How do RETURN and MLOAD work for high offsets?

Question

When trying to load the 32 bytes of memory that follow a high offset (say 0x12345678), the RETURN and MLOAD opcodes give an out of gas error. Why is that ?

Example

PUSH1 0x20
PUSH4 0x12345678
RETURN

I would expect the result to be 0x0000...00 since the memory isn't initialized there but it appears that another operation is ran, which costs gas.

What exactly is going on here ?

Answer 1

Your RETURN code needs to access a very high memory index (very high in the context of Ethereum contracts), which costs too much gas. The higher the memory cell that's accessed, the more gas it costs: https://ethereum.stackexchange.com/a/62732/7598