SIMD benchmark

[Peter Alexander]

On 16/01/12 10:36 PM, Iain Buclaw wrote:
> On 16 January 2012 21:57, Peter Alexander<peter.alexander.au at gmail.com>  wrote:
>> On 16/01/12 8:56 PM, Iain Buclaw wrote:
>>>
>>> On 16 January 2012 19:25, Walter Bright<newshound2 at digitalmars.com>
>>>   wrote:
>>>>
>>>> On 1/16/2012 11:16 AM, Iain Buclaw wrote:
>>>>
>>>>>>
>>>>>> But don't worry, I'm not planning on working on that at the moment :-)
>>>>>
>>>>>
>>>>> Leave that sort of optimisation for the backend to handle please. ;-)
>>>>
>>>>
>>>>
>>>> Of course.
>>>>
>>>> I suspect Intel's compiler does that one, does gcc?
>>>>
>>>
>>> There's auto-vectorisation for for(), foreach(), and foreach_reverse()
>>> loops that I have written support for.  I am not aware of GCC
>>> vectorising anything else.
>>>
>>> example:
>>>
>>> int a[256], b[256], c[256];
>>> void foo () {
>>>    for (int i=0; i<256; i++)
>>>      a[i] = b[i] + c[i];
>>> }
>>>
>>
>> Unfortunately, if the function was this:
>>
>> void foo(int[] a, int[] b, int[] c) {
>>
>>   for (int i=0; i<256; i++)
>>     a[i] = b[i] + c[i];
>> }
>>
>> Then it can't vectorize due to aliasing.
>
> Compile with -fstrict-aliasing then?

This has nothing to do with strict aliasing.

a[257];
foo(a[1..257], a[0..256], a[0..256]);

This doesn't break any strict aliasing rule, but the loop still cannot 
be (trivially) vectorized.

As Manu said, you need something like __restrict (or a linear type 
system) to solve this problem.

http://en.wikipedia.org/wiki/Linear_type_system
http://en.wikipedia.org/wiki/Uniqueness_typing