Update EIP-4200: Allow RJUMPV table sizes of up to 256

[Philogy]

Since RJUMPV table sizes of 0 are disallowed I'm suggesting a change that would make table sizes start at 1, meaning a count byte of 0 would indicate a table of size 1 and a count byte 0xff would indicate a table size of 256. This would remove an unnecessary piece of validation logic and allow tables that can map the full range of a 4-bit case value without 0xff resulting in the default path.

[eth-bot]

✅ All reviewers have approved.

[gumb0]

Originally we argued against this idea to avoid the spec stating "number count really means count + 1", which seemed quirky.

But we could also reframe it to mean max index of the table.

[chfast]

I think this is a good change. So far I was mostly focused on the encoding but realized having 256 element jump table should be very useful for implementing any bytecode interpreter.

[shemnon]

Perhaps instead of size it becomes "max index" - so jumpv of 1 has two indexes, 0 and 1.

Also, by that logic what's the value of jumpv 1, shouldn't 2 be the minimum? (please no, accept the inconsistency).

The big advantage I see is that we can do a full byte mapping to do things such as decoding large jump tables, with a full byte. Although nybbles (4-bits) will probably be more useful for sparser tables.

While we could do this pre-change it would require fallthrough handling of 0xFF.

[Philogy]

Originally we argued against this idea to avoid the spec stating "number count really means count + 1", which seemed quirky.

But we could also reframe it to mean max index of the table.

I agree that this definition can be confusing although I don't think that's a good enough reason not to include this change. I also think putting it in terms of "the byte represents the max index" is a good idea.

[axic]

I agree with the change.

[gumb0]

Some more fixes needed:

Specification, line 50 should say:

3. `RJUMPV max_index relative_offset+` pops a value (`case`) from the stack, and sets the `PC` to `PC_post_instruction + ((case > max_index) ? 0 : relative_offset[case])`.

Test cases, Invalid cases, remove line 138:

- `RJUMPV` with table size 0

Test cases, Execution, line 149 should say:

- `RJUMPV 0 relative_offset`

Line 155:

- `case` outside of table bounds (`case > max_index`, fallback case)

[lightclient]

This makes sense to me.

[lightclient]

bumping @gumb0 @axic @chfast

[eth-bot]

All Reviewers Have Approved; Performing Automatic Merge...