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Neither GCC nor Clang define an __arm64__ preprocessor macro, but use __aarch64__ (MSVC uses _MARM_64). Add a "64" suffix to the define, i.e. NETGEN_ARCH_ARM64 to make it more obvious in only refers to aarch64, and to be in line with NETGEN_ARCH_AMD64. Replace the (Clang specific) __builtin_readcyclecounter with inline asm: - The function return cycles (i.e. varies with CPU frequency), not time - It may return 0, depending on the PMU settings - It may cause an illegal instruction, in case it is not trapped by the kernel, e.g. on FreeBSD. Reading the generic timer/counter CNTVCT_EL0 instead of PMCCNTR_EL0 avoids these pitfalls. The inline asm works on GCC and Clang, instead of Clang only for the builtin.