分析 ERC721A 源码节省Gas优化思路
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分析ERC721A源码节省Gas优化思路
分析 ERC721A 源码节省Gas优化思路
Link: https://github.com/chiru-labs/ERC721A
"erc721a": "^4.3.0"
// Mapping from token ID to ownership details
// Bits Layout:
// - [0..159] `addr`
// - [160..223] `startTimestamp`
// - [224] `burned`
// - [225] `nextInitialized`
// - [232..255] `extraData`
mapping(uint256 => uint256) private _packedOwnerships;
// Mapping owner address to address data.
//
// Bits Layout:
// - [0..63] `balance`
// - [64..127] `numberMinted`
// - [128..191] `numberBurned`
// - [192..255] `aux`
mapping(address => uint256) private _packedAddressData;1. 分析 _mint(to, quantity)
function _mint(address to, uint256 quantity) internal virtual {
uint256 startTokenId = _currentIndex;
if (quantity == 0) _revert(MintZeroQuantity.selector);
_beforeTokenTransfers(address(0), to, startTokenId, quantity);
// Overflows are incredibly unrealistic.
// `balance` and `numberMinted` have a maximum limit of 2**64.
// `tokenId` has a maximum limit of 2**256.
unchecked {
// Updates:
// - `address` to the owner.
// - `startTimestamp` to the timestamp of minting.
// - `burned` to `false`.
// - `nextInitialized` to `quantity == 1`.
_packedOwnerships[startTokenId] = _packOwnershipData(
to,
_nextInitializedFlag(quantity) | _nextExtraData(address(0), to, 0)
);
// Updates:
// - `balance += quantity`.
// - `numberMinted += quantity`.
//
// We can directly add to the `balance` and `numberMinted`.
_packedAddressData[to] += quantity * ((1 << _BITPOS_NUMBER_MINTED) | 1);
// Mask `to` to the lower 160 bits, in case the upper bits somehow aren't clean.
uint256 toMasked = uint256(uint160(to)) & _BITMASK_ADDRESS;
if (toMasked == 0) _revert(MintToZeroAddress.selector);
uint256 end = startTokenId + quantity;
uint256 tokenId = startTokenId;
if (end - 1 > _sequentialUpTo()) _revert(SequentialMintExceedsLimit.selector);
do {
assembly {
// Emit the `Transfer` event.
log4(
0, // Start of data (0, since no data).
0, // End of data (0, since no data).
_TRANSFER_EVENT_SIGNATURE, // Signature.
0, // `address(0)`.
toMasked, // `to`.
tokenId // `tokenId`.
)
}
// The `!=` check ensures that large values of `quantity`
// that overflows uint256 will make the loop run out of gas.
} while (++tokenId != end);
_currentIndex = end;
}
_afterTokenTransfers(address(0), to, startTokenId, quantity);
}以 _mint(AAAA,10); _currentIndex = 1 为例,剖析执行流程
(1) 获取将要开始铸造的TokenId
uint256 startTokenId = _currentIndex;
if (quantity == 0) _revert(MintZeroQuantity.selector);
_beforeTokenTransfers(address(0), to, startTokenId, quantity);startTokenId = 1；
(2) 更新mapping(tokenId ==> (owner, startTime, ....))
// Updates:
// - `address` to the owner.
// - `startTimestamp` to the timestamp of minting.
// - `burned` to `false`.
// - `nextInitialized` to `quantity == 1`.
_packedOwnerships[startTokenId] = _packOwnershipData(
to,
_nextInitializedFlag(quantity) | _nextExtraData(address(0), to, 0)
);nextInitialized = (quantity == 1); 下一个tokenId有没有初始化。
_packedOwnerships[1] = _packed(AAAA, block.timestamp, false, nextInitialized, extraData);
将接收地址、铸造时间戳等信息打包后赋值给 _packedOwnerships[1]。
(3) 更新 mapping(owner => (balance, numberMinted, ...))
// Updates:
// - `balance += quantity`.
// - `numberMinted += quantity`.
//
// We can directly add to the `balance` and `numberMinted`.
_packedAddressData[to] += quantity * ((1 << _BITPOS_NUMBER_MINTED) | 1);更新 接收者 持有token的数量以及铸造token的数量。
(4) 循环生成 Transfer Event
uint256 end = startTokenId + quantity;
uint256 tokenId = startTokenId;
if (end - 1 > _sequentialUpTo()) _revert(SequentialMintExceedsLimit.selector);
do {
assembly {
// Emit the `Transfer` event.
log4(
0, // Start of data (0, since no data).
