On (03/20/19 10:13), Stanislav Fomichev wrote: > Tested them locally with the compiler I saw the initial issues with - all > fine, I don't see any errors with the older gcc. Thanks! > One last question I have is: what happens in the llvm+bpf case? Have > you tested that? I think LLVM has all the builtins required, but since > we are relying on the swab.h now (and it relies on > __HAVE_BUILTIN_BSWAP16__), I wonder whether this detection works > correctly on the llvm when targeting bpf. (sidenote: bpf_endian.h can be > used from both userspace and bpf programs). Honestly, I haven't, but I think we should be fine. For !__HAVE_BUILTIN_BSWAP16__ compilers we still do constant folding. swab16/swab32 turn into __builtin_constant_p((__u16)(x)) ? ___constant_swab16(x) : __fswab16(x)) and __builtin_constant_p((__u32)(x)) ? ___constant_swab32(x) : __fswab32(x)) clang/llvm support __builtin_constant_p GCC extension [1]: : Clang supports a number of builtin library functions with the same : syntax as GCC, including things like __builtin_nan, __builtin_constant_p, : __builtin_choose_expr, __builtin_types_compatible_p, : __builtin_assume_aligned, __sync_fetch_and_add, etc. So clang should be able to detect swab on a compile time constant and optimize it. [1] https://clang.llvm.org/docs/LanguageExtensions.html -ss
