path: root/ArrayUtils.cpp

/*
    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/>.
*/
/**
 * @author Christian <c@ethdev.com>
 * @date 2015
 * Code generation utils that handle arrays.
 */

#include <libsolidity/ArrayUtils.h>
#include <libevmcore/Instruction.h>
#include <libsolidity/CompilerContext.h>
#include <libsolidity/CompilerUtils.h>
#include <libsolidity/Types.h>
#include <libsolidity/Utils.h>
#include <libsolidity/LValue.h>

using namespace std;
using namespace dev;
using namespace solidity;

void ArrayUtils::copyArrayToStorage(ArrayType const& _targetType, ArrayType const& _sourceType) const
{
    // this copies source to target and also clears target if it was larger
    // need to leave "target_ref target_byte_off" on the stack at the end

    // stack layout: [source_ref] [source_byte_off] [source length] target_ref target_byte_off (top)
    solAssert(_targetType.getLocation() == ArrayType::Location::Storage, "");
    solAssert(
        _sourceType.getLocation() == ArrayType::Location::CallData ||
            _sourceType.getLocation() == ArrayType::Location::Storage,
        "Given array location not implemented."
    );

    IntegerType uint256(256);
    Type const* targetBaseType = _targetType.isByteArray() ? &uint256 : &(*_targetType.getBaseType());
    Type const* sourceBaseType = _sourceType.isByteArray() ? &uint256 : &(*_sourceType.getBaseType());

    // TODO unroll loop for small sizes

    bool sourceIsStorage = _sourceType.getLocation() == ArrayType::Location::Storage;
    bool directCopy = sourceIsStorage && sourceBaseType->isValueType() && *sourceBaseType == *targetBaseType;
    bool haveByteOffsetSource = !directCopy && sourceIsStorage && sourceBaseType->getStorageBytes() <= 16;
    bool haveByteOffsetTarget = !directCopy && targetBaseType->getStorageBytes() <= 16;
    unsigned byteOffsetSize = (haveByteOffsetSource ? 1 : 0) + (haveByteOffsetTarget ? 1 : 0);

    // stack: source_ref [source_byte_off] [source_length] target_ref target_byte_off
    // store target_ref
    // arrays always start at zero byte offset, pop offset
    m_context << eth::Instruction::POP;
    for (unsigned i = _sourceType.getSizeOnStack(); i > 0; --i)
        m_context << eth::swapInstruction(i);
    // stack: target_ref source_ref [source_byte_off] [source_length]
    if (sourceIsStorage)
        // arrays always start at zero byte offset, pop offset
        m_context << eth::Instruction::POP;
    // stack: target_ref source_ref [source_length]
    // retrieve source length
    if (_sourceType.getLocation() != ArrayType::Location::CallData || !_sourceType.isDynamicallySized())
        retrieveLength(_sourceType); // otherwise, length is already there
    // stack: target_ref source_ref source_length
    m_context << eth::Instruction::DUP3;
    // stack: target_ref source_ref source_length target_ref
    retrieveLength(_targetType);
    // stack: target_ref source_ref source_length target_ref target_length
    if (_targetType.isDynamicallySized())
        // store new target length
        m_context << eth::Instruction::DUP3 << eth::Instruction::DUP3 << eth::Instruction::SSTORE;
    if (sourceBaseType->getCategory() == Type::Category::Mapping)
    {
        solAssert(targetBaseType->getCategory() == Type::Category::Mapping, "");
        solAssert(_sourceType.getLocation() == ArrayType::Location::Storage, "");
        // nothing to copy
        m_context
            << eth::Instruction::POP << eth::Instruction::POP
            << eth::Instruction::POP << eth::Instruction::POP;
        m_context << u256(0);
        return;
    }
    // compute hashes (data positions)
    m_context << eth::Instruction::SWAP1;
    if (_targetType.isDynamicallySized())
        CompilerUtils(m_context).computeHashStatic();
    // stack: target_ref source_ref source_length target_length target_data_pos
    m_context << eth::Instruction::SWAP1;
    convertLengthToSize(_targetType);
    m_context << eth::Instruction::DUP2 << eth::Instruction::ADD;
    // stack: target_ref source_ref source_length target_data_pos target_data_end
    m_context << eth::Instruction::SWAP3;
    // stack: target_ref target_data_end source_length target_data_pos source_ref
    // skip copying if source length is zero
    m_context << eth::Instruction::DUP3 << eth::Instruction::ISZERO;
    eth::AssemblyItem copyLoopEndWithoutByteOffset = m_context.newTag();
    m_context.appendConditionalJumpTo(copyLoopEndWithoutByteOffset);

