LiveVariables.cpp revision 1681904cd1dd18b542611b9b9fd8a93d4c55ef02 
1//=- LiveVariables.cpp - Live Variable Analysis for Source CFGs -*- 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 implements Live Variables analysis for source-level CFGs.
11//
12//===----------------------------------------------------------------------===//
13
14#include "clang/Analysis/Analyses/LiveVariables.h"
15#include "clang/Basic/SourceManager.h"
16#include "clang/AST/ASTContext.h"
17#include "clang/AST/Expr.h"
18#include "clang/Analysis/CFG.h"
19#include "clang/Analysis/Visitors/CFGRecStmtDeclVisitor.h"
20#include "clang/Analysis/FlowSensitive/DataflowSolver.h"
21#include "clang/Analysis/Support/SaveAndRestore.h"
22#include "clang/Analysis/AnalysisContext.h"
23#include "llvm/ADT/SmallPtrSet.h"
24#include "llvm/ADT/SmallVector.h"
25#include "llvm/Support/raw_ostream.h"
26
27using namespace clang;
28
29//===----------------------------------------------------------------------===//
30// Useful constants.
31//===----------------------------------------------------------------------===//
32
33static const bool Alive = true;
34static const bool Dead = false;
35
36//===----------------------------------------------------------------------===//
37// Dataflow initialization logic.
38//===----------------------------------------------------------------------===//
39
40namespace {
41class RegisterDecls
42  : public CFGRecStmtDeclVisitor<RegisterDecls> {
43
44  LiveVariables::AnalysisDataTy& AD;
45
46  typedef llvm::SmallVector<VarDecl*, 20> AlwaysLiveTy;
47  AlwaysLiveTy AlwaysLive;
48
49
50public:
51  RegisterDecls(LiveVariables::AnalysisDataTy& ad) : AD(ad) {}
52
53  ~RegisterDecls() {
54
55    AD.AlwaysLive.resetValues(AD);
56
57    for (AlwaysLiveTy::iterator I = AlwaysLive.begin(), E = AlwaysLive.end();
58         I != E; ++ I)
59      AD.AlwaysLive(*I, AD) = Alive;
60  }
61
62  void VisitImplicitParamDecl(ImplicitParamDecl* IPD) {
63    // Register the VarDecl for tracking.
64    AD.Register(IPD);
65  }
66
67  void VisitVarDecl(VarDecl* VD) {
68    // Register the VarDecl for tracking.
69    AD.Register(VD);
70
71    // Does the variable have global storage?  If so, it is always live.
72    if (VD->hasGlobalStorage())
73      AlwaysLive.push_back(VD);
74  }
75
76  CFG& getCFG() { return AD.getCFG(); }
77};
78} // end anonymous namespace
79
80LiveVariables::LiveVariables(AnalysisContext &AC) {
81  // Register all referenced VarDecls.
82  CFG &cfg = *AC.getCFG();
83  getAnalysisData().setCFG(cfg);
84  getAnalysisData().setContext(AC.getASTContext());
85  getAnalysisData().AC = &AC;
86
87  RegisterDecls R(getAnalysisData());
88  cfg.VisitBlockStmts(R);
89
90  // Register all parameters even if they didn't occur in the function body.
91  if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(AC.getDecl()))
92    for (FunctionDecl::param_const_iterator PI = FD->param_begin(),
93           PE = FD->param_end(); PI != PE; ++PI)
94      getAnalysisData().Register(*PI);
95}
96
97//===----------------------------------------------------------------------===//
98// Transfer functions.
99//===----------------------------------------------------------------------===//
100
101namespace {
102
103class TransferFuncs : public CFGRecStmtVisitor<TransferFuncs>{
104  LiveVariables::AnalysisDataTy& AD;
105  LiveVariables::ValTy LiveState;
106public:
107  TransferFuncs(LiveVariables::AnalysisDataTy& ad) : AD(ad) {}
108
109  LiveVariables::ValTy& getVal() { return LiveState; }
110  CFG& getCFG() { return AD.getCFG(); }
111
112  void VisitDeclRefExpr(DeclRefExpr* DR);
113  void VisitBinaryOperator(BinaryOperator* B);
114  void VisitBlockExpr(BlockExpr *B);
115  void VisitAssign(BinaryOperator* B);
116  void VisitDeclStmt(DeclStmt* DS);
117  void BlockStmt_VisitObjCForCollectionStmt(ObjCForCollectionStmt* S);
118  void VisitUnaryOperator(UnaryOperator* U);
119  void Visit(Stmt *S);
120  void VisitTerminator(CFGBlock* B);
121
122  /// VisitConditionVariableInit - Handle the initialization of condition
123  ///  variables at branches.  Valid statements include IfStmt, ForStmt,
124  ///  WhileStmt, and SwitchStmt.
