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

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  Re: [PATCH] Optimize int_sqrt for small values for faster idle Peter Zijlstra <peterz@infradead.org> - 2017-07-20 12:20 +0200
    Re: [PATCH] Optimize int_sqrt for small values for faster idle Eric Dumazet <eric.dumazet@gmail.com> - 2017-07-24 15:30 +0200

#1692728 — Re: [PATCH] Optimize int_sqrt for small values for faster idle

On Mon, Feb 01, 2016 at 04:36:38PM -0800, Eric Dumazet 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;

It does not in fact do that :/ See commit:

  9e3d6223d209 ("math64, timers: Fix 32bit mul_u64_u32_shr() and friends")

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#1694708

On Thu, 2017-07-20 at 12:10 +0200, Peter Zijlstra wrote:
> On Mon, Feb 01, 2016 at 04:36:38PM -0800, Eric Dumazet 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;
> 
> It does not in fact do that :/ See commit:
> 
>   9e3d6223d209 ("math64, timers: Fix 32bit mul_u64_u32_shr() and friends")

Interesting :/

Thanks for letting me know !

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