[Python-Dev] Alignment assumptions (original) (raw)

Jack Jansen Jack.Jansen@oratrix.com
Thu, 28 Feb 2002 22:34:05 +0100

On donderdag, februari 28, 2002, at 07:57 , Tim Peters wrote:

[David Abrahams]

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

Include/dictobject.h:49: long aligner; This is in #ifdef USECACHEALIGNED 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.

MacPython uses it. At the time it was put in it caused a 15% increase in Pystones because dictionary entries were aligned in cache lines. But: this was in the PPC 601 and 604 era, I must say that I've never tested whether it made any difference on G3 and G4.

Put in a bug report in my name, and one day I'll get around to testing whether it still makes a difference on current hardware and rip it out if it doesn't.