linux-kernel - Re: [PATCH] tcp: don't use __constant_cpu_to

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Date:   Mon, 18 Mar 2019 09:28:22 -0700
From:   Stanislav Fomichev <sdf@...ichev.me>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
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