20160806

Uber Shader Unrolling

Looking at running a compute only pass, no graphics waves to contend with on the machine, so it becomes relatively easy to think about occupancy. Target 4 waves per SIMD via a 4 wave work-group (one work-group per SIMD unit). That provides 4 work-groups sharing a Compute Unit (CU) and L1 cache. This is only 16/40 occupancy, but enough in theory to maintain a good amount of multi-issue of waves to functional units. Each wave gets 64 VGPRs, each work-group gets 16KB LDS (16 32-bit words/invocation on average).

In this fixed context, one can leverage compile-time unrolling to manage variable register allocation for different sub-shaders in an uber-shader. Unrolling as in running more than one instance of the shader in the uber-shader at a given time.

Unroll 2 in parallel = 32 VGPRs, 8 words LDS
Unroll 3 in parallel = 21 VGPRs, 5 words LDS
Unroll 4 in parallel = 16 VGPRs, 4 words LDS

But it doesn't have to be this fixed. One can have variable blending of N parallel instances. Meaning as register usage starts to drain from one instance, start ramping up the other instance. Also enables instances to share intermediate computations.

This more "pinned task" model with unrolling in theory in some cases (maybe really short shaders like particle blending) would allow better utilization of the machine, than separate kernel launches for everything. During shader start as the shader ramps up, the VGPRs allocated to it are under utilized. During shader ramp down towards exit, VGPRs are also under utilized. Unrolling can blend the fill and drain.

Clearly there is also a question of unrolling out of instruction cache.

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