Messages in this thread | | | Date | Mon, 18 Mar 2019 09:28:22 -0700 | From | Stanislav Fomichev <> | Subject | Re: [PATCH] tcp: don't use __constant_cpu_to_be32 |
| |
On 03/16, Sergey Senozhatsky wrote: > 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. That sounds convincing, especially since we have the following in include/linux/compiler-gcc.h:
#if GCC_VERSION >= 40800 #define __HAVE_BUILTIN_BSWAP16__ #endif
So it does indeed look like it's not __builtin_constant_p, but rather missing __builtin_bswap16 for gcc < 4.8.
> > -ss
| |