test_bpf: Fix a new clang warning about xor-ing two numbers
diff mbox series

Message ID 20190819043419.68223-1-natechancellor@gmail.com
State Accepted
Delegated to: BPF Maintainers
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Series
  • test_bpf: Fix a new clang warning about xor-ing two numbers
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Commit Message

Nathan Chancellor Aug. 19, 2019, 4:34 a.m. UTC
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@gmail.com>
---

I highly doubt that 1e300 was intented but if it was (or something else
was), please let me know. Commit history wasn't entirely clear on why
this expression was used over just a raw number.

 lib/test_bpf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Yonghong Song Aug. 19, 2019, 5:51 a.m. UTC | #1
On 8/18/19 9:34 PM, 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@gmail.com>

Verified that latest trunk clang indeed has this warning, and below 
change indeed fixed the warning in the correct way.

Acked-by: Yonghong Song <yhs@fb.com>

> ---
> 
> I highly doubt that 1e300 was intented but if it was (or something else
> was), please let me know. Commit history wasn't entirely clear on why
> this expression was used over just a raw number.
> 
>   lib/test_bpf.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/test_bpf.c b/lib/test_bpf.c
> index c41705835cba..5ef3eccee27c 100644
> --- a/lib/test_bpf.c
> +++ b/lib/test_bpf.c
> @@ -867,7 +867,7 @@ static struct bpf_test tests[] = {
>   		},
>   		CLASSIC,
>   		{ },
> -		{ { 4, 10 ^ 300 }, { 20, 10 ^ 300 } },
> +		{ { 4, 0xA ^ 300 }, { 20, 0xA ^ 300 } },
>   	},
>   	{
>   		"SPILL_FILL",
>
Daniel Borkmann Aug. 20, 2019, 3:11 p.m. UTC | #2
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@gmail.com>

Applied, thanks!

Patch
diff mbox series

diff --git a/lib/test_bpf.c b/lib/test_bpf.c
index c41705835cba..5ef3eccee27c 100644
--- a/lib/test_bpf.c
+++ b/lib/test_bpf.c
@@ -867,7 +867,7 @@  static struct bpf_test tests[] = {
 		},
 		CLASSIC,
 		{ },
-		{ { 4, 10 ^ 300 }, { 20, 10 ^ 300 } },
+		{ { 4, 0xA ^ 300 }, { 20, 0xA ^ 300 } },
 	},
 	{
 		"SPILL_FILL",