SIMD benchmark

[Martin Nowak]

On Tue, 17 Jan 2012 09:20:43 +0100, Manu <turkeyman at gmail.com> wrote:

> On 17 January 2012 05:55, bearophile <bearophileHUGS at lycos.com> wrote:
>
>> Walter:
>>
>> > But don't worry, I'm not planning on working on that at the moment :-)
>>
>> Until better optimizations are implemented, I see a "simple"  
>> optimization
>> for vector ops. When the compiler knows an arrays are very small it  
>> unrolls
>> the operation in-place:
>>
>> int n = 5;
>> auto a = new int[n];
>> auto b = new int[n];
>> a[] += b[];
>>
>> ==>
>>
>> int n = 5;
>> auto a = new int[n]; // a and b are dynamic arrays,
>> auto b = new int[n]; // but their length is easy to find at compile-time
>> a[0] += b[0];
>> a[1] += b[1];
>> a[2] += b[2];
>> a[3] += b[4];
>> a[5] += b[5];
>>
>
> If this doesn't already exist, I think it's quite important. I was  
> thinking
> about needing to repeatedly specialise a template last night for a bunch  
> of
> short lengths of arrays, for this exact reason.
> Unrolling short loops must be one of the most trivial and worthwhile
> optimisations...

If the compiler knows it's a compile time constant
thus you could use a static foreach.