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Update EIP-4200: Allow RJUMPV table sizes of up to 256 #6546

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Merged
eth-bot merged 4 commits into from
May 11, 2023
Merged

Update EIP-4200: Allow RJUMPV table sizes of up to 256 #6546

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0b0e6f6
Update EIP-4200: Allow `RJUMPV` table sizes of up to 256
Philogy Feb 22, 2023
b2a29c2
Update EIPS/eip-4200.md
Philogy Mar 7, 2023
ffa1f07
Update EIP-4200
Philogy Mar 7, 2023
e8f0def
Update EIPS/eip-4200.md
Philogy Mar 8, 2023
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15 changes: 6 additions & 9 deletions EIPS/eip-4200.md
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Original file line number Diff line number Diff line change
Expand Up @@ -47,11 +47,11 @@ If the code is valid EOF1:

1. `RJUMP relative_offset` sets the `PC` to `PC_post_instruction + relative_offset`.
2. `RJUMPI relative_offset` pops a value (`condition`) from the stack, and sets the `PC` to `PC_post_instruction + ((condition == 0) ? 0 : relative_offset)`.
3. `RJUMPV count relative_offset+` pops a value (`case`) from the stack, and sets the `PC` to `PC_post_instruction + ((case >= count) ? 0 : relative_offset[case])`.
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])`.

The immediate argument `relative_offset` is encoded as a 16-bit **signed** (two's-complement) big-endian value. Under `PC_post_instruction` we mean the `PC` position after the entire immediate value.

The immediate encoding of `RJUMPV` is more special: the 8-bit `count` value determines the number of `relative_offset` values following. Validation algorithm of [EIP-3670](./eip-3670.md) is extended to verify that `count >= 1`. The encoding of `RJUMPV` must have at least one `relative_offset` and thus it will take at minimum 4 bytes. Furthermore, the `case >= count` condition falling through means that in many use cases one would place the *default* path following the `RJUMPV` instruction. An interesting feature is that `RJUMPV 1 relative_offset` is an inverted-`RJUMPI`, which can be used in many cases instead of `ISZERO RJUMPI relative_offset`.
The immediate encoding of `RJUMPV` is more special: the unsigned 8-bit `max_index` value determines the maximum index in the jump table. The number of `relative_offset` values following is `max_index+1`. This allows table sizes up to 256. The encoding of `RJUMPV` must have at least one `relative_offset` and thus it will take at minimum 4 bytes. Furthermore, the `case > max_index` condition falling through means that in many use cases, one would place the *default* path following the `RJUMPV` instruction. An interesting feature is that `RJUMPV 0 relative_offset` is an inverted-`RJUMPI`, which can be used in many cases instead of `ISZERO RJUMPI relative_offset`.

We also extend the validation algorithm of [EIP-3670](./eip-3670.md) to verify that each `RJUMP`/`RJUMPI`/`RJUMPV` has a `relative_offset` pointing to an instruction. This means it cannot point to an immediate data of `PUSHn`/`RJUMP`/`RJUMPI`/`RJUMPV`. It cannot point outside of code bounds. It is allowed to point to a `JUMPDEST`, but is not required to.

Expand Down Expand Up @@ -126,7 +126,7 @@ This change poses no risk to backwards compatibility, as it is introduced at the
- `relative_offset` is positive/negative/`0`
- `RJUMP`/`RJUMPI`/`RJUMPV` with instruction other than `JUMPDEST` as target
- `relative_offset` is positive/negative/`0`
- `RJUMPV` with various valid table sizes from 1 to 255
- `RJUMPV` with various valid table sizes from 1 to 256

#### Invalid cases

Expand All @@ -135,7 +135,6 @@ This change poses no risk to backwards compatibility, as it is introduced at the
- `RJUMP`/`RJUMPI`/`RJUMPV` target outside of code section bounds
- `RJUMP`/`RJUMPI`/`RJUMPV` target push data
- `RJUMP`/`RJUMPI`/`RJUMPV` target another `RJUMP`/`RJUMPI`/`RJUMPV` immediate argument
- `RJUMPV` with table size 0

### Execution

Expand All @@ -146,13 +145,13 @@ This change poses no risk to backwards compatibility, as it is introduced at the
- `relative_offset` is positive/negative/`0`
- `condition` equals `0`
- `condition` does not equal `0`
- `RJUMPV 1 relative_offset`
- `RJUMPV 0 relative_offset`
- `case` equals `0`
- `case` does not equal `0`
- `RJUMPV` with table containing positive, negative, `0` offsets
- `case` equals `0`
- `case` does not equal `0`
- `case` outside of table bounds (`case >= count`, fallback case)
- `case` outside of table bounds (`case > max_index`, fallback case)
- `case` > 255

## Reference Implementation
Expand Down Expand Up @@ -216,9 +215,7 @@ def validate_code(code: bytes):
elif opcode == 0x5e:
if pos + 1 > len(code):
raise ValidationException("truncated jump table")
jump_table_size = code[pos]
if jump_table_size == 0:
raise ValidationException("empty jump table")
jump_table_size = code[pos] + 1

pc_post_instruction = pos + 1 + 2 * jump_table_size
if pc_post_instruction > len(code):
Expand Down