What is the best practice for memory allocation in EVM assembly?

Question

As far as I know, this is the standard way of the memory allocation in assembly:

assembly {
    let firstFreeMemoryPointer := mload(0x40)
    mstore(0x40, add(firstFreeMemoryPointer, numberOfBytesToAllocate)
}

But I've recently noticed the following example code in the Yul docs:

function allocate(size) -> ptr {
    ptr := mload(0x40)
    if iszero(ptr) { ptr := 0x60 }
    mstore(0x40, add(ptr, size))
}

Does anyone know if this step with checking ptr for zero value (if iszero(ptr) { ptr := 0x60 }) is required and when can this pointer actually be zero? :)

Answer 1

Their code wants probably to keep the first two slots as scratch space, so they start the free memory at 0x60.

ptr := mload(0x40) is 0 the first time allocate is called. So without this line

if iszero(ptr) { ptr := 0x60 }

the code will write an object from the very first slot, overwriting the scratch space (and the free memory slot 0x40).

So this line is required only if you're writing a pure Yul object and not inline-assembly, because, as Ismael said, solidity already moves the free memory pointer before any other logic.

---

Note that to make this code Solidity-friendly, 0x60 should be 0x80. Check these commits:

https://github.com/ethereum/solidity/commit/6439955e834b1a1b6ae5f4a9aba857e04e4fefea

https://github.com/ethereum/solidity/commit/60d513c484c332dc816dad5ba17bf5c4da207e6f