Eddie Elizondo <eduardo.elizondoru...@gmail.com> added the comment:
> I'm somewhat puzzled how a version that does no more work and has no jumps is
> slower.
Oh I think I know why there's some confusion. I've updated the PR from the
initial version (which is probably the one that you saw). The branching does
less work in Py_INCREF and Py_DECREF for all instances marked with the immortal
bit by exiting early.
In the latest change, I added the immortal bit to a bunch of "known" immortal
objects by. For instance:
* All static types (i.e: PyType_Type, etc.)
* All small ints (-5 to 256)
* The following Singletons: PyTrue, PyFalse, PyNone
* And the heap after the runtime is initialized (in pymain_main)
Example 1)
```
PyObject _Py_NoneStruct = {
_PyObject_EXTRA_INIT
1, &_PyNone_Type
#ifdef Py_IMMORTAL_OBJECTS
_Py_IMMORTAL_BIT,
#else
1,
#endif /* Py_IMMORTAL_OBJECTS */
&_PyNone_Type
};
```
Example 2)
```
static int
pymain_main(_PyArgv *args)
{
PyStatus status = pymain_init(args);
if (_PyStatus_IS_EXIT(status)) {
pymain_free();
return status.exitcode;
}
if (_PyStatus_EXCEPTION(status)) {
pymain_exit_error(status);
}
#ifdef Py_IMMORTAL_OBJECTS
/* Most of the objects alive at this point will stay alive throughout the
* lifecycle of the runtime. Immortalize to avoid the GC and refcnt costs */
_PyGC_ImmortalizeHeap();
#endif /* Py_IMMORTAL_OBJECTS */
return Py_RunMain();
```
Therefore, you are now making Py_INCREF and Py_DECREF cheaper for things like
`Py_RETURN_NONE` and a bunch of other instances.
Let me know if that explains it! I could also send you patch of the branch-less
version so you can compare them.
> but making the object header immutable prevents changes like
Why would it prevent it? These changes are not mutually exclusive, you can
still have an immortal bit by:
1) Using a bit from `gc_bits`. Currently you only need 2 bits for the GC. Even
with a `short` you'll have space for the immortal bit.
2) Using a bit from the ob_refcnt. Separately, using this allows us to
experiment with a branch-less and test-less code by using saturated adds. For
example:
```
/* Branch-less incref with saturated add */
#define PY_REFCNT_MAX ((int)(((int)-1)>>1))
#define _Py_INCREF(op) ({
__asm__ (
"addl $0x1, %[refcnt]"
"cmovol %[refcnt_max], %[refcnt]"
: [refcnt] "+r" (((PyObject *)op)->ob_refcnt)
: [refcnt_max] "r" (PY_REFCNT_MAX)
);})
```
----------
_______________________________________
Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue40255>
_______________________________________
_______________________________________________
Python-bugs-list mailing list
Unsubscribe:
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
