Kernelspace C11 atomics for MIPS

[Adrian Chadd]

On 3 June 2013 20:55, Juli Mallett <jmallett at freebsd.org> wrote:

> To drain the pipeline on certain deficient (and mostly older) CPUs by way of
> guesswork and a little vague magic.  Most CPUs we support, I would guess, do
> not need this, and it continues to exist solely for hysterical reasons.

How can I turn it off for my compiles?

> I've certainly gotten rid of them and some other cargo cult synchronization
> on Octeon for testing and had it survive under considerable load, and
> occasionally with some slight speedups (for some more commonly-used or
> slower things than Just a Bunch Of NOPs.)

Right. Well, since it's happening on every inlined lock, it's a bit silly.

> The trouble is that proving they aren't necessary requires being rigorous
> and careful in understanding documentation and errata, and FUD about their
> possible necessity is somewhat-intimidating.  It's not an easy kind of
> corruption/unreliability/etc., to prove the lack of empirically.

I've checked the diassembly from gcc-4.mumble on linux; it doesn't
include NOPs like this as far as I can tell.



Adrian