1 //== PseudoConstantAnalysis.cpp - Find Pseudoconstants in the AST-*- C++ -*-==//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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.
14 //===----------------------------------------------------------------------===//
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"
22 using namespace clang;
24 // The number of ValueDecls we want to keep track of by default (per-function)
25 #define VARDECL_SET_SIZE 256
26 typedef llvm::SmallPtrSet<const VarDecl*, VARDECL_SET_SIZE> VarDeclSet;
28 PseudoConstantAnalysis::PseudoConstantAnalysis(const Stmt *DeclBody) :
29 DeclBody(DeclBody), Analyzed(false) {
30 NonConstantsImpl = new VarDeclSet;
31 UsedVarsImpl = new VarDeclSet;
34 PseudoConstantAnalysis::~PseudoConstantAnalysis() {
35 delete (VarDeclSet*)NonConstantsImpl;
36 delete (VarDeclSet*)UsedVarsImpl;
39 // Returns true if the given ValueDecl is never written to in the given DeclBody
40 bool PseudoConstantAnalysis::isPseudoConstant(const VarDecl *VD) {
41 // Only local and static variables can be pseudoconstants
42 if (!VD->hasLocalStorage() && !VD->isStaticLocal())
50 VarDeclSet *NonConstants = (VarDeclSet*)NonConstantsImpl;
52 return !NonConstants->count(VD);
55 // Returns true if the variable was used (self assignments don't count)
56 bool PseudoConstantAnalysis::wasReferenced(const VarDecl *VD) {
62 VarDeclSet *UsedVars = (VarDeclSet*)UsedVarsImpl;
64 return UsedVars->count(VD);
67 // Returns a Decl from a (Block)DeclRefExpr (if any)
68 const Decl *PseudoConstantAnalysis::getDecl(const Expr *E) {
69 if (const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(E))
75 void PseudoConstantAnalysis::RunAnalysis() {
76 std::deque<const Stmt *> WorkList;
77 VarDeclSet *NonConstants = (VarDeclSet*)NonConstantsImpl;
78 VarDeclSet *UsedVars = (VarDeclSet*)UsedVarsImpl;
80 // Start with the top level statement of the function
81 WorkList.push_back(DeclBody);
83 while (!WorkList.empty()) {
84 const Stmt *Head = WorkList.front();
87 if (const Expr *Ex = dyn_cast<Expr>(Head))
88 Head = Ex->IgnoreParenCasts();
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());
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
102 switch (BO->getOpcode()) {
103 // Self-assignments don't count as use of a variable
105 // Look for a DeclRef on the RHS
106 const Decl *RHSDecl = getDecl(BO->getRHS()->IgnoreParenCasts());
108 // If the Decls match, we have self-assignment
109 if (LHSDecl == RHSDecl)
110 // Do not visit the children
123 const VarDecl *VD = dyn_cast<VarDecl>(LHSDecl);
124 // The DeclRefExpr is being assigned to - mark it as non-constant
126 NonConstants->insert(VD);
136 // Case 2: Pre/post increment/decrement and address of
137 case Stmt::UnaryOperatorClass: {
138 const UnaryOperator *UO = cast<UnaryOperator>(Head);
140 // Look for a DeclRef in the subexpression
141 const Decl *D = getDecl(UO->getSubExpr()->IgnoreParenCasts());
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
148 switch (UO->getOpcode()) {
153 // The DeclRef is being changed - mark it as non-constant
155 // If we are taking the address of the DeclRefExpr, assume it is
157 const VarDecl *VD = dyn_cast<VarDecl>(D);
159 NonConstants->insert(VD);
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);
180 // We found a VarDecl; make sure it is a reference type
181 if (!VD->getType().getTypePtr()->isReferenceType())
184 // Try to find a Decl in the initializer
185 const Decl *D = getDecl(VD->getInit()->IgnoreParenCasts());
189 // If the reference is to another var, add the var to the non-constant
191 if (const VarDecl *RefVD = dyn_cast<VarDecl>(D)) {
192 NonConstants->insert(RefVD);
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);
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());
220 } // switch (head->getStmtClass())
222 // Add all substatements to the worklist
223 for (Stmt::const_child_range I = Head->children(); I; ++I)
225 WorkList.push_back(*I);
226 } // while (!WorkList.empty())