    if (_sourceType.getLocation() == ArrayType::Location::Storage && _sourceType.isDynamicallySized())
        CompilerUtils(m_context).computeHashStatic();
    // stack: target_ref target_data_end source_length target_data_pos source_data_pos
    m_context << eth::Instruction::SWAP2;
    convertLengthToSize(_sourceType);
    m_context << eth::Instruction::DUP3 << eth::Instruction::ADD;
    // stack: target_ref target_data_end source_data_pos target_data_pos source_data_end
    if (haveByteOffsetTarget)
        m_context << u256(0);
    if (haveByteOffsetSource)
        m_context << u256(0);
    // stack: target_ref target_data_end source_data_pos target_data_pos source_data_end [target_byte_offset] [source_byte_offset]
    eth::AssemblyItem copyLoopStart = m_context.newTag();
    m_context << copyLoopStart;
    // check for loop condition
    m_context
        << eth::dupInstruction(3 + byteOffsetSize) << eth::dupInstruction(2 + byteOffsetSize)
        << eth::Instruction::GT << eth::Instruction::ISZERO;
    eth::AssemblyItem copyLoopEnd = m_context.newTag();
    m_context.appendConditionalJumpTo(copyLoopEnd);
    // stack: target_ref target_data_end source_data_pos target_data_pos source_data_end [target_byte_offset] [source_byte_offset]
    // copy
    if (sourceBaseType->getCategory() == Type::Category::Array)
    {
        solAssert(byteOffsetSize == 0, "Byte offset for array as base type.");
        m_context << eth::Instruction::DUP3;
        if (sourceIsStorage)
            m_context << u256(0);
        m_context << eth::dupInstruction(sourceIsStorage ? 4 : 3) << u256(0);
        copyArrayToStorage(
            dynamic_cast<ArrayType const&>(*targetBaseType),
            dynamic_cast<ArrayType const&>(*sourceBaseType)
        );
        m_context << eth::Instruction::POP << eth::Instruction::POP;
    }
    else if (directCopy)
    {
        solAssert(byteOffsetSize == 0, "Byte offset for direct copy.");
        m_context
            << eth::Instruction::DUP3 << eth::Instruction::SLOAD
            << eth::Instruction::DUP3 << eth::Instruction::SSTORE;
    }
    else
    {
        // Note that we have to copy each element on its own in case conversion is involved.
        // We might copy too much if there is padding at the last element, but this way end
        // checking is easier.
        // stack: target_ref target_data_end source_data_pos target_data_pos source_data_end [target_byte_offset] [source_byte_offset]
        m_context << eth::dupInstruction(3 + byteOffsetSize);
        if (_sourceType.getLocation() == ArrayType::Location::Storage)
        {
            if (haveByteOffsetSource)
                m_context << eth::Instruction::DUP2;
            else
                m_context << u256(0);
            StorageItem(m_context, *sourceBaseType).retrieveValue(SourceLocation(), true);
        }
        else if (sourceBaseType->isValueType())
            CompilerUtils(m_context).loadFromMemoryDynamic(*sourceBaseType, true, true, false);
        else
            solAssert(false, "Copying of unknown type requested: " + sourceBaseType->toString());
        // stack: target_ref target_data_end source_data_pos target_data_pos source_data_end [target_byte_offset] [source_byte_offset] <source_value>...
        solAssert(2 + byteOffsetSize + sourceBaseType->getSizeOnStack() <= 16, "Stack too deep.");
        // fetch target storage reference
        m_context << eth::dupInstruction(2 + byteOffsetSize + sourceBaseType->getSizeOnStack());
        if (haveByteOffsetTarget)
            m_context << eth::dupInstruction(1 + byteOffsetSize + sourceBaseType->getSizeOnStack());
        else
            m_context << u256(0);
        StorageItem(m_context, *targetBaseType).storeValue(*sourceBaseType, SourceLocation(), true);
    }
    // stack: target_ref target_data_end source_data_pos target_data_pos source_data_end [target_byte_offset] [source_byte_offset]
    // increment source
    if (haveByteOffsetSource)
        incrementByteOffset(sourceBaseType->getStorageBytes(), 1, haveByteOffsetTarget ? 5 : 4);
    else
        m_context
            << eth::swapInstruction(2 + byteOffsetSize)
            << (sourceIsStorage ? sourceBaseType->getStorageSize() : sourceBaseType->getCalldataEncodedSize())
            << eth::Instruction::ADD
            << eth::swapInstruction(2 + byteOffsetSize);
    // increment target
    if (haveByteOffsetTarget)
        incrementByteOffset(targetBaseType->getStorageBytes(), byteOffsetSize, byteOffsetSize + 2);
    else
        m_context
            << eth::swapInstruction(1 + byteOffsetSize)
            << targetBaseType->getStorageSize()
            << eth::Instruction::ADD
            << eth::swapInstruction(1 + byteOffsetSize);
    m_context.appendJumpTo(copyLoopStart);
    m_context << copyLoopEnd;
    if (haveByteOffsetTarget)
    {
        // clear elements that might be left over in the current slot in target
        // stack: target_ref target_data_end source_data_pos target_data_pos source_data_end target_byte_offset [source_byte_offset]
        m_context << eth::dupInstruction(byteOffsetSize) << eth::Instruction::ISZERO;
        eth::AssemblyItem copyCleanupLoopEnd = m_context.appendConditionalJump();
        m_context << eth::dupInstruction(2 + byteOffsetSize) << eth::dupInstruction(1 + byteOffsetSize);
        StorageItem(m_context, *targetBaseType).setToZero(SourceLocation(), true);
        incrementByteOffset(targetBaseType->getStorageBytes(), byteOffsetSize, byteOffsetSize + 2);
        m_context.appendJumpTo(copyLoopEnd);

