08da97819a
This is a heuristic. Many switch statements, although they look covered
over an enum, may actually handle at runtime more values than in the enum.
This is overly conservative, as there are some cases that clearly
can be ruled as being clearly unreachable, e.g. 'switch (42) { case 1: ... }'.
We can refine this later.
llvm-svn: 202436
449 lines
14 KiB
C++
449 lines
14 KiB
C++
//=- ReachableCodePathInsensitive.cpp ---------------------------*- C++ --*-==//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements a flow-sensitive, path-insensitive analysis of
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// determining reachable blocks within a CFG.
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//
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//===----------------------------------------------------------------------===//
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#include "clang/Analysis/Analyses/ReachableCode.h"
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#include "clang/AST/Expr.h"
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#include "clang/AST/ExprCXX.h"
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#include "clang/AST/ExprObjC.h"
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#include "clang/AST/StmtCXX.h"
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#include "clang/Analysis/AnalysisContext.h"
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#include "clang/Analysis/CFG.h"
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#include "clang/Basic/SourceManager.h"
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#include "llvm/ADT/BitVector.h"
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#include "llvm/ADT/SmallVector.h"
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using namespace clang;
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namespace {
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class DeadCodeScan {
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llvm::BitVector Visited;
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llvm::BitVector &Reachable;
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SmallVector<const CFGBlock *, 10> WorkList;
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typedef SmallVector<std::pair<const CFGBlock *, const Stmt *>, 12>
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DeferredLocsTy;
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DeferredLocsTy DeferredLocs;
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public:
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DeadCodeScan(llvm::BitVector &reachable)
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: Visited(reachable.size()),
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Reachable(reachable) {}
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void enqueue(const CFGBlock *block);
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unsigned scanBackwards(const CFGBlock *Start,
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clang::reachable_code::Callback &CB);
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bool isDeadCodeRoot(const CFGBlock *Block);
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const Stmt *findDeadCode(const CFGBlock *Block);
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void reportDeadCode(const CFGBlock *B,
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const Stmt *S,
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clang::reachable_code::Callback &CB);
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};
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}
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void DeadCodeScan::enqueue(const CFGBlock *block) {
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unsigned blockID = block->getBlockID();
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if (Reachable[blockID] || Visited[blockID])
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return;
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Visited[blockID] = true;
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WorkList.push_back(block);
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}
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bool DeadCodeScan::isDeadCodeRoot(const clang::CFGBlock *Block) {
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bool isDeadRoot = true;
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for (CFGBlock::const_pred_iterator I = Block->pred_begin(),
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E = Block->pred_end(); I != E; ++I) {
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if (const CFGBlock *PredBlock = *I) {
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unsigned blockID = PredBlock->getBlockID();
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if (Visited[blockID]) {
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isDeadRoot = false;
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continue;
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}
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if (!Reachable[blockID]) {
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isDeadRoot = false;
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Visited[blockID] = true;
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WorkList.push_back(PredBlock);
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continue;
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}
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}
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}
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return isDeadRoot;
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}
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static bool isValidDeadStmt(const Stmt *S) {
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if (S->getLocStart().isInvalid())
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return false;
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if (const BinaryOperator *BO = dyn_cast<BinaryOperator>(S))
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return BO->getOpcode() != BO_Comma;
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return true;
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}
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const Stmt *DeadCodeScan::findDeadCode(const clang::CFGBlock *Block) {
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for (CFGBlock::const_iterator I = Block->begin(), E = Block->end(); I!=E; ++I)
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if (Optional<CFGStmt> CS = I->getAs<CFGStmt>()) {
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const Stmt *S = CS->getStmt();
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if (isValidDeadStmt(S))
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return S;
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}
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if (CFGTerminator T = Block->getTerminator()) {
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const Stmt *S = T.getStmt();
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if (isValidDeadStmt(S))
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return S;
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}
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return 0;
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}
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static int SrcCmp(const std::pair<const CFGBlock *, const Stmt *> *p1,
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const std::pair<const CFGBlock *, const Stmt *> *p2) {
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if (p1->second->getLocStart() < p2->second->getLocStart())
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return -1;
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if (p2->second->getLocStart() < p1->second->getLocStart())
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return 1;
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return 0;
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}
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unsigned DeadCodeScan::scanBackwards(const clang::CFGBlock *Start,
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clang::reachable_code::Callback &CB) {
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unsigned count = 0;
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enqueue(Start);
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while (!WorkList.empty()) {
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const CFGBlock *Block = WorkList.pop_back_val();
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// It is possible that this block has been marked reachable after
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// it was enqueued.
