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Test.sol

pragma solidity ^0.4.18;

contract Test {
    uint8 x;
    
    function get() public view returns (uint8) {
        return x;
    }
    
    function set(uint8 _x) public {
        x = _x;
    }
}

TestArray.sol

pragma solidity ^0.4.18;

contract TestArray {
    uint128[100] public foos;
    uint128[100] public bars;
    
    // * 40942
    // * 10962
    function set(uint idx, uint128 _foo, uint128 _bar) public {
        foos[idx] = _foo;
        bars[idx] = _bar;
    }
}

TestBytes.sol

pragma solidity ^0.4.18;

contract TestBytes {
    struct Test {
        uint128 foo;
        uint128 bar;
    }
    
    bytes32[100] public testBytes;
    
    // * 21348
    // * 6348
    function set(uint idx, uint128 _foo, uint128 _bar) public {
        Test memory test = fromBytes(testBytes[idx]);
        
        test.foo = _foo;
        test.bar = _bar;
        
        testBytes[idx] = toBytes(test);
    }

    function toBytes(Test test) internal pure returns (bytes32) {
        bytes32 foo = bytes32(test.foo) << 128;
        bytes32 bar = bytes32(test.bar);
        
        return foo | bar;
    }
    
    function fromBytes(bytes32 bs) internal pure returns (Test) {
        bytes16 fooBytes  = bytes16(bs);
        bytes16 barBytes  = bytes16(bs << 128);
    
        uint128 foo = uint128(fooBytes);
        uint128 bar = uint128(barBytes);
        
        return Test(foo, bar);
    }
    
    function getFoo(uint idx) public view returns (uint128) {
        return fromBytes(testBytes[idx]).foo;
    }
    
    function getBar(uint idx) public view returns (uint128) {
        return fromBytes(testBytes[idx]).bar;
    }
}

TestStruct1.sol

pragma solidity ^0.4.18;

contract TestStruct1 {
    struct Test {
        uint128 foo;
        uint128 bar;
    }
    
    Test[100] public tests;

    // * 25894
    // * 10894
    function set(uint idx, uint128 _foo, uint128 _bar) public {
        tests[idx].foo = _foo;
        tests[idx].bar = _bar;
    }
}

TestStruct2.sol

pragma solidity ^0.4.18;

contract TestStruct2 {
    struct Test {
        uint128 foo;
        uint128 bar;
    }
    
    Test[100] public tests;
    
    // * 26741
    // * 11741
    function set(uint idx, uint128 _foo, uint128 _bar) public {
        Test memory test = tests[idx];
        test.foo = _foo;
        test.bar = _bar;
        
        tests[idx] = test;
    }
}