compilerplugins/clang/constvars.cxx

/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
/*
 * This file is part of the LibreOffice project.
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 */

#if !defined _WIN32 //TODO, #include <sys/file.h>

#include <cassert>
#include <string>
#include <iostream>
#include <fstream>
#include <unordered_set>
#include <vector>
#include <algorithm>
#include <sys/file.h>
#include <unistd.h>
#include "plugin.hxx"
#include "compat.hxx"
#include "check.hxx"

/**
Look for static vars that are only assigned to once, and never written to, they can be const.
*/

namespace
{
/**
 * Wrap the different kinds of callable and callee objects in the clang AST so I can define methods that handle everything.
 */
class CallerWrapper
{
    const CallExpr* m_callExpr;
    const CXXConstructExpr* m_cxxConstructExpr;

public:
    CallerWrapper(const CallExpr* callExpr)
        : m_callExpr(callExpr)
        , m_cxxConstructExpr(nullptr)
    {
    }
    CallerWrapper(const CXXConstructExpr* cxxConstructExpr)
        : m_callExpr(nullptr)
        , m_cxxConstructExpr(cxxConstructExpr)
    {
    }
    unsigned getNumArgs() const
    {
        return m_callExpr ? m_callExpr->getNumArgs() : m_cxxConstructExpr->getNumArgs();
    }
    const Expr* getArg(unsigned i) const
    {
        return m_callExpr ? m_callExpr->getArg(i) : m_cxxConstructExpr->getArg(i);
    }
};
class CalleeWrapper
{
    const FunctionDecl* m_calleeFunctionDecl = nullptr;
    const CXXConstructorDecl* m_cxxConstructorDecl = nullptr;
    const FunctionProtoType* m_functionPrototype = nullptr;

public:
    explicit CalleeWrapper(const FunctionDecl* calleeFunctionDecl)
        : m_calleeFunctionDecl(calleeFunctionDecl)
    {
    }
    explicit CalleeWrapper(const CXXConstructExpr* cxxConstructExpr)
        : m_cxxConstructorDecl(cxxConstructExpr->getConstructor())
    {
    }
    explicit CalleeWrapper(const FunctionProtoType* functionPrototype)
        : m_functionPrototype(functionPrototype)
    {
    }
    unsigned getNumParams() const
    {
        if (m_calleeFunctionDecl)
            return m_calleeFunctionDecl->getNumParams();
        else if (m_cxxConstructorDecl)
            return m_cxxConstructorDecl->getNumParams();
        else if (m_functionPrototype->param_type_begin() == m_functionPrototype->param_type_end())
            // FunctionProtoType will assert if we call getParamTypes() and it has no params
            return 0;
        else
            return m_functionPrototype->getParamTypes().size();
    }
    const QualType getParamType(unsigned i) const
    {
        if (m_calleeFunctionDecl)
            return m_calleeFunctionDecl->getParamDecl(i)->getType();
        else if (m_cxxConstructorDecl)
            return m_cxxConstructorDecl->getParamDecl(i)->getType();
        else
            return m_functionPrototype->getParamTypes()[i];
    }
    std::string getNameAsString() const
    {
        if (m_calleeFunctionDecl)
            return m_calleeFunctionDecl->getNameAsString();
        else if (m_cxxConstructorDecl)
            return m_cxxConstructorDecl->getNameAsString();
        else
            return "";
    }
    CXXMethodDecl const* getAsCXXMethodDecl() const
    {
        if (m_calleeFunctionDecl)
            return dyn_cast<CXXMethodDecl>(m_calleeFunctionDecl);
        return nullptr;
    }
};

class ConstVars : public RecursiveASTVisitor<ConstVars>, public loplugin::Plugin
{
public:
    explicit ConstVars(loplugin::InstantiationData const& data)
        : Plugin(data)
    {
    }

    virtual void run() override;

    bool shouldVisitTemplateInstantiations() const { return true; }
    bool shouldVisitImplicitCode() const { return true; }