        m_context << copyCleanupLoopEnd;
        m_context << eth::Instruction::POP; // might pop the source, but then target is popped next
    }
    if (haveByteOffsetSource)
        m_context << eth::Instruction::POP;
    m_context << copyLoopEndWithoutByteOffset;

    // zero-out leftovers in target
    // stack: target_ref target_data_end source_data_pos target_data_pos_updated source_data_end
    m_context << eth::Instruction::POP << eth::Instruction::SWAP1 << eth::Instruction::POP;
    // stack: target_ref target_data_end target_data_pos_updated
    clearStorageLoop(*targetBaseType);
    m_context << eth::Instruction::POP;
    m_context << u256(0);
}

void ArrayUtils::clearArray(ArrayType const& _type) const
{
    unsigned stackHeightStart = m_context.getStackHeight();
    solAssert(_type.getLocation() == ArrayType::Location::Storage, "");
    if (_type.getBaseType()->getStorageBytes() < 32)
    {
        solAssert(_type.getBaseType()->isValueType(), "Invalid storage size for non-value type.");
        solAssert(_type.getBaseType()->getStorageSize() <= 1, "Invalid storage size for type.");
    }
    if (_type.getBaseType()->isValueType())
        solAssert(_type.getBaseType()->getStorageSize() <= 1, "Invalid size for value type.");

    m_context << eth::Instruction::POP; // remove byte offset
    if (_type.isDynamicallySized())
        clearDynamicArray(_type);
    else if (_type.getLength() == 0 || _type.getBaseType()->getCategory() == Type::Category::Mapping)
        m_context << eth::Instruction::POP;
    else if (_type.getBaseType()->isValueType() && _type.getStorageSize() <= 5)
    {
        // unroll loop for small arrays @todo choose a good value
        // Note that we loop over storage slots here, not elements.
        for (unsigned i = 1; i < _type.getStorageSize(); ++i)
            m_context
                << u256(0) << eth::Instruction::DUP2 << eth::Instruction::SSTORE
                << u256(1) << eth::Instruction::ADD;
        m_context << u256(0) << eth::Instruction::SWAP1 << eth::Instruction::SSTORE;
    }
    else if (!_type.getBaseType()->isValueType() && _type.getLength() <= 4)
    {
        // unroll loop for small arrays @todo choose a good value
        solAssert(_type.getBaseType()->getStorageBytes() >= 32, "Invalid storage size.");
        for (unsigned i = 1; i < _type.getLength(); ++i)
        {
            m_context << u256(0);
            StorageItem(m_context, *_type.getBaseType()).setToZero(SourceLocation(), false);
            m_context
                << eth::Instruction::POP
                << u256(_type.getBaseType()->getStorageSize()) << eth::Instruction::ADD;
        }
        m_context << u256(0);
        StorageItem(m_context, *_type.getBaseType()).setToZero(SourceLocation(), true);
    }
    else
    {
        m_context << eth::Instruction::DUP1 << _type.getLength();
        convertLengthToSize(_type);
        m_context << eth::Instruction::ADD << eth::Instruction::SWAP1;
        if (_type.getBaseType()->getStorageBytes() < 32)
            clearStorageLoop(IntegerType(256));
        else
            clearStorageLoop(*_type.getBaseType());
        m_context << eth::Instruction::POP;
    }
    solAssert(m_context.getStackHeight() == stackHeightStart - 2, "");
}

