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

Gregory P. Smith greg at krypto.org
Fri Dec 14 08:27:41 CET 2012

On Mon, Dec 10, 2012 at 11:16 PM, 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).

No they don't.

size_t and ssize_t exist in large part because they are often larger than
an int or long on 32bit platforms.  They are 64-bit on Linux regardless of
platform (i think there is a way to force a compile in ancient mode that
forces them and the APIs being used to be 32-bit size_t variants but nobody
does that).

> > 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.
> (*) GC-enabled PyObjects, anyway. Others will be naturally aligned
> thanks to the memory allocator.
> What's more, I think you shouldn't be doing this kind of change in a
> bugfix release. It might break compiled C extensions since you are
> changing some characteristics of object layout (although you would
> probably only break those extensions which access the GC header, which
> is probably not many of them). Resource consumption improvements
> generally go only into the next feature release.

This isn't a resource consumption improvement.  It is a compilation
correctness change with zero impact on the generated code or ABI
compatibility before and after.  The structure, as defined, is was flagged
as problematic by Clang's undefined behavior sanitizer because it contains
a 'long double' which requires 16-byte alignment but Python's own memory
allocator was using an 8 byte boundary.

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.

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