[Python-ideas] improving C structs layout

[Antoine Pitrou]

On Wed, 8 May 2013 09:17:27 -0700
Andrew Barnert <abarnert at yahoo.com> wrote:

> On May 8, 2013, at 6:52, Antoine Pitrou <solipsis at pitrou.net> wrote:
> 
> > On Wed, 08 May 2013 15:45:09 +0200
> > Alfredo Solano <asolano at icai.es> wrote:
> >> Hi,
> >> 
> >> Interesting observation, but isn't C struct alignment platform/compiler
> >> dependent?
> > 
> > The ABIs are standardized, so I would answer no.
> 
> What standard are you talking about? There's certainly no ABI standard that covers both Win64 and ARM7 Linux.

Per-platform ABI standards. Compilers aren't generally free to invent
things if they want to be interoperable with each other.

> There are very definitely different packing and alignment rules for different platforms that python runs on.
> 
> > Even if they weren't, there are common sense rules to minimize padding,
> > such as to put fields of the same width next to each other (e.g. put
> > chars together instead of intermingling them with ints and floats).
> 
> This is true... But you have to keep in mind that the width of different types is itself platform-dependent. If you've got an int, a long, a pointer, and a double, how do you pack them in a way that makes sense for all platforms, or even just the big 3 of x86_64 Mac/Linux/BSD, Win64, and Win32?

Well, you can be sure that int <= long, and in most cases you can assume
other inequalities such as int <= pointer and int <= double (and even
pointer <= double). The long <=> pointer relationship is less
predictable, but on common platforms long <= pointer.

So, double then pointer then long then int.

(of course, there may be other concerns such as ensuring proximity of
fields which are often looked up together)

Regards

Antoine.