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if (Reachable[Block->getBlockID()])
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continue;
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// Look for any dead code within the block.
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const Stmt *S = findDeadCode(Block);
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if (!S) {
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// No dead code. Possibly an empty block. Look at dead predecessors.
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for (CFGBlock::const_pred_iterator I = Block->pred_begin(),
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E = Block->pred_end(); I != E; ++I) {
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if (const CFGBlock *predBlock = *I)
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enqueue(predBlock);
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}
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continue;
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}
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// Specially handle macro-expanded code.
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if (S->getLocStart().isMacroID()) {
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count += clang::reachable_code::ScanReachableFromBlock(Block, Reachable);
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continue;
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}
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if (isDeadCodeRoot(Block)) {
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reportDeadCode(Block, S, CB);
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count += clang::reachable_code::ScanReachableFromBlock(Block, Reachable);
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}
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else {
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// Record this statement as the possibly best location in a
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// strongly-connected component of dead code for emitting a
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// warning.
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DeferredLocs.push_back(std::make_pair(Block, S));
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}
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}
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// If we didn't find a dead root, then report the dead code with the
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// earliest location.
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if (!DeferredLocs.empty()) {
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llvm::array_pod_sort(DeferredLocs.begin(), DeferredLocs.end(), SrcCmp);
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for (DeferredLocsTy::iterator I = DeferredLocs.begin(),
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E = DeferredLocs.end(); I != E; ++I) {
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const CFGBlock *Block = I->first;
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if (Reachable[Block->getBlockID()])
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continue;
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reportDeadCode(Block, I->second, CB);
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count += clang::reachable_code::ScanReachableFromBlock(Block, Reachable);
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}
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}
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return count;
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}
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static SourceLocation GetUnreachableLoc(const Stmt *S,
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SourceRange &R1,
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SourceRange &R2) {
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R1 = R2 = SourceRange();
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if (const Expr *Ex = dyn_cast<Expr>(S))
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S = Ex->IgnoreParenImpCasts();
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switch (S->getStmtClass()) {
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case Expr::BinaryOperatorClass: {
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const BinaryOperator *BO = cast<BinaryOperator>(S);
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return BO->getOperatorLoc();
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}
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case Expr::UnaryOperatorClass: {
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const UnaryOperator *UO = cast<UnaryOperator>(S);
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R1 = UO->getSubExpr()->getSourceRange();
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return UO->getOperatorLoc();
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}
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case Expr::CompoundAssignOperatorClass: {
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const CompoundAssignOperator *CAO = cast<CompoundAssignOperator>(S);
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R1 = CAO->getLHS()->getSourceRange();
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R2 = CAO->getRHS()->getSourceRange();
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return CAO->getOperatorLoc();
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}
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case Expr::BinaryConditionalOperatorClass:
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case Expr::ConditionalOperatorClass: {
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const AbstractConditionalOperator *CO =
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cast<AbstractConditionalOperator>(S);
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return CO->getQuestionLoc();
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}
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case Expr::MemberExprClass: {
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const MemberExpr *ME = cast<MemberExpr>(S);
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R1 = ME->getSourceRange();
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return ME->getMemberLoc();
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}
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case Expr::ArraySubscriptExprClass: {
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const ArraySubscriptExpr *ASE = cast<ArraySubscriptExpr>(S);
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R1 = ASE->getLHS()->getSourceRange();
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R2 = ASE->getRHS()->getSourceRange();
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return ASE->getRBracketLoc();
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}
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case Expr::CStyleCastExprClass: {
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const CStyleCastExpr *CSC = cast<CStyleCastExpr>(S);
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R1 = CSC->getSubExpr()->getSourceRange();
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return CSC->getLParenLoc();
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}