void ArrayUtils::clearDynamicArray(ArrayType const& _type) const
{
    solAssert(_type.getLocation() == ArrayType::Location::Storage, "");
    solAssert(_type.isDynamicallySized(), "");

    unsigned stackHeightStart = m_context.getStackHeight();
    // fetch length
    m_context << eth::Instruction::DUP1 << eth::Instruction::SLOAD;
    // set length to zero
    m_context << u256(0) << eth::Instruction::DUP3 << eth::Instruction::SSTORE;
    // stack: ref old_length
    convertLengthToSize(_type);
    // compute data positions
    m_context << eth::Instruction::SWAP1;
    CompilerUtils(m_context).computeHashStatic();
    // stack: len data_pos
    m_context << eth::Instruction::SWAP1 << eth::Instruction::DUP2 << eth::Instruction::ADD
        << eth::Instruction::SWAP1;
    // stack: data_pos_end data_pos
    if (_type.isByteArray() || _type.getBaseType()->getStorageBytes() < 32)
        clearStorageLoop(IntegerType(256));
    else
        clearStorageLoop(*_type.getBaseType());
    // cleanup
    m_context << eth::Instruction::POP;
    solAssert(m_context.getStackHeight() == stackHeightStart - 1, "");
}

void ArrayUtils::resizeDynamicArray(const ArrayType& _type) const
{
    solAssert(_type.getLocation() == ArrayType::Location::Storage, "");
    solAssert(_type.isDynamicallySized(), "");
    if (!_type.isByteArray() && _type.getBaseType()->getStorageBytes() < 32)
        solAssert(_type.getBaseType()->isValueType(), "Invalid storage size for non-value type.");

    unsigned stackHeightStart = m_context.getStackHeight();
    eth::AssemblyItem resizeEnd = m_context.newTag();

    // stack: ref new_length
    // fetch old length
    m_context << eth::Instruction::DUP2 << eth::Instruction::SLOAD;
    // stack: ref new_length old_length
    // store new length
    m_context << eth::Instruction::DUP2 << eth::Instruction::DUP4 << eth::Instruction::SSTORE;
    // skip if size is not reduced
    m_context << eth::Instruction::DUP2 << eth::Instruction::DUP2
        << eth::Instruction::ISZERO << eth::Instruction::GT;
    m_context.appendConditionalJumpTo(resizeEnd);

    // size reduced, clear the end of the array
    // stack: ref new_length old_length
    convertLengthToSize(_type);
    m_context << eth::Instruction::DUP2;
    convertLengthToSize(_type);
    // stack: ref new_length old_size new_size
    // compute data positions
    m_context << eth::Instruction::DUP4;
    CompilerUtils(m_context).computeHashStatic();
    // stack: ref new_length old_size new_size data_pos
    m_context << eth::Instruction::SWAP2 << eth::Instruction::DUP3 << eth::Instruction::ADD;
    // stack: ref new_length data_pos new_size delete_end
    m_context << eth::Instruction::SWAP2 << eth::Instruction::ADD;
    // stack: ref new_length delete_end delete_start
    if (_type.isByteArray() || _type.getBaseType()->getStorageBytes() < 32)
        clearStorageLoop(IntegerType(256));
    else
        clearStorageLoop(*_type.getBaseType());

