[neon] reciprocal square-root estimate

[gnzlbg]

No description provided.

[gnzlbg]

Do not merge. @alexcrichton for some reason cfg_target_feature is not detecting that neon is enabled (e.g. in aarch64 the neon tests are not running). If I disable the neon checks in arm/mod.rs the tests in aarch64 run and pass, but the arm-v6 and arm-v7 builds fail with a code-gen error. I thought that neon was orthogonal to aarch64 but from this error it seems that LLVM can only generate neon instructions for aarch64. @japaric do you have any experience with any of this?

[alexcrichton]

I think you can test this out locally via:

$ rustc +nightly -C target-feature=+neon --print cfg --target aarch64-unknown-linux-gnu
debug_assertions
target_arch="aarch64"
target_endian="little"
target_env="gnu"
target_family="unix"
target_has_atomic="128"
target_has_atomic="16"
target_has_atomic="32"
target_has_atomic="64"
target_has_atomic="8"
target_has_atomic="ptr"
target_os="linux"
target_pointer_width="64"
target_thread_local
target_vendor="unknown"
unix

(note the lack of neon)

unlike...

$ rustc +nightly -C target-feature=+neon --print cfg --target arm-unknown-linux-gnueabi
debug_assertions
target_arch="arm"
target_endian="little"
target_env="gnu"
target_family="unix"
target_feature="neon"
target_feature="vfp2"
target_feature="vfp3"
target_has_atomic="16"
target_has_atomic="32"
target_has_atomic="64"
target_has_atomic="8"
target_has_atomic="ptr"
target_os="linux"
target_pointer_width="32"
target_thread_local
target_vendor="unknown"
unix

Note the presence of neon!

Which leads me to conclude that this may be an upstream bug!

[gnzlbg]

@alexcrichton rust-lang/rust#45310

[alexcrichton]

👍

[alexcrichton]

Support is on master and looks like tests are green! I'll merge.