netdev - Re: [PATCHv2 bpf-next 1/3] bpf, tests: tweak endianness selection

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Date:   Thu, 21 Mar 2019 09:22:01 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Stanislav Fomichev <sdf@...ichev.me>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Shuah Khan <shuah@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        linux-kselftest@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCHv2 bpf-next 1/3] bpf, tests: tweak endianness selection

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