A question about fill_free_list(void) function

[Pedram]

On Jul 2, 1:11 pm, Peter Otten <__pete... at web.de> wrote:
> Pedram wrote:
> > On Jul 1, 10:01 pm, Christian Heimes <li... at cheimes.de> wrote:
> >> Pedram schrieb:
>
> >> > Hello community,
> >> > I'm reading the CPython interpreter source code,
> >> > first, if you have something that I should know for better reading
> >> > this source code, I would much appreciate that :)
> >> > second, in intobject.c file, I read the following code in
> >> > fill_free_list function that I couldn't understand:
> >> > <code>while (--q > p)
> >> > Py_TYPE(q) = (struct _typeobject *)(q-1);
> >> > Py_TYPE(q) = NULL;
> >> > </code>
> >> > What does it mean?
>
> >> The code is abusing the ob_type member to store a single linked, NULL
> >> terminated list of free integer objects. It's a tricky optimization you
> >> don't have to understand in order to understand the rest of the code.
>
> >> And no, the code in ctypes.h is totally unrelated to the free list in
> >> intobject.c.
>
> >> Christian
>
> > Thanks for reply.
> > I totally understood the fill_free_list() function. Thanks.
> > But I have problems in understanding the PyInt_FromLong(). Here's the
> > code:
>
> > PyObject *
> > PyInt_FromLong(long ival)
> > {
> > register PyIntObject *v;
> > #if NSMALLNEGINTS + NSMALLPOSINTS > 0
> > if (-NSMALLNEGINTS <= ival && ival < NSMALLPOSINTS) {
> > v = small_ints[ival + NSMALLNEGINTS];
> > Py_INCREF(v);
> > #ifdef COUNT_ALLOCS
> > if (ival >= 0)
> > quick_int_allocs++;
> > else
> > quick_neg_int_allocs++;
> > #endif
> > return (PyObject *) v;
> > }
> > #endif
> > if (free_list == NULL) {
> > if ((free_list = fill_free_list()) == NULL)
> > return NULL;
> > }
>
> > v = free_list; // <-- No problem till here :)
> > free_list = (PyIntObject *)Py_TYPE(v);
> > PyObject_INIT(v, &PyInt_Type);
> > v->ob_ival = ival;
> > return (PyObject *) v;
> > }
>
> > I understood that small numbers (between -5 and 256) are referenced to
> > small_ints list. Others have to store in PyIntBlock, am I right? OK,
> > no problem till 'v = free_list;' but can't understand the line
> > 'free_list = (PyIntObject *)Py_TYPE(v);' :(. Where free_list
> > referenced to? Don't this correct that free_list have to be referenced
> > to another free block so next time the function called v store in this
> > location?
> > Sorry for my bad English. I hope you could understand me :)
>
> Well, if I understand fill_free_list() correctly, you haven't ;) It only
> makes sense together with the allocation code in PyInt_FromLong().
>
> Py_TYPE(v) translates to v->ob_type, and in fill_free_list() that field was
> abused(*) to hold a pointer to the previous item in the array of free
> integers, thereby temporarily turning it into a linked list.
>
> Assuming that free_list pointed to block->objects[10] in the current
> PyIntBlock it will now point to block->objects[9].
>
> When block->objects[0] is reached free_list will be set to NULL as block-
>
> >objects[0]->ob_type was initialized with NULL by the line
>
> Py_TYPE(q) = NULL; /* in fill_free_list() */
>
> The next call of PyInt_FromLong() will then trigger the allocation of a new
> block.
>
> Peter
>
> (*) It may help if you think of the objects array as a
>
> union linked {
>     linked *next;
>     PyInt_Object obj;
>
>
>
>
>
> };
>
>

Oh, I got it! What a wonderful implementation! :o
Thanks so much Peter. You made my day :) I didn't read the code
carefully.

>
> ;)
>