Tell ptxas the largest block size a kernel can be launched with - #9284
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abadams wants to merge 3 commits into
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Tell ptxas the largest block size a kernel can be launched with#9284abadams wants to merge 3 commits into
abadams wants to merge 3 commits into
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Without it ptxas has to assume a block could hold the maximum number of threads, which caps it at 65536/1024 = 64 registers per thread. The kernel in apps/cuda_mat_mul is launched with 16x2 threads and wants 70 registers, so it spilled: 20 bytes of spill stores and 16 of spill loads. Giving ptxas the bound removes the spills, and the app goes from 0.314 ms to 0.233 ms. This has to be a function attribute. The equivalent nvvm.annotations entry, which is how the kernel annotation just above is written, is upgraded to one of these when a module is read from a file, but that never happens to a module we built ourselves, and the backend only looks at the attribute. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
alexreinking
approved these changes
Aug 5, 2026
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Without it ptxas has to assume a block could hold the maximum number of threads, which caps it at 65536/1024 = 64 registers per thread. The kernel in apps/cuda_mat_mul is launched with 16x2 threads and wants 70 registers, so it spilled: 20 bytes of spill stores and 16 of spill loads. Giving ptxas the bound removes the spills, and the app goes from 0.314 ms to 0.233 ms.
I believe this is the underlying reason we found a benefit in manually messing with the number of registers with an env var - code that was backed out in #9260.