Messages in this thread | | | Date | Fri, 9 Dec 2016 06:41:44 +0100 | From | Peter Zijlstra <> | Subject | Re: [patch 5/6] [RFD] timekeeping: Provide optional 128bit math |
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On Fri, Dec 09, 2016 at 06:22:03AM +0100, Ingo Molnar wrote: > > * Peter Zijlstra <peterz@infradead.org> wrote: > > > On Fri, Dec 09, 2016 at 05:08:26AM +0100, Ingo Molnar wrote: > > > > +#if defined(CONFIG_ARCH_SUPPORTS_INT128) && defined(__SIZEOF_INT128__) > > > > +static inline u64 timekeeping_delta_to_ns(struct tk_read_base *tkr, u64 delta) > > > > +{ > > > > + unsigned __int128 nsec; > > > > + > > > > + nsec = ((unsigned __int128)delta * tkr->mult) + tkr->xtime_nsec; > > > > + return (u64) (nsec >> tkr->shift); > > > > +} > > > > > > Actually, 128-bit multiplication shouldn't be too horrible - at least on 64-bit > > > architectures. (128-bit division is another matter, but there's no division here.) > > > > IIRC there are 64bit architectures that do not have a 64x64->128 mult, > > only a 64x64->64 mult instruction. Its not immediately apparent using > > __int128 will generate optimal code for those, nor is it a given GCC > > will not require libgcc functions for those. > > Well, if the overflow case is rare (which it is in this case) then it should still > be relatively straightforward, something like: > > X and Y are 64-bit: > > X = Xh*2^32 + Xl > Y = Yh*2^32 + Yl > > X*Y = (Xh*2^32 + Xl)*(Yh*2^32 + Yl) > > = Xh*2^32*(Yh*2^32 + Yl) > + Xl*(Yh*2^32 + Yl) > > = Xh*Yh*2^64 > + Xh*Yl*2^32 > + Xl*Yh*2^32 > + XL*Yl > > Which is four 32x32->64 multiplications in the worst case.
Yeah, that's the full 64x64->128 mult on 3bit. Luckily we only need 64x32->96, which reduces to 2 32x32->64 mults.
But my point was that unconditionally using __int128 might not be the right thing.
> Where a valid overflow threshold is relatively easy to determine in a hot path > compatible fashion: > > if (Xh != 0 || Yh != 0) > slow_path(); > > And this simple and fast overflow check should still cover the overwhelming > majority of 'sane' systems. (A more involved 'could it overflow' check of counting > the high bits with 8 bit granularity by looking at the high bytes not at the words > could be done in the slow path - to still avoid the 4 multiplications in most > cases.) > > Am I missing something?
Yeah, the fact that we only need the 2 mults and that the fallback already does the second multiply conditionally :-) But then look at the email where I said that that condition actually makes the thing vastly more expensive on some archs (like tilegx).
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