atrosinenko wrote:
I tried adding a few more lines to
`clang/test/Preprocessor/aarch64-target-features.c` and turned out, clang
accepts `-march=armv8-a+jscvt` but not `-march=armv8-a+dpb`. Brief debugging
suggests this is because in the
```cpp
{"dpb", AArch64::AEK_NONE, {}, {}, FEAT_DPB, "+ccpp", 190},
```
line, `Feature` and `NegFeature` fields are empty.
I wonder if there is an overview of all the "feature-related" AArch64 stuff
anywhere. As far as I understand, there are several kinds of identifiers:
* feature names defined in the chapter "A2: Armv8-A Architecture Extensions" of
the Arm Architecture Reference Manual (such as `FEAT_JSCVT` mentioned in "A2.5:
The Armv8.3 architecture extension")
* Function Multi-Versioning CPU features defined in `enum CPUFeatures` (such as
`FEAT_JSCVT`)
* `AEK_*` constants defined in `enum ArchExtKind` (such as `AEK_JSCVT`)
* `Name`s defined in the `Extensions` array in AArch64TargetParser.h
* `Feature` / `NegFeature` defined in the same array
* `DependentFeatures` defined in the same array
I guess, each kind of identifiers belongs to one of logical "namespaces". Here
is what I see:
* the names of FMV features looks identical to feature names defined in Arm
ARM, but the _sets_ of identifiers are different - the former is a strict
subset of the latter at best. Obviously, there is a number of features hardly
related to compiler development (such as various properties of hardware debug
interfaces) and some other are not related to runtime function selection by FMV
machinery (or at least the rules are not yet written in ACLE for them). It is
just a bit misleading that Arm ARM defines FEAT_PAUTH and this feature is
mentioned as AEK_PAUTH, "+pauth" literal, etc. but not as FEAT_PAUTH in the
enumeration with constants named `FEAT_*` :)
* after `AEK_*` constants were recently made not restricted by the bitsize of
`uint64_t` anymore, these look like the best mirror of `FEAT_*` identifiers
defined in Arm ARM (and there is an explicit [naming
guideline](https://github.com/llvm/llvm-project/blob/7f409cd82b322038f08a984a07377758e76b0e4c/llvm/include/llvm/TargetParser/AArch64TargetParser.h#L106))
* `Name` fields are used when parsing `-march=...+feature` arguments of Clang.
These are kind of user-visible interface
* `Feature`/`NegFeature` and `DependentFeatures` are used when parsing
`-target-feature`s passed to `clang -cc1` and are likely to contain string
identifiers from the same namespace, according to seemingly the [only in-tree
user](https://github.com/llvm/llvm-project/blob/7fdb932c4e78706cec3468a1f149e5a54a865d36/clang/lib/Basic/Targets/AArch64.cpp#L1013)
of `DependentFeatures` contents, but duplication like `{"jscvt",
AArch64::AEK_JSCVT, "+jsconv", "-jsconv", FEAT_JSCVT,
"+fp-armv8,+neon,+jsconv", 210}` (E.Feature additionally listed in
E.DependentFeatures) looks redundant
https://github.com/llvm/llvm-project/pull/79311
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