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enter only via jumpsub #2646

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merged 1 commit into from May 15, 2020
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enter only via jumpsub #2646

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14 changes: 7 additions & 7 deletions EIPS/eip-2315.md
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Expand Up @@ -27,7 +27,7 @@ We introduce one more stack into the EVM, called `return_stack`. The `return_sta

##### `BEGINSUB`

Marks the entry point to a subroutine.
Marks the entry point to a subroutine. Subroutines can only be entered via `JUMPSUB`. Execution of BEGINSUB causes an exception (OOG: all gas consumed) and terminates execution.

pops: `0`
pushes: `0`
Expand All @@ -38,15 +38,13 @@ pushes: `0`
- 1.1 If the opcode at `location` is not a `BEGINSUB`, abort with error.
2. Pushes the current `pc+1` to the `return_stack`. (See Note 1 below)
- 2.1 If the `return_stack` already has `1023` items, abort with error.
3. Sets the `pc` to `location`.
3. Sets the `pc` to `location + 1`.

**Note 1:** If the resulting `pc` is beyond the last instruction then the opcode is implicitly a `STOP`, which is not an error.

pops: `1`
pushes: `0` (`return_stack` pushes: `1`)

**Note 1:** The description above lays out the _semantics_ of the `JUMPSUB`. 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.

##### `RETURNSUB`

1. Pops `1` value form the `return_stack`.
Expand All @@ -59,6 +57,8 @@ pushes: `0`
**Note 2:** Values popped from `return_stack` do not need to be validated, since they cannot be set arbitrarily from code, only implicitly by the evm.
**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`. 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

This is the smallest possible change that provides native subroutines without breaking backwards compatibility.
Expand Down Expand Up @@ -259,4 +259,4 @@ MULTIPLY:
returnsub
```

**Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).**
**Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).**