EIPs/EIPS/eip-2315.md

---
eip: 2315
title: Simple Subroutines for the EVM
status: Draft
type: Standards Track
category: Core
author: Greg Colvin (greg@colvin.org), Martin Holst Swende (@holiman)
discussions-to: https://ethereum-magicians.org/t/eip-2315-simple-subroutines-for-the-evm/3941
created: 2019-10-17
---

## Abstract

This proposal introduces three opcodes to support subroutines:  `BEGINSUB`, `JUMPSUB` and `RETURNSUB`.

## Motivation

The EVM does not provide subroutines as a primitive.  Instead, calls can be synthesized by fetching and pushing the current program counter on the data stack and jumping to the subroutine address; returns can be synthesized by contriving to get the return address back to the top of stack and jumping back to it.  Complex calling conventions are then needed to use the same stack for computation and control flow.  Code becomes harder to read and write, and tools may need to pattern-match the conventions to identify the use of subroutines.  Complex calling conventions like these can be avoided using memory, but regardless, it costs a lot of gas.

Having opcodes to directly support subroutines can eliminate this complexity and cost, just as for other physical and virtual machines going back at least 50 years.

In the Appendix we show example solc output for a simple program that uses over three times as much gas just calling and returning from subroutines as comparable code using these opcodes.

## Specification

We introduce one more stack into the EVM, called `return_stack`. The `return_stack` is limited to `1023` items.

##### `BEGINSUB`

Marks the entry point to a subroutine.

pops: `0`
pushes: `0`

##### `JUMPSUB`

1. Pops `1` value from the `stack`, hereafter referred to as `location`. 
  - 1.1 If the opcode at `location` is not a `BEGINSUB`, abort with error.
2. Pushes the current `pc+1` to the `return_stack`. 
  - 2.1 If the `return_stack` already has `1023` items, abort with error.
3. Sets the `pc` to `location`.

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


##### `RETURNSUB`

1. Pops `1` value form the `return_stack`.
1.1 If the `return_stack` is empty, abort with error
2. Sets `pc` to the popped value

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

**Note:** Values popped from `return_stack` do not need to be validated, since they cannot be set arbitrarily from code, only implicitly by the evm. 
**Note2:** 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.

## Rationale

This is the smallest possible change that provides native subroutines without breaking backwards compatibility.

## Backwards Compatibility

These changes do not affect the semantics of existing EVM code.

## Alternative variants

One possible variant, would be to add an extra clause to the `BEGINSUB` opcode. 

- A `BEGINSUB` opcode may only be reached via a `JUMPSUB`. 

This would make `walking` into a subroutine an error. The rationale for this would be to possibly improve static analysis, being able 
to make stronger assertions about the code flow. 

This is not part of the current specification, since code-generators can trivially implement these guarantees by always prepending `STOP` opcode before
any `BEGINSUB` operation. 

## Test Cases

### Simple routine

This should jump into a subroutine, back out and stop.

Bytecode: `0x6004b300b5b7`


|  Pc   |      Op     | Cost |   Stack   |   RStack  |
|-------|-------------|------|-----------|-----------|
|    0  |      PUSH1  |    3 |        [] |        [] |
|    2  |    JUMPSUB  |    8 |       [4] |        [] |
|    4  |   BEGINSUB  |    1 |        [] |      [ 2] |
|    5  |  RETURNSUB  |    2 |        [] |      [ 2] |
|    3  |       STOP  |    0 |        [] |        [] |

