[dart2wasm] Support `try`/`catch`/`finally` in `sync*` functions

[askeksa-google]

The sync* implementation in dart2wasm currently does not support try statements.

To implement these via the sync* CFG, each try block needs to be split into individual Wasm try blocks covering each of the sync* CFG blocks that are covered by the original try.

The catch blocks need not be duplicated. They can be separate CFG blocks targeted by the try blocks.

Also, finally blocks can probably be reused between the various control flow chains (normal completion, exception thrown, return from function) rather than being duplicated as in the normal implementation. One local for each finally block could contain the target index of where the execution needs to go after the execution of the finally block completes. Care must be taken to make this interact properly with normal finally blocks (those not containing any yield or yield* and therefore translated by the normal code generator). Either the CFG-based finally statements must properly maintain the stack of finalizers for the normal code generator, such that it properly duplicates these finalizers on return or exceptions, or all try/finally statements in a sync* functions must be translated via the CFG, even if they do not contain any yield or yield*.

[osa1]

This is being implemented in https://dart-review.googlesource.com/c/sdk/+/294480.

[osa1]

One local for each finally block could contain the target index of where the execution needs to go after the execution of the finally block completes

In this implementation do we need separate blocks for return, break N, and rethrow?

An alternative is to have two variables (which can be combined into one) for the whole CFG:

1. One for how many parent finalizers to run
2. One for how to continue after the last finalizer

In a finalizer, after running the finalizer code, we check (1). If it's non-zero we decrement it and continue with the parent finalizer. If it's zero we check (2). If the value is 0 we return. If it's 1 we rethrow. Otherwise the value minus one gives us the target block index (used in break).

I don't know how easy it is to create a synthetic local currently (can I update Closures.captures and expect the context struct to be updated/generated as expected?) but depending on that this may be easier to implement, we just add these two variables as fields in _SuspendState.

We can also combine these two variables into one by adding a single int field and using low and high bits for the variables.

[osa1]

We discussed this with @askeksa-google offline, I'm implementing the plan with two suspend state varibles described in my comment above.

[osa1]

The approach I describe above does not work, we need a stack of continuation variables for nested finalizers. For example:

(Code below is async but try-finally implementation is the same one in my sync* CL above)

void main() async {
  print('1');
  try {
    print('2');
  } finally {
    print('3');
    try {
      print('4');
    } finally {
      print('5');
    }
    print('6');
  }
  print('7');
}

Here in the nested finalizer we need to save parent finalizer's continuation and restore it on fallthrough (i.e. when the nested finalizer does not override the continuation with throw, return, or break/continue). With the idea described above we have one continuation variable which we override in the nested finalizer. Because the nested finalizer does not override the control flow, after the finalizer block we should continue according to the parent finalizer's continuation, but that continuation is lost because we've overridden it when entering the nested finalizer.

So we need a stack of continuation variables. This can be implemented by (conceptually) adding a local for each finally block. When we enter a finally block we set the continuation local the current continuation (return, break, re-throw). The variable will then be used if the current finalizer block does not override the control flow (fallthrough).

I think we still need a global (within a function) "number of finalizers to run" variable as described in my previous comment, to be able to implement break N when there are finalizers between the labeled block and the current statements. In these cases the continuation of the first finalizers will be "run X finalizers, then jump to block N". If the finalizer doesn't override it the fallthrough code will decrement X by one, set the parent finalizer block's fallthrough continuation, and jump to it.

The code that jumps to a finalizer block will set the finalizer's continuation variable. The only difference here is that we will have a variable specific to the finalizer block, not a single variable.

Normally these continuation variables don't need to be in the context (heap-allocated sync/async function state) always, but currently we put all locals in the context, so I'll try a simpler implementation that doesn't require creating a kernel variable declaration for finally blocks and instead try to generate context fields for them before code generation.

[osa1]

I think we still need a global (within a function) "number of finalizers to run" variable as described in my previous comment

We can avoid this variable if we set continuation variables of all parent finalizers when changing control flow (instead of just the continuation variable of the parent finalizer). For example, break N can set continuation of the next finalizer as "jump to <block of parent finalizer>", and then continuation of the next finalizer's parent as "jump to <block for continuation of N>".

[osa1]

We have a correct implementation of this in AsyncCodeGenerator, we should either reuse the logic somehow or copy-paste-modify the code from AsyncCodeGenerator.

[meg4cyberc4t]

Hello @osa1!
Now (when building web assembly in stable version) I catch this unimplemented exception in my project.
Libraries that use try/catch in sync* cannot be build for wasm.
I see that the development of this is abandoned, but could you please return to it?

[osa1]

Hi @meg4cyberc4t.

This is not really abandoned, we just had better things to work on as sync* is not that widely used. I'll try to get back to this soon.

[osa1]

I'm fixing the remaining sync* missing features and issues in https://dart-review.googlesource.com/c/sdk/+/366663.