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

[Jacob Lifshay]

the algorithm i was going to use would get the union of the sets of live
variables at the barriers (union over barriers), create an array of structs
that holds them all, then for each barrier, insert the code to store all
live variables, then end the for loop over tid_in_workgroup, then run the
memory barrier, then start another for loop over tid_in_workgroup, then
load all live variables.
Jacob Lifshay

On Feb 13, 2017 08:45, "Nicolai Hähnle" <nhaehnle at gmail.com> wrote:

> [ 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
>
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