Change __ARM_NEON_SVE_BRIDGE to refer to the availability of the header

[SpencerAbson]

Afterthought: Another way of looking at this is that the user should not expect to be able to use intrinsics after specifying the relevant target features via anything other than the command line, it's unclear to me if this is the case.

The ACLE suggests the use of the predefined __ARM_NEON_SVE_BRIDGE macro to gaurd the inclusion of arm_neon_sve_bridge.h.

<arm_neon_sve_bridge.h> defines intrinsics for moving data between Neon and SVE vector types; see NEON-SVE Bridge for details. Before including the header, you should test the __ARM_NEON_SVE_BRIDGE macro.

The current definition of this macro is

__ARM_NEON_SVE_BRIDGE is defined to 1 if NEON-SVE Bridge
intrinsics are available. This implies that the following macros are nonzero

• __ARM_NEON
• __ARM_NEON_FP
• __ARM_FEATURE_SVE

The intrinsics described here are not preprocessor guarded (See change for LLVM). We should expect to be able to use them in any function with the necessary features, whether they are supplied globally on the command line or via a target attribute.

However, since we cannot make assumptions about the order in which the predefined feature macros are evaluated (see relevant ACLE), we cannot use the __ARM_NEON_SVE_BRIDGE macro to guard the inclusion of arm_neon_sve_bridge.h and expect to use it's builtins in unless the required features are supplied globally on the command line.

See an example of this issue (in LLVM Vs. GCC) from @georges-arm - https://godbolt.org/z/6YPvqdjTv.

The proposal of this PR is to change the meaning of __ARM_NEON_SVE_BRIDGE to refer to the availability of the arm_neon_sve_bridge.h header file only, such that it can be unconditionally defined in supporting compilers and it's builtins can be safely used in the context of the example above.

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[rsandifo-arm]

LGTM!

[vhscampos]

@all-contributors please add @SpencerAbson for content

[allcontributors]

@vhscampos

I've put up a pull request to add @SpencerAbson! 🎉