This is more an observation and question than anything else, but
perhaps it will stimulate some ideas from the experts. Consider this
trivial generator function:

    def gen(a):
        yield a

When the YIELD_VALUE instruction is executed, it executes (in the
non-async case):

    retval = POP();
    f->f_stacktop = stack_pointer;
    goto exiting;

This is fine as far as it goes. However, execution eventually leads to
Objects/genobject.c where we hit this code (I think after falling off
the YIELD_VALUE instruction, but perhaps virtual machine execution
reaches RETURN_VALUE):

    /* If the generator just returned (as opposed to yielding), signal
     * that the generator is exhausted. */
    if (result && f->f_stacktop == NULL) {

There are several other references to f->f_stacktop in genobject.c.
I've not yet investigated all of them.

As I'm working on a register-based virtual machine implementation, I
don't fiddle with the stack at all, so it's a bit problematic that the
generator implementation is so intimate with the stack. As this is an
area of the core which is completely new to me, I wonder if someone
can suggest alternate ways of achieving the same effect without
relying on the state of the stack. It seems to me that from within
PyEval_EvalFrameDefault the implementations of relevant instructions
could reference the generator object via f->f_gen and call some (new?)
private API on generators which toggles the relevant bit of state in
the generator.

I think it's worse that this though, as it seems that in gen_send_ex()
it actually pushes a value onto the stack. That can't be solved by
simply adding a state attribute to the generator object struct.

Skip
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