[Mesa-dev] software implementation of vulkan for gsoc/evoc

[Nicolai Hähnle]

[ re-adding mesa-dev on the assumption that it got dropped by accident ]

On 13.02.2017 17:27, Jacob Lifshay wrote:
>         I would start a thread for each cpu, then have each thread run the
>         compute shader a number of times instead of having a thread per
>         shader
>         invocation.
>
>
>     This will not work.
>
>     Please, read again what the barrier() instruction does: When the
>     barrier() call is reached, _all_ threads within the workgroup are
>     supposed to be run until they reach that barrier() call.
>
>
> to clarify, I had meant that each os thread would run the sections of
> the shader between the barriers for all the shaders in a work group,
> then, when it finished the work group, it would go to the next work
> group assigned to the os thread.
>
> so, if our shader is:
> a = b + tid;
> barrier();
> d = e + f;
>
> and our simd width is 4, our work-group size is 128, and we have 16 os
> threads, then it will run for each os thread:
> for(workgroup = os_thread_index; workgroup < workgroup_count; workgroup++)
> {
>     for(tid_in_workgroup = 0; tid_in_workgroup < 128; tid_in_workgroup += 4)
>     {
>         ivec4 tid = ivec4(0, 1, 2, 3) + ivec4(tid_in_workgroup +
> workgroup * 128);
>         a[tid_in_workgroup / 4] = ivec_add(b[tid_in_workgroup / 4], tid);
>     }
>     memory_fence(); // if needed
>     for(tid_in_workgroup = 0; tid_in_workgroup < 128; tid_in_workgroup += 4)
>     {
>         d[tid_in_workgroup / 4] = vec_add(e[tid_in_workgroup / 4],
> f[tid_in_workgroup / 4]);
>     }
> }
> // after this, we run the next rendering or compute job

Okay good, that's the right concept.

Actually doing that is not at all straightforward though: consider that 
the barrier() might occur inside a loop in the shader.

So if you implemented that within the framework of llvmpipe, you'd make 
a lot of people very happy: it would allow finally adding compute shader 
support to llvmpipe. Mind you, that in itself would already be a pretty 
decent-sized project for GSoC!

Cheers,
Nicolai