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

[Peter Zijlstra]

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")