125  void VisitConditionVariableInit(Stmt *S);
126
127  void SetTopValue(LiveVariables::ValTy& V) {
128    V = AD.AlwaysLive;
129  }
130
131};
132
133void TransferFuncs::Visit(Stmt *S) {
134
135  if (S == getCurrentBlkStmt()) {
136
137    if (AD.Observer)
138      AD.Observer->ObserveStmt(S,AD,LiveState);
139
140    if (getCFG().isBlkExpr(S))
141      LiveState(S, AD) = Dead;
142
143    StmtVisitor<TransferFuncs,void>::Visit(S);
144  }
145  else if (!getCFG().isBlkExpr(S)) {
146
147    if (AD.Observer)
148      AD.Observer->ObserveStmt(S,AD,LiveState);
149
150    StmtVisitor<TransferFuncs,void>::Visit(S);
151
152  }
153  else {
154    // For block-level expressions, mark that they are live.
155    LiveState(S,AD) = Alive;
156  }
157}
158
159void TransferFuncs::VisitConditionVariableInit(Stmt *S) {
160  assert(!getCFG().isBlkExpr(S));
161  CFGRecStmtVisitor<TransferFuncs>::VisitConditionVariableInit(S);
162}
163
164void TransferFuncs::VisitTerminator(CFGBlock* B) {
165
166  const Stmt* E = B->getTerminatorCondition();
167
168  if (!E)
169    return;
170
171  assert (getCFG().isBlkExpr(E));
172  LiveState(E, AD) = Alive;
173}
174
175void TransferFuncs::VisitDeclRefExpr(DeclRefExpr* DR) {
176  if (VarDecl* V = dyn_cast<VarDecl>(DR->getDecl()))
177    LiveState(V, AD) = Alive;
178}
179
180void TransferFuncs::VisitBlockExpr(BlockExpr *BE) {
181  AnalysisContext::referenced_decls_iterator I, E;
182  llvm::tie(I, E) = AD.AC->getReferencedBlockVars(BE->getBlockDecl());
183  for ( ; I != E ; ++I) {
184    DeclBitVector_Types::Idx i = AD.getIdx(*I);
185    if (i.isValid())
186      LiveState.getBit(i) = Alive;
187  }
188}
189
190void TransferFuncs::VisitBinaryOperator(BinaryOperator* B) {
191  if (B->isAssignmentOp()) VisitAssign(B);
192  else VisitStmt(B);
193}
194
195void
196TransferFuncs::BlockStmt_VisitObjCForCollectionStmt(ObjCForCollectionStmt* S) {
197
198  // This is a block-level expression.  Its value is 'dead' before this point.
199  LiveState(S, AD) = Dead;
200
201  // This represents a 'use' of the collection.
202  Visit(S->getCollection());
203
204  // This represents a 'kill' for the variable.
205  Stmt* Element = S->getElement();
206  DeclRefExpr* DR = 0;
207  VarDecl* VD = 0;
208
209  if (DeclStmt* DS = dyn_cast<DeclStmt>(Element))
210    VD = cast<VarDecl>(DS->getSingleDecl());
211  else {
212    Expr* ElemExpr = cast<Expr>(Element)->IgnoreParens();
213    if ((DR = dyn_cast<DeclRefExpr>(ElemExpr)))
214      VD = cast<VarDecl>(DR->getDecl());
215    else {
216      Visit(ElemExpr);
217      return;
218    }
219  }
220
221  if (VD) {
222    LiveState(VD, AD) = Dead;
223    if (AD.Observer && DR) { AD.Observer->ObserverKill(DR); }
224  }
225}
226
227
228void TransferFuncs::VisitUnaryOperator(UnaryOperator* U) {
229  Expr *E = U->getSubExpr();
230
231  switch (U->getOpcode()) {
232  case UnaryOperator::PostInc:
233  case UnaryOperator::PostDec:
234  case UnaryOperator::PreInc:
235  case UnaryOperator::PreDec:
236    // Walk through the subexpressions, blasting through ParenExprs
237    // until we either find a DeclRefExpr or some non-DeclRefExpr
238    // expression.
239    if (DeclRefExpr* DR = dyn_cast<DeclRefExpr>(E->IgnoreParens()))
240      if (VarDecl* VD = dyn_cast<VarDecl>(DR->getDecl())) {
241        // Treat the --/++ operator as a kill.
242        if (AD.Observer) { AD.Observer->ObserverKill(DR); }
243        LiveState(VD, AD) = Alive;
244        return VisitDeclRefExpr(DR);
245      }
246
247    // Fall-through.
248
249  default:
250    return Visit(E);
251  }
252}
253
254void TransferFuncs::VisitAssign(BinaryOperator* B) {
255  Expr* LHS = B->getLHS();
256
257  // Assigning to a variable?
258  if (DeclRefExpr* DR = dyn_cast<DeclRefExpr>(LHS->IgnoreParens())) {
259
260    // Update liveness inforamtion.
