On 23/09/2020 9:47 am, Ivan Levkivskyi wrote:
On Wed, 23 Sep 2020 at 01:19, Guido van Rossum <gu...@python.org
<mailto:gu...@python.org>> wrote:
On Tue, Sep 22, 2020 at 4:58 PM Greg Ewing
<greg.ew...@canterbury.ac.nz <mailto:greg.ew...@canterbury.ac.nz>>
wrote:
What are you trying to achieve by using tagged pointers?
It seems to me that in a dynamic environment like Python, tagged
pointer tricks are only ever going to reduce memory usage, not
make anything faster, and in fact can only make things slower
if applied everywhere.
Hm... mypyc (an experimental Python-to-C compiler bundled with mypy)
uses tagged pointers to encode integers up to 63 bits. I think it's
done for speed, and it's probably faster in part because it avoids
slow memory accesses. But (a) I don't think there's overflow
checking, and (b) mypyc is very careful that tagged integers are
never passed to the CPython runtime (since mypyc apps link with an
unmodified CPython runtime for data types, compatibility with
extensions and pure Python code). Nevertheless I think it puts your
blanket claim in some perspective.
FWIW mypyc does overflow checking, see e.g.
https://github.com/python/mypy/blob/master/mypyc/lib-rt/CPy.h#L168, also
tagged pointers did bring some speed wins, not big however. My
expectation is that speed wins may be even more modest without static
information that mypyc has.
// offtopic below, sorry
In general, I think any significant perf wins will require some static
info given to the Python compiler. I was thinking a long while ago about
defining some kind of a standard protocol so that static type checkers
can optionally provide some info to the Python compiler (e.g.
pre-annotated ASTs and pre-annotated symbol tables), and having a lot of
specialized bytecodes. For example, if we know x is a list and y is an
int, we can emit a special byte code for x[y] that will call
PyList_GetItem, and will fall back to PyObject_GetItem in rare cases
when type-checker didn't infer right (or there were some Any involved).
Another example is having special byte codes for direct pointer access
to instance attributes, etc. The main downside of such ideas is it will
take a lot of work to implement.
Performance improvements do not need static annotations, otherwise PyPy,
V8 and luajit wouldn't exist.
Even HotSpot was originally based on a VM for SmallTalk.
Cheers,
Mark.
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