Rename invalid to verbatim

[axic]

(invalid #xaabbccff) - sequence of bytes that represent a sequence of arbitrary invalid instructions

This may be confused with the invalid instruction. Solidity/Yul has a verbatim instruction, which may be a good naming in this case as well, i.e (verbatim #5c0001) to encode RJUMP 1 (EIP-4200.

Btw, does the literal has any length limitation?

[dvush]

Hi, thanks for the comment.

1. In the current form, invalid works as a verbatim, and it does not have any length limitation - its just any blob of bytes.
2. I often refer to any instruction that is not defined as invalid, but you are right - special 'invalid' opcode may be different because users will always be sure that it is 'invalid' unlike generic 'verbatim' sequence that can become valid after update.
3. When I thought about naming it invalid, I was thinking about an experience that I had when I tried symbolical execution of code. I decided to lump all instruction after 'undefined' one but before valid jumpdest into one blob because it makes reading asm easier and bytecode inside this blob is unreachable. Also, even valid jumpdest can be lumped together with 'invalid' blob if it immediately followed by it - because jump to that location will fail anyway. ecfg panics on certain contracts quilt/etk#99 (comment)

Currently, 'invalid' does not have checks described above (i.e. we can write (invalid #x00) and it's the same as stop, but maybe it makes sense to be implemented: asm may check if invalid blocks inside are indeed unexecutable.

But now that I think about it – checking that (invalid ..) is indeed unexecutable while writing code may hurt someone because some compact data (i.e. jump table) can have executable parts (like accidental jumpdest stop) but on the other hand I can imagine someone using 'invalid' to have extra protection against suddenly making data executable.

In the spirit of lisp the good solution would be to give user (for this particular project it's probably just me) helper function that can check if some verbatim code is indeed executable data.

Anyway, you have a good point, I'll think about it!

[dvush]

renamed, why not