[Python-Dev] cpython: Using 'long double' to force this structure to be worst case aligned is no (original) (raw)

Gregory P. Smith greg at krypto.org
Fri Dec 14 10:14:04 CET 2012

Yes, see the followup. My comments before were all misinterpreting size_t.

Same result on x86_64 linux. On a 64-bit platform the 24 byte structure now occupies 24 bytes instead of being padded to 32. Nice. On a 32-bit platform it should remain 16 bytes.

The PyGC_Head union structure is NOT part of the ABI laid out in http://www.python.org/dev/peps/pep-0384/ and is accurately excluded from the .h file when Py_LIMITED_API is defined so changing this in 3.4 should not be a problem. This structure occupies the space gc tracked PyObject* pointers.

-gps

On Fri, Dec 14, 2012 at 12:42 AM, Mark Dickinson <dickinsm at gmail.com> wrote:

On Fri, Dec 14, 2012 at 7:27 AM, Gregory P. Smith <greg at krypto.org> wrote: > So changing the definition of the dummy side of the union makes zero > difference to already compiled code as it (a) doesn't change the structure's > size and (b) all existing implementations already align these on an 8 byte > boundary.

It looks to me as though the struct size is changed, at least on some platforms. Before this commit, I get (OS X 10.6, 64-bit non-debug build): Python 3.4.0a0 (default:b4c383f31881+, Dec 14 2012, 08:30:39) [GCC 4.2.1 (Apple Inc. build 5664)] on darwin Type "help", "copyright", "credits" or "license" for more information. >>> class A(object): pass ... >>> a = A() >>> import sys >>> sys.getsizeof(a) 64

After it: Python 3.4.0a0 (default:76bc92fb90c1+, Dec 14 2012, 08:33:48) [GCC 4.2.1 (Apple Inc. build 5664)] on darwin Type "help", "copyright", "credits" or "license" for more information. >>> class A(object): pass ... >>> a = A() >>> import sys >>> sys.getsizeof(a) 56 -- Mark -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20121214/1e21b679/attachment.html>

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