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Use unchecked for ERC721 balance updates #3524

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merged 8 commits into from Aug 23, 2022
Merged

Use unchecked for ERC721 balance updates #3524

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61a0037
Use unchecked to improve gas usage of ERC721
Amxx Jul 1, 2022
48bea11
add unchecked comments
Amxx Jul 6, 2022
ffae7c2
fix lint
Amxx Jul 6, 2022
5db7f76
Apply suggestions from code review
Amxx Jul 7, 2022
1bbc2c9
Merge branch 'master' into optimization/erc721
frangio Aug 23, 2022
073bb8f
fix typo transfered -> transferred
frangio Aug 23, 2022
d100459
fix typos
frangio Aug 23, 2022
96f6857
update changelog
frangio Aug 23, 2022
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25 changes: 21 additions & 4 deletions contracts/token/ERC721/ERC721.sol
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Expand Up @@ -282,7 +282,13 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata {

_beforeTokenTransfer(address(0), to, tokenId);

_balances[to] += 1;
unchecked {
// Will not overflow unless all 2**256 token ids are minted to the same owner.
// Given that tokens are minted one by one, it is impossible in practice that
// this ever happens. Might change if allow batch minting.
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// The ERC fails to describe this case.
_balances[to] += 1;
}
_owners[tokenId] = to;

emit Transfer(address(0), to, tokenId);
Expand All @@ -308,7 +314,11 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata {
// Clear approvals
_approve(address(0), tokenId);

_balances[owner] -= 1;
unchecked {
// Cannot overflow, as that would require more tokens to be burned/transfered
// out then the owner initialy received through minting and transfering in.
_balances[owner] -= 1;
}
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delete _owners[tokenId];

emit Transfer(owner, address(0), tokenId);
Expand Down Expand Up @@ -340,8 +350,15 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata {
// Clear approvals from the previous owner
_approve(address(0), tokenId);

_balances[from] -= 1;
_balances[to] += 1;
unchecked {
// `_balances[from]` cannot overflow for the same reason as described in `_burn`:
// `from`'s balance is the number of token held, which is at least one before the current
// transfer.
// `_balances[to]` could overflow in the conditions described in `_mint`. That would require
// all 2**256 token ids to be minted, which in practice is impossible.
_balances[from] -= 1;
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_balances[to] += 1;
}
_owners[tokenId] = to;

emit Transfer(from, to, tokenId);
Expand Down