    bool VisitVarDecl(const VarDecl*);
    bool VisitCXXForRangeStmt(const CXXForRangeStmt*);
    bool VisitDeclRefExpr(const DeclRefExpr*);
    bool TraverseCXXConstructorDecl(CXXConstructorDecl*);
    bool TraverseCXXMethodDecl(CXXMethodDecl*);
    bool TraverseFunctionDecl(FunctionDecl*);
    bool TraverseIfStmt(IfStmt*);

private:
    void check(const VarDecl* varDecl, const Expr* memberExpr);
    bool isSomeKindOfZero(const Expr* arg);
    bool IsPassedByNonConst(const VarDecl* varDecl, const Stmt* child, CallerWrapper callExpr,
                            CalleeWrapper calleeFunctionDecl);
    llvm::Optional<CalleeWrapper> getCallee(CallExpr const*);

    RecordDecl* insideMoveOrCopyDeclParent = nullptr;
    // For reasons I do not understand, parentFunctionDecl() is not reliable, so
    // we store the parent function on the way down the AST.
    FunctionDecl* insideFunctionDecl = nullptr;
    std::vector<VarDecl const*> insideConditionalCheckOfVarSet;

    std::set<VarDecl const*> cannotBeConstSet;
    std::set<VarDecl const*> definitionSet;
};

void ConstVars::run()
{
    // clang::Expr::isCXX11ConstantExpr only works for C++
    if (!compiler.getLangOpts().CPlusPlus)
        return;

    TraverseDecl(compiler.getASTContext().getTranslationUnitDecl());

    SourceManager& SM = compiler.getSourceManager();
    for (VarDecl const* v : definitionSet)
    {
        if (cannotBeConstSet.find(v) != cannotBeConstSet.end())
            continue;
        llvm::StringRef sourceString(SM.getCharacterData(v->getSourceRange().getEnd()), 50);
        // Implement a marker that disables this plugins warning at a specific site
        if (sourceString.contains("loplugin:constvars:ignore"))
            continue;
        report(DiagnosticsEngine::Warning, "var can be const", compat::getBeginLoc(v));
    }
}

bool ConstVars::VisitVarDecl(const VarDecl* varDecl)
{
    varDecl = varDecl->getCanonicalDecl();
    if (varDecl->getLocation().isValid() && ignoreLocation(varDecl))
        return true;
    if (!varDecl->hasGlobalStorage())
        return true;
    if (isa<ParmVarDecl>(varDecl))
        return true;
    if (varDecl->getLinkageAndVisibility().getLinkage() == ExternalLinkage)
        return true;
    if (varDecl->getType().isConstQualified())
        return true;
    if (isa<ConstantArrayType>(varDecl->getType()))
        return true;
    if (loplugin::TypeCheck(varDecl->getType()).Pointer().Const())
        return true;
    // ignore stuff that forms part of the stable URE interface
    if (isInUnoIncludeFile(compiler.getSourceManager().getSpellingLoc(varDecl->getLocation())))
        return true;

    if (!varDecl->getInit())
        return true;
    if (varDecl->getInit()->isInstantiationDependent())
        return true;
    if (!varDecl->getInit()->isCXX11ConstantExpr(compiler.getASTContext()))
        return true;

    definitionSet.insert(varDecl);
    return true;
}

bool ConstVars::VisitCXXForRangeStmt(const CXXForRangeStmt* forStmt)
{
    if (compat::getBeginLoc(forStmt).isValid() && ignoreLocation(forStmt))
        return true;
    const VarDecl* varDecl = forStmt->getLoopVariable();
    if (!varDecl)
        return true;
    // we don't handle structured assignment properly
    if (isa<DecompositionDecl>(varDecl))
        return true;
    auto tc = loplugin::TypeCheck(varDecl->getType());
    if (!tc.LvalueReference())
        return true;
    if (tc.LvalueReference().Const())
        return true;

