ConstantOptimiser.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/>.
*/
// SPDX-License-Identifier: GPL-3.0
/** @file ConstantOptimiser.cpp
 * @author Christian <c@ethdev.com>
 * @date 2015
 */

#include <libevmasm/ConstantOptimiser.h>
#include <libevmasm/Assembly.h>
#include <libevmasm/GasMeter.h>

using namespace std;
using namespace solidity;
using namespace solidity::evmasm;

unsigned ConstantOptimisationMethod::optimiseConstants(
	bool _isCreation,
	size_t _runs,
	langutil::EVMVersion _evmVersion,
	Assembly& _assembly
)
{
	// TODO: design the optimiser in a way this is not needed
	AssemblyItems& _items = _assembly.items();

	unsigned optimisations = 0;
	map<AssemblyItem, size_t> pushes;
	for (AssemblyItem const& item: _items)
		if (item.type() == Push)
			pushes[item]++;
	map<u256, AssemblyItems> pendingReplacements;
	for (auto it: pushes)
	{
		AssemblyItem const& item = it.first;
		if (item.data() < 0x100)
			continue;
		Params params;
		params.multiplicity = it.second;
		params.isCreation = _isCreation;
		params.runs = _runs;
		params.evmVersion = _evmVersion;
		LiteralMethod lit(params, item.data());
		bigint literalGas = lit.gasNeeded();
		CodeCopyMethod copy(params, item.data());
		bigint copyGas = copy.gasNeeded();
		ComputeMethod compute(params, item.data());
		bigint computeGas = compute.gasNeeded();
		AssemblyItems replacement;
		if (copyGas < literalGas && copyGas < computeGas)
		{
			replacement = copy.execute(_assembly);
			optimisations++;
		}
		else if (computeGas < literalGas && computeGas <= copyGas)
		{
			replacement = compute.execute(_assembly);
			optimisations++;
		}
		if (!replacement.empty())
			pendingReplacements[item.data()] = replacement;
	}
	if (!pendingReplacements.empty())
		replaceConstants(_items, pendingReplacements);
	return optimisations;
}

bigint ConstantOptimisationMethod::simpleRunGas(AssemblyItems const& _items)
{
	bigint gas = 0;
	for (AssemblyItem const& item: _items)
		if (item.type() == Push)
			gas += GasMeter::runGas(Instruction::PUSH1);
		else if (item.type() == Operation)
		{
			if (item.instruction() == Instruction::EXP)
				gas += GasCosts::expGas;
			else
				gas += GasMeter::runGas(item.instruction());
		}
	return gas;
}

bigint ConstantOptimisationMethod::dataGas(bytes const& _data) const
{
	assertThrow(_data.size() > 0, OptimizerException, "Empty bytecode generated.");
	return bigint(GasMeter::dataGas(_data, m_params.isCreation, m_params.evmVersion));
}

size_t ConstantOptimisationMethod::bytesRequired(AssemblyItems const& _items)
{
	return evmasm::bytesRequired(_items, 3, Precision::Approximate); // assume 3 byte addresses
}

void ConstantOptimisationMethod::replaceConstants(
	AssemblyItems& _items,
	map<u256, AssemblyItems> const& _replacements
)
{
	AssemblyItems replaced;
	for (AssemblyItem const& item: _items)
	{
		if (item.type() == Push)
		{
			auto it = _replacements.find(item.data());
			if (it != _replacements.end())
			{
				replaced += it->second;
				continue;
			}
		}
		replaced.push_back(item);
	}
	_items = std::move(replaced);
}

bigint LiteralMethod::gasNeeded() const
{
	return combineGas(
		simpleRunGas({Instruction::PUSH1}),
		// PUSHX plus data
		(m_params.isCreation ? GasCosts::txDataNonZeroGas(m_params.evmVersion) : GasCosts::createDataGas) + dataGas(toCompactBigEndian(m_value, 1)),
		0
	);
}

bigint CodeCopyMethod::gasNeeded() const
{
	return combineGas(
		// Run gas: we ignore memory increase costs
		simpleRunGas(copyRoutine()) + GasCosts::copyGas,
		// Data gas for copy routines: Some bytes are zero, but we ignore them.
		bytesRequired(copyRoutine()) * (m_params.isCreation ? GasCosts::txDataNonZeroGas(m_params.evmVersion) : GasCosts::createDataGas),
		// Data gas for data itself
		dataGas(toBigEndian(m_value))
	);
}

AssemblyItems CodeCopyMethod::execute(Assembly& _assembly) const
{
	bytes data = toBigEndian(m_value);
	assertThrow(data.size() == 32, OptimizerException, "Invalid number encoding.");
	AssemblyItems actualCopyRoutine = copyRoutine();
	actualCopyRoutine[4] = _assembly.newData(data);
	return actualCopyRoutine;
}

AssemblyItems const& CodeCopyMethod::copyRoutine()
{
	AssemblyItems static copyRoutine{
		// constant to be reused 3+ times
		u256(0),

		// back up memory
		// mload(0)
		Instruction::DUP1,
		Instruction::MLOAD,

		// codecopy(0, <offset>, 32)
		u256(32),
		AssemblyItem(PushData, u256(1) << 16), // replaced above in actualCopyRoutine[4]
		Instruction::DUP4,
		Instruction::CODECOPY,

