PseudoConstantAnalysis.cpp revision 967fea6cd9ae60ea31d27d440967990d2c705729 
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 if (const BlockDeclRefExpr *BDR = dyn_cast<BlockDeclRefExpr>(E))
72    return BDR->getDecl();
73  else
74    return 0;
75}
76
77void PseudoConstantAnalysis::RunAnalysis() {
78  std::deque<const Stmt *> WorkList;
79  VarDeclSet *NonConstants = (VarDeclSet*)NonConstantsImpl;
80  VarDeclSet *UsedVars = (VarDeclSet*)UsedVarsImpl;
81
82  // Start with the top level statement of the function
83  WorkList.push_back(DeclBody);
84
85  while (!WorkList.empty()) {
86    const Stmt* Head = WorkList.front();
87    WorkList.pop_front();
88
89    switch (Head->getStmtClass()) {
90    // Case 1: Assignment operators modifying VarDecls
91    case Stmt::BinaryOperatorClass: {
92      const BinaryOperator *BO = cast<BinaryOperator>(Head);
93      // Look for a Decl on the LHS
94      const Decl *LHSDecl = getDecl(BO->getLHS()->IgnoreParenCasts());
95
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
143      // We found a unary operator with a DeclRef as a subexpression. We now
144      // check for any of the increment/decrement operators, as well as
145      // addressOf.
146      switch (UO->getOpcode()) {
147      case UO_PostDec:
148      case UO_PostInc:
149      case UO_PreDec:
150      case UO_PreInc:
151        // The DeclRef is being changed - mark it as non-constant
152      case UO_AddrOf: {
153        // If we are taking the address of the DeclRefExpr, assume it is
154        // non-constant.
155        const VarDecl *VD = dyn_cast<VarDecl>(D);
156        if (VD)
157          NonConstants->insert(VD);
158        break;
159      }
160
161      default:
162        break;
163      }
164      break;
165    }
166
167    // Case 3: Reference Declarations
168    case Stmt::DeclStmtClass: {
169      const DeclStmt *DS = cast<DeclStmt>(Head);
170      // Iterate over each decl and see if any of them contain reference decls
171      for (DeclStmt::const_decl_iterator I = DS->decl_begin(),
172          E = DS->decl_end(); I != E; ++I) {
173        // We only care about VarDecls
174        const VarDecl *VD = dyn_cast<VarDecl>(*I);
175        if (!VD)
176          continue;
177
178        // We found a VarDecl; make sure it is a reference type
179        if (!VD->getType().getTypePtr()->isReferenceType())
180          continue;
181
182        // Try to find a Decl in the initializer
183        const Decl *D = getDecl(VD->getInit()->IgnoreParenCasts());
184
185        // If the reference is to another var, add the var to the non-constant
186        // list
187        if (const VarDecl *RefVD = dyn_cast<VarDecl>(D)) {
188          NonConstants->insert(RefVD);
189          continue;
190        }
191      }
192      break;
193    }
194
195    // Case 4: Block variable references
196    case Stmt::BlockDeclRefExprClass: {
197      const BlockDeclRefExpr *BDR = cast<BlockDeclRefExpr>(Head);
198      if (const VarDecl *VD = dyn_cast<VarDecl>(BDR->getDecl())) {
199        // Add the Decl to the used list
200        UsedVars->insert(VD);
201        continue;
202      }
203      break;
204    }
205
206    // Case 5: Variable references
207    case Stmt::DeclRefExprClass: {
208      const DeclRefExpr *DR = cast<DeclRefExpr>(Head);
209      if (const VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl())) {
210        // Add the Decl to the used list
211        UsedVars->insert(VD);
212        continue;
213      }
214      break;
215    }
216
217    // Case 6: Block expressions
218    case Stmt::BlockExprClass: {
219      const BlockExpr *B = cast<BlockExpr>(Head);
220      // Add the body of the block to the list
221      WorkList.push_back(B->getBody());
222      continue;
223    }
224
225      default:
226        break;
227    } // switch (head->getStmtClass())
228
229    // Add all substatements to the worklist
230    for (Stmt::const_child_iterator I = Head->child_begin(),
231        E = Head->child_end(); I != E; ++I)
232      if (*I)
233        WorkList.push_back(*I);
234  } // while (!WorkList.empty())
235}
236