linux-kernel - Re: [PATCH] test_bpf: Fix a new clang warning about xor-ing two numbers

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Message-ID: <636449fa-9bfb-2c1e-af20-2ddac83a2143@iogearbox.net>
Date:   Tue, 20 Aug 2019 17:11:25 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Nathan Chancellor <natechancellor@...il.com>,
        Alexei Starovoitov <ast@...nel.org>
Cc:     Martin KaFai Lau <kafai@...com>, Song Liu <songliubraving@...com>,
        Yonghong Song <yhs@...com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] test_bpf: Fix a new clang warning about xor-ing two
 numbers

On 8/19/19 6:34 AM, Nathan Chancellor wrote:
> r369217 in clang added a new warning about potential misuse of the xor
> operator as an exponentiation operator:
> 
> ../lib/test_bpf.c:870:13: warning: result of '10 ^ 300' is 294; did you
> mean '1e300'? [-Wxor-used-as-pow]
>                  { { 4, 10 ^ 300 }, { 20, 10 ^ 300 } },
>                         ~~~^~~~~
>                         1e300
> ../lib/test_bpf.c:870:13: note: replace expression with '0xA ^ 300' to
> silence this warning
> ../lib/test_bpf.c:870:31: warning: result of '10 ^ 300' is 294; did you
> mean '1e300'? [-Wxor-used-as-pow]
>                  { { 4, 10 ^ 300 }, { 20, 10 ^ 300 } },
>                                           ~~~^~~~~
>                                           1e300
> ../lib/test_bpf.c:870:31: note: replace expression with '0xA ^ 300' to
> silence this warning
> 
> The commit link for this new warning has some good logic behind wanting
> to add it but this instance appears to be a false positive. Adopt its
> suggestion to silence the warning but not change the code. According to
> the differential review link in the clang commit, GCC may eventually
> adopt this warning as well.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/643
> Link: https://github.com/llvm/llvm-project/commit/920890e26812f808a74c60ebc14cc636dac661c1
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>

Applied, thanks!