[Python-Dev] cpython: Using 'long double' to force this structure to be worst case aligned is no
solipsis at pitrou.net
Tue Dec 11 08:21:30 CET 2012
On Tue, 11 Dec 2012 08:16:27 +0100
Antoine Pitrou <solipsis at pitrou.net> wrote:
> On Tue, 11 Dec 2012 03:05:19 +0100 (CET)
> gregory.p.smith <python-checkins at python.org> wrote:
> > Using 'long double' to force this structure to be worst case aligned is no
> > longer required as of Python 2.5+ when the gc_refs changed from an int (4
> > bytes) to a Py_ssize_t (8 bytes) as the minimum size is 16 bytes.
> > The use of a 'long double' triggered a warning by Clang trunk's
> > Undefined-Behavior Sanitizer as on many platforms a long double requires
> > 16-byte alignment but the Python memory allocator only guarantees 8 byte
> > alignment.
> > So our code would allocate and use these structures with technically improper
> > alignment. Though it didn't matter since the 'dummy' field is never used.
> > This silences that warning.
> > Spelunking into code history, the double was added in 2001 to force better
> > alignment on some platforms and changed to a long double in 2002 to appease
> > Tru64. That issue should no loner be present since the upgrade from int to
> > Py_ssize_t where the minimum structure size increased to 16 (unless anyone
> > knows of a platform where ssize_t is 4 bytes?)
> What?? Every 32-bit platform has a 4 bytes ssize_t (and size_t).
> > We can probably get rid of the double and this union hack all together today.
> > That is a slightly more invasive change that can be left for later.
> How do you suggest to get rid of it? Some platforms still have strict
> alignment rules and we must enforce that PyObjects (*) are always
> aligned to the largest possible alignment, since a PyObject-derived
> struct can hold arbitrary C types.
Ok, I hadn't seen your proposal. I find it reasonable:
“A more correct non-hacky alternative if any alignment issues are still
found would be to use a compiler specific alignment declaration on the
structure and determine which value to use at configure time.”
However, the commit is still problematic, and I think it should be
reverted. We can't remove the alignment hack just because it seems to
be useless on x86(-64).
More information about the Python-Dev