[HiderX]

Summary

RISCV64_ZVL256B builds DSDOT from the scalar ../generic/dot.c. This patch
switches it to dot_rvv.c, compiled with -DDSDOT, which RISCV64_ZVL128B
already uses. It is a one-line change in KERNEL.RISCV64_ZVL256B.

Performance

Platform: SpacemiT K3, one X100 core (2.2 GHz), single thread, GCC 15.2,
cblas_dsdot with unit stride. Each number is the median ns per call, built
exactly from this patch:

n	develop	patched	speed-up
1024	1235.2	249.7	4.9x
32768	38841	11315	3.4x
4194304	5019082	4426346	1.13x

At every size, all patched processes were faster than all develop processes.
Two earlier jobs ran the same DSDOT kernel object in a larger kernel-selection
experiment: an exploratory job and a separate confirmation job, each with
4 processes per build, alternating. They measured 4.2x, 3.4x and 1.14–1.17x.

Testing

• OpenBLAS tests. The default make goal's tests give identical results
  before and after the patch: utest 141/141, extension tests 1479/1479, and all
  BLAS/CBLAS tests PASSED.

• Accuracy checks against a long-double reference. The result must be
  within 2·n·eps·Σ|x·y| of the reference. The cases cover:

  • tail-heavy lengths from 1 to 32775;
  • strides 1, 2 and 3;
  • zero vectors;
  • magnitudes around 1e30 and 1e-30, and mixtures of the two;
  • Inf/NaN propagation.

  There were no failures, before or after the patch.

Disclosure: an AI coding agent (Anthropic Claude) prepared this patch, its
validation runs and this description on behalf of the account owner. All
numbers above come from archived runs on the board.