[llvm] Remove inline constexpr from AArch64TargetParser.h

[rsuderman]

The use of inline constexpr appears to be a premature optimization that can cause failures on some platforms. StringRef has a heavy constructor that can be missed on dylib loading causing the Extension table values to be incorrect. Switching from inline constexpr to const results in no increase in binary size of llc indicating the optimization is unnecessary.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[topperc]

Does using StringLiteral instead of StringRef in the struct help? It has a more specialized constructor for constexpr.

[rsuderman]

Does using StringLiteral instead of StringRef in the struct help? It has a more specialized constructor for constexpr.

I attempted to replace with StringLiteral instead of StringRef however some of the constructors use empty list initializers which is not supported by the StringLiteral constructors. I performed the replacement for the cases where it was possible and it still did not avoid the Extension tables corruption issue.

It is worth noting I was able to simply drop the inline keyword to achieve the same effect as const if that is preferable.

[topperc]

Does using StringLiteral instead of StringRef in the struct help? It has a more specialized constructor for constexpr.

I attempted to replace with StringLiteral instead of StringRef however some of the constructors use empty list initializers which is not supported by the StringLiteral constructors.

Does using "" instead of the empty list initializer work?

I performed the replacement for the cases where it was possible and it still did not avoid the Extension tables corruption issue.

What does where it was possible mean? What change did you make?

[qcolombet]

Given other targets use const in similar situation, that sounds reasonable, but I don't know that part of the code base at all.
So take my comment with a grain of salt.

[topperc]

Why are these arrays declared in the .h file? They appear to only be used by AArch64TargetParser.cpp

[rsuderman]

Why are these arrays declared in the .h file? They appear to only be used by AArch64TargetParser.cpp

Sadly they do have uses outside of the target parser. E.g. in llvm-project/clang/lib/Basic/Targets/AArch64.cpp

[tmatheson-arm]

Just to add some context: IIRC, inline constexpr was used to guarantee that these objects had unique addresses, which was necessary because some equality checks were based on address. That turned out to be unworkable and was removed, so I think the inline constexpr is no longer necessary. It wasn't for optimisation.