csrc

• A .cu is one unit of build parallelism, and it is compiled once per GPU architecture (-gencode on CUDA, --offload-arch on ROCm), so a file instantiating every variant of an op can be the build's whole critical path. Before adding instantiations, find the slowest unit of a cold build from .ninja_log (end minus start per object, MAX_JOBS fixed).
• Split on the instantiation axis (head size, output width, dtype, block size), never on line count: kernels in an include-only header, one thin .cu per variant, and an entry-point unit that only declares and dispatches, so every entry it declares is defined in a variant unit. Keep <torch/all.h> out of kernel headers. Copy an existing split in csrc/ and its CMake wiring, including the arch-gated source list of the file it came from.
• Never vary a shared function body with a per-unit #define. Identical symbol names with different bodies are an ODR violation, and the linker keeps one copy, so the other variants silently run the wrong code or fail: give each variant its own symbol instead. Derive the variant list, the dispatch arms and the CMake sources from one place, generating them where you can so no unit compiles without a dispatch arm.
• Call every variant from one test, and measure cold. A build-time claim needs ccache off, MAX_JOBS fixed, before and after, naming the unit that was the critical path: a split only pays off once its unit is, or is about to become, that path, and a compiler flag is no substitute without a measurement.