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Automatically merged updates to draft EIP(s) 2315 (#2656)

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Hi, I'm a bot! This change was automatically merged because:

 - It only modifies existing Draft or Last Call EIP(s)
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Martin Holst Swende 2020-05-19 15:42:07 +02:00 committed by GitHub
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EIPS/eip-2315.md
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@ -60,10 +60,8 @@ pushes: `0`
**Note 3:** A value popped from `return_stack` _may_ be outside of the code length if the last `JUMPSUB` was the last byte of the `code`. In this case the next opcode is implicitly a `STOP`, which is not an error.
**Note 4:** The description above lays out the _semantics_ of this feature in terms of a `return_stack`. But the actual state of the `return_stack` is not observable to EVM code or consensus-critical to the protocol.
> For example, a node implementor may code `JUMPSUB` to unobservably push `pc` on the `return_stack` rather than `pc+1`, which is allowed so long as `RETURNSUB` observably returns control to the `pc+1` location.
**Note 4:** The description above lays out the _semantics_ of this feature in terms of a `return_stack`. But the actual state of the `return_stack` is not observable to EVM code or consensus-critical to the protocol. For example, a node implementor may code `JUMPSUB` to unobservably push `pc` on the `return_stack` rather than `pc+1`, which is allowed so long as `RETURNSUB` observably returns control to the `pc+1` location.
**Note 4:** The description above lays out the _semantics_ of this feature in terms of a `return_stack`. It's up to node implementations to decide the internal representation. For example, a node may decide to place `PC` on the `return_stack` at `JUMPSUB`, as long as the `RETURNSUB` correctly returns to the `PC+1` location. The internals of the `return_stack` is not one of the "observable"/consensus-critical parts of the EVM.
## Rationale
@ -81,7 +79,6 @@ This should jump into a subroutine, back out and stop.
Bytecode: `0x6004b300b2b7`
| Pc | Op | Cost | Stack | RStack |
|-------|-------------|------|-----------|-----------|
| 0 | PUSH1 | 3 | [] | [] |
@ -104,6 +101,7 @@ Bytecode: `0x6800000000000000000cb300b26011b3b7b2b7`
| 16 | RETURNSUB | 2 | [] | [10] |
| 11 | STOP | 0 | [] | [] |
Consumed gas: `26`
### Failure 1: invalid jump
@ -149,11 +147,23 @@ Bytecode: `0x600556b2b75b6003b3`
| 5 | JUMPDEST | 1 | [] | [] |
| 6 | PUSH1 | 3 | [] | [] |
| 8 | JUMPSUB | 8 | [3] | [] |
| 3 | BEGINSUB | 1 | [] | [ 8] |
| 4 | RETURNSUB | 2 | [] | [ 8] |
| 9 | STOP | 0 | [] | [] |
Consumed gas: `26`
Consumed gas: `25`
### Error on walk-into-subroutine
In this example, the code 'walks' into a subroutine, which is not allowed, and causes an error
| Pc | Op | Cost | Stack | RStack |
|-------|-------------|------|-----------|-----------|
| 0 | BEGINSUB | 1 | [] | [] |
```
Error: at pc=0, op=BEGINSUB: invalid subroutine entry
```
## Implementations
@ -167,7 +177,7 @@ The changes for the current version are trivial.
### Costs and Codes
We suggest that the cost of `JUMPSUB` be _low_, and `RETURNSUB` be _verylow_.
We suggest that the cost of `BEGINSUB` be _base_, `JUMPSUB` be _low_, and `RETURNSUB` be _verylow_.
Measurement will tell. We suggest the following opcodes:
```
@ -175,6 +185,9 @@ We suggest that the cost of `JUMPSUB` be _low_, and `RETURNSUB` be _verylow_.
0xb3 JUMPSUB
0xb7 RETURNSUB
```
**Note 5**: Although specified at _base_, the cost of `BEGINSUB` does not matter in practice, since `BEGINSUB` never executes without error.
## Security Considerations
These changes do introduce new flow control instructions, so any software which does static/dynamic analysis of evm-code