0, // End of data (0, since no data).
_TRANSFER_EVENT_SIGNATURE, // Signature.
0, // `address(0)`.
toMasked, // `to`.
tokenId // `tokenId`.
)
}
// The `!=` check ensures that large values of `quantity`
// that overflows uint256 will make the loop run out of gas.
} while (++tokenId != end);(5) 更新 _currentIndex
_currentIndex = end;
_afterTokenTransfers(address(0), to, startTokenId, quantity);_currentIndex = 11；
总结
无论铸造几个 NFT，都只更新 3 个 Slot，外加 N 个 Transfer 事件（必须），这就是 ERC721 批量 mint 节省 GAS 的精髓。
2. 分析 ownferOf(tokenId)
function ownerOf(uint256 tokenId) public view virtual override returns (address) {
return address(uint160(_packedOwnershipOf(tokenId)));
} function _packedOwnershipOf(uint256 tokenId) private view returns (uint256 packed) {
if (_startTokenId() <= tokenId) {
packed = _packedOwnerships[tokenId];
if (tokenId > _sequentialUpTo()) {
if (_packedOwnershipExists(packed)) return packed;
_revert(OwnerQueryForNonexistentToken.selector);
}
// If the data at the starting slot does not exist, start the scan.
if (packed == 0) {
if (tokenId >= _currentIndex) _revert(OwnerQueryForNonexistentToken.selector);
// Invariant:
// There will always be an initialized ownership slot
// (i.e. `ownership.addr != address(0) && ownership.burned == false`)
// before an unintialized ownership slot
// (i.e. `ownership.addr == address(0) && ownership.burned == false`)
// Hence, `tokenId` will not underflow.
//
// We can directly compare the packed value.
// If the address is zero, packed will be zero.
for (;;) {
unchecked {
packed = _packedOwnerships[--tokenId];
}
if (packed == 0) continue;
if (packed & _BITMASK_BURNED == 0) return packed;
// Otherwise, the token is burned, and we must revert.
// This handles the case of batch burned tokens, where only the burned bit
// of the starting slot is set, and remaining slots are left uninitialized.
_revert(OwnerQueryForNonexistentToken.selector);
}
}
// Otherwise, the data exists and we can skip the scan.
// This is possible because we have already achieved the target condition.
// This saves 2143 gas on transfers of initialized tokens.
// If the token is not burned, return `packed`. Otherwise, revert.
if (packed & _BITMASK_BURNED == 0) return packed;
}
_revert(OwnerQueryForNonexistentToken.selector);
}以 ownerOf(5) 为例
_startTokenId() <= tokenId < _currentIndex
if (packed == 0) {
if (tokenId >= _currentIndex) _revert(OwnerQueryForNonexistentToken.selector);
for (;;) {
unchecked {
packed = _packedOwnerships[--tokenId];
}
if (packed == 0) continue;
if (packed & _BITMASK_BURNED == 0) return packed;
_revert(OwnerQueryForNonexistentToken.selector);
}
}tokenId 的传参范围：1 <= tokenId <11；
获取 tokenId 的打包数据，packed = _packedOwnerships[5];
如果 packed 为空，依次往下获取 （tokenId - 1） 的打包数据，直到 packed不为空。
最终， packed = _packedOwnerships[1]；解析packed 后，owner = AAAA。
总结
查询 tokenId 的 owner 是一个循环往下遍历的过程，直到数据不为空。
3. 分析 transferFrom(from, to, tokenId)
function transferFrom(
address from,
address to,
uint256 tokenId
) public payable virtual override {
uint256 prevOwnershipPacked = _packedOwnershipOf(tokenId);
// Mask `from` to the lower 160 bits, in case the upper bits somehow aren't clean.
from = address(uint160(uint256(uint160(from)) & _BITMASK_ADDRESS));
if (address(uint160(prevOwnershipPacked)) != from) _revert(TransferFromIncorrectOwner.selector);
(uint256 approvedAddressSlot, address approvedAddress) = _getApprovedSlotAndAddress(tokenId);
// The nested ifs save around 20+ gas over a compound boolean condition.
if (!_isSenderApprovedOrOwner(approvedAddress, from, _msgSenderERC721A()))
if (!isApprovedForAll(from, _msgSenderERC721A())) _revert(TransferCallerNotOwnerNorApproved.selector);
_beforeTokenTransfers(from, to, tokenId, 1);
// Clear approvals from the previous owner.
assembly {
if approvedAddress {
// This is equivalent to `delete _tokenApprovals[tokenId]`.
sstore(approvedAddressSlot, 0)
}
}
// Underflow of the sender's balance is impossible because we check for
// ownership above and the recipient's balance can't realistically overflow.
