Messages in this thread | | | From | Sergey Senozhatsky <> | Date | Sat, 16 Mar 2019 14:19:02 +0900 | Subject | Re: [PATCH] tcp: don't use __constant_cpu_to_be32 |
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On (03/15/19 10:24), Stanislav Fomichev wrote: > On 03/15, Sergey Senozhatsky wrote: > > On (03/14/19 05:42), Eric Dumazet wrote: > > > > cpu_to_be32() is capable enough to detect __builtin_constant_p() > > > > and to use an appropriate compile time ___constant_swahb32() > > > > function. > See my recent commit a0517a0f7ef23 ("selftests/bpf: use > __bpf_constant_htons in test_prog.c") where compiler was not smart > enough to figure it out (you can google similar issues for GCC < 4.8).
Allow me to disagree.
The error in a0517a0f7ef23 says
error: implicit declaration of function '__builtin_bswap16'
which doesn't sound like "the compiler was not smart".
Let's look at __swab16
100 #ifdef __HAVE_BUILTIN_BSWAP16__ 101 #define __swab16(x) (__u16)__builtin_bswap16((__u16)(x)) 102 #else 103 #define __swab16(x) \ 104 (__builtin_constant_p((__u16)(x)) ? \ 105 ___constant_swab16(x) : \ 106 __fswab16(x)) 107 #endif
We can call __builtin_bswap16() only when the compiler has that builtin, which is what #ifdef __HAVE_BUILTIN_BSWAP16__ for.
But this is not what tools/testing/selftests/bpf/bpf_endian.h does
# define __bpf_ntohs(x) __builtin_bswap16(x) # define __bpf_htons(x) __builtin_bswap16(x)
So I sort of suspect that what should have been done was that __HAVE_BUILTIN_BSWAP16__ ifdef, just like what include/uapi/linux/swab.h does.
-ss
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