path: root/compilerplugins/clang/passparamsbyref.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/.
 */
#ifndef LO_CLANG_SHARED_PLUGINS

#include <string>
#include <unordered_set>

#include "check.hxx"
#include "compat.hxx"
#include "plugin.hxx"

// Find places where parameters are passed by value
// It's not very efficient, because we generally end up copying it twice - once into the parameter and
// again into the destination.
// They should rather be passed by reference.
//
// Generally recommending lambda capture by-ref rather than by-copy is even more
// problematic than with function parameters, as a lambda instance can easily
// outlive a referenced variable. So once lambdas start to get used in more
// sophisticated ways than passing them into standard algorithms, this plugin's
// advice, at least for explicit captures, will need to be revisited.

namespace {

class PassParamsByRef:
    public loplugin::FilteringPlugin<PassParamsByRef>
{
public:
    explicit PassParamsByRef(loplugin::InstantiationData const & data): FilteringPlugin(data), mbInsideFunctionDecl(false) {}

    virtual void run() override { TraverseDecl(compiler.getASTContext().getTranslationUnitDecl()); }

    // When warning about function params of primitive type that could be passed
    // by value instead of by reference, make sure not to warn if the parameter
    // is ever bound to a reference; on the one hand, this needs scaffolding in
    // all Traverse*Decl functions (indirectly) derived from FunctionDecl; and
    // on the other hand, use a hack of ignoring just the DeclRefExprs nested in
    // LValueToRValue ImplicitCastExprs when determining whether a param is
    // bound to a reference:
    bool PreTraverseFunctionDecl(FunctionDecl *);
    bool PostTraverseFunctionDecl(FunctionDecl *, bool);
    bool TraverseFunctionDecl(FunctionDecl *);
    bool PreTraverseCXXMethodDecl(CXXMethodDecl *);
    bool PostTraverseCXXMethodDecl(CXXMethodDecl *, bool);
    bool TraverseCXXMethodDecl(CXXMethodDecl *);
    bool PreTraverseCXXConstructorDecl(CXXConstructorDecl *);
    bool PostTraverseCXXConstructorDecl(CXXConstructorDecl *, bool);
    bool TraverseCXXConstructorDecl(CXXConstructorDecl *);
    bool PreTraverseImplicitCastExpr(ImplicitCastExpr *);
    bool TraverseImplicitCastExpr(ImplicitCastExpr *);

    bool VisitBinaryOperator(BinaryOperator const *);
    bool VisitCXXOperatorCallExpr(const CXXOperatorCallExpr *);
    bool VisitCallExpr(const CallExpr *);
    bool VisitCXXMemberCallExpr(const CXXMemberCallExpr *);

private:
    bool isFat(QualType type);

    bool mbInsideFunctionDecl;
    std::unordered_set<ParmVarDecl const *> mParamExclusions;
};

bool PassParamsByRef::PreTraverseFunctionDecl(FunctionDecl* functionDecl)
{
    if (ignoreLocation(functionDecl))
        return false;
    if (functionDecl->isDeleted()
        || functionDecl->isFunctionTemplateSpecialization())
    {
        return false;
    }
    // Ignore virtual methods, sometimes we want to pass by value, and we cannot tell from
    // the limited info available at an individual site.
    const CXXMethodDecl * methodDecl = dyn_cast<CXXMethodDecl>(functionDecl);
    if (methodDecl && methodDecl->isVirtual())
        return false;

