Gas optimization, Flexible code

[abhi3700]

Background

In this line:

solmate/src/tokens/ERC721.sol

Line 36 in c93f771

require((owner = _ownerOf[id]) != address(0), "NOT_MINTED");

solmate/src/tokens/ERC721.sol

Line 40 in c93f771

require(owner != address(0), "ZERO_ADDRESS");

although they are view functions, it's cheaper to simply return _ownerOf[id] & _balanceOf[owner]. Also, it's more developer-friendly or flexible I think.
Like in this test code:

/// address who owns the nft token id, can only burn
function testBurnWorks() public {
    vm.expectEmit(true, true, true, true);
    emit Burned(ALICE, 1);
    vm.prank(ALICE);
    myNFT.burn(1);
    // assertEq(myNFT.ownerOf(1), address(0));
}

I could call simply ownerOf function after burning NFT to verify. Otherwise, i have to use new_balance - old_balance.

Analysis

ownerOf

The current code costs 622 gas:

 ├─ [622] MyNFT::ownerOf(2) [staticcall]
    │   └─ ← [Return] ECRecover: [0x0000000000000000000000000000000000000001]

Modifying the code with:

- require((owner = _ownerOf[id]) != address(0), "NOT_MINTED");
+ return _ownerOf[id];

Function code (NEW):

function ownerOf(uint256 id) public view virtual returns (address) {
    return _ownerOf[id];
}

The new code would cost 588 gas:

├─ [588] MyNFT::ownerOf(2) [staticcall]
    │   └─ ← [Return] ECRecover: [0x0000000000000000000000000000000000000001]

balanceOf

Same reasoning in balanceOf function with new function code:

function balanceOf(address owner) public view virtual returns (uint256) {
    return _balanceOf[owner];
}

├─ [634] MyNFT::balanceOf(ECRecover: [0x0000000000000000000000000000000000000001]) [staticcall]
    │   └─ ← [Return] 4

├─ [582] MyNFT::balanceOf(ECRecover: [0x0000000000000000000000000000000000000001]) [staticcall]
    │   └─ ← [Return] 4
    └─ ← [Stop]

Summary

• In ownerOf function, we can save by 52 gas per call.
• In balanceOf function, we can save by 34 gas per call.
• Also, the code becomes more developer friendly or flexible.