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Date: Tue, 02 Feb 2016 13:30:10 -0800
From: Eric Dumazet <eric.dumazet@...il.com>
To: Rasmus Villemoes <linux@...musvillemoes.dk>
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, 2016-02-02 at 21:46 +0100, Rasmus Villemoes wrote:
> 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
crypto/vmac.c has this, you could make it generic maybe.
#define MUL32(i1, i2) ((u64)(u32)(i1)*(u32)(i2))
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