Header-only `__device__` function `sp4_histogram_p99<BLOCK_SIZE>(buf, count)`
returning p99(|buf|) via three-pass single-block algorithm:
Pass 1: block-wide max-reduce → step_max (0.0 short-circuit on degenerate)
Pass 2: linear-spaced [0, step_max] binning into 256 bins via per-warp
tiles (no atomicAdd; lane-collisions cost <0.012% precision)
Pass 3: cumulative-from-top → p99 = bin upper-edge
Returns 0.0 for degenerate step_max=0 (caller skips ISV update). Linear
spacing chosen over log because SP4 producers care about resolution at
the top of the distribution — top bin width = step_max/256 ≈ 0.39%, well
within the 1% quantile precision budget.
Wrapper kernel `sp4_histogram_p99_test_kernel` exposes the device fn for
Rust testing; mapped-pinned scalar output with __threadfence_system() so
host read_volatile sees the result (no memcpy_dtoh). Build.rs
registration mirrors thompson_test_kernel.cu pattern + adds
rerun-if-changed for the .cuh header.
Unit test: 4096 deterministic |N(0,1)| Box-Muller samples, sorted
ground-truth p99 ≈ 2.576 z-score one-tailed, asserts rel_err < 5%
against device output. GPU-gated (#[ignore]). Local L40S run:
true_p99=2.59758, computed_p99=2.59858, rel_err=0.039% — passes.
No producer wired yet — header is library code included only by the
test wrapper; SP4 Tasks A5-A9 add the magnitude-bound producer kernels
that #include "sp4_histogram_p99.cuh" directly. Behaviour unchanged.
cargo check --lib --tests clean.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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