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

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Message-ID: <20161209044849.GY3045@worktop.programming.kicks-ass.net>
Date:   Fri, 9 Dec 2016 05:48:49 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        John Stultz <john.stultz@...aro.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 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.