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88 lines
2.6 KiB
Markdown
88 lines
2.6 KiB
Markdown
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---
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eip: 2315
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title: Simple Subroutines for the EVM
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status: Draft
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type: Standards Track
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category: Core
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author: Greg Colvin
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discussions-to: https://ethereum-magicians.org/t/eip-2315-simple-subroutines-for-the-evm/3941
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created: 2019-10-17
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---
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## Abstract
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This proposal introduces two opcodes to support subroutines: `JUMPSUB` and `RETSUB`.
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## Motivation
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The EVM does not provide subroutines as a primitive. Instead, calls must be synthesized by fetching and pushing the current program counter on the data stack and jumping to the subroutine address; returns must be synthesized by getting the return address to the top of stack and jumping back to it.
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## Specification
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##### `JUMPSUB`
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Jumps to the address on top of the stack, which must be the offset of a `JUMPDEST`.
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##### `RETSUB`
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Returns to the instruction after the most recently executed `JUMPSUB` instruction, which must be a `JUMPDEST`
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A program may jump at most 1024 times without returning.
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## Rationale
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This is the smallest possible change that provides native subroutines without breaking backwards compatibility.
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## Backwards Compatibility
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These changes do not affect the semantics of existing EVM code.
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## Test Cases
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```
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step op stack
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0 PUSH1 3 []
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1 JUMPSUB [3]
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4 JUMPDEST []
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5 STOP []
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2 JUMPDEST []
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3 RETSUB []
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```
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This code should terminate after 5 steps with an empty stack.
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## Implementations
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No clients have implemented this proposal as of yet.
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The new operators proposed here are implemented by the following pseudocode, which adds cases for `JUMPSUB` and `RETSUB` to a simple loop-and-switch interpreter.
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```
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bytecode[code_size]
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data_stack[1024]
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return_stack[1024]
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push(return_stack, PC)
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PC = 0
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while PC < code_size {
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opcode = bytecode[PC]
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...
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switch opcode {
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...
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case JUMPSUB:
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push(return_stack, PC + 1)
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PC = pop(data_stack)
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continue
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case RETSUB:
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PC = pop(return_stack)
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continue
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}
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++PC
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}
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```
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Execution of EVM bytecode begins with one value on the return stack—the size of the bytecode. The virtual byte of 0 at this offset is the EVM `STOP` opcode, so executing a `RETSUB` with no prior `JUMPSUB` executes a `STOP`. A `STOP` or `RETURN` ends the execution of the subroutine and the program.
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We suggest the cost of `JUMPSUB` should be _low_, and `RETSUB` should be _verylow_.
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Measurement will tell. We suggest the following opcodes:
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```
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0xbe JUMPSUB
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0xbf RETSUB
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```
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## Security Considerations
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This proposal introduces no new security considerations to the EVM.
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**Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).**
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