    m_context << resizeEnd;
    // cleanup
    m_context << eth::Instruction::POP << eth::Instruction::POP << eth::Instruction::POP;
    solAssert(m_context.getStackHeight() == stackHeightStart - 2, "");
}

void ArrayUtils::clearStorageLoop(Type const& _type) const
{
    unsigned stackHeightStart = m_context.getStackHeight();
    if (_type.getCategory() == Type::Category::Mapping)
    {
        m_context << eth::Instruction::POP;
        return;
    }
    // stack: end_pos pos
    eth::AssemblyItem loopStart = m_context.newTag();
    m_context << loopStart;
    // check for loop condition
    m_context << eth::Instruction::DUP1 << eth::Instruction::DUP3
               << eth::Instruction::GT << eth::Instruction::ISZERO;
    eth::AssemblyItem zeroLoopEnd = m_context.newTag();
    m_context.appendConditionalJumpTo(zeroLoopEnd);
    // delete
    m_context << u256(0);
    StorageItem(m_context, _type).setToZero(SourceLocation(), false);
    m_context << eth::Instruction::POP;
    // increment
    m_context << u256(1) << eth::Instruction::ADD;
    m_context.appendJumpTo(loopStart);
    // cleanup
    m_context << zeroLoopEnd;
    m_context << eth::Instruction::POP;
    solAssert(m_context.getStackHeight() == stackHeightStart - 1, "");
}

void ArrayUtils::convertLengthToSize(ArrayType const& _arrayType, bool _pad) const
{
    if (_arrayType.getLocation() == ArrayType::Location::Storage)
    {
        if (_arrayType.getBaseType()->getStorageSize() <= 1)
        {
            unsigned baseBytes = _arrayType.getBaseType()->getStorageBytes();
            if (baseBytes == 0)
                m_context << eth::Instruction::POP << u256(1);
            else if (baseBytes <= 16)
            {
                unsigned itemsPerSlot = 32 / baseBytes;
                m_context
                    << u256(itemsPerSlot - 1) << eth::Instruction::ADD
                    << u256(itemsPerSlot) << eth::Instruction::SWAP1 << eth::Instruction::DIV;
            }
        }
        else
            m_context << _arrayType.getBaseType()->getStorageSize() << eth::Instruction::MUL;
    }
    else
    {
        if (!_arrayType.isByteArray())
            m_context << _arrayType.getBaseType()->getCalldataEncodedSize() << eth::Instruction::MUL;
        else if (_pad)
            m_context << u256(31) << eth::Instruction::ADD
                << u256(32) << eth::Instruction::DUP1
                << eth::Instruction::SWAP2 << eth::Instruction::DIV << eth::Instruction::MUL;
    }
}

void ArrayUtils::retrieveLength(ArrayType const& _arrayType) const
{
    if (!_arrayType.isDynamicallySized())
        m_context << _arrayType.getLength();
    else
    {
        m_context << eth::Instruction::DUP1;
        switch (_arrayType.getLocation())
        {
        case ArrayType::Location::CallData:
            // length is stored on the stack
            break;
        case ArrayType::Location::Memory:
            m_context << eth::Instruction::MLOAD;
            break;
        case ArrayType::Location::Storage:
            m_context << eth::Instruction::SLOAD;
            break;
        }
    }
}

void ArrayUtils::accessIndex(ArrayType const& _arrayType) const
{
    ArrayType::Location location = _arrayType.getLocation();
    eth::Instruction load =
        location == ArrayType::Location::Storage ? eth::Instruction::SLOAD :
        location == ArrayType::Location::Memory ? eth::Instruction::MLOAD :
        eth::Instruction::CALLDATALOAD;

    // retrieve length
    if (!_arrayType.isDynamicallySized())
        m_context << _arrayType.getLength();
    else if (location == ArrayType::Location::CallData)
        // length is stored on the stack
        m_context << eth::Instruction::SWAP1;
    else
        m_context << eth::Instruction::DUP2 << load;
    // stack: <base_ref> <index> <length>
    // check out-of-bounds access
    m_context << eth::Instruction::DUP2 << eth::Instruction::LT;
    eth::AssemblyItem legalAccess = m_context.appendConditionalJump();
    // out-of-bounds access throws exception (just STOP for now)
    m_context << eth::Instruction::STOP;

