The kernels that were there: where a hand kernel beats the library, and where it can't

Sequel to "The kernel that wasn't there" . Last time I profiled my way out of writing a kernel and shipped streaming instead. This time I wrote them: a skinny bf16 GEMV that beats cuBLAS by up to 1.36x, and a four-stage SSD scan that loses to Triton by about 2x. The two outcomes together are the point. Last post ended on a promise. Profiling a Mamba2 decode step told me the scan everyone points at…