PseudoConstantAnalysis.cpp revision f4b88a45902af1802a1cb42ba48b1c474474f228 
1//== PseudoConstantAnalysis.cpp - Find Pseudoconstants in the AST-*- 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 tracks the usage of variables in a Decl body to see if they are
11// never written to, implying that they constant. This is useful in static
12// analysis to see if a developer might have intended a variable to be const.
13//
14//===----------------------------------------------------------------------===//
15
16#include "clang/Analysis/Analyses/PseudoConstantAnalysis.h"
17#include "clang/AST/Decl.h"
18#include "clang/AST/Expr.h"
19#include "clang/AST/Stmt.h"
20#include <deque>
21
22using namespace clang;
23
24// The number of ValueDecls we want to keep track of by default (per-function)
25#define VARDECL_SET_SIZE 256
26typedef llvm::SmallPtrSet<const VarDecl*, VARDECL_SET_SIZE> VarDeclSet;
27
28PseudoConstantAnalysis::PseudoConstantAnalysis(const Stmt *DeclBody) :
29      DeclBody(DeclBody), Analyzed(false) {
30  NonConstantsImpl = new VarDeclSet;
31  UsedVarsImpl = new VarDeclSet;
32}
33
34PseudoConstantAnalysis::~PseudoConstantAnalysis() {
35  delete (VarDeclSet*)NonConstantsImpl;
36  delete (VarDeclSet*)UsedVarsImpl;
37}
38
39// Returns true if the given ValueDecl is never written to in the given DeclBody
40bool PseudoConstantAnalysis::isPseudoConstant(const VarDecl *VD) {
41  // Only local and static variables can be pseudoconstants
42  if (!VD->hasLocalStorage() && !VD->isStaticLocal())
43    return false;
44
45  if (!Analyzed) {
46    RunAnalysis();
47    Analyzed = true;
48  }
49
50  VarDeclSet *NonConstants = (VarDeclSet*)NonConstantsImpl;
51
52  return !NonConstants->count(VD);
53}
54
55// Returns true if the variable was used (self assignments don't count)
56bool PseudoConstantAnalysis::wasReferenced(const VarDecl *VD) {
57  if (!Analyzed) {
58    RunAnalysis();
59    Analyzed = true;
60  }
61
62  VarDeclSet *UsedVars = (VarDeclSet*)UsedVarsImpl;
63
64  return UsedVars->count(VD);
65}
66
67// Returns a Decl from a (Block)DeclRefExpr (if any)
68const Decl *PseudoConstantAnalysis::getDecl(const Expr *E) {
69  if (const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(E))
70    return DR->getDecl();
71  else
72    return 0;
73}
74
75void PseudoConstantAnalysis::RunAnalysis() {
76  std::deque<const Stmt *> WorkList;
77  VarDeclSet *NonConstants = (VarDeclSet*)NonConstantsImpl;
78  VarDeclSet *UsedVars = (VarDeclSet*)UsedVarsImpl;
79
80  // Start with the top level statement of the function
81  WorkList.push_back(DeclBody);
82
83  while (!WorkList.empty()) {
84    const Stmt *Head = WorkList.front();
85    WorkList.pop_front();
86
87    if (const Expr *Ex = dyn_cast<Expr>(Head))
88      Head = Ex->IgnoreParenCasts();
89
90    switch (Head->getStmtClass()) {
91    // Case 1: Assignment operators modifying VarDecls
92    case Stmt::BinaryOperatorClass: {
93      const BinaryOperator *BO = cast<BinaryOperator>(Head);
94      // Look for a Decl on the LHS
95      const Decl *LHSDecl = getDecl(BO->getLHS()->IgnoreParenCasts());
96      if (!LHSDecl)
97        break;
98
99      // We found a binary operator with a DeclRefExpr on the LHS. We now check
100      // for any of the assignment operators, implying that this Decl is being
101      // written to.