    definitionSet.insert(varDecl);
    return true;
}

bool ConstVars::TraverseCXXConstructorDecl(CXXConstructorDecl* cxxConstructorDecl)
{
    auto copy = insideMoveOrCopyDeclParent;
    if (!ignoreLocation(cxxConstructorDecl) && cxxConstructorDecl->isThisDeclarationADefinition())
    {
        if (cxxConstructorDecl->isCopyOrMoveConstructor())
            insideMoveOrCopyDeclParent = cxxConstructorDecl->getParent();
    }
    bool ret = RecursiveASTVisitor::TraverseCXXConstructorDecl(cxxConstructorDecl);
    insideMoveOrCopyDeclParent = copy;
    return ret;
}

bool ConstVars::TraverseCXXMethodDecl(CXXMethodDecl* cxxMethodDecl)
{
    auto copy1 = insideMoveOrCopyDeclParent;
    auto copy2 = insideFunctionDecl;
    if (!ignoreLocation(cxxMethodDecl) && cxxMethodDecl->isThisDeclarationADefinition())
    {
        if (cxxMethodDecl->isCopyAssignmentOperator() || cxxMethodDecl->isMoveAssignmentOperator())
            insideMoveOrCopyDeclParent = cxxMethodDecl->getParent();
    }
    insideFunctionDecl = cxxMethodDecl;
    bool ret = RecursiveASTVisitor::TraverseCXXMethodDecl(cxxMethodDecl);
    insideMoveOrCopyDeclParent = copy1;
    insideFunctionDecl = copy2;
    return ret;
}

bool ConstVars::TraverseFunctionDecl(FunctionDecl* functionDecl)
{
    auto copy2 = insideFunctionDecl;
    insideFunctionDecl = functionDecl;
    bool ret = RecursiveASTVisitor::TraverseFunctionDecl(functionDecl);
    insideFunctionDecl = copy2;
    return ret;
}

bool ConstVars::TraverseIfStmt(IfStmt* ifStmt)
{
    VarDecl const* varDecl = nullptr;
    Expr const* cond = ifStmt->getCond()->IgnoreParenImpCasts();
    if (auto declRefExpr = dyn_cast<DeclRefExpr>(cond))
    {
        if ((varDecl = dyn_cast<VarDecl>(declRefExpr->getDecl())))
            insideConditionalCheckOfVarSet.push_back(varDecl);
    }
    bool ret = RecursiveASTVisitor::TraverseIfStmt(ifStmt);
    if (varDecl)
        insideConditionalCheckOfVarSet.pop_back();
    return ret;
}

bool ConstVars::VisitDeclRefExpr(const DeclRefExpr* declRefExpr)
{
    const VarDecl* varDecl = dyn_cast<VarDecl>(declRefExpr->getDecl());
    if (!varDecl)
        return true;
    varDecl = varDecl->getCanonicalDecl();
    if (ignoreLocation(varDecl))
        return true;
    // ignore stuff that forms part of the stable URE interface
    if (isInUnoIncludeFile(compiler.getSourceManager().getSpellingLoc(varDecl->getLocation())))
        return true;

    if (definitionSet.find(varDecl) != definitionSet.end())
        check(varDecl, declRefExpr);

    return true;
}

void ConstVars::check(const VarDecl* varDecl, const Expr* memberExpr)
{
    auto parentsRange = compiler.getASTContext().getParents(*memberExpr);
    const Stmt* child = memberExpr;
    const Stmt* parent
        = parentsRange.begin() == parentsRange.end() ? nullptr : parentsRange.begin()->get<Stmt>();

