[Python-Dev] Alignment assumptions

[Guido van Rossum]

> This is in
> 
> #ifdef USE_CACHE_ALIGNED
> 	long	aligner;
> #endif
> 
> and AFAIK nobody ever defines the symbol.  It's a cache-line
> optimization gimmick, but is effectively a nop (except to waste
> memory) on "almost all" machines.  IIRC, the author never measured
> any improvement by using it (not surprising, since I believe almost
> all mallocs at least 8-byte align now).  I vote we delete it.

The malloc 8-byte align argument doesn't apply, since this struct is
used in an array.  Since the struct itself doesn't require alignment
beyond 4 bytes, the array entries can be 12 bytes apart.  So I don't
think this is a nop -- I think it would waste 4 bytes per hash table
entry on most machines.

This was added by Jack Jansen ages ago -- I think he did measure a
speedup on an old Mac compiler, or he wouldn't have added it, and I
bet there was a #define USE_CACHE_ALIGNED in his config.h then.

But that's all history; I agree it should be deleted.

--Guido van Rossum (home page: http://www.python.org/~guido/)