[Python-Dev] Alignment assumptions

[David Abrahams]

A quick grep-find through the Python-2.2 sources reveals the following:

Include/dictobject.h:49: long aligner;
Include/objimpl.h:275: double dummy;  /* force worst-case alignment */
Modules/addrinfo.h:162: LONG_LONG __ss_align; /* force desired structure
storage alignment */
Modules/addrinfo.h:164: double __ss_align; /* force desired structure
storage alignment */

At first glance, there appear to be different assumptions at work here about
what constitutes maximal alignment on any given platform. I've been using a
little C++ metaprogram to find a type which will properly align any other
given type. Because of limitations of one compiler, I had to disable the
computation and instead used the objimpl.h assumption that double was
maximally aligned, but also added a compile-time assertion to check that the
alignment is always greater than or equal to that of the target type. Well,
it failed today on Tru64 Unix with the latest compaq CXX 6.5 prerelease
compiler; it appears that the alignment of long double is greater than that
of double on that platform.

I thought someone might want to know,
Dave


+---------------------------------------------------------------+
                  David Abrahams
      C++ Booster (http://www.boost.org)               O__  ==
      Pythonista (http://www.python.org)              c/ /'_ ==
  resume: http://users.rcn.com/abrahams/resume.html  (*) \(*) ==
          email: david.abrahams@rcn.com
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