There is no known (real) hardware with V and without the complete B
extension. B was indeed required in the RISC-V application profile from
2022, earlier than V. There should not be any relevant hardware in the
future either.
In practice, different R-V Vector optimisations in FFmpeg already depend on
every constituent of the B extension anyhow, so it would not work well.
The code was blindly assuming that Zbb or V implied Zba. While the
earlier is practically always true, the later broke some QEMU setups,
as V was introduced earlier than Zba.
RV64G supports MIN & MAX instructions natively only on floating point
registers, not general purpose ones. The later would require the Zbb
extension. Due to that, it is actually faster to perform the clipping
"properly" in FPU.
Benchmarks on SiFive U74-MC (courtesy of Shanghai StarFive Tech):
audiodsp.vector_clipf_c: 29551.5
audiodsp.vector_clipf_rvf: 17871.0
Also tried unrolling with 2 or 8 elements but it gets worse either way.