    // Only warn on the definition of the function:
    if (!functionDecl->doesThisDeclarationHaveABody())
        return false;

    mbInsideFunctionDecl = true;
    mParamExclusions.clear();
    return true;
}

bool PassParamsByRef::PostTraverseFunctionDecl(FunctionDecl* functionDecl, bool)
{
    mbInsideFunctionDecl = false;

    unsigned n = functionDecl->getNumParams();
    for (unsigned i = 0; i != n; ++i) {
        const ParmVarDecl * pvDecl = functionDecl->getParamDecl(i);
        auto const t = pvDecl->getType();
        if (!isFat(t))
            continue;
        if (mParamExclusions.find(pvDecl) != mParamExclusions.end())
            continue;
        report(
            DiagnosticsEngine::Warning,
            ("passing %0 by value, rather pass by const lvalue reference"),
            pvDecl->getLocation())
            << t << pvDecl->getSourceRange();
        auto can = functionDecl->getCanonicalDecl();
        if (can->getLocation() != functionDecl->getLocation()) {
            report(
                DiagnosticsEngine::Note, "function is declared here:",
                can->getLocation())
                << can->getSourceRange();
        }
    }
    return true;
}

bool PassParamsByRef::TraverseFunctionDecl(FunctionDecl* functionDecl)
{
    bool ret = true;
    if (PreTraverseFunctionDecl(functionDecl))
    {
        ret = RecursiveASTVisitor::TraverseFunctionDecl(functionDecl);
        PostTraverseFunctionDecl(functionDecl, ret);
    }
    return ret;
}

bool PassParamsByRef::PreTraverseCXXMethodDecl(CXXMethodDecl* functionDecl)
{
    return PreTraverseFunctionDecl(functionDecl);
}

bool PassParamsByRef::PostTraverseCXXMethodDecl(CXXMethodDecl* functionDecl, bool b)
{
    return PostTraverseFunctionDecl(functionDecl, b);
}

bool PassParamsByRef::TraverseCXXMethodDecl(CXXMethodDecl* functionDecl)
{
    bool ret = true;
    if (PreTraverseCXXMethodDecl(functionDecl))
    {
        ret = RecursiveASTVisitor::TraverseCXXMethodDecl(functionDecl);
        PostTraverseCXXMethodDecl(functionDecl, ret);
    }
    return ret;
}

bool PassParamsByRef::PreTraverseCXXConstructorDecl(CXXConstructorDecl* functionDecl)
{
    return PreTraverseFunctionDecl(functionDecl);
}

bool PassParamsByRef::PostTraverseCXXConstructorDecl(CXXConstructorDecl* functionDecl, bool b)
{
    return PostTraverseFunctionDecl(functionDecl, b);
}

bool PassParamsByRef::TraverseCXXConstructorDecl(CXXConstructorDecl* functionDecl)
{
    bool ret = true;
    if (PreTraverseCXXConstructorDecl(functionDecl))
    {
        ret = RecursiveASTVisitor::TraverseCXXConstructorDecl(functionDecl);
        PostTraverseCXXConstructorDecl(functionDecl, ret);
    }
    return ret;
}

bool PassParamsByRef::PreTraverseImplicitCastExpr(ImplicitCastExpr * expr)
{
    if (ignoreLocation(expr))
        return false;
    if (expr->getCastKind() == CK_LValueToRValue
         && isa<DeclRefExpr>(expr->getSubExpr()->IgnoreParenImpCasts()))
        return false;
    return true;
}

bool PassParamsByRef::TraverseImplicitCastExpr(ImplicitCastExpr * expr)
{
    bool ret = true;
    if (PreTraverseImplicitCastExpr(expr))
    {
        ret = RecursiveASTVisitor::TraverseImplicitCastExpr(expr);
    }
    return ret;
}

bool PassParamsByRef::VisitBinaryOperator(const BinaryOperator * binaryOperator)
{
    if (binaryOperator->getOpcode() != BO_Assign) {
        return true;
    }
    if (!mbInsideFunctionDecl)
        return true;
    // if we are assigning to a parameter, it can be inconvenient to make the param pass-by-ref
    if (auto declRefExpr = dyn_cast<DeclRefExpr>(binaryOperator->getLHS()))
    {
        if (auto parmVarDecl = dyn_cast<ParmVarDecl>(declRefExpr->getDecl()))
            mParamExclusions.emplace(parmVarDecl);
    }
    return true;
}

