[clang] [llvm] [AArch64] Decouple feature dependency expansion. (PR #94279)

[David Green via llvm-commits]

davemgreen wrote:

Yeah I had just seen that error message before you edited your comment. There are some examples of neon I found in a quick search, which were presumably added for AArch32:
https://github.com/aaru-dps/Aaru.Checksums.Native/blob/bd5051ce181b225a7662bfb764ebcc5cbe7542b2/simd.h#L112
https://github.com/mooch443/commons/blob/30dc797430968831959d77d7f2503cec3518a13a/common/misc/PVBlob.cpp#L385
I'm not sure if that is reason enough to still support it.

But like I said, if I try this patch locally then `target("neon")` seems to be accepted fine (no errors). It is the same for other features like `target("fullfp16")`, which seem to enable `+fullfp16` in the backend. `"noneon"` is no longer accepted, which might be fine as I don't believe negative features are commonly used. (For aarch64 from a baseline of armv8 they are mostly additive. They are likely to become more common going forward but new users can use the "right" attribute names).

https://github.com/llvm/llvm-project/pull/94279