AnalysisBasedWarnings.cpp revision 69a171bc0a3ffed1931471a024d33b2fd00291bb 
1//=- AnalysisBasedWarnings.cpp - Sema warnings based on libAnalysis -*- C++ -*-=//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file defines analysis_warnings::[Policy,Executor].
11// Together they are used by Sema to issue warnings based on inexpensive
12// static analysis algorithms in libAnalysis.
13//
14//===----------------------------------------------------------------------===//
15
16#include "Sema.h"
17#include "AnalysisBasedWarnings.h"
18#include "clang/Basic/SourceManager.h"
19#include "clang/AST/ExprObjC.h"
20#include "clang/AST/ExprCXX.h"
21#include "clang/AST/StmtObjC.h"
22#include "clang/AST/StmtCXX.h"
23#include "clang/Analysis/AnalysisContext.h"
24#include "clang/Analysis/CFG.h"
25#include "clang/Analysis/Analyses/ReachableCode.h"
26#include "llvm/ADT/BitVector.h"
27#include "llvm/Support/Casting.h"
28
29using namespace clang;
30
31//===----------------------------------------------------------------------===//
32// Unreachable code analysis.
33//===----------------------------------------------------------------------===//
34
35namespace {
36  class UnreachableCodeHandler : public reachable_code::Callback {
37    Sema &S;
38  public:
39    UnreachableCodeHandler(Sema &s) : S(s) {}
40
41    void HandleUnreachable(SourceLocation L, SourceRange R1, SourceRange R2) {
42      S.Diag(L, diag::warn_unreachable) << R1 << R2;
43    }
44  };
45}
46
47/// CheckUnreachable - Check for unreachable code.
48static void CheckUnreachable(Sema &S, AnalysisContext &AC) {
49  UnreachableCodeHandler UC(S);
50  reachable_code::FindUnreachableCode(AC, UC);
51}
52
53//===----------------------------------------------------------------------===//
54// Check for missing return value.
55//===----------------------------------------------------------------------===//
56
57enum ControlFlowKind { NeverFallThrough = 0, MaybeFallThrough = 1,
58  AlwaysFallThrough = 2, NeverFallThroughOrReturn = 3 };
59
60/// CheckFallThrough - Check that we don't fall off the end of a
61/// Statement that should return a value.
62///
63/// \returns AlwaysFallThrough iff we always fall off the end of the statement,
64/// MaybeFallThrough iff we might or might not fall off the end,
65/// NeverFallThroughOrReturn iff we never fall off the end of the statement or
66/// return.  We assume NeverFallThrough iff we never fall off the end of the
67/// statement but we may return.  We assume that functions not marked noreturn
68/// will return.
69static ControlFlowKind CheckFallThrough(AnalysisContext &AC) {
70  CFG *cfg = AC.getCFG();
71  if (cfg == 0)
72    // FIXME: This should be NeverFallThrough
73    return NeverFallThroughOrReturn;
74
75  // The CFG leaves in dead things, and we don't want the dead code paths to
76  // confuse us, so we mark all live things first.
77  llvm::BitVector live(cfg->getNumBlockIDs());
78  unsigned count = reachable_code::ScanReachableFromBlock(cfg->getEntry(),
79                                                          live);
80
81  bool AddEHEdges = AC.getAddEHEdges();
82  if (!AddEHEdges && count != cfg->getNumBlockIDs())
83    // When there are things remaining dead, and we didn't add EH edges
84    // from CallExprs to the catch clauses, we have to go back and
85    // mark them as live.
86    for (CFG::iterator I = cfg->begin(), E = cfg->end(); I != E; ++I) {
87      CFGBlock &b = **I;
88      if (!live[b.getBlockID()]) {
89        if (b.pred_begin() == b.pred_end()) {
90          if (b.getTerminator() && isa<CXXTryStmt>(b.getTerminator()))
91            // When not adding EH edges from calls, catch clauses
92            // can otherwise seem dead.  Avoid noting them as dead.
