Improve Initializable readability using intermediate variables

[Amxx]

Fixes LIB-1041

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Latest commit: 175b1e9

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Name	Type
openzeppelin-solidity	Minor

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[0xVolosnikov]

$._initialized = 1;
$._initializing = isTopLevelCall;

May be cheaper here, removing redundant SLOAD operation in case of !isTopLevelCall

[Amxx]

I'm not sure I understand your proposal. Before doing _;, we want to make sure the initializing is true, this would possibly invert it to false.

Also I'm not sure where the duplicated sload is.

[0xVolosnikov]

Also I'm not sure where the duplicated sload is.

Since $ is a struct with two fields packed in one storage slot, compiler need to load this slot if you change only one field. If you write all the fields of the structure at once, the compiler does not need to read this slot before SSTORE

[0xVolosnikov]

So it could (in theory) save around 100 gas here

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

Lines 122 to 125 in 175b1e9

	$._initialized = 1;
	if (isTopLevelCall) {
	$._initializing = true;
	}

If do it like

$._initialized = 1;
$._initializing = isTopLevelCall;

[0xVolosnikov]

this would possibly invert it to false

Upd: yes, in this case my proposal should be rather:

  $._initialized = 1;
  $._initializing = true;

And here:

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

[frangio]

Writing it that way requires proving that the _; part doesn't change the value of $._initialized.

I think it is true, because $._initializing = true prevents that value from changing in the rest of the contract, but it's more difficult to see.

I think I prefer to keep the current code at this stage (audit fixes) and come back to this for a future release. @vladyan18 can you open an issue?