bool PassParamsByRef::VisitCXXOperatorCallExpr(const CXXOperatorCallExpr * cxxOperatorCallExpr )
{
    if (!mbInsideFunctionDecl)
        return true;
    // if we are assigning to a parameter, it can be inconvenient to make the param pass-by-ref
    auto op = cxxOperatorCallExpr->getOperator();
    if ( op != clang::OverloadedOperatorKind::OO_Equal
         && op != clang::OverloadedOperatorKind::OO_SlashEqual
         && op != clang::OverloadedOperatorKind::OO_StarEqual
         && op != clang::OverloadedOperatorKind::OO_MinusEqual
         && op != clang::OverloadedOperatorKind::OO_PlusEqual)
        return true;
    auto declRefExpr = dyn_cast<DeclRefExpr>(cxxOperatorCallExpr->getArg(0));
    if (!declRefExpr)
        return true;
    if (auto parmVarDecl = dyn_cast<ParmVarDecl>(declRefExpr->getDecl()))
        mParamExclusions.emplace(parmVarDecl);
    return true;
}

bool PassParamsByRef::VisitCallExpr(const CallExpr * callExpr )
{
    if (!mbInsideFunctionDecl)
        return true;
    if (loplugin::DeclCheck(callExpr->getCalleeDecl()).Function("move").StdNamespace())
        if (auto declRefExpr = dyn_cast<DeclRefExpr>(callExpr->getArg(0)))
        {
            if (auto parmVarDecl = dyn_cast<ParmVarDecl>(declRefExpr->getDecl()))
                mParamExclusions.emplace(parmVarDecl);
        }
    if (auto const fun = callExpr->getDirectCallee())
    {
        unsigned const n = std::min(fun->getNumParams(), callExpr->getNumArgs());
        for (unsigned i = 0; i != n; ++i)
        {
            if (!loplugin::TypeCheck(fun->getParamDecl(i)->getType())
                     .LvalueReference()
                     .NonConstVolatile())
                continue;
            auto a = callExpr->getArg(i)->IgnoreParenImpCasts();
            if (auto declRefExpr = dyn_cast<DeclRefExpr>(a))
                if (auto parmVarDecl = dyn_cast<ParmVarDecl>(declRefExpr->getDecl()))
                    mParamExclusions.emplace(parmVarDecl);
        }
    }
    return true;
}

bool PassParamsByRef::VisitCXXMemberCallExpr(const CXXMemberCallExpr * callExpr )
{
    if (!mbInsideFunctionDecl)
        return true;
    // exclude cases where we call a non-const method on the parameter i.e. potentially mutating it
    if (auto const e1 = callExpr->getMethodDecl())
        if (!e1->isConst())
        {
            auto a = callExpr->getImplicitObjectArgument()->IgnoreParenImpCasts();
            if (auto declRefExpr = dyn_cast<DeclRefExpr>(a))
                if (auto parmVarDecl = dyn_cast<ParmVarDecl>(declRefExpr->getDecl()))
                    mParamExclusions.emplace(parmVarDecl);
        }
    return true;
}

bool PassParamsByRef::isFat(QualType type) {
    if (!type->isRecordType()) {
        return false;
    }
    loplugin::TypeCheck tc(type);
    if ((tc.Class("Reference").Namespace("uno").Namespace("star")
            .Namespace("sun").Namespace("com").GlobalNamespace())
        || (tc.Class("Sequence").Namespace("uno").Namespace("star")
            .Namespace("sun").Namespace("com").GlobalNamespace())
        || tc.Class("OString").Namespace("rtl").GlobalNamespace()
        || tc.Class("OUString").Namespace("rtl").GlobalNamespace()
        || tc.Class("Reference").Namespace("rtl").GlobalNamespace())
    {
        return true;
    }
    if (type->isIncompleteType()) {
        return false;
    }
    clang::Type const * t2 = type.getTypePtrOrNull();
    return t2 != nullptr
        && compiler.getASTContext().getTypeSizeInChars(t2).getQuantity() > 64;
}

loplugin::Plugin::Registration< PassParamsByRef > passparamsbyref("passparamsbyref");

} // namespace

#endif // LO_CLANG_SHARED_PLUGINS

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