// Counter overflow is incredibly unrealistic as `tokenId` would have to be 2**256.
unchecked {
// We can directly increment and decrement the balances.
--_packedAddressData[from]; // Updates: `balance -= 1`.
++_packedAddressData[to]; // Updates: `balance += 1`.
// Updates:
// - `address` to the next owner.
// - `startTimestamp` to the timestamp of transfering.
// - `burned` to `false`.
// - `nextInitialized` to `true`.
_packedOwnerships[tokenId] = _packOwnershipData(
to,
_BITMASK_NEXT_INITIALIZED | _nextExtraData(from, to, prevOwnershipPacked)
);
// If the next slot may not have been initialized (i.e. `nextInitialized == false`) .
if (prevOwnershipPacked & _BITMASK_NEXT_INITIALIZED == 0) {
uint256 nextTokenId = tokenId + 1;
// If the next slot's address is zero and not burned (i.e. packed value is zero).
if (_packedOwnerships[nextTokenId] == 0) {
// If the next slot is within bounds.
if (nextTokenId != _currentIndex) {
// Initialize the next slot to maintain correctness for `ownerOf(tokenId + 1)`.
_packedOwnerships[nextTokenId] = prevOwnershipPacked;
}
}
}
}
// Mask `to` to the lower 160 bits, in case the upper bits somehow aren't clean.
uint256 toMasked = uint256(uint160(to)) & _BITMASK_ADDRESS;
assembly {
// Emit the `Transfer` event.
log4(
0, // Start of data (0, since no data).
0, // End of data (0, since no data).
_TRANSFER_EVENT_SIGNATURE, // Signature.
from, // `from`.
toMasked, // `to`.
tokenId // `tokenId`.
)
}
if (toMasked == 0) _revert(TransferToZeroAddress.selector);
_afterTokenTransfers(from, to, tokenId, 1);
}以 transferFrom(AAAA, BBBB, 5) 为例
(1) 获取 tokenId 的打包数据（owner）
uint256 prevOwnershipPacked = _packedOwnershipOf(tokenId);
// Mask `from` to the lower 160 bits, in case the upper bits somehow aren't clean.
from = address(uint160(uint256(uint160(from)) & _BITMASK_ADDRESS));
if (address(uint160(prevOwnershipPacked)) != from) _revert(TransferFromIncorrectOwner.selector);uint256 prevOwnershipPacked = _packedOwnershipOf(5)；
依次往下找，prevOwnershipPacked = _packedOwnerships[1];
解析数据，拿到 tokenId 的owner，与 from 地址做校验。
(2) 校验 from 是否将 tokenId 授权给 _msgSender
(uint256 approvedAddressSlot, address approvedAddress) = _getApprovedSlotAndAddress(tokenId);
// The nested ifs save around 20+ gas over a compound boolean condition.
if (!_isSenderApprovedOrOwner(approvedAddress, from, _msgSenderERC721A()))
if (!isApprovedForAll(from, _msgSenderERC721A())) _revert(TransferCallerNotOwnerNorApproved.selector);
_beforeTokenTransfers(from, to, tokenId, 1);
// Clear approvals from the previous owner.
assembly {
if approvedAddress {
// This is equivalent to `delete _tokenApprovals[tokenId]`.
sstore(approvedAddressSlot, 0)
}
}校验 from 是否将 tokenId 授权给 调用者；之后清除 tokenId 的授权信息。
(3) 更新 form, to 的余额信息，及 tokenId 的打包信息
unchecked {
// We can directly increment and decrement the balances.
--_packedAddressData[from]; // Updates: `balance -= 1`.
++_packedAddressData[to]; // Updates: `balance += 1`.
// Updates:
// - `address` to the next owner.
// - `startTimestamp` to the timestamp of transfering.
// - `burned` to `false`.
// - `nextInitialized` to `true`.