    // walk up the tree until we find something interesting

    bool bCannotBeConst = false;
    bool bDump = false;
    auto walkUp = [&]() {
        child = parent;
        auto parentsRange = compiler.getASTContext().getParents(*parent);
        parent = parentsRange.begin() == parentsRange.end() ? nullptr
                                                            : parentsRange.begin()->get<Stmt>();
    };
    do
    {
        if (!parent)
        {
            // check if we have an expression like
            //    int& r = var;
            auto parentsRange = compiler.getASTContext().getParents(*child);
            if (parentsRange.begin() != parentsRange.end())
            {
                auto varDecl = dyn_cast_or_null<VarDecl>(parentsRange.begin()->get<Decl>());
                if (varDecl)
                {
                    if (varDecl->isImplicit())
                    {
                        // so we can walk up from inside a for-range stmt
                        parentsRange = compiler.getASTContext().getParents(*varDecl);
                        if (parentsRange.begin() != parentsRange.end())
                            parent = parentsRange.begin()->get<Stmt>();
                    }
                    else if (loplugin::TypeCheck(varDecl->getType()).LvalueReference().NonConst())
                    {
                        bCannotBeConst = true;
                        break;
                    }
                }
            }
        }
        if (!parent)
            break;
        if (isa<CXXReinterpretCastExpr>(parent))
        {
            // once we see one of these, there is not much useful we can know
            bCannotBeConst = true;
            break;
        }
        else if (isa<CastExpr>(parent) || isa<MemberExpr>(parent) || isa<ParenExpr>(parent)
                 || isa<ParenListExpr>(parent) || isa<ArrayInitLoopExpr>(parent)
                 || isa<ExprWithCleanups>(parent))
        {
            walkUp();
        }
        else if (auto unaryOperator = dyn_cast<UnaryOperator>(parent))
        {
            UnaryOperator::Opcode op = unaryOperator->getOpcode();
            if (op == UO_AddrOf || op == UO_PostInc || op == UO_PostDec || op == UO_PreInc
                || op == UO_PreDec)
            {
                bCannotBeConst = true;
            }
            walkUp();
        }
        else if (auto operatorCallExpr = dyn_cast<CXXOperatorCallExpr>(parent))
        {
            auto callee = getCallee(operatorCallExpr);
            if (callee)
            {
                // if calling a non-const operator on the var
                auto calleeMethodDecl = callee->getAsCXXMethodDecl();
                if (calleeMethodDecl && operatorCallExpr->getArg(0) == child
                    && !calleeMethodDecl->isConst())
                {
                    bCannotBeConst = true;
                }
                else if (IsPassedByNonConst(varDecl, child, operatorCallExpr, *callee))
                {
                    bCannotBeConst = true;
                }
            }
            else
                bCannotBeConst = true; // conservative, could improve
            walkUp();
        }
        else if (auto cxxMemberCallExpr = dyn_cast<CXXMemberCallExpr>(parent))
        {
            const CXXMethodDecl* calleeMethodDecl = cxxMemberCallExpr->getMethodDecl();
            if (calleeMethodDecl)
            {
                // if calling a non-const method on the var
                const Expr* tmp = dyn_cast<Expr>(child);
                if (tmp->isBoundMemberFunction(compiler.getASTContext()))
                {
                    tmp = dyn_cast<MemberExpr>(tmp)->getBase();
                }
                if (cxxMemberCallExpr->getImplicitObjectArgument() == tmp
                    && !calleeMethodDecl->isConst())
                {
                    bCannotBeConst = true;
                    break;
                }
                if (IsPassedByNonConst(varDecl, child, cxxMemberCallExpr,
                                       CalleeWrapper(calleeMethodDecl)))
                    bCannotBeConst = true;
            }
            else
                bCannotBeConst = true; // can happen in templates
            walkUp();
        }
        else if (auto cxxConstructExpr = dyn_cast<CXXConstructExpr>(parent))
        {
            if (IsPassedByNonConst(varDecl, child, cxxConstructExpr,
                                   CalleeWrapper(cxxConstructExpr)))
                bCannotBeConst = true;
            walkUp();
        }
        else if (auto callExpr = dyn_cast<CallExpr>(parent))
        {
            auto callee = getCallee(callExpr);
            if (callee)
            {
                if (IsPassedByNonConst(varDecl, child, callExpr, *callee))
                    bCannotBeConst = true;
            }
            else
                bCannotBeConst = true; // conservative, could improve
            walkUp();
        }
        else if (auto binaryOp = dyn_cast<BinaryOperator>(parent))
        {
            BinaryOperator::Opcode op = binaryOp->getOpcode();
            const bool assignmentOp = op == BO_Assign || op == BO_MulAssign || op == BO_DivAssign
                                      || op == BO_RemAssign || op == BO_AddAssign
                                      || op == BO_SubAssign || op == BO_ShlAssign
                                      || op == BO_ShrAssign || op == BO_AndAssign
                                      || op == BO_XorAssign || op == BO_OrAssign;
            if (assignmentOp)
            {
                if (binaryOp->getLHS() == child)
                    bCannotBeConst = true;
                else if (loplugin::TypeCheck(binaryOp->getLHS()->getType())
                             .LvalueReference()
                             .NonConst())
                    // if the LHS is a non-const reference, we could write to the var later on
                    bCannotBeConst = true;
            }
            walkUp();
        }
        else if (isa<ReturnStmt>(parent))
        {
            if (insideFunctionDecl)
            {
                auto tc = loplugin::TypeCheck(insideFunctionDecl->getReturnType());
                if (tc.LvalueReference().NonConst())
                    bCannotBeConst = true;
            }
            break;
        }
        else if (auto rangeStmt = dyn_cast<CXXForRangeStmt>(parent))
        {
            if (rangeStmt->getRangeStmt() == child)
            {
                auto tc = loplugin::TypeCheck(rangeStmt->getLoopVariable()->getType());
                if (tc.LvalueReference().NonConst())
                    bCannotBeConst = true;
            }
            break;
        }
        else if (isa<SwitchStmt>(parent) || isa<WhileStmt>(parent) || isa<ForStmt>(parent)
                 || isa<IfStmt>(parent) || isa<DoStmt>(parent) || isa<DefaultStmt>(parent))
        {
            break;
        }
        else
        {
            walkUp();
        }
    } while (true);

