Re: [PATCH] Optimize int_sqrt for small values for faster idle

From	Eric Dumazet <eric.dumazet@gmail.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH] Optimize int_sqrt for small values for faster idle
Date	2016-02-02 22:40 +0100
Message-ID	<qXQQ4-KX-41@gated-at.bofh.it> (permalink)
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On Tue, 2016-02-02 at 21:46 +0100, Rasmus Villemoes wrote:
> On Tue, Feb 02 2016, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> 
> > On Tue, 2016-02-02 at 00:08 +0100, Rasmus Villemoes wrote:
> >
> >> Thanks. (Is there a good way to tell gcc that avg*avg is actually a
> >> 32x32->64 multiplication?)
> >
> > If avg is 32bit, compiler does that for you.
> >
> > u32 avg = ...
> >
> > u64 result = (u64)avg * avg;
> 
> Yeah, but in this case avg is u64 because it is used to temporarily
> contain the sum of a bunch of u32s, before being divided by #bunch. So
> I'd have to write that as (u64)(u32)avg * (u32)avg, which isn't very
> readable :-/
> 
> I just thought the scenario of a u64 known to be holding a value < 2^32
> was common enough that some utility macros already existed.
> 
> Rasmus

crypto/vmac.c has this, you could make it generic maybe.

#define MUL32(i1, i2)   ((u64)(u32)(i1)*(u32)(i2))

Thread

[PATCH] Optimize int_sqrt for small values for faster idle Andi Kleen <andi@firstfloor.org> - 2016-01-28 22:50 +0100
  Re: [PATCH] Optimize int_sqrt for small values for faster idle kbuild test robot <lkp@intel.com> - 2016-01-28 23:10 +0100
  Re: [PATCH] Optimize int_sqrt for small values for faster idle Joe Perches <joe@perches.com> - 2016-01-28 23:20 +0100
    Re: [PATCH] Optimize int_sqrt for small values for faster idle Andi Kleen <andi@firstfloor.org> - 2016-01-28 23:50 +0100
  Re: [PATCH] Optimize int_sqrt for small values for faster idle kbuild test robot <lkp@intel.com> - 2016-01-28 23:20 +0100
  Re: [PATCH] Optimize int_sqrt for small values for faster idle Joe Perches <joe@perches.com> - 2016-01-28 23:30 +0100
  Re: [PATCH] Optimize int_sqrt for small values for faster idle Andi Kleen <andi@firstfloor.org> - 2016-01-28 23:40 +0100
  Re: [PATCH] Optimize int_sqrt for small values for faster idle Eric Dumazet <eric.dumazet@gmail.com> - 2016-01-28 23:40 +0100
  Re: [PATCH] Optimize int_sqrt for small values for faster idle "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2016-01-29 05:00 +0100
  Re: [PATCH] Optimize int_sqrt for small values for faster idle Thomas Rohwer <trohwer85@gmail.com> - 2016-01-31 08:30 +0100
  Re: [PATCH] Optimize int_sqrt for small values for faster idle Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2016-02-01 22:30 +0100
    Re: [PATCH] Optimize int_sqrt for small values for faster idle Andi Kleen <ak@linux.intel.com> - 2016-02-01 22:40 +0100
      Re: [PATCH] Optimize int_sqrt for small values for faster idle Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2016-02-02 00:10 +0100
        Re: [PATCH] Optimize int_sqrt for small values for faster idle Andi Kleen <ak@linux.intel.com> - 2016-02-02 01:10 +0100
        Re: [PATCH] Optimize int_sqrt for small values for faster idle Eric Dumazet <eric.dumazet@gmail.com> - 2016-02-02 01:40 +0100
          Re: [PATCH] Optimize int_sqrt for small values for faster idle Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2016-02-02 21:50 +0100
            Re: [PATCH] Optimize int_sqrt for small values for faster idle Eric Dumazet <eric.dumazet@gmail.com> - 2016-02-02 22:40 +0100
      Re: [PATCH] Optimize int_sqrt for small values for faster idle Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2016-02-07 22:40 +0100

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