caching result of reading storage variable to save gas

[molly-ting]

Caching the result of reading storage variable $._initialized to stack could reduce one sload, thus saving gas.

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[0xVolosnikov]

Hello! Have you checked the difference in gas? It seems to me that the compiler is capable of optimizing this case on its own.

[frangio]

There is some improvement according to our gas measurements: https://github.com/OpenZeppelin/openzeppelin-contracts/actions/runs/5930652468

Close to 100 gas, so it does sound like the compiler is not optimizing the redundant SLOAD.