    if (bDump)
    {
        report(DiagnosticsEngine::Warning, "oh dear, what can the matter be? writtenTo=%0",
               compat::getBeginLoc(memberExpr))
            << bCannotBeConst << memberExpr->getSourceRange();
        if (parent)
        {
            report(DiagnosticsEngine::Note, "parent over here", compat::getBeginLoc(parent))
                << parent->getSourceRange();
            parent->dump();
        }
        memberExpr->dump();
        varDecl->getType()->dump();
    }

    if (bCannotBeConst)
        cannotBeConstSet.insert(varDecl);
}

bool ConstVars::IsPassedByNonConst(const VarDecl* varDecl, const Stmt* child,
                                   CallerWrapper callExpr, CalleeWrapper calleeFunctionDecl)
{
    unsigned len = std::min(callExpr.getNumArgs(), calleeFunctionDecl.getNumParams());
    // if it's an array, passing it by value to a method typically means the
    // callee takes a pointer and can modify the array
    if (varDecl->getType()->isConstantArrayType())
    {
        for (unsigned i = 0; i < len; ++i)
            if (callExpr.getArg(i) == child)
                if (loplugin::TypeCheck(calleeFunctionDecl.getParamType(i)).Pointer().NonConst())
                    return true;
    }
    else
    {
        for (unsigned i = 0; i < len; ++i)
            if (callExpr.getArg(i) == child)
            {
                auto tc = loplugin::TypeCheck(calleeFunctionDecl.getParamType(i));
                if (tc.LvalueReference().NonConst() || tc.Pointer().NonConst())
                    return true;
            }
    }
    return false;
}

llvm::Optional<CalleeWrapper> ConstVars::getCallee(CallExpr const* callExpr)
{
    FunctionDecl const* functionDecl = callExpr->getDirectCallee();
    if (functionDecl)
        return CalleeWrapper(functionDecl);

    // Extract the functionprototype from a type
    clang::Type const* calleeType = callExpr->getCallee()->getType().getTypePtr();
    if (auto pointerType = calleeType->getUnqualifiedDesugaredType()->getAs<clang::PointerType>())
    {
        if (auto prototype = pointerType->getPointeeType()
                                 ->getUnqualifiedDesugaredType()
                                 ->getAs<FunctionProtoType>())
        {
            return CalleeWrapper(prototype);
        }
    }

    return llvm::Optional<CalleeWrapper>();
}

/** off by default because it is very expensive, it walks up the AST a lot */
loplugin::Plugin::Registration<ConstVars> X("constvars", false);
}

#endif

/* vim:set shiftwidth=4 softtabstop=4 expandtab: */