jhorstmann opened a new issue, #1829:
URL: https://github.com/apache/arrow-rs/issues/1829
**Is your feature request related to a problem or challenge? Please describe
what you are trying to do.**
Related to extending the tests for different features flags #1822, I wanted
to take another look at the avx512 feature and its performance. Benchmarks were
run on an i9-11900KB @ 3Ghz (turbo disabled) with
```
RUSTFLAGS="-Ctarget-cpu=native -Ctarget-feature=-prefer-256-bit"
```
(the second flag might require some explanation, it **disables** the
`prefer-256-bit` feature, which makes llvm use the full 512 bit vectors)
For some reason the second benchmark is always significantly slower when run
together, running them separately gives the same (higher) performance and the
assembly looks identical except for the and/or. I'm guessing branch predictor
or allocator related.
## Default features
```
$ cargo +nightly bench --bench buffer_bit_ops
buffer_bit_ops/buffer_bit_ops and
time: [134.57 ns 134.90 ns 135.28 ns]
thrpt: [105.74 GiB/s 106.04 GiB/s 106.30 GiB/s]
buffer_bit_ops/buffer_bit_ops or
time: [275.55 ns 276.22 ns 277.03 ns]
thrpt: [51.637 GiB/s 51.789 GiB/s 51.914 GiB/s]
```
## Simd feature
```
$ cargo +nightly bench --features simd --bench buffer_bit_ops
buffer_bit_ops/buffer_bit_ops and
time: [168.90 ns 169.10 ns 169.32 ns]
thrpt: [84.486 GiB/s 84.596 GiB/s 84.697 GiB/s]
buffer_bit_ops/buffer_bit_ops or
time: [303.13 ns 303.27 ns 303.45 ns]
thrpt: [47.142 GiB/s 47.169 GiB/s 47.192 GiB/s]
```
## Avx512 feature
```
$ cargo +nightly bench --features avx512 --bench buffer_bit_ops --
buffer_bit_ops/buffer_bit_ops and
time: [165.46 ns 165.95 ns 166.83 ns]
thrpt: [85.745 GiB/s 86.203 GiB/s 86.458 GiB/s]
buffer_bit_ops/buffer_bit_ops or
time: [310.63 ns 311.32 ns 312.04 ns]
thrpt: [45.844 GiB/s 45.950 GiB/s 46.052 GiB/s]
```
Generated assembly for `simd` and `avx512` looks identical, the loop
calculates 512bits/64 bytes. The auto-vectorized version instead gets unrolled
4 times, which reduces the loop overhead, so each iteration processes 4x512bits.
**Describe the solution you'd like**
With these benchmark results it seems that we can remove the `avx512`
feature and simplify the buffer code.
**Describe alternatives you've considered**
An `avx512` feature for other kernels would still be very useful. Avx512 for
example has instructions that basically implement the filter kernel for
primitives in a single instruction and it is unlikely that these will be
supported in a portable way soon
(https://github.com/rust-lang/portable-simd/issues/240).
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