Store dynamic bytes to call swap function with solidity assembly about v2-core HOT 3 OPEN

fallback() external payable {
assembly {
let pair := shr(96, calldataload(0))
let tokenAmountOut := calldataload(4)
mstore(0, PAIR_SWAP)
mstore(4, tokenAmountOut)
mstore(0x24, 0)
mstore(0x44, address())
mstore8(0x64, 0)
let result := call(gas(), pair, callvalue(), 0, 0x84, 0, 0)
returndatacopy(0, 0, returndatasize())
switch result
case 0 {revert(0, returndatasize())}
default {return (0, returndatasize())}
}
}

from v2-core.

In order to pass a zero-length bytes value to the swap function of the UniswapV2 pair, you can simply store a single zero byte at memory location 0x64. This can be done using the mstore8 opcode, which stores a single byte at the specified memory location.

Here's the modified assembly code with the mstore8 instruction to set the length of the data parameter to zero:

fallback() external payable { assembly { let pair := shr(96, calldataload(0)) let tokenAmountOut := calldataload(4) mstore(0, PAIR_SWAP) mstore(4, tokenAmountOut) mstore(0x24, 0) mstore(0x44, address()) mstore8(0x64, 0) let result := call(gas(), pair, callvalue(), 0, 0x84, 0, 0) returndatacopy(0, 0, returndatasize()) switch result case 0 {revert(0, returndatasize())} default {return (0, returndatasize())} } }
Note that in addition to setting the length of the data parameter to zero, this code also includes some additional changes to handle the result of the call to the swap function, and to copy the return data back to the caller.

from v2-core.

much appreciated, this works!

from v2-core.