93            count += reachable_code::ScanReachableFromBlock(b, live);
94          continue;
95        }
96      }
97    }
98
99  // Now we know what is live, we check the live precessors of the exit block
100  // and look for fall through paths, being careful to ignore normal returns,
101  // and exceptional paths.
102  bool HasLiveReturn = false;
103  bool HasFakeEdge = false;
104  bool HasPlainEdge = false;
105  bool HasAbnormalEdge = false;
106  for (CFGBlock::pred_iterator I=cfg->getExit().pred_begin(),
107       E = cfg->getExit().pred_end();
108       I != E;
109       ++I) {
110    CFGBlock& B = **I;
111    if (!live[B.getBlockID()])
112      continue;
113    if (B.size() == 0) {
114      if (B.getTerminator() && isa<CXXTryStmt>(B.getTerminator())) {
115        HasAbnormalEdge = true;
116        continue;
117      }
118
119      // A labeled empty statement, or the entry block...
120      HasPlainEdge = true;
121      continue;
122    }
123    Stmt *S = B[B.size()-1];
124    if (isa<ReturnStmt>(S)) {
125      HasLiveReturn = true;
126      continue;
127    }
128    if (isa<ObjCAtThrowStmt>(S)) {
129      HasFakeEdge = true;
130      continue;
131    }
132    if (isa<CXXThrowExpr>(S)) {
133      HasFakeEdge = true;
134      continue;
135    }
136    if (const AsmStmt *AS = dyn_cast<AsmStmt>(S)) {
137      if (AS->isMSAsm()) {
138        HasFakeEdge = true;
139        HasLiveReturn = true;
140        continue;
141      }
142    }
143    if (isa<CXXTryStmt>(S)) {
144      HasAbnormalEdge = true;
145      continue;
146    }
147
148    bool NoReturnEdge = false;
149    if (CallExpr *C = dyn_cast<CallExpr>(S)) {
150      if (std::find(B.succ_begin(), B.succ_end(), &cfg->getExit())
151            == B.succ_end()) {
152        HasAbnormalEdge = true;
153        continue;
154      }
155      Expr *CEE = C->getCallee()->IgnoreParenCasts();
156      if (getFunctionExtInfo(CEE->getType()).getNoReturn()) {
157        NoReturnEdge = true;
158        HasFakeEdge = true;
159      } else if (DeclRefExpr *DRE = dyn_cast<DeclRefExpr>(CEE)) {
160        ValueDecl *VD = DRE->getDecl();
161        if (VD->hasAttr<NoReturnAttr>()) {
162          NoReturnEdge = true;
163          HasFakeEdge = true;
164        }
165      }
166    }
167    // FIXME: Add noreturn message sends.
168    if (NoReturnEdge == false)
169      HasPlainEdge = true;
170  }
171  if (!HasPlainEdge) {
172    if (HasLiveReturn)
173      return NeverFallThrough;
174    return NeverFallThroughOrReturn;
175  }
176  if (HasAbnormalEdge || HasFakeEdge || HasLiveReturn)
177    return MaybeFallThrough;
178  // This says AlwaysFallThrough for calls to functions that are not marked
179  // noreturn, that don't return.  If people would like this warning to be more
180  // accurate, such functions should be marked as noreturn.
181  return AlwaysFallThrough;
182}
183
184struct CheckFallThroughDiagnostics {
185  unsigned diag_MaybeFallThrough_HasNoReturn;
186  unsigned diag_MaybeFallThrough_ReturnsNonVoid;
187  unsigned diag_AlwaysFallThrough_HasNoReturn;
188  unsigned diag_AlwaysFallThrough_ReturnsNonVoid;
189  unsigned diag_NeverFallThroughOrReturn;
190  bool funMode;
191
192  static CheckFallThroughDiagnostics MakeForFunction(const Decl *Func) {
193    CheckFallThroughDiagnostics D;
194    D.diag_MaybeFallThrough_HasNoReturn =
195      diag::warn_falloff_noreturn_function;
196    D.diag_MaybeFallThrough_ReturnsNonVoid =
197      diag::warn_maybe_falloff_nonvoid_function;
198    D.diag_AlwaysFallThrough_HasNoReturn =
199      diag::warn_falloff_noreturn_function;
200    D.diag_AlwaysFallThrough_ReturnsNonVoid =
201      diag::warn_falloff_nonvoid_function;
202
203    // Don't suggest that virtual functions be marked "noreturn", since they
204    // might be overridden by non-noreturn functions.
