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Date: Fri, 9 Dec 2016 10:11:49 +0100 From: Peter Zijlstra <peterz@...radead.org> To: Thomas Gleixner <tglx@...utronix.de> Cc: LKML <linux-kernel@...r.kernel.org>, John Stultz <john.stultz@...aro.org>, Ingo Molnar <mingo@...nel.org>, David Gibson <david@...son.dropbear.id.au>, Liav Rehana <liavr@...lanox.com>, Chris Metcalf <cmetcalf@...lanox.com>, Richard Cochran <richardcochran@...il.com>, Parit Bhargava <prarit@...hat.com>, Laurent Vivier <lvivier@...hat.com>, "Christopher S. Hall" <christopher.s.hall@...el.com> Subject: Re: [patch 5/6] [RFD] timekeeping: Provide optional 128bit math On Fri, Dec 09, 2016 at 09:30:11AM +0100, Peter Zijlstra wrote: > > > 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. > Only when using x86_64 instructions, once I fixed the i386 variant it > was slower, probably due to register pressure and the like. OK, maybe I messed up on i386, although I've yet to try running that on an actual 32bit machine. I also need to dig up a small core, who knows what atoms do. Results are in cycles, average over 1e6 loops. I think the 128 results are around 1 cycle, measurements are maybe a tad wobbly because I compare against an empty loop to correct measurement overhead. root@...-ep:~/tmp# for i in -m64 -m32 -mx32; do echo $i ; gcc -O3 $i -o mult mult.c -lm; ./mult ; done -m64 cond: avg: 5.487738 +- 0.004152 uncond: avg: 4.495690 +- 0.006009 128: avg: 0.634496 +- 0.004795 -m32 cond: avg: 14.807630 +- 0.006890 uncond: avg: 11.601985 +- 0.009722 -mx32 cond: avg: 5.027696 +- 0.005766 uncond: avg: 4.038013 +- 0.008069 128: avg: 0.009928 +- 0.005730 root@hsw:~/tmp# for i in -m64 -m32 -mx32; do echo $i ; gcc -O3 $i -o mult mult.c -lm; ./mult ; done -m64 cond: avg: 1.998718 +- 0.008775 uncond: avg: 2.004795 +- 0.009865 128: avg: 0.991947 +- 0.007607 -m32 cond: avg: 12.981868 +- 0.011239 uncond: avg: 13.000566 +- 0.011668 -mx32 cond: avg: 2.005437 +- 0.006840 uncond: avg: 3.001631 +- 0.004786 128: avg: 1.990425 +- 0.003880
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