More efficient GST2 implementation

[fulldecent]

Just wanted to say this is a cute project. I have been studying and wanted to share my notes.

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Current contract call costs 22 gas.

https://github.com/projectchicago/gastoken/blob/master/contract/GST2_ETH.sol#L106-L114

Gas	Set	Inst
3	VERYLOW	PUSH15 0xb3f879cb30fe243b4dfee438691c04
2	BASE	CALLER
3	VERYLOW	XOR
2	BASE	PC
10	HIGH	JUMPI
2	BASE	CALLER
—	—	SELFDESTRUCT

Proposed update requires 19 gas.

Gas	Set	Inst
3	VERYLOW	PUSH15 0xb3f879cb30fe243b4dfee438691cXX
2	BASE	CALLER
3	VERYLOW	XOR
8	MID	JUMP
1	JUMPDEST	JUMPDEST
2	BASE	CALLER
—	—	SELFDESTRUCT

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With this new technique, the comment on the homepage can be updated:

The address of our GST2 GasToken, 0x0000000000b3F879cb30FE243b4Dfee438691c04, is hardcoded. Note that this address is very short (only 15 non-zero bytes instead of 20). We generated our GasToken contract at such an address on purpose, so as to save some gas when creating contracts. The above child contract uses 22 bytes of code, and thus costs roughly 32000 + 22 * 200 = 36400 gas to create (the additional overhead of calling mint is only about 150 gas). If we could make use of address 0x0 for GasToken, we could save an extra 3000 gas or so for every created contract!

Don't worry about the 0x0 address. You want the 0xPC address. Then you can additionally save the PUSH15 and XOR operations.

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This proposed update also affects the efficiency ratio and other parts of the web page.

[lorenzb]

@fulldecent
Thanks for letting us know! ❤️
If I understand you correctly, this technique would shave off 3 gas per minted contract?

[fulldecent]

Yes, that is correct.

[k06a]

@fulldecent can you clarify what XX means?

[fulldecent]

XX = 0x04 ^ 0x13 = 0x17.

This is chosen so that

1. 0xb3f879cb30fe243b4dfee438691c04 is the beneficiary (msg.caller)
2. 0xb3f879cb30fe243b4dfee438691c04 ^ 0xb3f879cb30fe243b4dfee438691c17 = 0x13
3. JUMP 0x13 lands on the JUMPDEST