EIP-6206: EOF - JUMPF and non-returning functions Ethereum Improvement Proposals AllCoreNetworkingInterfaceERCMetaInformational ⚠️ Draft Standards Track: Core EIP-6206: EOF - JUMPF and non-returning functions Introduces instruction for chaining function calls. Authors Andrei Maiboroda (@gumb0), Alex Beregszaszi (@axic), Paweł Bylica (@chfast), Matt Garnett (@lightclient) Created 2022-12-21 Discussion Link https://ethereum-magicians.org/t/eip-4750-eof-functions/8195 Requires EIP-4750, EIP-5450 Table of Contents Abstract Motivation Specification Type section changes Execution Semantics Code Validation Rationale Allowing JUMPF to section with less outputs Backwards Compatibility Security Considerations Copyright 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. Additionally the format of the type sections is extended to allow declaring sections as non-returning, with simplified stack validation for JUMPF to such section. 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. Knowing at validation time that a function will never return control allows for JUMPF to such function to be treated similar to terminating instructions, where extra items may be left on the operand stack at execution termination. This provides opportunities for compilers to generate more optimal code, both in code size and in spent gas. It is particularly benefitial for small error handling helpers, that end execution with REVERT: they are commonly reused in multiple branches and extracting them into a helper function is efficient, when there is no need to pop extra stack items before JUMPF to such helper. Specification Type section changes We define non-returning section as the one that can not return control (via RETF instruction) to the caller section. Type section outputs field contains a special value 0x80 when corresponding code section is non-returning. See Non-returning status validation below for validation details. The first code section MUST have 0 inputs and be non-returning. Execution Semantics A new instruction, JUMPF (0xe5), is introduced. JUMPF has one immediate argument, target_section_index, encoded as a 16-bit unsigned big-endian value. If the operand stack size exceeds 1024 - type[target_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 target function does not exceed global stack height limit. JUMPF sets current_section_index to target_section_index and PC to 0, but does not change the return stack. Execution continues in the target section. JUMPF costs 5 gas. 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: either type[current_section_index].outputs MUST be greater or equal type[target_section_index].outputs, or type[target_section_index].outputs MUST be 0x80 The stack height validation at JUMPF depends on whether the target section is non-returning: JUMPF into returning section (type[target_section_index].outputs does not equal 0x80): stack height MUST be equal to type[current_section_index].outputs + type[target_section_index].inputs - type[target_section_index].outputs. This means that target section can output less stack elements than the original code section called by the top element on the return stack, if the current code section leaves the delta type[current_section_index].outputs - type[target_section_index].outputs element(s) on the stack. JUMPF into non-returning section (type[target_section_index].outputs equals 0x80): stack height must be greater or equal than type[target_section_index].inputs. JUMPF is considered terminating instruction, i.e. does not have successor instructions in code validation and MAY be final instruction in the section. 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. CALLF instruction validation is extended to include the rule: Code section is invalid in case an immediate argument target_section_index of any CALLF targets a non-returning section, i.e. type[target_section_index equals 0x80. Non-returning status validation Section type MUST be non-returning in case the section contains no RETF instructions and no JUMPF instructions targeting returning sections (target section’s status is checked via its output value in type section.) Note: This implies that section containing only JUMPFs into non-returning sections is non-returning itself. Rationale Allowing JUMPF to section with less outputs As long as JUMPF prepares the delta type[current_section_index].outputs - type[target_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. Citation Please cite this document as: Andrei Maiboroda (@gumb0), Alex Beregszaszi (@axic), Paweł Bylica (@chfast), Matt Garnett (@lightclient), "EIP-6206: EOF - JUMPF and non-returning functions [DRAFT]," Ethereum Improvement Proposals, no. 6206, December 2022. [Online serial]. Available: https://eips.ethereum.org/EIPS/eip-6206. Ethereum Improvement Proposals Ethereum Improvement Proposals ethereum/EIPs Ethereum Improvement Proposals (EIPs) describe standards for the Ethereum platform, including core protocol specifications, client APIs, and contract standards.