annotated-code | A series of short , heavily annotated JavaScript programs | Natural Language Processing library

Each storage read (opcode sload) of the same slot costs 2,100 gas the first time during a transaction, and then 100 gas each other time during the same transaction. (After the EIP-2929 implemented in the Berlin hardfork. Before that, it was 800 for each read no matter how many times you performed the reads.)

Each memory write (opcode mstore) and each memory read (opcode mload) cost 3 gas.

So in traditional programming, _kLast would be a reference to kLast

In this particular Solidity snippet, _kLast is not a reference pointing to the storage. It's a memory variable that has value assigned from the storage variable.

So 3 storage reads - not creating the memory variable - would cost 2,300 gas (= 2,100 + 100 + 100).

But because the code creates the memory variable, it performs one storage read (2,100 gas), one memory write (3 gas), and three memory reads (3 x 3 gas) - totalling 2,112 gas. Which is a cheaper option.

Some other EVM-compatible networks, such as BSC, might still use the original gas calculation of 800 per each sload. Which would make even larger difference - non-optimized 2,400 gas (3 x 800), and optimized 812 gas (800 + 3 + 3x3).