evm interface overhaul

[debris]

Current evm interface has been great for us, but as the ethereum virtual machine gets more complicated, we need to revisit how it's done. We need to introduce changes which improve code readability and not decrease the performance.

assumptions:

• evm should never do recursive calls
• executive should not use crossbeam to reset the stack
• we should get rid of Externalities interface
• evm should be executed in a loop. the result of the execution should contain state changes, information if execution is done and what should be executed in the next iteration of the loop
• evm should not use callbacks
• executive public interface should not be changed
• builtins should be separated from executive

I think we could target 1.10 with those changes, so 3 months from now.

cc @paritytech/core-devs

[tomusdrw]

Is it also possible to clearly separate builtins from executive?

[debris]

@tomusdrw I've added it to the list

[pepyakin]

I thought that these wasm vm use-cases might be useful to keep in mind if and when performing the overhaul:

For now, the wasm engine performs validation of the wasm code each time before executing a contract. The validation run time depends on the code size. Since it depends only on a module itself and nothing else (i.e pure), it doesn't make sense to perform validation each times. It's sufficient to validate once and cache the result of the validation somewhere. Basically it's just a tri-state: not validated, invalid and valid.

Apart from that, there is an instrumentation (such as gas metering) of the wasm code that is also performed on each run. It's not so pure as validation, since it might depend on the exact algorithms used (i.e you can do metering by counting each instruction or by summing all instructions in a block. Same result but different algos), but IMO still something worthwhile caching.

And, of course, wasm→native compilation: recompiling each time before the execution might be a no-go and definitely needs some sort of caching.

So this boils down to the need of caching some data by the vm: ranging from a single flag to a big byte blob.

[sorpaas]

I would like to take this refactoring, if that's okay.

I think I'll start with "evm should be executed in a loop"/"executive should not use crossbeam to reset the stack" refactoring -- changing CALL/CREATE to trap execution, loop to the next callstack item, and then feed result back in. We probably won't gain any memory improvement in this process, because it's basically just moving callstack from program stack to heap. But after that, we can introduce a "runtime" notion that is global to all current callstack items. This allows us to optimize RETURNDATA buffer to avoid allocating additional buffers per callstack.

Regarding "the result of the execution should contain state changes" -- if we don't write state changes directly to the merkle trie but to a hashmap cache, it gives a significant advantage for parallel transaction execution even without ethereum/EIPs#648. But this will introduce a little bit memory overhead.

(And full disclaimer, the above design is similar to what we have in SputnikVM. 😄)

I'm not sure how we can get rid of Externalities interface. @debris @tomusdrw Would it be possible to provide more backgrounds on that?

[sorpaas]

Another refactoring that might be beneficial is to use c style enum #[repr(u8)] for instruction instead of a raw u8. This allows us to type-check match branches.

[sorpaas]

@pepyakin mentioned an issue that if we eliminate recursive calls for vm, it might break some of the wasm usage:

1. To eliminate recursive calls (and get rid of crossbeam), we would need the vm to be able to "pause" execution, during which it execute subsequent calls and feed result back in.
2. Currently, we cannot pause execution of wasm. This can be implemented in wasmi (interpreter pauses wasmi-labs/wasmi#85). If we do that, our current implementation will work fine.
3. However, the issue is that in the future, we may want to switch from using a wasm interpreter to a wasm compiler. The compiler might not support pausing execution. So if we eliminate recursive calls, it might not be "future-proof" -- we may make it really hard when doing the wasm interpreter-to-compiler migration. This might not be good.
4. Another way to handle this is that we allocate stack dynamically, using something like (https://github.com/alexcrichton/stacker). In this way, we get rid of crossbeam but keep recursive calls. A possible drawback on this is that it's not quite portable -- there're only support for win/linux/macos.

@debris @tomusdrw Do you have any thoughts on this?

[tomusdrw]

Can't we keep recursive calls for wasm but get rid of them for EVM? It probably means maintaining two different interfaces for the VMs (larger surface for bugs), but might still be worth it to boost EVM perf?

[sorpaas]

Yeah sounds good. I'll try to get the design based on that.

[insipx]

Hey guys,

I'm actually working with the Parity EVM implementation as a library in a project of mine. Was wondering if there was any possibility of adding a public 'step hook' to the VM. This could even be similar to the current trace functionality, but could be used for real-time debugging. IE, instead of executing the entirety of the code, the user has the choice of executing only one instruction at a time.

created an separate issue for this, since I don't think it's very related here: #9035

[tomusdrw]

@InsidiousMind maybe just block the thread in the tracer?

[sorpaas]

@InsidiousMind @tomusdrw I think there is a way to do that. We basically just need to add an additional "initialize" lifecycle to Interpreter, and then replace the while loop by a step function. With proper inlining the performance shouldn't be of difference too much. (And shameless self-advertising -- SputnikVM can already step individual opcodes as of today. 😄)

I mostly got the design for callstack. What I was planning to do is to add in Ext::call/create an extra parameter can_trap: bool. For EVM, we set can_trap = true, and for WASM we set can_trap = false. With can_trap, the Ext would return a Trap struct error which EVM should propagate outside of Interpreter. Executive will then handle Trap and manage the callstack, and call Interpreter::feed_return_data when it's available.

This would actually require step functionality, which is a prerequisite for VM resume.

[sorpaas]

Just some update on removing crossbeam/recursive stack: This turns out to be a much bigger refactoring than I thought. In addition, I'm having some local commit mess while trying to pull master... So I'm planning to split it and submit smaller PRs. Here's the list of refactoring to be done before we can change executive to be resumable:

• Make VmFactory creation not dependent on Ext, but only on Schedule and depth.
• Make Vm::new not fail-able, otherwise the control flow for resumable in Executive would be really complicated.
• Move VmFactory reference out of State. Having Factories::trie and Factories::accountdb there makes sense, but VmFactory is totally independent on State.
• Handle ReturnData out of Externalities::ret. Otherwise making reference of call stack to work is really difficult. ( Remove pass-by-reference return data value from executive  #9211)
• Refactor Tracer. I think we would need to move it so that we only need reference on it after Vm is initialized. That's currently one of the major pain point for the refactoring.

For the actual refactoring on removing crossbeam/recursive stack:

• Change Vm interface to have an extra Vm::resume, which takes a ResumeValue. On wasm, this will just be unimplemented!().
• In Externalities, have two other versions of call and create which return Trap instead of directly executing the VM.
• Split Executive. Have CallCreateExecutive which works on call/create level, and TransactExecutive which works on transaction level. CallCreateExecutive is resumable, and have a consume function (which takes self instead of &mut self) that changes it to non-resumable. TransactExecutive is a wrapper on CallCreateExecutive.

This is as far as where I'm now at right now. We may also need some additional refactorings along the way.