Re: [PATCH bpf-next v3 2/2] selftests/bpf: Fix "expression result unused" warnings with icecc

[Yonghong Song <yonghong.song@linux.dev>]

On 8/27/25 12:46 PM, Ilya Leoshkevich wrote:
> icecc is a compiler wrapper that distributes compile jobs over a build
> farm [1]. It works by sending toolchain binaries and preprocessed
> source code to remote machines.
>
> Unfortunately using it with BPF selftests causes build failures due to
> a clang bug [2]. The problem is that clang suppresses the
> -Wunused-value warning if the unused expression comes from a macro
> expansion. Since icecc compiles preprocessed source code, this
> information is not available. This leads to -Wunused-value false
> positives.
>
> arena_spin_lock_slowpath() uses two macros that produce values and
> ignores the results. Add (void) cast to explicitly indicate that this
> is intentional and suppress the warning.
>
> An alternative solution is to change the macros to not produce values.
> This would work today, but in the future there may appear users who
> need them. Another potential solution is to replace these macros with
> functions. Unfortunately this would not work, because these macros
> work with unknown types and control flow.
>
> [1] https://github.com/icecc/icecream
> [2] https://github.com/llvm/llvm-project/issues/142614

As you described in [1] and [2]. The failure is due to the compilation from .i file to .o file.

$ cat run2.sh
echo '.c -> .i -> .o'
clang  -g -Wall -Werror -D__TARGET_ARCH_x86 -mlittle-endian -I/home/yhs/work/bpf-next/tools/testing/selftests/bpf/tools/include -I/home/yhs/work/bpf-next/tools/testing/selftests/bpf -I/home/yhs/work/bpf-next/tools/include/uapi -I/home/yhs/work/bpf-next/tools/testing/selftests/usr/include -std=gnu11 -fno-strict-aliasing -Wno-compare-distinct-pointer-types -idirafter /home/yhs/work/llvm-project/llvm/build.21/Release/lib/clang/21/include -idirafter /usr/local/include -idirafter /usr/include    -DENABLE_ATOMICS_TESTS   -O2 --target=bpfel -E progs/arena_spin_lock.c -mcpu=v4 -o /home/yhs/work/bpf-next/tools/testing/selftests/bpf/arena_spin_lock.i
clang  -g -Wall -Werror -mlittle-endian -std=gnu11 -fno-strict-aliasing -Wno-compare-distinct-pointer-types -O2 --target=bpfel -c arena_spin_lock.i -mcpu=v4 -o /home/yhs/work/bpf-next/tools/testing/selftests/bpf/cpuv4/arena_spin_lock.bpf.o

echo '.c -> .o'
clang  -g -Wall -Werror -D__TARGET_ARCH_x86 -mlittle-endian -I/home/yhs/work/bpf-next/tools/testing/selftests/bpf/tools/include -I/home/yhs/work/bpf-next/tools/testing/selftests/bpf -I/home/yhs/work/bpf-next/tools/include/uapi -I/home/yhs/work/bpf-next/tools/testing/selftests/usr/include -std=gnu11 -fno-strict-aliasing -Wno-compare-distinct-pointer-types -idirafter /home/yhs/work/llvm-project/llvm/build.21/Release/lib/clang/21/include -idirafter /usr/local/include -idirafter /usr/include    -DENABLE_ATOMICS_TESTS   -O2 --target=bpfel -c progs/arena_spin_lock.c -mcpu=v4 -o /home/yhs/work/bpf-next/tools/testing/selftests/bpf/cpuv4/arena_spin_lock.bpf.o

$ ./run2.sh
.c -> .i -> .o
In file included from progs/arena_spin_lock.c:7:
progs/bpf_arena_spin_lock.h:305:1765: error: expression result unused [-Werror,-Wunused-value]
   305 |   ...unsigned long __val; __sync_fetch_and_add(&__val, 0); }); else asm volatile("" ::: "memory"); }); }); (typeof(*(&lock->locked)))__val; });
       |                                                                                                            ^                         ~~~~~
progs/bpf_arena_spin_lock.h:383:1769: error: expression result unused [-Werror,-Wunused-value]
   383 |   ...unsigned long __val; __sync_fetch_and_add(&__val, 0); }); else asm volatile("" ::: "memory"); }); }); (typeof(*(&node->locked)))__val; });
       |                                                                                                            ^                         ~~~~~
2 errors generated.
.c -> .o
$

I am not sure whether we should do anything with pahole or libbpf. Essentially this is a clang
bug. We should push clang community to fix the problem.

>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
>   tools/testing/selftests/bpf/progs/bpf_arena_spin_lock.h | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/progs/bpf_arena_spin_lock.h b/tools/testing/selftests/bpf/progs/bpf_arena_spin_lock.h
> index d67466c1ff77..f90531cf3ee5 100644
> --- a/tools/testing/selftests/bpf/progs/bpf_arena_spin_lock.h
> +++ b/tools/testing/selftests/bpf/progs/bpf_arena_spin_lock.h
> @@ -302,7 +302,7 @@ int arena_spin_lock_slowpath(arena_spinlock_t __arena __arg_arena *lock, u32 val
>   	 * barriers.
>   	 */
>   	if (val & _Q_LOCKED_MASK)
> -		smp_cond_load_acquire_label(&lock->locked, !VAL, release_err);
> +		(void)smp_cond_load_acquire_label(&lock->locked, !VAL, release_err);
>   
>   	/*
>   	 * take ownership and clear the pending bit.
> @@ -380,7 +380,7 @@ int arena_spin_lock_slowpath(arena_spinlock_t __arena __arg_arena *lock, u32 val
>   		/* Link @node into the waitqueue. */
>   		WRITE_ONCE(prev->next, node);
>   
> -		arch_mcs_spin_lock_contended_label(&node->locked, release_node_err);
> +		(void)arch_mcs_spin_lock_contended_label(&node->locked, release_node_err);
>   
>   		/*
>   		 * While waiting for the MCS lock, the next pointer may have