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From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Stanislav Fomichev <sdf@fomichev.me>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Shuah Khan <shuah@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Stanislav Fomichev <sdf@google.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	linux-kselftest@vger.kernel.org, netdev@vger.kernel.org,
	bpf@vger.kernel.org
Subject: Re: [PATCHv2 bpf-next 1/3] bpf, tests: tweak endianness selection
Date: Thu, 21 Mar 2019 09:22:01 +0900	[thread overview]
Message-ID: <20190321002201.GA2097@jagdpanzerIV> (raw)
In-Reply-To: <20190320171332.GJ7431@mini-arch.hsd1.ca.comcast.net>

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

  parent reply	other threads:[~2019-03-21  0:22 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-20 12:53 [PATCHv2 bpf-next 1/3] bpf, tests: tweak endianness selection Sergey Senozhatsky
2019-03-20 12:53 ` [PATCHv2 bpf-next 2/3] bpf, tests: drop unused __bpf_constant_foo defines Sergey Senozhatsky
2019-03-20 12:56   ` Sergey Senozhatsky
2019-03-20 12:53 ` [PATCHv2 bpf-next 3/3] bpf, tests: don't use __bpf_constant_htons() Sergey Senozhatsky
2019-03-20 12:55   ` Sergey Senozhatsky
2019-03-20 17:13 ` [PATCHv2 bpf-next 1/3] bpf, tests: tweak endianness selection Stanislav Fomichev
2019-03-20 22:20   ` Yonghong Song
2019-03-20 22:27     ` Stanislav Fomichev
2019-03-20 22:45       ` Yonghong Song
2019-03-20 23:08         ` Daniel Borkmann
2019-03-21  0:03           ` Stanislav Fomichev
2019-03-21  0:23             ` Yonghong Song
2019-03-21  0:34       ` Sergey Senozhatsky
2019-03-21  0:22   ` Sergey Senozhatsky [this message]
2019-03-21  3:24 ` Alexei Starovoitov
2019-03-21  5:08   ` Sergey Senozhatsky
2019-03-21 15:49     ` Stanislav Fomichev
2019-03-22  2:46       ` Sergey Senozhatsky
2019-03-22  3:12         ` Alexei Starovoitov

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