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

[Eric Dumazet]

On Tue, 2016-02-02 at 21:46 +0100, Rasmus Villemoes wrote:
> On Tue, Feb 02 2016, Eric Dumazet <eric.dumazet@xxxxxxxxx> 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))