path: root/libyul/optimiser/RedundantAssignEliminator.cpp

/*
    This file is part of solidity.

    solidity is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    solidity is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with solidity.  If not, see <http://www.gnu.org/licenses/>.
*/
/**
 * Optimiser component that removes assignments to variables that are not used
 * until they go out of scope or are re-assigned.
 */

#include <libyul/optimiser/RedundantAssignEliminator.h>

#include <libyul/optimiser/Semantics.h>
#include <libyul/AsmData.h>

#include <libdevcore/CommonData.h>

#include <boost/range/algorithm_ext/erase.hpp>

using namespace std;
using namespace dev;
using namespace yul;
using namespace dev::solidity;

void RedundantAssignEliminator::operator()(Identifier const& _identifier)
{
    changeUndecidedTo(_identifier.name, State::Used);
}

void RedundantAssignEliminator::operator()(VariableDeclaration const& _variableDeclaration)
{
    ASTWalker::operator()(_variableDeclaration);

    for (auto const& var: _variableDeclaration.variables)
        m_declaredVariables.emplace(var.name);
}

void RedundantAssignEliminator::operator()(Assignment const& _assignment)
{
    visit(*_assignment.value);
    for (auto const& var: _assignment.variableNames)
        changeUndecidedTo(var.name, State::Unused);

    if (_assignment.variableNames.size() == 1)
        // Default-construct it in "Undecided" state if it does not yet exist.
        m_assignments[_assignment.variableNames.front().name][&_assignment];
}

void RedundantAssignEliminator::operator()(If const& _if)
{
    visit(*_if.condition);

    RedundantAssignEliminator branch{*this};
    branch(_if.body);

    join(branch);
}

void RedundantAssignEliminator::operator()(Switch const& _switch)
{
    visit(*_switch.expression);

    bool hasDefault = false;
    vector<RedundantAssignEliminator> branches;
    for (auto const& c: _switch.cases)
    {
        if (!c.value)
            hasDefault = true;
        branches.emplace_back(*this);
        branches.back()(c.body);
    }

    if (hasDefault)
    {
        *this = std::move(branches.back());
        branches.pop_back();
    }
    for (auto& branch: branches)
        join(branch);
}

void RedundantAssignEliminator::operator()(FunctionDefinition const& _functionDefinition)
{
    (*this)(_functionDefinition.body);

    for (auto const& param: _functionDefinition.parameters)
    {
        changeUndecidedTo(param.name, State::Unused);
        finalize(param.name);
    }
    for (auto const& retParam: _functionDefinition.returnVariables)
    {
        changeUndecidedTo(retParam.name, State::Used);
        finalize(retParam.name);
    }
}

void RedundantAssignEliminator::operator()(ForLoop const& _forLoop)
{
    // This will set all variables that are declared in this
    // block to "unused" when it is destroyed.
    BlockScope scope(*this);

    // We need to visit the statements directly because of the
    // scoping rules.
    walkVector(_forLoop.pre.statements);

    // We just run the loop twice to account for the
    // back edge.
    // There need not be more runs because we only have three different states.

    visit(*_forLoop.condition);

    RedundantAssignEliminator zeroRuns{*this};

    (*this)(_forLoop.body);
    (*this)(_forLoop.post);

    visit(*_forLoop.condition);

    RedundantAssignEliminator oneRun{*this};

    (*this)(_forLoop.body);
    (*this)(_forLoop.post);

    visit(*_forLoop.condition);

    // Order does not matter because "max" is commutative and associative.
    join(oneRun);
    join(zeroRuns);
}

void RedundantAssignEliminator::operator()(Block const& _block)
{
    // This will set all variables that are declared in this
    // block to "unused" when it is destroyed.
    BlockScope scope(*this);

    ASTWalker::operator()(_block);
}

void RedundantAssignEliminator::run(Block& _ast)
{
    RedundantAssignEliminator rae;
    rae(_ast);

    AssignmentRemover remover{rae.m_assignmentsToRemove};
    remover(_ast);
}

template <class K, class V, class F>
void joinMap(std::map<K, V>& _a, std::map<K, V>&& _b, F _conflictSolver)
{
    // TODO Perhaps it is better to just create a sorted list
    // and then use insert(begin, end)

    auto ita = _a.begin();
    auto aend = _a.end();
    auto itb = _b.begin();
    auto bend = _b.end();

    for (; itb != bend; ++ita)
    {
        if (ita == aend)
            ita = _a.insert(ita, std::move(*itb++));
        else if (ita->first < itb->first)
            continue;
        else if (itb->first < ita->first)
            ita = _a.insert(ita, std::move(*itb++));
        else
        {
            _conflictSolver(ita->second, std::move(itb->second));
            ++itb;
        }
    }
}

void RedundantAssignEliminator::join(RedundantAssignEliminator& _other)
{
    m_assignmentsToRemove.insert(begin(_other.m_assignmentsToRemove), end(_other.m_assignmentsToRemove));

    joinMap(m_assignments, std::move(_other.m_assignments), [](
        map<Assignment const*, State>& _assignmentHere,
        map<Assignment const*, State>&& _assignmentThere
    )
    {
        return joinMap(_assignmentHere, std::move(_assignmentThere), State::join);
    });
}

void RedundantAssignEliminator::changeUndecidedTo(YulString _variable, RedundantAssignEliminator::State _newState)
{
    for (auto& assignment: m_assignments[_variable])
        if (assignment.second == State{State::Undecided})
            assignment.second = _newState;
}

void RedundantAssignEliminator::finalize(YulString _variable)
{
    for (auto& assignment: m_assignments[_variable])
    {
        assertThrow(assignment.second != State::Undecided, OptimizerException, "");
        if (assignment.second == State{State::Unused} && MovableChecker{*assignment.first->value}.movable())
            // TODO the only point where we actually need this
            // to be a set is for the for loop
            m_assignmentsToRemove.insert(assignment.first);
    }
    m_assignments.erase(_variable);
}

void AssignmentRemover::operator()(Block& _block)
{
    boost::range::remove_erase_if(_block.statements, [=](Statement const& _statement) -> bool {
        return _statement.type() == typeid(Assignment) && m_toRemove.count(&boost::get<Assignment>(_statement));
    });

    ASTModifier::operator()(_block);
}