Refactor/reentrancy guard gas optimization #1056

[Eenae]

Fixes #1056

🚀 Description

Implemented solution proposed in the issue. Using integer constants to represent state of the guard. Not using enum {Dummy, Free, Locked} because solidity documentation as far as I can see does't provide necessary guarantees on underlying integer values of enum members (again, our goal is to never touch 0).

It could be seen in Remix that execution cost of each empty nonReentrant call is 10390 gas now.

• 📘 I've reviewed the OpenZeppelin Contributor Guidelines
• ✅ I've added tests where applicable to test my new functionality.
• 📖 I've made sure that my contracts are well-documented.
• 🎨 I've run the JS/Solidity linters and fixed any issues (npm run lint:all:fix).

[BrendanChou]

I believe changing to uint8 will save storage costs in the case that other storage variables can fit within the same 32bit word

[Eenae]

I believe changing to uint8 will save storage costs in the case that other storage variables can fit within the same 32bit word

Yes, but with ~500 gas overhead on read-write. Was't sure it worths it in a long run.

[BrendanChou]

Where does that ~500 gas overhead come from?

[Eenae]

Where does that ~500 gas overhead come from?

Before writing 1 byte solidity has to read (200 gas) current 32-bytes slot and merge 1 byte into it and finally write back 32-bytes slot. Mechanics is the same as in partial writes of disk sectors. This extra gas overhead is noted in solidity docs afair.

[BrendanChou]

Either way, you have to SSTORE and SLOAD a whole word (32 bytes). I'm not sure why 500 gas would be needed to do a bitmask.

[Eenae]

You don't need SLOAD to write uint. But this is not the case for uint8.

[BrendanChou]

Ah makes sense, you're right

[nventuro]

Great work @Eenae, thanks! I'm puzzled by Solidity not providing any guarantees regarding enums, specially considering there's (AFAIK) no way to retrieve those values from outside Solidity (i.e. they are not specified in the ABI, nor anywhere else).

[axic]

no way to retrieve those values from outside Solidity (i.e. they are not specified in the ABI, nor anywhere else).

It is on the roadmap though: ethereum/solidity#1602