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

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Date:	Tue, 02 Feb 2016 21:46:05 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	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 Tue, Feb 02 2016, Eric Dumazet <eric.dumazet@...il.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