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

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Message-ID: <20170720101001.kubsvi5mpzwp3ujd@hirez.programming.kicks-ass.net>
Date:   Thu, 20 Jul 2017 12:10:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andi Kleen <ak@...ux.intel.com>,
        Andi Kleen <andi@...stfloor.org>, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, davidlohr.bueso@...com,
        rafael.j.wysocki@...el.com, lenb@...nel.org
Subject: 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")