On 03/20, Sergey Senozhatsky wrote: > Not all compilers have __builtin_bswap16() and __builtin_bswap32(), > thus not all compilers are able to compile the following code: > > (__builtin_constant_p(x) ? \ > ___constant_swab16(x) : __builtin_bswap16(x)) > > That's the reason why bpf_ntohl() doesn't work on GCC < 4.8, for > instance: > > error: implicit declaration of function '__builtin_bswap16' > > We can use __builtin_bswap16() only if compiler has this built-in, > that is, only if __HAVE_BUILTIN_BSWAP16__ is defined. Standard UAPI > __swab16()/__swab32() take care of that, and, additionally, handle > __builtin_constant_p() cases as well: > > #ifdef __HAVE_BUILTIN_BSWAP16__ > #define __swab16(x) (__u16)__builtin_bswap16((__u16)(x)) > #else > #define __swab16(x) \ > (__builtin_constant_p((__u16)(x)) ? \ > ___constant_swab16(x) : \ > __fswab16(x)) > #endif > > So we can tweak selftests/bpf/bpf_endian.h and use UAPI > __swab16()/__swab32(). > > Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx> > --- > > v2: fixed build error, reshuffled patches (Stanislav Fomichev) Tested them locally with the compiler I saw the initial issues with - all fine, I don't see any errors with the older gcc. 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). > > tools/testing/selftests/bpf/bpf_endian.h | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > > diff --git a/tools/testing/selftests/bpf/bpf_endian.h b/tools/testing/selftests/bpf/bpf_endian.h > index b25595ea4a78..1ed268b2002b 100644 > --- a/tools/testing/selftests/bpf/bpf_endian.h > +++ b/tools/testing/selftests/bpf/bpf_endian.h > @@ -20,12 +20,12 @@ > * use different targets. > */ > #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ > -# define __bpf_ntohs(x) __builtin_bswap16(x) > -# define __bpf_htons(x) __builtin_bswap16(x) > +# define __bpf_ntohs(x) __swab16(x) > +# define __bpf_htons(x) __swab16(x) > # define __bpf_constant_ntohs(x) ___constant_swab16(x) > # define __bpf_constant_htons(x) ___constant_swab16(x) > -# define __bpf_ntohl(x) __builtin_bswap32(x) > -# define __bpf_htonl(x) __builtin_bswap32(x) > +# define __bpf_ntohl(x) __swab32(x) > +# define __bpf_htonl(x) __swab32(x) > # define __bpf_constant_ntohl(x) ___constant_swab32(x) > # define __bpf_constant_htonl(x) ___constant_swab32(x) > #elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__ > -- > 2.21.0 >
