DCIPs/EIPS/eip-6206.md

eip	title	description	author	discussions-to	status	type	category	created	requires
6206	EOF - JUMPF instruction	Introduces instruction for chaining function calls.	Andrei Maiboroda (@gumb0), Alex Beregszaszi (@axic), Paweł Bylica (@chfast), Matt Garnett (@lightclient)	https://ethereum-magicians.org/t/eip-4750-eof-functions/8195	Draft	Standards Track	Core	2022-12-21	4750, 5450

Abstract

This EIP allows for tail call optimizations in EOF functions (EIP-4750) by introducing a new instruction JUMPF, which jumps to a code section without adding a new return stack frame.

Motivation

It is common for functions to make a call at the end of the routine only to then return. JUMPF optimizes this behavior by changing code sections without needing to update the return stack.

Specification

A new instruction, JUMPF (0xb2), is introduced.

Execution Semantics

1. JUMPF has one immediate argument, code_section_index, encoded as a 16-bit unsigned big-endian value.
2. If the operand stack size exceeds 1024 - type[code_section_index].max_stack_height (i.e. if the called function may exceed the global stack height limit), execution results in an exceptional halt. This guarantees that the stack height after the call is within the limits.
3. JUMPF costs 5 gas.
4. JUMPF neither pops nor pushes anything to the operand stack.

Code Validation

Let the definition of type[i] be inherited from EIP-4750 and define stack_height to be the height of the stack at a certain instruction during the instruction flow traversal if the operand stack at the start of the function were equal to type[i].inputs.

• The immediate argument of JUMPF MUST be less than the total number of code sections.
• For each JUMPF instruction type[current_section_index].outputs MUST be greater or equal type[code_section_index].outputs.
• The stack height at JUMPF MUST be equal to type[current_section_index].outputs + type[code_section_index].inputs - type[code_section_index].outputs. This means that code_section_index can output less stack elements than the original code section called by the top element on the return stack, if the current_section_index code section leaves the delta type[current_section_index].outputs - type[code_section_index].outputs element(s) on the stack.
• The code validation defined in EIP-4200 also fails if any RJUMP* offset points to one of the two bytes directly following a JUMPF instruction.

Rationale

Allowing JUMPF to section with less outputs

As long as JUMPF prepares the delta type[current_section_index].outputs - type[code_section_index].outputs stack elements before changing code sections, it is possible to jump to a section with less outputs than was originally entered via CALLF. This will reduce duplicated code as it will allow compilers more flexibility during code generation such that certain helpers can be used generically by functions, regardless of their output values.

Backwards Compatibility

This change is backward compatible as EOF does not allow undefined instructions to be used or deployed, meaning no contracts will be affected.

Security Considerations

Needs discussion.

Copyright

Copyright and related rights waived via CC0.