Why EVM does CALLDATALOAD and MSTORE an array parameter and its elements individually instead of CALLDATACOPY all of it?

Question

I debugged a simple function execution on Remix:

  function megaTest(uint256[] memory p) external pure returns(uint256[] memory r) {}

What I noticed is that CALLDATALOAD and MSTORE were used to copy calldata to memory.

1. Is CALLDATACOPY a set of CALLDATALOAD AND MSTORE of 32bytes until end is reached ?
2. Why not use CALLDATACOPY ?

Furthermore, I had the following calldata when I called it with params [1,2,3]:

0x5308ae31
0000000000000000000000000000000000000000000000000000000000000020
0000000000000000000000000000000000000000000000000000000000000003
0000000000000000000000000000000000000000000000000000000000000001
0000000000000000000000000000000000000000000000000000000000000002
0000000000000000000000000000000000000000000000000000000000000003

1. What is the 1st word (0x20) representing after 4-bytes of selector ? Is it pointer ?
2. After selector's 4-bytes, isn't 2nd word which represents length of array to be copied sufficient for EVM ? Can't the EVM interpret what is to be expected in the calldata from selector matching ?

Answer 1

• Represents pointer. Pointing is absolute not relative to the slot itself. Absolute indexing starts after the first 4 bytes selector.
• Not sufficient. Arguments will consume their slots as how they are defined. Dynamic types will only point to a location after the arguments in memory where length and actual elements are stored.