__builtin_cpu_supports should accept unknown options

[DanielKristofKiss]

Backward\forward compatibility reasons any string shaped parameter in __builtin_cpu_supports should be accepted.
A given codebase might use a valid features that is supported in a new toolchain but not in the previous version.

__builtin_cpu_supports("unsupported-features")

It is valid to turn this into a constant false when the feature is not supported by the compiler.

see #82378

[llvmbot]

@llvm/issue-subscribers-backend-powerpc

Author: Dani (DanielKristofKiss)

Backward\forward compatibility reasons any string shaped parameter in __builtin_cpu_supports should be accepted. A given codebase might use a valid features that is supported in a new toolchain but not in the previous version.

__builtin_cpu_supports("unsupported-features")

It is valid to turn this into a constant false when the feature is not supported by the compiler.

see #82378

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Dani (DanielKristofKiss)

Backward\forward compatibility reasons any string shaped parameter in __builtin_cpu_supports should be accepted. A given codebase might use a valid features that is supported in a new toolchain but not in the previous version.

__builtin_cpu_supports("unsupported-features")

It is valid to turn this into a constant false when the feature is not supported by the compiler.

see #82378

[llvmbot]

@llvm/issue-subscribers-backend-arm

Author: Dani (DanielKristofKiss)

Backward\forward compatibility reasons any string shaped parameter in __builtin_cpu_supports should be accepted. A given codebase might use a valid features that is supported in a new toolchain but not in the previous version.

__builtin_cpu_supports("unsupported-features")

It is valid to turn this into a constant false when the feature is not supported by the compiler.

see #82378

[jroelofs]

ISTM this should come with a warning like you get for __attribute__((target_version("unsupported-feature"))). Then codebases that want to turn those back into an error can -Werror= it.

[KanRobert]

CC @FreddyLeaf