SIMD benchmark

[Peter Alexander]

On 17/01/12 11:34 PM, Walter Bright wrote:
> On 1/17/2012 3:23 PM, Peter Alexander wrote:
>> On 17/01/12 10:55 PM, Walter Bright wrote:
>>> On 1/17/2012 1:47 PM, Peter Alexander wrote:
>>>> On 17/01/12 9:24 PM, Walter Bright wrote:
>>>>> On 1/17/2012 1:20 PM, Peter Alexander wrote:
>>>>>> As Manu said, you need something like __restrict (or a linear type
>>>>>> system) to
>>>>>> solve this problem.
>>>>>
>>>>> No, you don't. It can be done with a runtime check, like array bounds
>>>>> checking is done.
>>>>
>>>> So you'd change it to this, even in release builds?
>>>
>>> No. Like array bounds, if they overlap, an exception is thrown.
>>>
>>> Remember, the D spec says that overlapping arrays are illegal.
>>
>> The D spec says that overlapping arrays are illegal for vector ops. The
>> foo(int[], int[], int[]) function does not use vector ops.
>>
>> Or am I missing something really major?
>>
>> For example, is this legal code?
>>
>> int[100] a;
>> int[] b = a[0..100];
>> int[] c = a[10..90]; // Illegal? b and c overlap...
>
> No, not illegal.
>
>>
>> foreach (i; 0..80)
>> c[i] = b[i]; // Illegal?
>
> No, not illegal.
>
>> I know that b[] = c[] would be illegal, but that has nothing to do
>> with the
>> prior discussion.
>
> Yes, b[]=c[] is illegal.

So, my original point still stands, you can't vectorise this function:

void foo(int[] a, int[] b, int[] c)
{
   foreach (i; 0..256)
     a[i] = b[i] + c[i];
}

Those slices are allowed to overlap, so this cannot be automatically 
vectorised (without inlining to get better context about those arrays).

Without inlining, you need something along the lines of __restrict or 
uniqueness typing.