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case Expr::CXXFunctionalCastExprClass: {
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const CXXFunctionalCastExpr *CE = cast <CXXFunctionalCastExpr>(S);
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R1 = CE->getSubExpr()->getSourceRange();
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return CE->getLocStart();
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}
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case Stmt::CXXTryStmtClass: {
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return cast<CXXTryStmt>(S)->getHandler(0)->getCatchLoc();
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}
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case Expr::ObjCBridgedCastExprClass: {
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const ObjCBridgedCastExpr *CSC = cast<ObjCBridgedCastExpr>(S);
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R1 = CSC->getSubExpr()->getSourceRange();
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return CSC->getLParenLoc();
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}
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default: ;
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}
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R1 = S->getSourceRange();
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return S->getLocStart();
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}
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static bool bodyEndsWithNoReturn(const CFGBlock *B) {
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for (CFGBlock::const_reverse_iterator I = B->rbegin(), E = B->rend();
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I != E; ++I) {
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if (Optional<CFGStmt> CS = I->getAs<CFGStmt>()) {
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if (const CallExpr *CE = dyn_cast<CallExpr>(CS->getStmt())) {
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QualType CalleeType = CE->getCallee()->getType();
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if (getFunctionExtInfo(*CalleeType).getNoReturn())
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return true;
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}
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break;
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}
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}
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return false;
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}
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static bool bodyEndsWithNoReturn(const CFGBlock::AdjacentBlock &AB) {
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const CFGBlock *Pred = AB.getPossiblyUnreachableBlock();
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assert(!AB.isReachable() && Pred);
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return bodyEndsWithNoReturn(Pred);
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}
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static bool isBreakPrecededByNoReturn(const CFGBlock *B,
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const Stmt *S) {
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if (!isa<BreakStmt>(S) || B->pred_empty())
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return false;
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assert(B->empty());
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assert(B->pred_size() == 1);
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return bodyEndsWithNoReturn(*B->pred_begin());
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}
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static bool isEnumConstant(const Expr *Ex) {
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const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(Ex);
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if (!DR)
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return false;
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return isa<EnumConstantDecl>(DR->getDecl());
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}
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static bool isTrivialExpression(const Expr *Ex) {
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return isa<IntegerLiteral>(Ex) || isa<StringLiteral>(Ex) ||
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isEnumConstant(Ex);
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}
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static bool isTrivialReturnPrecededByNoReturn(const CFGBlock *B,
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const Stmt *S) {
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if (B->pred_empty())
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return false;
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const Expr *Ex = dyn_cast<Expr>(S);
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if (!Ex)
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return false;
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Ex = Ex->IgnoreParenCasts();
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if (!isTrivialExpression(Ex))
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return false;
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// Look to see if the block ends with a 'return', and see if 'S'
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// is a substatement. The 'return' may not be the last element in
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// the block because of destructors.
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assert(!B->empty());
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for (CFGBlock::const_reverse_iterator I = B->rbegin(), E = B->rend();
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I != E; ++I) {
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if (Optional<CFGStmt> CS = I->getAs<CFGStmt>()) {
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if (const ReturnStmt *RS = dyn_cast<ReturnStmt>(CS->getStmt())) {
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const Expr *RE = RS->getRetValue();
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if (RE && RE->IgnoreParenCasts() == Ex)
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break;
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}
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return false;
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}
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}
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assert(B->pred_size() == 1);
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return bodyEndsWithNoReturn(*B->pred_begin());
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}
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void DeadCodeScan::reportDeadCode(const CFGBlock *B,
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const Stmt *S,
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clang::reachable_code::Callback &CB) {
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// Suppress idiomatic cases of calling a noreturn function just
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// before executing a 'break'. If there is other code after the 'break'
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// in the block then don't suppress the warning.
