Tune the cost of EXTCALL with value

[chfast]

There is the table comparing the gas and costs of all legacy and EOF calls.

	Cost (burned)	Callee min	Callee max	Min retained
CALL V=0	100	0	g - g/64	g/64
CALL V≠0	100+6700	2300	g - g/64	g/64
DELEGATECALL	100	0	g - g/64	g/64
STATICCALL	100	0	g - g/64	g/64
EXTCALL V=0	100	2300	g - max(g/64, 5000)	5000
EXTCALL V≠0	100+9000	2300	g - max(g/64, 5000)	5000
EXTDELEGATECALL	100	2300	g - max(g/64, 5000)	5000
EXTSTATICCALL	100	2300	g - max(g/64, 5000)	5000

Note that in legacy the additional value transfer cost in CALL is 6700 while in EXTCALL this is 9000.

This is because of how stipend is applied in legacy and how we do it in EXTCALL. I think it is just a coincidence and we should fix it.
I.e. in legacy CALL the stipend gas (2300) is added from nowhere so the actual value transfer is 6700 not 9000. Try sending a value to an empty contract, you will get the 2300 back.
In EOF we kinda implemented it properly so the cost is actually 9000.

[axic]

How about also defining the retained gas not by a fixed number, but rather as i.e. "maximum cost of SSTORE * 1.5x" ?

Also left this comment on #95.

[chfast]

I reviewed the implementation of CALL and EXTCALL and there are some important details:

1. In CALL relation between the value transfer cost (9000) and the stipend (2300) is more convoluted than presented in the table above. Some checks and calculations (including 63/64 rule) are performed in the context where 9000 cost is already burned but the 2300 stipend is not added back yet. E.g. a CALL can fail of OOG if it cannot cover the 9000 cost although the total gas balance would be positive if the callee returned any of its stipend back.
2. In EXTCALL the stipend mechanism is replaced with "min callee gas". This is worth mentioning in https://github.com/ethereum/EIPs/blob/master/EIPS/eip-7069.md#stipend-and-6364th-rule.

So the conclusion is: if the value transfer in EXTCALL is lowered from 9000 to 6700 its behavior will be closer to CALL but not identical. Main differences are:

• in EXTCALL the callee will receive more gas because the 63/64 rule is calculated out of higher value.
• there are cases where CALL fails of OOG while EXTCALL can go though.

In my opinion keeping the cost of 9000 is still slightly better than changing to 6700.

[chfast]

Updated table. What do you think?

	Caller required gas	Caller cost (burned gas)	Caller min retained gas	Callee min gas
CALL V=0	100	100	0	0
CALL V≠0	100+9000	100+6700	0	2300
DELEGATECALL	100	100	0	0
STATICCALL	100	100	0	0
EXTCALL V=0	100	100	5000	2300
EXTCALL V≠0	100+9000	100+9000	5000	2300
EXTDELEGATECALL	100	100	5000	2300
EXTSTATICCALL	100	100	5000	2300

• caller required gas: the minimum amount of gas a caller is required to have to execute a call instruction, lower value causes caller's OOG,
• caller cost: the amount of gas deducted from the caller to execute the instruction, this amount is not available to the callee,
• caller min retained gas: the minimum amount of gas the caller is guaranteed to have after the call, if this cannot be guaranteed the call fails with 0 without even reaching the callee,
• callee min gas: the minimum gas limit for the callee's execution.

[chfast]

Posted the EIP update: ethereum/EIPs#8506

[chfast]

The "rough consensus" is not to change the 9000 value.