I am experimenting with some gas savings and I found out that inside a function declaring a variable first and then assigning a value is more gas efficient than making the declaration and assignment in one line. My question is that why does this happen? I would think that the shorter version produces less code and less code costs less in this scenario.

checkVariableDeclaration1 >> 23466 gas
checkVariableDeclaration2 >> 23412 gas
difference >> 44 gas

function checkVariableDeclaration1(uint[] calldata nums) public pure returns (uint) {
    uint n1 = nums[0];
    uint n2 = nums[1];
    uint n3 = nums[2];
    uint n4 = nums[3];
    return n1 + n2 + n3 + n4;


function checkVariableDeclaration2(uint[] calldata nums) public pure returns (uint) {
    uint n1;
    uint n2;
    uint n3;
    uint n4;
    n1 = nums[0];
    n2 = nums[1];
    n3 = nums[2];
    n4 = nums[3];
    return n1 + n2 + n3 + n4;


1 Answer 1


In the second example, you assign zero to n1 - n4 before assigning the final value. This can be seen if you compile the contract using intermediate representation

solc Check.sol -ir


/// @src 0:477:484 "uint n1"
let var_n1_51
let null_t_uint256_11 := null_value_for_split_t_uint256()
var_n1_51 := null_t_uint256_11
/// @src 0:494:501 "uint n2"
let var_n2_54
let null_t_uint256_12 := null_value_for_split_t_uint256()
var_n2_54 := null_t_uint256_12
/// @src 0:511:518 "uint n3"
let var_n3_57
let null_t_uint256_13 := null_value_for_split_t_uint256()
var_n3_57 := null_t_uint256_13
/// @src 0:528:535 "uint n4"
let var_n4_60
let null_t_uint256_14 := null_value_for_split_t_uint256()
var_n4_60 := null_t_uint256_14

  • 1
    Any insight on why assigning default values first makes it cheaper?
    – Ryan Sea
    Aug 22, 2022 at 1:36
  • That is a good question. Can you share your compiler settings (version, number of optimization runs, etc..)? Aug 22, 2022 at 13:47
  • This was a simple test using the default settings of Remix. After doubling the number of variables the difference increased even more. With 8 variables the difference was around 120 gas. Do you think this would work on the mainnet as well or there is a chance that its produced by a Remix compiler setting?
    – rihekopo
    Aug 24, 2022 at 12:12
  • Yes, I'm pretty sure this doesn't just happen in the Remix VM (remix-ide.readthedocs.io/en/latest/run.html#environment). As a test you could try deploying to a public test network. The compiler setting can be found under Advanced Configuration. Aug 24, 2022 at 13:18

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