When converting to split-sources, we disgorged ReverseHashBufferIfNeeded from Intel CLMUL and ARM PMULL operations. The problem is, they are linked. The only time a buffer needs reversing is when CLMUL or PMULL is in effect.
However, we made GCM_ReverseHashBufferIfNeeded_CLMUL and GCM_ReverseHashBufferIfNeeded_PMULL available wheneever SSSE3 or NEON was available, which was incorrect. They should only be used when CLMUL or PMULL is being used
getauxval() is the recommended way to determine features on Linux. Its likely less expensive than CPU probing for SIGILLs. We gave up portability, but some gained stability
We only need to base it on the compiler in config.h. config.h activates the code path guarded by HasNEON(). The source file that actially provides the NEON implementation will be compiled with -fpu=neon or -march=armv8-a.
Since we are providing the specialized implementation in a sequestered source file (and not a header file), we can probably avoid the defines like CRYPTOPP_ARM_NEON_AVAILABLE altogether.
Jeffrey Walton