Gas for Execution Cost doesn't add up about remix-project

[tomw1808]

I found what causes the difference between the values shown and the values calculated: MSTORE is marked as "W_verylow" in the Yellowpaper (3 gas), shows 3 gas in the remix debugger, but takes up 12 gas on chain. I have released a similar contract on rinkeby, its's the third instruction and pretty obvious: http://rinkeby.etherscan.io/remix?txhash=0xb837a742727706e9b8f57f33477d3b5dfd3959f705f15f041c1ed6357178917b

I'd be very happy if someone can explain it to me before closing the issue. Thank you.

[yann300]

That's a bug in Remix:
The cost of MSTORE is 3 gas, but the general cost if the step can be higher if the memory needs to be extended.

[seandotau]

Thanks for this @yann300 . Could you elaborate under what situation would the memory need to be extended? Is it a linear increase as well? Just curious.

[yann300]

this is has been fixed for sometimes.
Sorry for not responding here.
you'll find the information here: https://github.com/wolflo/evm-opcodes/blob/main/gas.md#a0-1-memory-expansion
The following opcodes incur a memory expansion cost in addition to their otherwise static gas cost: RETURN, REVERT, MLOAD, MSTORE, MSTORE8.

[tomw1808]

@yann300 appreciate it. I already forgot about the issue, been 5 years. Again, really appreciate getting back to it and referencing more information here.