[pypy-dev] Question about extension support

Kevin Modzelewski kmod at dropbox.com
Thu Mar 27 05:17:23 CET 2014

On Wed, Mar 26, 2014 at 1:52 PM, Benjamin Peterson <benjamin at python.org>wrote:

> There are several reasons. Two of the most important are
> 1) PyPy's internal representation of objects is different from
> CPython's, so a conversion cost must be payed every time objects pass
> between pure Python and C. Unlike CPython, extensions with PyPy can't
> poke around directly in data structures. Macros like PyList_SET_ITEM
> have to become function calls.

Hmm interesting... I'm not sure I follow, though, why the calling
PyList_SET_ITEM on a PyPy list can't know about the PyPy object
representation.  Again, I understand how it's not necessarily going to be
as fast as pure-python code, but I don't understand why PyList_SET_ITEM on
PyPy needs to be slower than on CPython.  Is it because PyPy uses more
complicated internal representations, expecting the overhead to be elided
by the JIT?

Also, I'm assuming that CPyExt gets to do a recompilation of the extension
module; I could definitely understand how there could be significant
overhead if this was being done as an ABI compatibility layer.

2) Bridging the gap between PyPy's GC and CPython's ref counting

requires a lot of bookkeeping.

>From a personal standpoint I'm also curious about how much of this overhead
is fundamental, and how much could be alleviated with (potentially
significant) implementation effort.  I know PyPy has a precise GC, but I
wonder if using a conservative GC could change the situation dramatically
if you were able to hook the extension module's allocator and switch it to
using the conservative GC.  That's my plan, at least, which is one of the
reasons I've been curious about the issues that PyPy has been running into
since I'm curious about how much will be applicable.

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