Checker {
public:
static void *getTag();
void VisitEndAnalysis(ExplodedGraph &G,
BugReporter &B,
ExprEngine &Eng);
private:
static inline const Stmt *getUnreachableStmt(const CFGBlock *CB);
void FindUnreachableEntryPoints(const CFGBlock *CB);
static bool isInvalidPath(const CFGBlock *CB, const ParentMap &PM);
static inline bool isEmptyCFGBlock(const CFGBlock *CB);
llvm::SmallSet<unsigned, DEFAULT_CFGBLOCKS> reachable;
llvm::SmallSet<unsigned, DEFAULT_CFGBLOCKS> visited;
};
}
void *UnreachableCodeChecker::getTag() {
static int x = 0;
return &x;
}
void ento::RegisterUnreachableCodeChecker(ExprEngine &Eng) {
Eng.registerCheck(new UnreachableCodeChecker());
}
void UnreachableCodeChecker::VisitEndAnalysis(ExplodedGraph &G,
BugReporter &B,
ExprEngine &Eng) {
// Bail out if we didn't cover all paths
if (Eng.hasWorkRemaining())
return;
CFG *C = 0;
ParentMap *PM = 0;
// Iterate over ExplodedGraph
for (ExplodedGraph::node_iterator I = G.nodes_begin(), E = G.nodes_end();
I != E; ++I) {
const ProgramPoint &P = I->getLocation();
const LocationContext *LC = P.getLocationContext();
// Save the CFG if we don't have it already
if (!C)
C = LC->getAnalysisContext()->getUnoptimizedCFG();
if (!PM)
PM = &LC->getParentMap();
if (const BlockEntrance *BE = dyn_cast<BlockEntrance>(&P)) {
const CFGBlock *CB = BE->getBlock();
reachable.insert(CB->getBlockID());
}
}
// Bail out if we didn't get the CFG or the ParentMap.
if (!C || !PM)
return;
ASTContext &Ctx = B.getContext();
// 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()))
continue;
// Check if the block is empty (an artificial block)
if (isEmptyCFGBlock(CB))
continue;
// Find the entry points for this block
if (!visited.count(CB->getBlockID()))
FindUnreachableEntryPoints(CB);
// This block may have been pruned; check if we still want to report it
if (reachable.count(CB->getBlockID()))
continue;
// Check for false positives
if (CB->size() > 0 && isInvalidPath(CB, *PM))
continue;
// Special case for __builtin_unreachable.
// FIXME: This should be extended to include other unreachable markers,
// such as llvm_unreachable.
if (!CB->empty()) {
CFGElement First = CB->front();
if (CFGStmt S = First.getAs<CFGStmt>()) {
if (const CallExpr *CE = dyn_cast<CallExpr>(S.getStmt())) {
if (CE->isBuiltinCall(Ctx) == Builtin::BI__builtin_unreachable)
continue;
}
}
}
// We found a block that wasn't covered - find the statement to report
SourceRange SR;
SourceLocation SL;
if (const Stmt *S = getUnreachableStmt(CB)) {
SR = S->getSourceRange();
SL = S->getLocStart();
if (SR.isInvalid() || SL.isInvalid())
continue;
}
else
continue;
// Check if the SourceLocation is in a system header
const SourceManager &SM = B.getSourceManager();
if (SM.isInSystemHeader(SL) || SM.isInExternCSystemHeader(SL))
continue;
B.EmitBasicReport("Unreachable code", "Dead code", "This statement is never"
" executed", SL, SR);
}
}
// Recursively finds the entry point(s) for this dead CFGBlock.
void UnreachableCodeChecker::FindUnreachableEntryPoints(const CFGBlock *CB) {
visited.insert(CB->getBlockID());
for (CFGBlock::const_pred_iterator I = CB->pred_begin(), E = CB->pred_end();
I != E; ++I) {
if (!reachable.count((*I)->getBlockID())) {
// If we find an unreachable predecessor, mark this block as reachable so
// we don't report this block
reachable.insert(CB->getBlockID());
if (!visited.count((*I)->getBlockID()))
// If we haven't previously visited the unreachable predecessor, recurse
FindUnreachableEntryPoints(*I);
}
}
}
// 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 (CFGStmt S = I->getAs<CFGStmt>())
return S;
}
if (const Stmt *S = CB->getTerminator())
return S;
else
return 0;
}
// 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();
// 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
if (containsMacro(cond) || containsEnum(cond)
|| containsStaticLocal(cond) || containsBuiltinOffsetOf(cond)
|| containsStmt<SizeOfAlignOfExpr>(cond))
return true;
return false;
}
// Returns true if the given CFGBlock is empty
bool UnreachableCodeChecker::isEmptyCFGBlock(const CFGBlock *CB) {
return CB->getLabel() == 0 // No labels
&& CB->size() == 0 // No statements
&& CB->getTerminator() == 0; // No terminator
}
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