(Re-)Introduce ARMv7+NEON Android and GNU/Linux targets

[hsivonen]

Even though ARMv7 Android doesn't guarantee the presence of NEON, most devices have it and e.g. Firefox opts not to support ARMv7 devices that don't have it. (Notably the Tegra 2-based ones.)

To allow Rust code, including the standard library, to unconditionally use NEON in such a scenario, it would help a lot if upstream Rust supported an ARMv7+NEON Android target. (NEON support was explicitly removed from armv7-linux-androideabi.

In order to test code on a system that has all the usual tooling, it's useful to have a corresponding GNU/Linux target, too. NEON was explicitly removed from armv7-unknown-linux-gnueabihf and also from armv7-unknown-linux-musleabihf.

Please add new androideabi and gnueabihf ARMv7 targets that have NEON enabled and a NEON-enabled standard library shipped via rustup so that std::simd ends up compiled with NEON enabled. (See the issue about boolean reductions there.)

How to name these is a total bikeshed, but as a starting point, I suggest armv7neon-unknown-linux-gnueabihf and armv7neon-linux-androideabi.

[gnzlbg]

Note that aarch64 has the same issue, so it might be worth it to add the aarch64neon-unknown-linux-gnu and aarch64neon-linux-android targets as well. Maybe also aarch64neon-apple-ios :/

[hsivonen]

I thought that ARMv8 guaranteed 128-bit SIMD support like x86_64 guarantees SSE2. Is that not the case?

[hsivonen]

It seems that Rust's aarch64-unknown-linux-gnu target feels eligible to generate ldr q0, [x0] for loading an u8x16 from memory.

Programmer’s Guide for ARMv8-A says:

Both floating-point and NEON are required in all standard ARMv8 implementations. However, implementations targeting specialized markets may support the following combinations:

• No NEON or floating-point.
  ...

I would hope that the Rust built-in aarch64 targets were for "standard implementations" and not for "specialized markets", so if they fail to allow some NEON instructions, we should probably edit those targets in place instead of introducing aarch64neon variants. But, as noted, aarch64 in Rust seems to allow 128-bit SIMD already.

[gnzlbg]

I just tried it on stdsimd and aarch64-unknown-linux-gnu does indeed have neon/asimd always enabled :)

[hsivonen]

These were added in #49902.