102      switch (BO->getOpcode()) {
103      // Self-assignments don't count as use of a variable
104      case BO_Assign: {
105        // Look for a DeclRef on the RHS
106        const Decl *RHSDecl = getDecl(BO->getRHS()->IgnoreParenCasts());
107
108        // If the Decls match, we have self-assignment
109        if (LHSDecl == RHSDecl)
110          // Do not visit the children
111          continue;
112
113      }
114      case BO_AddAssign:
115      case BO_SubAssign:
116      case BO_MulAssign:
117      case BO_DivAssign:
118      case BO_AndAssign:
119      case BO_OrAssign:
120      case BO_XorAssign:
121      case BO_ShlAssign:
122      case BO_ShrAssign: {
123        const VarDecl *VD = dyn_cast<VarDecl>(LHSDecl);
124        // The DeclRefExpr is being assigned to - mark it as non-constant
125        if (VD)
126          NonConstants->insert(VD);
127        break;
128      }
129
130      default:
131        break;
132      }
133      break;
134    }
135
136    // Case 2: Pre/post increment/decrement and address of
137    case Stmt::UnaryOperatorClass: {
138      const UnaryOperator *UO = cast<UnaryOperator>(Head);
139
140      // Look for a DeclRef in the subexpression
141      const Decl *D = getDecl(UO->getSubExpr()->IgnoreParenCasts());
142      if (!D)
143        break;
144
145      // We found a unary operator with a DeclRef as a subexpression. We now
146      // check for any of the increment/decrement operators, as well as
147      // addressOf.
148      switch (UO->getOpcode()) {
149      case UO_PostDec:
150      case UO_PostInc:
151      case UO_PreDec:
152      case UO_PreInc:
153        // The DeclRef is being changed - mark it as non-constant
154      case UO_AddrOf: {
155        // If we are taking the address of the DeclRefExpr, assume it is
156        // non-constant.
157        const VarDecl *VD = dyn_cast<VarDecl>(D);
158        if (VD)
159          NonConstants->insert(VD);
160        break;
161      }
162
163      default:
164        break;
165      }
166      break;
167    }
168
169    // Case 3: Reference Declarations
170    case Stmt::DeclStmtClass: {
171      const DeclStmt *DS = cast<DeclStmt>(Head);
172      // Iterate over each decl and see if any of them contain reference decls
173      for (DeclStmt::const_decl_iterator I = DS->decl_begin(),
174          E = DS->decl_end(); I != E; ++I) {
175        // We only care about VarDecls
176        const VarDecl *VD = dyn_cast<VarDecl>(*I);
177        if (!VD)
178          continue;
179
180        // We found a VarDecl; make sure it is a reference type
181        if (!VD->getType().getTypePtr()->isReferenceType())
182          continue;
183
184        // Try to find a Decl in the initializer
185        const Decl *D = getDecl(VD->getInit()->IgnoreParenCasts());
186        if (!D)
187          break;
188
189        // If the reference is to another var, add the var to the non-constant
190        // list
191        if (const VarDecl *RefVD = dyn_cast<VarDecl>(D)) {
192          NonConstants->insert(RefVD);
193          continue;
194        }
195      }
196      break;
197    }
198
199    // Case 4: Variable references
200    case Stmt::DeclRefExprClass: {
201      const DeclRefExpr *DR = cast<DeclRefExpr>(Head);
202      if (const VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl())) {
203        // Add the Decl to the used list
204        UsedVars->insert(VD);
205        continue;
206      }
207      break;
208    }
209
210    // Case 5: Block expressions
211    case Stmt::BlockExprClass: {
212      const BlockExpr *B = cast<BlockExpr>(Head);
213      // Add the body of the block to the list
214      WorkList.push_back(B->getBody());
215      continue;
216    }
217
218    default:
219      break;
220    } // switch (head->getStmtClass())
221
222    // Add all substatements to the worklist
223    for (Stmt::const_child_range I = Head->children(); I; ++I)
224      if (*I)
225        WorkList.push_back(*I);
226  } // while (!WorkList.empty())
227}
228