GPGPUs

[John Colvin]

On Friday, 16 August 2013 at 22:11:41 UTC, luminousone wrote:
> On Friday, 16 August 2013 at 20:07:32 UTC, John Colvin wrote:
>> We have a[] = b[] * c[] - 5; etc. which could work very neatly 
>> perhaps?
>
> While this in fact could work, given the nature of GPGPU it 
> would
> not be very effective.
>
> In a non shared memory and non cache coherent setup, the 
> entirety
> of all 3 arrays have to be copied into GPU memory, had that
> statement ran as gpu bytecode, and then copied back to complete
> the operation.
>
> GPGPU doesn't make sense on small code blocks, both in
> instruction count, and by how memory bound a particular 
> statement
> would be.
>
> The compiler needs to either be explicitly told what can/should
> be ran as a GPU function, or have some intelligence about what 
> to
> or not to run as a GPU function.
>
> This will get better in the future, APU's using the full HSA
> implementation will drastically reduce the "buyin" latency/cycle
> cost of using a GPGPU function, and make them more practical for
> smaller(in instruction count/memory boundess) operations.

I didn't literally mean automatically inserting GPU code.

I was more imagining this:

void foo(T)(T[] arr)
{
     useArray(arr);
}

auto a = someLongArray;
auto b = someOtherLongArray;

gpu
{
     auto aGPU = toGPUMem(a);
     auto bGPU = toGPUMem(b);

     auto c = GPUArr(a.length);

     c[] = a[] * b[];

     auto cCPU = toCPUMem(c);
     c.foo();

     dot(c, iota(c.length).array().toGPUMem())
         .foo();
}

gpu T dot(T)(T[] a, T[] b)
{
     //gpu dot product
}


with cpu arrays and gpu arrays identified separately in the type 
system. Automatic conversions could be possible, but of course 
that would allow carelessness.

Obviously there is some cpu code mixed in with the gpu code 
there, which should be executed asynchronously if possible. You 
could also have
onlyGPU
{
     //only code that can all be executed on the GPU.
}



Just ideas off the top of my head. Definitely full of holes and I 
haven't really considered the detail :)