Saving gas costs by combining variables?

Question

Lets say I have 4 variables which I have for some reason declared as uint256's:

uint256 a = ...;
uint256 b = ...;
uint256 c = ...;
uint256 d = ...;

Lets say for my usecase, these variables actually never go higher than 2^64-1 in practice (that is each variable only needs a size of uint64).

What are the gas cost savings implications for if I were to combine all four variables into a single uint256 which I can split back to the four variables when needed? Thanks!

Answer 1

Think of solidity storage is an array of slots. each slot stores 32 bytes.

 [[],[],[],[],.......[]]

If you define your variables with uint256 which is 32 bytes, first 4 slots will be reserved.

But if you declare them uint64 which is 8 bytes, the solidity will store all of them in the first slot. You will save gas because you will not be using extra 3 slots