It looks it is using this to save a separate, explicit bit that says "we
left this frame by yielding rather than by returning". Note that f_stacktop
is cleared when the frame is re-entered:
stack_pointer = f->f_stacktop;
assert(stack_pointer != NULL);
f->f_stacktop = NULL; /* remains NULL unless yield suspends frame
*/
f->f_executing = 1;
On Sun, Apr 26, 2020 at 7:57 PM Skip Montanaro <skip.montan...@gmail.com>
wrote:
> 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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--
--Guido van Rossum (python.org/~guido)
*Pronouns: he/him **(why is my pronoun here?)*
<http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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