On Wed, 2012-10-24 at 16:18 -0700, Linus Torvalds wrote:
> On Wed, Oct 24, 2012 at 2:53 PM, Juri Lelli <juri.le...@gmail.com> wrote:
> > From: Peter Zijlstra <a.p.zijls...@chello.nl>
> >
> > Grow rudimentary u128 support without relying on gcc/libgcc.
>
> I missed the part where somebody explains why and what needs this?
> It's going to be very expensive indeed on some platforms, so the fact
> that it is *sometimes* cheap doesn't necessarily imply it should ever
> be used.
>
> So please, explain what the pressing need is that is so worthwhile
> that this is worth it. Maybe it was in a 00/16 cover letter, but not
> only was that not sent out to the people who got 01, you'd still want
> it in the commit message.
There's two use cases:
1) the proposed SCHED_DEADLINE needs to do some u64xu64 math, it
ends up having to multiply a deadline (in usec) with runtime (also
in usec).
2) the infrastructure adds mul_u64_u32_shr(), which is something we
do a lot of with all the time manipulation, apply a multiplier to
some u64 clock value.
We can do better on some archs than we can in generic, so this
interface could give a win there.
But yes, in general people should be very very reluctant to use this.
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