Re: [patch 5/6] [RFD] timekeeping: Provide optional 128bit math

Peter Zijlstra
2016-12-09T06:38:59Z

On Fri, Dec 09, 2016 at 06:26:38AM +0100, Peter Zijlstra wrote:
> On Thu, Dec 08, 2016 at 08:49:39PM -0000, Thomas Gleixner wrote:
>
> > +static inline u64 timekeeping_delta_to_ns(struct tk_read_base *tkr, u64 delta)
> > +{
> > + u32 dh, dl;
> > + u64 nsec;
> > +
> > + dl = delta;
> > + dh = delta >> 32;
> > +
> > + nsec = ((u64)dl * tkr->mult) + tkr->xtime_nsec;
> > + nsec >>= tkr->shift;
> > + if (unlikely(dh))
> > + nsec += ((u64)dh * tkr->mult) << (32 - tkr->shift);
> > + return nsec;
> > +}
>
> Just for giggles, on tilegx the branch is actually slower than doing the
> mult unconditionally.
>
> The problem is that the two multiplies would otherwise completely
> pipeline, whereas with the conditional you serialize them.

On my Haswell laptop the unconditional version is faster too.

> (came to light while talking about why the mul_u64_u32_shr() fallback
> didn't work right for them, which was a combination of the above issue
> and the fact that their compiler 'lost' the fact that these are
> 32x32->64 mults and did 64x64 ones instead).

Turns out using GCC-6.2.1 we have the same problem on i386, GCC doesn't
recognise the 32x32 mults and generates crap.

This used to work :/