----- Original Message ----- From: "Tim Peters" <tim.one@comcast.net>
Include/objimpl.h:275: double dummy; /* force worst-case alignment */
One branch of a union, forces enough padding in the gc header so that whatever follows the gc header is "aligned enough". This is sufficient for all core gc types, but may not be sufficient for user-defined gc types. I'm happy enough to view it as a restriction on what user-defined gc'able types can contain.
As I read the code, it affects all types (doesn't this header begin every object, regardless of its GC flags?) and I think that's a very unhappy circumstance for your numeric community. Remember, the type that raised the alarm here was just a long double.
At first glance, there appear to be different assumptions at work here about what constitutes maximal alignment on any given platform.
Only the objimpl.h trick might benefit from maximal alignment.
I'm not actually after maximal alignment; I look for a minimally-sized/aligned type whose alignment is a multiple of the target type's alignment. In any case, I was just using the assumption that double was maximally aligned since I was linking with Python code and the EDG front-end was too slow to handle the metaprogram -- I figured that if the assumption was good enough for Python and my clients were depending on it anyway, it was good enough for my code (not!).
If you ever compile on a KSR machine, you'll discover there's no std C type that captures maximal alignment.
I was aware that this was a theoretical possibility, but not that it was a practical one. What's KSR?
You'd have to guess it's an extension type named "_subpage". I'm not sure that even C++ template metaprogramming could manage that bit of channeling <wink>
Nope; we can only look through a list of likely candidates to try to find a match. We're hoping to address this for the next standard -- I'm pushing for allowing non-POD types in unions, leaving construction/destruction up to the user.
(FYI, _subpage required 128-byte alignment).
I guess that strictly speaking, requiring maximal alignment wouldn't be appropriate for objimpl ;-)
Stupid trick: If you can compute this at run time, do malloc(1) a few times, count the number of trailing 0 bits in the returned addresses, and take the minimum. Since malloc has to return memory "suitably aligned so that it may be assigned to a pointer to any type of object and then used to access such an object or an array of such objects", you'd soon discover you always got at least 7 trailing zero bits back from KSR malloc(), and presumably at least 4 under Tru64.
Sounds like a good candidate for your autoconf script. Seriously, though, I think it would be reasonable to stick to aligning the standard builtin types, in which can you can do the test without calling malloc, FWIW.
there's-the-standard-and-then-there's-real-life<wink>-ly y'rs - tim
in-theory-theory-and-practice-are-the-same-and-to-hell-with-what-happens-in- practice-ly y'rs -Dave