[cfe-dev] -Wunused-value gets triggered only when storing/using intermediate outputs

[Nehal J Wani via cfe-dev]

On Mon, Dec 30, 2019 at 2:03 PM Nehal J Wani <nehaljw.kkd1 at gmail.com> wrote:
>
> Dear Clang Devs,
>
> Consider the following code snippet:
>
> #include <sched.h>
> void whatever() {
>   cpu_set_t d, s1, s2;
>   CPU_AND(&d, &s1, &s2);
> }
>
> On compiling normally, I see no warnings being emitted:
> $ clang++ -c test.cpp -o test.o -Wall -Wextra -Wunused-value -Werror
> $
>
> However, if I ask Clang to save the temporary files:
> $ clang++ -c test.cpp -o test.o -Wall -Wextra -Wunused-value -Werror --save-temps
> test.cpp:5:308: error: expression result unused [-Werror,-Wunused-value]
>   (__extension__ ({ cpu_set_t *__dest = (&d); __const __cpu_mask *__arr1 = (&s1)->__bits; __const __cpu_mask *__arr2 = (&s2)->__bits; size_t __imax = (sizeof (cpu_set_t)) / sizeof (__cpu_mask); size_t __i; for (__i = 0; __i < __imax; ++__i) ((__cpu_mask *) __dest->__bits)[__i] = __arr1[__i] & __arr2[__i]; __dest; }));
>                                                                                                                                                                                                                                                                                                                    ^~~~~~
> 1 error generated.
>
> Another way to look at this problem is by first storing the intermediate output and then trying to use it:
> $ clang++ -E -c test.cpp -o test.ii -Wall -Wextra -Wunused-value -Werror
> $ clang++ -c test.ii -o test.o -Wall -Wextra -Wunused-value -Werror
> test.cpp:5:308: error: expression result unused [-Werror,-Wunused-value]
>   (__extension__ ({ cpu_set_t *__dest = (&d); __const __cpu_mask *__arr1 = (&s1)->__bits; __const __cpu_mask *__arr2 = (&s2)->__bits; size_t __imax = (sizeof (cpu_set_t)) / sizeof (__cpu_mask); size_t __i; for (__i = 0; __i < __imax; ++__i) ((__cpu_mask *) __dest->__bits)[__i] = __arr1[__i] & __arr2[__i]; __dest; }));
>                                                                                                                                                                                                                                                                                                                    ^~~~~~
> 1 error generated.
>
>
> Questions:
> - Is this report of unused value legit? GCC doesn't warn anything on the same snippet.
> - Is this behavior expected? Why does the warning throw up only when the end user wants to do anything with the intermediate outputs? I ended up here because this was failing my project when I was trying to use distcc.
>
>
>
> Cheers!
>
> Nehal J Wani

I was suggested at #llvm to send a ping! :-)