bpo-46409: Make generators in bytecode

[markshannon]

Adds a RETURN_GENERATOR bytecode which makes a generator (or coroutine) from the current frame.
This has a number of advantages:

1. Generator frames cannot be incomplete, as the frame is fully initialized before creating the generator
2. It simplifies the code for calling a Python function, as there is no need for a special case for generator functions.
3. It removes the need to specialize calls to generator functions, they can be specialized as any other call to a Python function.

This PR also adds a JUMP_NO_INTERRUPT bytecode. This is the same as JUMP_ABSOLUTE without any checks for interrupts. Checking for interrupts in yield from would destroy the flaky pretense that Python coroutines are stackful. (They are not, but PEPs 380 and 489 like to pretend that they are), so we need the extra, seemingly redundant instruction.

We use RETURN_GENERATOR rather than MAKE_GENERATOR; RETURN_VALUE as the RETURN_VALUE would make the compiler treat all the following code as unreachable. We could fix that using artificial instructions, but I think that is best left to another PR.

https://bugs.python.org/issue46409

[markshannon]

Fixes https://bugs.python.org/issue46389 and https://bugs.python.org/issue46374, but does not include tests for those issues.
I'll add the tests in separate PRs to close those issues.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @markshannon for commit c33abbf 🤖

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[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @markshannon for commit 243e441 🤖

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[gvanrossum]

I'm still trying to follow the rest of the C code here, but in case I fail, here's the rest of my review. :-)

[gvanrossum]

I'd name the new bytecode(s) explicitly here.

[gvanrossum]

I have a question about gi_iframe. Below (L31) it is defined as an array of length 1 of object pointers. But everywhere it's used, it is cast to InterpreterFrame *. That's not an object or object pointer AFAICT. So what's going on here? Is the declaration wrong? An intentional lie?

[markshannon]

It is a lie.

We don't want to expose InterpreterFrame in public headers, so we can't do the sensible thing and declare gi_frame as:

InterpreterFramegi_frame;

as C won't allow incomplete types in structs, even as the last member.

We could improve this by breaking up the header, so the public API sees a different definition. Still a lie, but a more elegant one.
Public header:

typedefstruct{_PyGenObject_HEAD(gi) } PyGenObject;

Private header:

typedefstruct{_PyGenObject_HEAD(gi) InterpreterFramegi_frame} PyGenObject;

Probably best done in a different PR, though.

[gvanrossum]

Gotcha. Any type other than PyObject* would be better though :-). And if you replace the casts with a macro it’s easier to fix later. I have an idea for the macro but it’s too painful to type on a phone.

[markshannon]

The first field of InterpreterFrame is PyFunctionObject * and the first few fields are all PyObject *, so declaring gi_frame as PyObject * gi_frame[1] seemed safe. Do let me know what your macro is, once you at a PC.

[gvanrossum]

Do let me know what your macro is, once you at a PC.

See https://bugs.python.org/issue40120#msg365465

[gvanrossum]

PS. What do you mean by "coroutines are stackful"? (I was hoping that reading the code would clarify this, but either I didn't get to that point in the code yet or it's hidden by details.)

[markshannon]

PS. What do you mean by "coroutines are stackful"? (I was hoping that reading the code would clarify this, but either I didn't get to that point in the code yet or it's hidden by details.)

Stackful coroutines have their own stack, like Lua coroutines or greenlets.
async def coroutines are "stackless", since they do not have their own stack.