path: root/libevmasm/BlockDeduplicator.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/>.
*/
/**
 * @file BlockDeduplicator.cpp
 * @author Christian <c@ethdev.com>
 * @date 2015
 * Unifies basic blocks that share content.
 */

#include <libevmasm/BlockDeduplicator.h>

#include <libevmasm/AssemblyItem.h>
#include <libevmasm/SemanticInformation.h>

#include <functional>
#include <set>

using namespace std;
using namespace dev;
using namespace dev::eth;


bool BlockDeduplicator::deduplicate()
{
    // Compares indices based on the suffix that starts there, ignoring tags and stopping at
    // opcodes that stop the control flow.

    // Virtual tag that signifies "the current block" and which is used to optimise loops.
    // We abort if this virtual tag actually exists.
    AssemblyItem pushSelf(PushTag, u256(-4));
    if (
        std::count(m_items.cbegin(), m_items.cend(), pushSelf.tag()) ||
        std::count(m_items.cbegin(), m_items.cend(), pushSelf.pushTag())
    )
        return false;

    function<bool(size_t, size_t)> comparator = [&](size_t _i, size_t _j)
    {
        if (_i == _j)
            return false;

        // To compare recursive loops, we have to already unify PushTag opcodes of the
        // block's own tag.
        AssemblyItem pushFirstTag(pushSelf);
        AssemblyItem pushSecondTag(pushSelf);

        if (_i < m_items.size() && m_items.at(_i).type() == Tag)
            pushFirstTag = m_items.at(_i).pushTag();
        if (_j < m_items.size() && m_items.at(_j).type() == Tag)
            pushSecondTag = m_items.at(_j).pushTag();

        BlockIterator first(m_items.begin() + _i, m_items.end(), &pushFirstTag, &pushSelf);
        BlockIterator second(m_items.begin() + _j, m_items.end(), &pushSecondTag, &pushSelf);
        BlockIterator end(m_items.end(), m_items.end());

        if (first != end && (*first).type() == Tag)
            ++first;
        if (second != end && (*second).type() == Tag)
            ++second;

        return std::lexicographical_compare(first, end, second, end);
    };

    size_t iterations = 0;
    for (; ; ++iterations)
    {
        //@todo this should probably be optimized.
        set<size_t, function<bool(size_t, size_t)>> blocksSeen(comparator);
        for (size_t i = 0; i < m_items.size(); ++i)
        {
            if (m_items.at(i).type() != Tag)
                continue;
            auto it = blocksSeen.find(i);
            if (it == blocksSeen.end())
                blocksSeen.insert(i);
            else
                m_replacedTags[m_items.at(i).data()] = m_items.at(*it).data();
        }

        if (!applyTagReplacement(m_items, m_replacedTags))
            break;
    }
    return iterations > 0;
}

bool BlockDeduplicator::applyTagReplacement(
    AssemblyItems& _items,
    map<u256, u256> const& _replacements,
    size_t _subId
)
{
    bool changed = false;
    for (AssemblyItem& item: _items)
        if (item.type() == PushTag)
        {
            size_t subId;
            size_t tagId;
            tie(subId, tagId) = item.splitForeignPushTag();
            if (subId != _subId)
                continue;
            auto it = _replacements.find(tagId);
            if (it != _replacements.end())
            {
                changed = true;
                item.setPushTagSubIdAndTag(subId, size_t(it->second));
            }
        }
    return changed;
}

BlockDeduplicator::BlockIterator& BlockDeduplicator::BlockIterator::operator++()
{
    if (it == end)
        return *this;
    if (SemanticInformation::altersControlFlow(*it) && *it != AssemblyItem(Instruction::JUMPI))
        it = end;
    else
    {
        ++it;
        while (it != end && it->type() == Tag)
            ++it;
    }
    return *this;
}

AssemblyItem const& BlockDeduplicator::BlockIterator::operator*() const
{
    if (replaceItem && replaceWith && *it == *replaceItem)
        return *replaceWith;
    else
        return *it;
}