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/*
This file is part of cpp-ethereum.
cpp-ethereum 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.
cpp-ethereum 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 cpp-ethereum. If not, see <http://www.gnu.org/licenses/>.
*/
/** @file PathGasMeter.cpp
* @author Christian <c@ethdev.com>
* @date 2015
*/
#include "PathGasMeter.h"
#include <libevmasm/KnownState.h>
#include <libevmasm/SemanticInformation.h>
using namespace std;
using namespace dev;
using namespace dev::eth;
PathGasMeter::PathGasMeter(AssemblyItems const& _items):
m_items(_items)
{
for (size_t i = 0; i < m_items.size(); ++i)
if (m_items[i].type() == Tag)
m_tagPositions[m_items[i].data()] = i;
}
GasMeter::GasConsumption PathGasMeter::estimateMax(
size_t _startIndex,
shared_ptr<KnownState> const& _state
)
{
auto path = unique_ptr<GasPath>(new GasPath());
path->index = _startIndex;
path->state = _state->copy();
m_queue.push_back(move(path));
GasMeter::GasConsumption gas;
while (!m_queue.empty() && !gas.isInfinite)
gas = max(gas, handleQueueItem());
return gas;
}
GasMeter::GasConsumption PathGasMeter::handleQueueItem()
{
assertThrow(!m_queue.empty(), OptimizerException, "");
unique_ptr<GasPath> path = move(m_queue.back());
m_queue.pop_back();
shared_ptr<KnownState> state = path->state;
GasMeter meter(state, path->largestMemoryAccess);
ExpressionClasses& classes = state->expressionClasses();
GasMeter::GasConsumption gas = path->gas;
size_t index = path->index;
if (index >= m_items.size() || (index > 0 && m_items.at(index).type() != Tag))
// Invalid jump usually provokes an out-of-gas exception, but we want to give an upper
// bound on the gas that is needed without changing the behaviour, so it is fine to
// return the current gas value.
return gas;
set<u256> jumpTags;
for (; index < m_items.size() && !gas.isInfinite; ++index)
{
bool branchStops = false;
jumpTags.clear();
AssemblyItem const& item = m_items.at(index);
if (item.type() == Tag || item == AssemblyItem(Instruction::JUMPDEST))
{
// Do not allow any backwards jump. This is quite restrictive but should work for
// the simplest things.
if (path->visitedJumpdests.count(index))
return GasMeter::GasConsumption::infinite();
path->visitedJumpdests.insert(index);
}
else if (item == AssemblyItem(Instruction::JUMP))
{
branchStops = true;
jumpTags = state->tagsInExpression(state->relativeStackElement(0));
if (jumpTags.empty()) // unknown jump destination
return GasMeter::GasConsumption::infinite();
}
else if (item == AssemblyItem(Instruction::JUMPI))
{
ExpressionClasses::Id condition = state->relativeStackElement(-1);
if (classes.knownNonZero(condition) || !classes.knownZero(condition))
{
jumpTags = state->tagsInExpression(state->relativeStackElement(0));
if (jumpTags.empty()) // unknown jump destination
return GasMeter::GasConsumption::infinite();
}
branchStops = classes.knownNonZero(condition);
}
else if (SemanticInformation::altersControlFlow(item))
branchStops = true;
gas += meter.estimateMax(item);
for (u256 const& tag: jumpTags)
{
auto newPath = unique_ptr<GasPath>(new GasPath());
newPath->index = m_items.size();
if (m_tagPositions.count(tag))
newPath->index = m_tagPositions.at(tag);
newPath->gas = gas;
newPath->largestMemoryAccess = meter.largestMemoryAccess();
newPath->state = state->copy();
newPath->visitedJumpdests = path->visitedJumpdests;
m_queue.push_back(move(newPath));
}
if (branchStops)
break;
}
return gas;
}
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