[Bug target/94396] New: [8/9/10 Regression] fp16 feature bits not passed on to assembler from Armv8.4-a and up.

[tnfchris at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94396

            Bug ID: 94396
           Summary: [8/9/10 Regression] fp16 feature bits not passed on to
                    assembler from Armv8.4-a and up.
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: tnfchris at gcc dot gnu.org
          Reporter: tnfchris at gcc dot gnu.org
  Target Milestone: ---
            Target: aarch64-*

At Arm8.2-a using `+fp16fml` enables both `+fp` and `+fp16` as such the
smallest canonical option to use is `armv8.2-a+fp16fml`.

For `Armv8.4-a` and up `+fp16fml` is mandatory if `+fp16` but optional
otherwise. This requirement confuses the canonization.

This confusion makes `armv8.4-a+fp16` canonicalize to `armv8.4-a` which is
incorrect and so it's impossible to use `fp16` and `fp16fml` at `armv8.4-a` and
higher.