Redundant SLOADs in Initializable

[0xVolosnikov]

Opening issue as mentioned in #4576 (comment)

📝 Details

Since InitializableStorage is a struct with two fields packed in one storage slot, compiler need to load this slot when writing new value only for one of the fields, like here:

openzeppelin-contracts/contracts/proxy/utils/Initializable.sol

Lines 127 to 130 in 175b1e9

	if (isTopLevelCall) {
	$._initializing = false;
	emit Initialized(1);
	}

Since (when) we know values for other fields, explicit usage of known values can let the compiler to prevent excessive SLOADs before SSTORE. Example:

$._initialized = 1;
$._initializing = false;

PS: this is applicable not only in this place, but also in some other contracts.