		// mload(0)
		Instruction::DUP2,
		Instruction::MLOAD,

		// restore original memory
		Instruction::SWAP2,
		Instruction::MSTORE
	};
	return copyRoutine;
}

AssemblyItems ComputeMethod::findRepresentation(u256 const& _value)
{
	if (_value < 0x10000)
		// Very small value, not worth computing
		return AssemblyItems{_value};
	else if (numberEncodingSize(~_value) < numberEncodingSize(_value))
		// Negated is shorter to represent
		return findRepresentation(~_value) + AssemblyItems{Instruction::NOT};
	else
	{
		// Decompose value into a * 2**k + b where abs(b) << 2**k
		// Is not always better, try literal and decomposition method.
		AssemblyItems routine{u256(_value)};
		bigint bestGas = gasNeeded(routine);
		for (unsigned bits = 255; bits > 8 && m_maxSteps > 0; --bits)
		{
			unsigned gapDetector = unsigned((_value >> (bits - 8)) & 0x1ff);
			if (gapDetector != 0xff && gapDetector != 0x100)
				continue;

			u256 powerOfTwo = u256(1) << bits;
			u256 upperPart = _value >> bits;
			bigint lowerPart = _value & (powerOfTwo - 1);
			if ((powerOfTwo - lowerPart) < lowerPart)
			{
				lowerPart = lowerPart - powerOfTwo; // make it negative
				upperPart++;
			}
			if (upperPart == 0)
				continue;
			if (abs(lowerPart) >= (powerOfTwo >> 8))
				continue;

			AssemblyItems newRoutine;
			if (lowerPart != 0)
				newRoutine += findRepresentation(u256(abs(lowerPart)));
			if (m_params.evmVersion.hasBitwiseShifting())
			{
				newRoutine += findRepresentation(upperPart);
				newRoutine += AssemblyItems{u256(bits), Instruction::SHL};
			}
			else
			{
				newRoutine += AssemblyItems{u256(bits), u256(2), Instruction::EXP};
				if (upperPart != 1)
					newRoutine += findRepresentation(upperPart) + AssemblyItems{Instruction::MUL};
			}
			if (lowerPart > 0)
				newRoutine += AssemblyItems{Instruction::ADD};
			else if (lowerPart < 0)
				newRoutine.push_back(Instruction::SUB);

			if (m_maxSteps > 0)
				m_maxSteps--;
			bigint newGas = gasNeeded(newRoutine);
			if (newGas < bestGas)
			{
				bestGas = std::move(newGas);
				routine = std::move(newRoutine);
			}
		}
		return routine;
	}
}

bool ComputeMethod::checkRepresentation(u256 const& _value, AssemblyItems const& _routine) const
{
	// This is a tiny EVM that can only evaluate some instructions.
	vector<u256> stack;
	for (AssemblyItem const& item: _routine)
	{
		switch (item.type())
		{
		case Operation:
		{
			if (stack.size() < item.arguments())
				return false;
			u256* sp = &stack.back();
			switch (item.instruction())
			{
			case Instruction::MUL:
				sp[-1] = sp[0] * sp[-1];
				break;
			case Instruction::EXP:
				if (sp[-1] > 0xff)
					return false;
				sp[-1] = boost::multiprecision::pow(sp[0], unsigned(sp[-1]));
				break;
			case Instruction::ADD:
				sp[-1] = sp[0] + sp[-1];
				break;
			case Instruction::SUB:
				sp[-1] = sp[0] - sp[-1];
				break;
			case Instruction::NOT:
				sp[0] = ~sp[0];
				break;
			case Instruction::SHL:
				assertThrow(
					m_params.evmVersion.hasBitwiseShifting(),
					OptimizerException,
					"Shift generated for invalid EVM version."
				);
				assertThrow(sp[0] <= u256(255), OptimizerException, "Invalid shift generated.");
				sp[-1] = u256(bigint(sp[-1]) << unsigned(sp[0]));
				break;
			case Instruction::SHR:
				assertThrow(
					m_params.evmVersion.hasBitwiseShifting(),
					OptimizerException,
					"Shift generated for invalid EVM version."
				);
				assertThrow(sp[0] <= u256(255), OptimizerException, "Invalid shift generated.");
				sp[-1] = sp[-1] >> unsigned(sp[0]);
				break;
			default:
				return false;
			}
			stack.resize(stack.size() + item.deposit());
			break;
		}
		case Push:
			stack.push_back(item.data());
			break;
		default:
			return false;
		}
	}
	return stack.size() == 1 && stack.front() == _value;
}

bigint ComputeMethod::gasNeeded(AssemblyItems const& _routine) const
{
	auto numExps = static_cast<size_t>(count(_routine.begin(), _routine.end(), Instruction::EXP));
	return combineGas(
		simpleRunGas(_routine) + numExps * (GasCosts::expGas + GasCosts::expByteGas(m_params.evmVersion)),
		// Data gas for routine: Some bytes are zero, but we ignore them.
		bytesRequired(_routine) * (m_params.isCreation ? GasCosts::txDataNonZeroGas(m_params.evmVersion) : GasCosts::createDataGas),
		0
	);
}