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authorKenneth Graunke <[email protected]>2019-05-03 14:57:54 -0700
committerKenneth Graunke <[email protected]>2019-05-14 13:16:30 -0700
commit646924cfa1bb51c9091a8cf0079f00759cbea372 (patch)
treee69a694db5d38d01cc4ba05a659a3729cddffc88 /doxygen/math_subset.doxy
parent076159b40b96096ba01413abc011a26c9acf7176 (diff)
intel/compiler: Implement TCS 8_PATCH mode and INTEL_DEBUG=tcs8
Our tessellation control shaders can be dispatched in several modes. - SINGLE_PATCH (Gen7+) processes a single patch per thread, with each channel corresponding to a different patch vertex. PATCHLIST_N will launch (N / 8) threads. If N is less than 8, some channels will be disabled, leaving some untapped hardware capabilities. Conditionals based on gl_InvocationID are non-uniform, which means that they'll often have to execute both paths. However, if there are fewer than 8 vertices, all invocations will happen within a single thread, so barriers can become no-ops, which is nice. We also burn a maximum of 4 registers for ICP handles, so we can compile without regard for the value of N. It also works in all cases. - DUAL_PATCH mode processes up to two patches at a time, where the first four channels come from patch 1, and the second group of four come from patch 2. This tries to provide better EU utilization for small patches (N <= 4). It cannot be used in all cases. - 8_PATCH mode processes 8 patches at a time, with a thread launched per vertex in the patch. Each channel corresponds to the same vertex, but in each of the 8 patches. This utilizes all channels even for small patches. It also makes conditions on gl_InvocationID uniform, leading to proper jumps. Barriers, unfortunately, become real. Worse, for PATCHLIST_N, the thread payload burns N registers for ICP handles. This can burn up to 32 registers, or 1/4 of our register file, for URB handles. For Vulkan (and DX), we know the number of vertices at compile time, so we can limit the amount of waste. In GL, the patch dimension is dynamic state, so we either would have to waste all 32 (not reasonable) or guess (badly) and recompile. This is unfortunate. Because we can only spawn 16 thread instances, we can only use this mode for PATCHLIST_16 and smaller. The rest must use SINGLE_PATCH. This patch implements the new 8_PATCH TCS mode, but leaves us using SINGLE_PATCH by default. A new INTEL_DEBUG=tcs8 flag will switch to using 8_PATCH mode for testing and benchmarking purposes. We may want to consider using 8_PATCH mode in Vulkan in some cases. The data I've seen shows that 8_PATCH mode can be more efficient in some cases, but SINGLE_PATCH mode (the one we use today) is faster in other cases. Ultimately, the TES matters much more than the TCS for performance, so the decision may not matter much. Reviewed-by: Jason Ekstrand <[email protected]>
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