205    bool isVirtualMethod = false;
206    if (const CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(Func))
207      isVirtualMethod = Method->isVirtual();
208
209    if (!isVirtualMethod)
210      D.diag_NeverFallThroughOrReturn =
211        diag::warn_suggest_noreturn_function;
212    else
213      D.diag_NeverFallThroughOrReturn = 0;
214
215    D.funMode = true;
216    return D;
217  }
218
219  static CheckFallThroughDiagnostics MakeForBlock() {
220    CheckFallThroughDiagnostics D;
221    D.diag_MaybeFallThrough_HasNoReturn =
222      diag::err_noreturn_block_has_return_expr;
223    D.diag_MaybeFallThrough_ReturnsNonVoid =
224      diag::err_maybe_falloff_nonvoid_block;
225    D.diag_AlwaysFallThrough_HasNoReturn =
226      diag::err_noreturn_block_has_return_expr;
227    D.diag_AlwaysFallThrough_ReturnsNonVoid =
228      diag::err_falloff_nonvoid_block;
229    D.diag_NeverFallThroughOrReturn =
230      diag::warn_suggest_noreturn_block;
231    D.funMode = false;
232    return D;
233  }
234
235  bool checkDiagnostics(Diagnostic &D, bool ReturnsVoid,
236                        bool HasNoReturn) const {
237    if (funMode) {
238      return (D.getDiagnosticLevel(diag::warn_maybe_falloff_nonvoid_function)
239              == Diagnostic::Ignored || ReturnsVoid)
240        && (D.getDiagnosticLevel(diag::warn_noreturn_function_has_return_expr)
241              == Diagnostic::Ignored || !HasNoReturn)
242        && (D.getDiagnosticLevel(diag::warn_suggest_noreturn_block)
243              == Diagnostic::Ignored || !ReturnsVoid);
244    }
245
246    // For blocks.
247    return  ReturnsVoid && !HasNoReturn
248            && (D.getDiagnosticLevel(diag::warn_suggest_noreturn_block)
249                == Diagnostic::Ignored || !ReturnsVoid);
250  }
251};
252
253/// CheckFallThroughForFunctionDef - Check that we don't fall off the end of a
254/// function that should return a value.  Check that we don't fall off the end
255/// of a noreturn function.  We assume that functions and blocks not marked
256/// noreturn will return.
257static void CheckFallThroughForBody(Sema &S, const Decl *D, const Stmt *Body,
258                                    QualType BlockTy,
259                                    const CheckFallThroughDiagnostics& CD,
260                                    AnalysisContext &AC) {
261
262  bool ReturnsVoid = false;
263  bool HasNoReturn = false;
264
265  if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D)) {
266    ReturnsVoid = FD->getResultType()->isVoidType();
267    HasNoReturn = FD->hasAttr<NoReturnAttr>() ||
268       FD->getType()->getAs<FunctionType>()->getNoReturnAttr();
269  }
270  else if (const ObjCMethodDecl *MD = dyn_cast<ObjCMethodDecl>(D)) {
271    ReturnsVoid = MD->getResultType()->isVoidType();
272    HasNoReturn = MD->hasAttr<NoReturnAttr>();
273  }
274  else if (isa<BlockDecl>(D)) {
275    if (const FunctionType *FT =
276          BlockTy->getPointeeType()->getAs<FunctionType>()) {
277      if (FT->getResultType()->isVoidType())
278        ReturnsVoid = true;
279      if (FT->getNoReturnAttr())
280        HasNoReturn = true;
281    }
282  }
283
284  Diagnostic &Diags = S.getDiagnostics();
285
286  // Short circuit for compilation speed.
