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

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Thu, 2025-05-08 at 11:38 -0700, Alexei Starovoitov wrote:
> On Thu, May 8, 2025 at 4:38 AM Ilya Leoshkevich <iii@linux.ibm.com>
> wrote:
> > 
> > clang-21 complains about unused expressions in a few progs.
> > Fix by explicitly casting the respective expressions to void.
> 
> ...
> >         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);
> 
> Hmm. I'm on clang-21 too and I don't see them.
> What warnings do you see ?

In file included from progs/arena_spin_lock.c:7:
progs/bpf_arena_spin_lock.h:305:1756: error: expression result unused
[-Werror,-Wunused-value]
  305 |   ({ typeof(_Generic((*&lock->locked), char: (char)0, unsigned
char : (unsigned char)0, signed char : (signed char)0, unsigned short :
(unsigned short)0, signed short : (signed short)0, unsigned int :
(unsigned int)0, signed int : (signed int)0, unsigned long : (unsigned
long)0, signed long : (signed long)0, unsigned long long : (unsigned
long long)0, signed long long : (signed long long)0, default:
(typeof(*&lock->locked))0)) __val = ({ typeof(&lock->locked) __ptr =
(&lock->locked); typeof(_Generic((*(&lock->locked)), char: (char)0,
unsigned char : (unsigned char)0, signed char : (signed char)0,
unsigned short : (unsigned short)0, signed short : (signed short)0,
unsigned int : (unsigned int)0, signed int : (signed int)0, unsigned
long : (unsigned long)0, signed long : (signed long)0, unsigned long
long : (unsigned long long)0, signed long long : (signed long long)0,
default: (typeof(*(&lock->locked)))0)) VAL; for (;;) { VAL =
(typeof(_Generic((*(&lock->locked)), char: (char)0, unsigned char :
(unsigned char)0, signed char : (signed char)0, unsigned short :
(unsigned short)0, signed short : (signed short)0, unsigned int :
(unsigned int)0, signed int : (signed int)0, unsigned long : (unsigned
long)0, signed long : (signed long)0, unsigned long long : (unsigned
long long)0, signed long long : (signed long long)0, default:
(typeof(*(&lock->locked)))0)))(*(volatile typeof(*__ptr) *)&(*__ptr));
if (!VAL) break; ({ __label__ l_break, l_continue; asm volatile
goto("may_goto %l[l_break]" :::: l_break); goto l_continue; l_break:
goto release_err; l_continue:; }); ({}); } (typeof(*(&lock-
>locked)))VAL; }); ({ ({ if (!CONFIG_X86_64) ({ unsigned long __val;
__sync_fetch_and_add(&__val, 0); }); else asm volatile("" :::
"memory"); }); }); (typeof(*(&lock->locked)))__val; });
      |                                                               
^                         ~~~~~
1 error generated.

It started today.
Here is the full compiler version:

$ clang-21 --version
Debian clang version 21.0.0 (++20250501112544+75d1cceb9486-
1~exp1~20250501112558.1422)
Target: s390x-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-21/bin

Best regards,
Ilya