261    unsigned bit = AD.getIdx(DR->getDecl());
262    LiveState.getDeclBit(bit) = Dead | AD.AlwaysLive.getDeclBit(bit);
263
264    if (AD.Observer) { AD.Observer->ObserverKill(DR); }
265
266    // Handle things like +=, etc., which also generate "uses"
267    // of a variable.  Do this just by visiting the subexpression.
268    if (B->getOpcode() != BinaryOperator::Assign)
269      VisitDeclRefExpr(DR);
270  }
271  else // Not assigning to a variable.  Process LHS as usual.
272    Visit(LHS);
273
274  Visit(B->getRHS());
275}
276
277void TransferFuncs::VisitDeclStmt(DeclStmt* DS) {
278  // Declarations effectively "kill" a variable since they cannot
279  // possibly be live before they are declared.
280  for (DeclStmt::decl_iterator DI=DS->decl_begin(), DE = DS->decl_end();
281       DI != DE; ++DI)
282    if (VarDecl* VD = dyn_cast<VarDecl>(*DI)) {
283      // Update liveness information by killing the VarDecl.
284      unsigned bit = AD.getIdx(VD);
285      LiveState.getDeclBit(bit) = Dead | AD.AlwaysLive.getDeclBit(bit);
286
287      // The initializer is evaluated after the variable comes into scope, but
288      // before the DeclStmt (which binds the value to the variable).
289      // Since this is a reverse dataflow analysis, we must evaluate the
290      // transfer function for this expression after the DeclStmt.  If the
291      // initializer references the variable (which is bad) then we extend
292      // its liveness.
293      if (Expr* Init = VD->getInit())
294        Visit(Init);
295
296      if (const VariableArrayType* VT =
297            AD.getContext().getAsVariableArrayType(VD->getType())) {
298        StmtIterator I(const_cast<VariableArrayType*>(VT));
299        StmtIterator E;
300        for (; I != E; ++I) Visit(*I);
301      }
302    }
303}
304
305} // end anonymous namespace
306
307//===----------------------------------------------------------------------===//
308// Merge operator: if something is live on any successor block, it is live
309//  in the current block (a set union).
310//===----------------------------------------------------------------------===//
311
312namespace {
313  typedef StmtDeclBitVector_Types::Union Merge;
314  typedef DataflowSolver<LiveVariables, TransferFuncs, Merge> Solver;
315} // end anonymous namespace
316
317//===----------------------------------------------------------------------===//
318// External interface to run Liveness analysis.
319//===----------------------------------------------------------------------===//
320
321void LiveVariables::runOnCFG(CFG& cfg) {
322  Solver S(*this);
323  S.runOnCFG(cfg);
324}
325
326void LiveVariables::runOnAllBlocks(const CFG& cfg,
327                                   LiveVariables::ObserverTy* Obs,
328                                   bool recordStmtValues) {
329  Solver S(*this);
330  SaveAndRestore<LiveVariables::ObserverTy*> SRObs(getAnalysisData().Observer,
331                                                   Obs);
332  S.runOnAllBlocks(cfg, recordStmtValues);
333}
334
335//===----------------------------------------------------------------------===//
336// liveness queries
337//
338
339bool LiveVariables::isLive(const CFGBlock* B, const VarDecl* D) const {
340  DeclBitVector_Types::Idx i = getAnalysisData().getIdx(D);
341  return i.isValid() ? getBlockData(B).getBit(i) : false;
342}
343
344bool LiveVariables::isLive(const ValTy& Live, const VarDecl* D) const {
345  DeclBitVector_Types::Idx i = getAnalysisData().getIdx(D);
346  return i.isValid() ? Live.getBit(i) : false;
347}
348
349bool LiveVariables::isLive(const Stmt* Loc, const Stmt* StmtVal) const {
350  return getStmtData(Loc)(StmtVal,getAnalysisData());
351}
352
353bool LiveVariables::isLive(const Stmt* Loc, const VarDecl* D) const {
354  return getStmtData(Loc)(D,getAnalysisData());
355}
356
357//===----------------------------------------------------------------------===//
358// printing liveness state for debugging
359//
360
361void LiveVariables::dumpLiveness(const ValTy& V, const SourceManager& SM) const {
362  const AnalysisDataTy& AD = getAnalysisData();
363
364  for (AnalysisDataTy::decl_iterator I = AD.begin_decl(),
365                                     E = AD.end_decl(); I!=E; ++I)
366    if (V.getDeclBit(I->second)) {
367      llvm::errs() << "  " << I->first->getIdentifier()->getName() << " <";
368      I->first->getLocation().dump(SM);
369      llvm::errs() << ">\n";
370    }
371}
372
373void LiveVariables::dumpBlockLiveness(const SourceManager& M) const {
374  for (BlockDataMapTy::const_iterator I = getBlockDataMap().begin(),
375       E = getBlockDataMap().end(); I!=E; ++I) {
376    llvm::errs() << "\n[ B" << I->first->getBlockID()
377                 << " (live variables at block exit) ]\n";
378    dumpLiveness(I->second,M);
379  }
380
381  llvm::errs() << "\n";
382}
383