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@xxxxxxxxxxxxx>
---
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