287  if (CD.checkDiagnostics(Diags, ReturnsVoid, HasNoReturn))
288      return;
289
290  // FIXME: Function try block
291  if (const CompoundStmt *Compound = dyn_cast<CompoundStmt>(Body)) {
292    switch (CheckFallThrough(AC)) {
293      case MaybeFallThrough:
294        if (HasNoReturn)
295          S.Diag(Compound->getRBracLoc(),
296                 CD.diag_MaybeFallThrough_HasNoReturn);
297        else if (!ReturnsVoid)
298          S.Diag(Compound->getRBracLoc(),
299                 CD.diag_MaybeFallThrough_ReturnsNonVoid);
300        break;
301      case AlwaysFallThrough:
302        if (HasNoReturn)
303          S.Diag(Compound->getRBracLoc(),
304                 CD.diag_AlwaysFallThrough_HasNoReturn);
305        else if (!ReturnsVoid)
306          S.Diag(Compound->getRBracLoc(),
307                 CD.diag_AlwaysFallThrough_ReturnsNonVoid);
308        break;
309      case NeverFallThroughOrReturn:
310        if (ReturnsVoid && !HasNoReturn && CD.diag_NeverFallThroughOrReturn)
311          S.Diag(Compound->getLBracLoc(),
312                 CD.diag_NeverFallThroughOrReturn);
313        break;
314      case NeverFallThrough:
315        break;
316    }
317  }
318}
319
320//===----------------------------------------------------------------------===//
321// AnalysisBasedWarnings - Worker object used by Sema to execute analysis-based
322//  warnings on a function, method, or block.
323//===----------------------------------------------------------------------===//
324
325clang::sema::AnalysisBasedWarnings::Policy::Policy() {
326  enableCheckFallThrough = 1;
327  enableCheckUnreachable = 0;
328}
329
330clang::sema::AnalysisBasedWarnings::AnalysisBasedWarnings(Sema &s) : S(s) {
331  Diagnostic &D = S.getDiagnostics();
332  DefaultPolicy.enableCheckUnreachable = (unsigned)
333    (D.getDiagnosticLevel(diag::warn_unreachable) != Diagnostic::Ignored);
334}
335
336void clang::sema::
337AnalysisBasedWarnings::IssueWarnings(sema::AnalysisBasedWarnings::Policy P,
338                                     const Decl *D, QualType BlockTy) {
339
340  assert(BlockTy.isNull() || isa<BlockDecl>(D));
341
342  // We avoid doing analysis-based warnings when there are errors for
343  // two reasons:
344  // (1) The CFGs often can't be constructed (if the body is invalid), so
345  //     don't bother trying.
346  // (2) The code already has problems; running the analysis just takes more
347  //     time.
348  Diagnostic &Diags = S.getDiagnostics();
349
350  if (Diags.hasErrorOccurred() || Diags.hasFatalErrorOccurred())
351    return;
352
353  // Do not do any analysis for declarations in system headers if we are
354  // going to just ignore them.
355  if (Diags.getSuppressSystemWarnings() &&
356      S.SourceMgr.isInSystemHeader(D->getLocation()))
357    return;
358
359  if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D)) {
360    // For function templates, class templates and member function templates
361    // we'll do the analysis at instantiation time.
362    if (FD->isDependentContext())
363      return;
364  }
365
366  const Stmt *Body = D->getBody();
367  assert(Body);
368
369  // Don't generate EH edges for CallExprs as we'd like to avoid the n^2
370  // explosion for destrutors that can result and the compile time hit.
371  AnalysisContext AC(D, false);
372
373  // Warning: check missing 'return'
374  if (P.enableCheckFallThrough) {
375    const CheckFallThroughDiagnostics &CD =
376      (isa<BlockDecl>(D) ? CheckFallThroughDiagnostics::MakeForBlock()
377                         : CheckFallThroughDiagnostics::MakeForFunction(D));
378    CheckFallThroughForBody(S, D, Body, BlockTy, CD, AC);
379  }
380
381  // Warning: check for unreachable code
382  if (P.enableCheckUnreachable)
383    CheckUnreachable(S, AC);
384}
385