commit 3a3c1a4fd5e397f9860530598f064322a475c35f
author bjacob <benoitjacob@google.com> Wed Nov 08 12:29:25 2023 -0500
committer GitHub <noreply@github.com> Wed Nov 08 12:29:25 2023 -0500
tree 2e8e92ae66f8bb172c41730f47bd9eb6ec79fbd5
parent 87ed5fc589ff7501a4dad96888e30ecf4890eae6

Fix `fp16` feature on arm64: the proper feature name is `fullfp16`, not `fp16`. (#15479)

The symptom being fixed here is these warnings while building
`iree-test-deps` on arm64:

```
'+fp16' is not a recognized feature for this target (ignoring feature)
```

This was saying that bytecode modules that are meant to be compiled with
fp16 native arithmetic instructions were actually compiled without them,
falling back on slower paths converting to fp32.

The problem was that we were mixing up two concepts in the LLVM AArch64
target: "extension" vs "feature". The features are the individual,
mutually orthogonal things that we want to be working with here. The
feature name here is `fullfp16`. Extensions are a higher-level
construct, a group of features. Many extensions contain only one
feature, and in that case, most of the time, the extension name equals
the feature name. But `fp16` is an exception:


https://github.com/llvm/llvm-project/blob/26ab444e88fc8fdd554e5a9381a68b7b5e63b6fd/llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp#L3639

I think the most important dimension here is orthogonality. Features are
mutually orthogonal, so they are what we want to reflect in our own CPU
feature bits.

The one place where we need to keep working with extensions is in Clang
copts, when building either native code or bitcode in ukernels. There,
the `-march=armv8.2-a+...` wants extensions, not features.