_packedOwnerships[tokenId] = _packOwnershipData(
to,
_BITMASK_NEXT_INITIALIZED | _nextExtraData(from, to, prevOwnershipPacked)
);
// If the next slot may not have been initialized (i.e. `nextInitialized == false`) .
if (prevOwnershipPacked & _BITMASK_NEXT_INITIALIZED == 0) {
uint256 nextTokenId = tokenId + 1;
// If the next slot's address is zero and not burned (i.e. packed value is zero).
if (_packedOwnerships[nextTokenId] == 0) {
// If the next slot is within bounds.
if (nextTokenId != _currentIndex) {
// Initialize the next slot to maintain correctness for `ownerOf(tokenId + 1)`.
_packedOwnerships[nextTokenId] = prevOwnershipPacked;
}
}
}
}更新 AAAA, BBBB 的余额信息；balance_AAAA = 9, balance_BBBB = 1;
// Mapping from token ID to ownership details
// Bits Layout:
// - [0..159] `addr`
// - [160..223] `startTimestamp`
// - [224] `burned`
// - [225] `nextInitialized`
// - [232..255] `extraData`更新tokenId = 5 的打包信息；_packedOwnerships[5] =(BBBB, block.timestamp, false, true, extraData); 此时，需要 nextInitialized = true。
如果 prevOwnershipPacked 的 nextInitialized = false，即 下一个tokenId的打包信息没有初始化；那么就要对现在tokenId = 5的下一个进行初始化。
_packedOwnerships[6] = prevOwnershipPacked。
（也可以这么理解，在转移TokenId为 N 的NFT时，如果TokenId为 N + 1 的NFT没有被初始化过，就要将要TokenId为 N 的打包信息赋值给TokenId为 N + 1；然后更新TokenId为 N 的打包信息）
为什要这样呢？ 因为需要将 prevOwnershipPacked 赋值给 _packedOwnerships[6]，初始化tokenId = 6 打包数据 。不然你查找owner(10)时将查到 _packedOwnerships[5]的 owner 是 BBBB; AAAA 的 NFT 无缘无故的丢了，肯定大哭不愿意啊!
(4) 生成 Transfer Event
// Mask `to` to the lower 160 bits, in case the upper bits somehow aren't clean.
uint256 toMasked = uint256(uint160(to)) & _BITMASK_ADDRESS;
assembly {
// Emit the `Transfer` event.
log4(
0, // Start of data (0, since no data).
0, // End of data (0, since no data).
_TRANSFER_EVENT_SIGNATURE, // Signature.
from, // `from`.
toMasked, // `to`.
tokenId // `tokenId`.
)
}
if (toMasked == 0) _revert(TransferToZeroAddress.selector);
_afterTokenTransfers(from, to, tokenId, 1);总结
在 NFT的一次转移过程中，最多更新两个token（N和 N + 1）的打包数据。
4. 有这么一个问题，CCCC批量铸造了5个NFT，tokenId 分别为 11,12,13,14,15。现在他要将这5个NFT分5次转给DDDD，那么他应该从tokenId为11还是15开始呢？
首先肯定是要执行5次 transferFrom(CCCC, DDDD, tokenId)。
假如从15 ==> 11, transferFrom(CCCC, DDDD, 15),查询owner执行了 5次,初始化 15 的打包信息； transferFrom(CCCC, DDDD, 14),查询owner执行了 4次,初始化 14 的打包信息； transferFrom(CCCC, DDDD, 13),查询owner执行了 3次,初始化 13 的打包信息； transferFrom(CCCC, DDDD, 12),查询owner执行了 2次,初始化 12 的打包信息； transferFrom(CCCC, DDDD, 11),查询owner执行了 1次,更新 11 的打包信息。
假如从11 ==> 15, transferFrom(CCCC, DDDD, 11),查询owner执行了 1次,更新 11 的打包信息,初始化12的打包信息； transferFrom(CCCC, DDDD, 12),查询owner执行了 1次,更新 12 的打包信息,初始化13的打包信息； transferFrom(CCCC, DDDD, 13),查询owner执行了 1次,更新 13 的打包信息,初始化14的打包信息； transferFrom(CCCC, DDDD, 14),查询owner执行了 1次,更新 14 的打包信息,初始化15的打包信息； transferFrom(CCCC, DDDD, 15),查询owner执行了 1次,更新 15 的打包信息。
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