[ 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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