    m_context << legalAccess;
    // stack: <base_ref> <index>
    m_context << eth::Instruction::SWAP1;
    if (_arrayType.isDynamicallySized())
    {
        if (location == ArrayType::Location::Storage)
            CompilerUtils(m_context).computeHashStatic();
        else if (location == ArrayType::Location::Memory)
            m_context << u256(32) << eth::Instruction::ADD;
    }
    // stack: <index> <data_ref>
    switch (location)
    {
    case ArrayType::Location::CallData:
        if (!_arrayType.isByteArray())
            m_context
                << eth::Instruction::SWAP1
                << _arrayType.getBaseType()->getCalldataEncodedSize()
                << eth::Instruction::MUL;
        m_context << eth::Instruction::ADD;
        if (_arrayType.getBaseType()->isValueType())
            CompilerUtils(m_context).loadFromMemoryDynamic(
                *_arrayType.getBaseType(),
                true,
                !_arrayType.isByteArray(),
                false
            );
        break;
    case ArrayType::Location::Storage:
        m_context << eth::Instruction::SWAP1;
        if (_arrayType.getBaseType()->getStorageBytes() <= 16)
        {
            // stack: <data_ref> <index>
            // goal:
            // <ref> <byte_number> = <base_ref + index / itemsPerSlot> <(index % itemsPerSlot) * byteSize>
            unsigned byteSize = _arrayType.getBaseType()->getStorageBytes();
            solAssert(byteSize != 0, "");
            unsigned itemsPerSlot = 32 / byteSize;
            m_context << u256(itemsPerSlot) << eth::Instruction::SWAP2;
            // stack: itemsPerSlot index data_ref
            m_context
                << eth::Instruction::DUP3 << eth::Instruction::DUP3
                << eth::Instruction::DIV << eth::Instruction::ADD
            // stack: itemsPerSlot index (data_ref + index / itemsPerSlot)
                << eth::Instruction::SWAP2 << eth::Instruction::SWAP1
                << eth::Instruction::MOD;
            if (byteSize != 1)
                m_context << u256(byteSize) << eth::Instruction::MUL;
        }
        else
        {
            if (_arrayType.getBaseType()->getStorageSize() != 1)
                m_context << _arrayType.getBaseType()->getStorageSize() << eth::Instruction::MUL;
            m_context << eth::Instruction::ADD << u256(0);
        }
        break;
    case ArrayType::Location::Memory:
        solAssert(false, "Memory lvalues not yet implemented.");
    }
}

void ArrayUtils::incrementByteOffset(unsigned _byteSize, unsigned _byteOffsetPosition, unsigned _storageOffsetPosition) const
{
    solAssert(_byteSize < 32, "");
    solAssert(_byteSize != 0, "");
    // We do the following, but avoiding jumps:
    // byteOffset += byteSize
    // if (byteOffset + byteSize > 32)
    // {
    //     storageOffset++;
    //     byteOffset = 0;
    // }
    if (_byteOffsetPosition > 1)
        m_context << eth::swapInstruction(_byteOffsetPosition - 1);
    m_context << u256(_byteSize) << eth::Instruction::ADD;
    if (_byteOffsetPosition > 1)
        m_context << eth::swapInstruction(_byteOffsetPosition - 1);
    // compute, X := (byteOffset + byteSize - 1) / 32, should be 1 iff byteOffset + bytesize > 32
    m_context
        << u256(32) << eth::dupInstruction(1 + _byteOffsetPosition) << u256(_byteSize - 1)
        << eth::Instruction::ADD << eth::Instruction::DIV;
    // increment storage offset if X == 1 (just add X to it)
    // stack: X
    m_context
        << eth::swapInstruction(_storageOffsetPosition) << eth::dupInstruction(_storageOffsetPosition + 1)
        << eth::Instruction::ADD << eth::swapInstruction(_storageOffsetPosition);
    // stack: X
    // set source_byte_offset to zero if X == 1 (using source_byte_offset *= 1 - X)
    m_context << u256(1) << eth::Instruction::SUB;
    // stack: 1 - X
    if (_byteOffsetPosition == 1)
        m_context << eth::Instruction::MUL;
    else
        m_context
            << eth::dupInstruction(_byteOffsetPosition + 1) << eth::Instruction::MUL
            << eth::swapInstruction(_byteOffsetPosition) << eth::Instruction::POP;
}