Enable wasm32 as a target architecture for the SIMD feature

[roee88]

Which issue does this PR close?

Closes #316

Rationale for this change

packed_simd compiles to wasm32-wasi and wasm32-unknown-unknown targets (and includes CI against it). It will only work on WASM runtimes that support the WASM SIMD proposal but that's no reason to block it from arrow-rs side.

What changes are included in this PR?

Same as #303 but without limiting to specific architectures. The rationale is described in #316:

Optional features should not be limited to specific architectures in arrow-rs itself. The arrow-rs CI shouldn't run tests with combinations that are not known to work, but as a user I would expect that if I enable a feature then it's enabled (and might create a failure on its own if it doesn't work yet).

If this turns out to be a problem with the CI then I will revert to just adding wasm32 to build.rs.

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

Merging #324 (c5788c5) into master (c863a2c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #324   +/-   ##
=======================================
  Coverage   82.52%   82.52%           
=======================================
  Files         162      162           
  Lines       44007    44007           
=======================================
  Hits        36316    36316           
  Misses       7691     7691           

Impacted Files	Coverage Δ
arrow/src/buffer/ops.rs	96.96% <ø> (ø)
arrow/src/compute/kernels/aggregate.rs	73.40% <ø> (ø)
arrow/src/compute/kernels/arithmetic.rs	87.36% <ø> (ø)
arrow/src/compute/kernels/comparison.rs	95.84% <ø> (ø)
arrow/src/util/bit_util.rs	100.00% <ø> (ø)

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[jorgecarleitao]

cc @houqp

[alamb]

This seems like a fine change to me. As @jorgecarleitao probably found, this configuration was added in 73b7576 by @houqp so I think it would be good if he wanted to weigh in

[roee88]

Sure. Though I don't think that the link you shared actually affects it. Originally there was no feature flag for simd and it was always enabled for x86 and x86_64. Then, a feature flag was added on top to allow disabling simd, it was then set to disabled by default, then cfg_aliases was used (no functionality change), and finally aarch64 was added to the list of architectures.

So I might be wrong but it all seems to be for historic reasons. However, this applies only as long as it's disabled by default. If at some point it will change to enable by default it would need to be enabled by default only for known supported architectures and disabled by default otherwise. Is that something that's possible to implement easily?

[alamb]

Yeah, this looks good to me

I tested (on a mac, with the most recently updated nightly buid)

cargo +nightly build  --target=wasm32-unknown-unknown  --features=simd   -p arrow

and it built 👍

[houqp]

Apologize for missing the ping, yes, the cfg_aliases was added with the goal to not introduce any functioning change at the time. I am all for simplifying it to use the native feature flag and let build fail for unsupported architectures :)