From: Eric Wong <normalperson@...>
Date: 2017-04-18T20:10:02+00:00
Subject: [ruby-core:80775] Re: [Ruby trunk Feature#13434] better method definition in C API

naruse@airemix.jp wrote:
> Issue #13434 has been updated by naruse (Yui NARUSE).
> 
> 
> I agree with the concept.
> 
> From r55102, rb_scan_args is statically resolved by C compilers on some environment, rb_get_kwargs is still inefficient.
> To allow C compilers statically resolve them, Ruby method in C should be defined in more machine friendly way.

Cool, I forgot about that rb_scan_args optimization.  Maybe we
can use similar optimization for defining methods, too,
to speed up VM startup.

> I thought the new API should use C struct (write rb_method_info by hand?).

I think the new API should resemble pure Ruby method definition
for ease-of-learning.  But, we will need a new way to mark
unused/readonly...

> Anyway we should list up the requirement of the new API, for example
> * readonly/unused flag for arguments.
> * whether the caller requires return single value, multiple value (like Perl's wantarray), or not.

Agreed on all of these.

For wantarray, I think we can support returning klass==0
(hidden) array from C methods.  We then teach the VM to treat
klass==0 Array return values as a special case: the VM will set
klass=rb_cArray lazily if capturing the array is required.

In other words, this example:

    static VALUE cfunc(VALUE self)
    {
	VALUE ret = rb_ary_tmp_new(3);
	rb_ary_push(ret, INT2FIX(1));
	rb_ary_push(ret, INT2FIX(2));
	rb_ary_push(ret, INT2FIX(3));

	return ret; /* klass == 0 */
    }

Example 1, return value is discarded immediately:

    a, b, c = cfunc

In the above case, the VM calls:

    rb_ary_clear(ret);
    rb_gc_force_recycle(ret)

after assigning a, b, c since the temporary array is no longer
used.


Example 2, return value is preserved:

    a, b, c = ary = cfunc

In the above case, the VM calls:

    rb_obj_reveal(ret, rb_cArray)

since it is assigned to `ary`.

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