### Two levels of subroutines

This should execute fine, going into one two depths of subroutines
Bytecode: `0x6800000000000000000cb300b56011b3b7b5b7`

|  Pc   |      Op     | Cost |   Stack   |   RStack  |
|-------|-------------|------|-----------|-----------|
|    0  |      PUSH9  |    3 |        [] |        [] |
|   10  |    JUMPSUB  |    8 |      [12] |        [] |
|   12  |   BEGINSUB  |    1 |        [] |      [10] |
|   13  |      PUSH1  |    3 |        [] |      [10] |
|   15  |    JUMPSUB  |    8 |      [17] |      [10] |
|   17  |   BEGINSUB  |    1 |        [] |   [10,15] |
|   18  |  RETURNSUB  |    2 |        [] |   [10,15] |
|   16  |  RETURNSUB  |    2 |        [] |      [10] |
|   11  |       STOP  |    0 |        [] |        [] |


### Failure 1: invalid jump

This should fail, since the given `location` is outside of the code-range. The code is the same as previous example, 
except that the pushed `location` is `0x01000000000000000c` instead of `0x0c`.

Bytecode: `0x6801000000000000000cb300b56011b3b7b5b7`

|  Pc   |      Op     | Cost |   Stack   |   RStack  |
|-------|-------------|------|-----------|-----------|
|    0  |      PUSH9  |    3 |        [] |        [] |
|   10  |    JUMPSUB  |    8 |[18446744073709551628] |        [] |

```
Error: at pc=10, op=JUMPSUB: evm: invalid jump destination
```

### Failure 2: shallow `return_stack`

This should fail at first opcode, due to shallow `return_stack`

Bytecode: `0xb75858` (`RETURNSUB`, `PC`, `PC`)


|  Pc   |      Op     | Cost |   Stack   |   RStack  |
|-------|-------------|------|-----------|-----------|
|    0  |  RETURNSUB  |    2 |        [] |        [] |

```
Error: at pc=0, op=RETURNSUB: evm: invalid retsub
```

## Implementations

No clients have implemented this proposal as of yet, but there are Draft PRs for 

- [evmone](https://github.com/ethereum/evmone/pull/229), and 
- [geth](https://github.com/ethereum/go-ethereum/pull/20619) .


### Costs and Codes

We suggest that the cost of `BEGINSUB` be _base_, `JUMPSUB` be _low_, and `RETURNSUB` be _verylow_. 
 Measurement will tell.  We suggest the following opcodes:

```
0xb2 BEGINSUB
0xb3 JUMPSUB
0xb7 RETURNSUB
```
## Security Considerations

These changes do introduce new flow control instructions, so any software which does static/dynamic analysis of evm-code 
needs to be modified accordingly. The `JUMPSUB` semantics are similar to `JUMP` (but jumping to a `BEGINSUB`), whereas the `RETURNSUB` instruction
is different, since it can 'land' on any opcode (but the possible destinations can be statically inferred).


## Appendix: Comparative costs.

```
contract fun {
    function test(uint x, uint y) public returns (uint) {
        return test_mul(2,3);
    }
    function test_mul(uint x, uint y) public returns (uint) {
        return multiply(x,y);
    }
    function multiply(uint x, uint y) public returns (uint) {
        return x * y;
    }
}

```
Here is solc 0.6.3 assembly code with labeled destinations.
```
TEST:
     jumpdest
     0x00
     RTN
     0x02
     0x03
     TEST_MUL
     jump
TEST_MUL:
     jumpdest
     0x00
     RTN
     dup4
     dup4
     MULTIPLY
     jump
RTN:
     jumpdest
     swap4
     swap3
     pop
     pop
     pop
     jump
MULTIPLY:   
     jumpdest
     mul
     swap1
     jump
```
solc does a good job with the multiply() function, which is a leaf.  Non-leaf functions are more awkward to get out of.  Calling `fun.test()` will cost _118 gas_, plus 5 for the `mul`.

  This is the same code written using `jumpsub` and `returnsub`.  Calling `fun.test()` will cost _34 gas_ (plus 5).
```
TEST:
     beginsub
     0x02
     0x03
     TEST_MUL
     jumpsub
     returnsub
TEST_MUL:
     beginsub
     MULTIPLY
     jumpsub
     returnsub
MULTIPLY:
     beginsub
     mul
     returnsub
```

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