This proposal introduces three opcodes to support subroutines: `BEGINSUB`, `JUMPSUB` and `RETURNSUB`. (The smallest possible change would do without `BEGINSUB`).
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 getting the return address to the top of the stack and jumping back to it.
Facilities to directly support subroutines are provided in some form by most physical and virtual machines going back at least fifty years. In whatever form, these operations provide for capturing the current context of execution, transferring control to a new context, and returning to the original context.
We propose a simple _return-stack_ mechanism, known to work well for stack machines, which we specify here. Note that this specification is entirely semantic. It constrains only stack usage and control flow and imposes no syntax on code beyond being a sequence of bytes to be executed.
In the future, amenability to static analysis equivalent to [EIP-615](https://eips.ethereum.org/EIPS/eip-615) could be ensured by enforcing a few simple rules, and validated with the provided algorithm, still without imposing syntactic constraints.
We introduce one more stack into the EVM in addition to the existing `data stack` which we call the `return stack`. The `return stack` is limited to `1024` items.
_Note 3: The description above lays out the semantics of this feature in terms of a `return stack`. But the actual state of the `return stack` is not observable by EVM code or consensus-critical to the protocol. (For example, a node implementor may code `JUMPSUB` to unobservably push `pc` on the `return stack` rather than `pc + 1`, which is allowed so long as `RETURNSUB` observably returns control to the `pc + 1` location.)_
If [EIP-2327: BEGINDATA](https://eips.ethereum.org/EIPS/eip-2327) or similar is implemented then the indirect jumps from [EIP-615](https://eips.ethereum.org/EIPS/eip-615) -- `JUMPV` and `JUMPSUBV` -- can be implemented. These could take two arguments on the stack: a constant offset relative to `BEGINDATA` to a jump table, and a variable index into that table.
We modeled this design on the simple, proven, archetypal Forth virtual machine of 1970. It is a two-stack design -- the data stack is supplemented with a return stack to support jumping into and returning from subroutines, as specified above. The separate return stack ensures that the return address cannot be overwritten or mislaid, and obviates any need to swap the return address past the arguments on the stack. Importantly, a dynamic jump is not needed to implement subroutine returns, allowing for deprecation of dynamic uses of JUMP and JUMPI. Eventually deprecating dynamic jumps is key to practical static analysis of code.
These changes are compatible with using [EIP-3337](https://eips.ethereum.org/EIPS/eip-3337) to provide stack frames, by associating a frame with each subroutine.
- This is the same as `JUMPI`, and `2` more than `JUMP`.
-`RETURNSUB` be _low_ (`5`).
Benchmarking might be needed to tell if the costs are well-balanced.
We suggest the following opcodes:
```
0x5c BEGINSUB
0x5d RETURNSUB
0x5e JUMPSUB
```
## 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).
The safety and amenability to static analysis of valid programs can be made comparable to [EIP-615](https://eips.ethereum.org/EIPS/eip-615), but without imposing syntactic constraints, and thus with minimal impact on low-level optimizations. Validity can ensured by following the rules given in the next section, and programs can be validated with the provided algorithm. The validation algorithm is simple and bounded by the size of the code, allowing for validation at deploy time or at load time.
While it is crucial going forward that it be possible to validate programs, this EIP does propose that validity be enforced. Note that much value for people doing static analysis (e.g. for proofs that bytecode meets formal specifications of a contract) can be had without enforcement. Code can be scanned in linear time to ensure that the rules are or are not followed before analysis begins. And compilers can easily follow the rules up front.
_Execution_ is as defined in the [Yellow Paper](https://ethereum.github.io/yellowpaper/paper.pdf)—a sequence of changes in the EVM state. The conditions on valid code are preserved by state changes. At runtime, if execution of an instruction would violate a condition the execution is in an exceptional halting state. The Yellow Paper defines five such states.
We would like to consider EVM code valid iff no execution of the program can lead to an exceptional halting state, but we must be able to validate code in linear time to avoid denial of service attacks. So in practice, we can only partially meet these requirements. Our validation algorithm does not consider the code’s data and computations, only its control flow and stack use. This means we will reject programs with any invalid code paths, even if those paths are not reachable at runtime. Further, conditions 1 and 2 —Insufficient gas and stack overflow—must in general be checked at runtime. Conditions 3, 4, and 5 cannot occur if the code conforms to the following rules.
Rule 3, requiring a `PUSH` before each `JUMP*` would forbid dynamic jumps. Absent dynamic jumps another mechanism is needed for subroutine returns, as provided here.
For rules 4 and 5 we need to define `stack depth`. The Yellow Paper has the `stack pointer` or `SP` pointing just past the top item on the `data stack`. We define the `stack base` as where the `SP` pointed at the most recent `JUMPSUB`, or `0` on program entry. So we can define the `stack depth` as the number of stack elements between the current `SP` and the current `stack base`.
Given our definition of `stack depth` Rule 4 ensures that control flows which return to the same place with a different `stack depth` are invalid. These can be caused by irreducible paths like jumping into loops and subroutines, and calling subroutines with different numbers of arguments. Taken together, these rules allow for code to be validated by following the control-flow graph, traversing each edge only once.
The following is a pseudo-Go specification of an algorithm for enforcing program validity. It recursively traverses the bytecode, following its control flow and stack use and checking for violations of the rules above. (For simplicity we ignore the issue of JUMPDEST or BEGINSUB bytes in PUSH data.) It runs in time == O(vertices + edges) in the program's control-flow graph, where vertices represent control-flow instructions and the edges represent basic blocks.
In this example. the JUMPSUB is on the last byte of code. When the subroutine returns, it should hit the 'virtual stop' _after_ the bytecode, and not exit with error
Gavin Wood, [Ethereum: A Secure Decentralized Generalized Transaction Ledger](https://ethereum.github.io/yellowpaper/paper.pdf), 2014-2021
Greg Colvin, Brooklyn Zelenka, Paweł Bylica, Christian Reitwiessner, [EIP-615: Subroutines and Static Jumps for the EVM](https://eips.ethereum.org/EIPS/eip-615), 2016-2019