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if (isBreakPrecededByNoReturn(B, S))
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return;
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// Suppress trivial 'return' statements that are dead.
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if (isTrivialReturnPrecededByNoReturn(B, S))
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return;
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SourceRange R1, R2;
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SourceLocation Loc = GetUnreachableLoc(S, R1, R2);
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CB.HandleUnreachable(Loc, R1, R2);
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}
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namespace clang { namespace reachable_code {
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void Callback::anchor() { }
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unsigned ScanReachableFromBlock(const CFGBlock *Start,
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llvm::BitVector &Reachable) {
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unsigned count = 0;
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// Prep work queue
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SmallVector<const CFGBlock*, 32> WL;
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// The entry block may have already been marked reachable
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// by the caller.
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if (!Reachable[Start->getBlockID()]) {
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++count;
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Reachable[Start->getBlockID()] = true;
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}
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WL.push_back(Start);
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// Find the reachable blocks from 'Start'.
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while (!WL.empty()) {
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const CFGBlock *item = WL.pop_back_val();
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// Look at the successors and mark then reachable.
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for (CFGBlock::const_succ_iterator I = item->succ_begin(),
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E = item->succ_end(); I != E; ++I) {
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const CFGBlock *B = *I;
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if (!B) {
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//
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// For switch statements, treat all cases as being reachable.
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// There are many cases where a switch can contain values that
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// are not in an enumeration but they are still reachable because
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// other values are possible.
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//
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// Note that this is quite conservative. If one saw:
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//
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// switch (1) {
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// case 2: ...
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//
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// we should be able to say that 'case 2' is unreachable. To do
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// this we can either put more heuristics here, or possibly retain
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// that information in the CFG itself.
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//
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if (const CFGBlock *UB = I->getPossiblyUnreachableBlock()) {
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const Stmt *Label = UB->getLabel();
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if (Label && isa<SwitchCase>(Label)) {
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B = UB;
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}
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}
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}
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if (B) {
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unsigned blockID = B->getBlockID();
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if (!Reachable[blockID]) {
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Reachable.set(blockID);
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WL.push_back(B);
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++count;
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}
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}
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}
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}
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return count;
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}
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void FindUnreachableCode(AnalysisDeclContext &AC, Callback &CB) {
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CFG *cfg = AC.getCFG();
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if (!cfg)
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return;
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// Scan for reachable blocks from the entrance of the CFG.
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// If there are no unreachable blocks, we're done.
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llvm::BitVector reachable(cfg->getNumBlockIDs());
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unsigned numReachable = ScanReachableFromBlock(&cfg->getEntry(), reachable);
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if (numReachable == cfg->getNumBlockIDs())
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return;
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// If there aren't explicit EH edges, we should include the 'try' dispatch
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// blocks as roots.
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if (!AC.getCFGBuildOptions().AddEHEdges) {
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for (CFG::try_block_iterator I = cfg->try_blocks_begin(),
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E = cfg->try_blocks_end() ; I != E; ++I) {
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numReachable += ScanReachableFromBlock(*I, reachable);
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}
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if (numReachable == cfg->getNumBlockIDs())
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return;
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}
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// There are some unreachable blocks. We need to find the root blocks that
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// contain code that should be considered unreachable.
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for (CFG::iterator I = cfg->begin(), E = cfg->end(); I != E; ++I) {
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const CFGBlock *block = *I;
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// A block may have been marked reachable during this loop.
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if (reachable[block->getBlockID()])
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continue;
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DeadCodeScan DS(reachable);
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numReachable += DS.scanBackwards(block, CB);
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if (numReachable == cfg->getNumBlockIDs())
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return;
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}
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}
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}} // end namespace clang::reachable_code
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