[RFC] Rework mem_barrier() definitions in compiler.h

[Mark Kettenis]

> Date: Tue, 4 Aug 2009 02:14:55 -0400
> From: Matt Turner <mattst88 at gmail.com>
>
> As noted by the numerous XXX markers, there are quite a few issues,
> outlined here.
> 
> o what the heck is the stuff wrapped in 'defined(__powerpc__) &&
> !defined(__OpenBSD__)'?

No idea; Alan Coopersmith added that check fairly recently, but I fail
to see why.

> o any particular reason that no BSDs are allowed for the __ia64__ section?

OpenBSD doesn't run on ia64 (yet).

> o no Open/NetBSD for x86/64?

Sure it does, except that we prefer to call it OpenBSD/amd64.

> o no BSD checks on MIPS?

I don't think OpenBSD/sgi supports any accelerated graphics drivers
yet, but that may change.

> o as with {s,m}fence, how to check for MIPS III 'sync' instruction?

OpenBSD/sgi is 64-bit, so using MIPS III instructions is probably safe
there.