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Subject: [Bug target/103069] cmpxchg isn't optimized
Date: Tue, 15 Feb 2022 08:59:44 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103069

--- Comment #11 from Hongyu Wang <wwwhhhyyy333 at gmail dot com> ---


For the case with atomic_compare_exchange_weak_release, it can be expanded =
as

loop: mov    %eax,%r8d
      and    $0xfff80000,%r8d
      mov    (%r8),%rsi <--- load lock first
      cmp    %rsi,%rax <--- compare with expected input
      jne    .L2 <--- lock ne expected
      lock cmpxchg %r8d,(%rdi)
      mov    %rsi,%rax <--- perform the behavior of failed cmpxchg
      jne    loop

But this is not suitable for atomic_compare_exchange_strong, as the document
said

Unlike atomic_compare_exchange_weak, this strong version is required to alw=
ays
return true when expected indeed compares equal to the contained object, not
allowing spurious failures. If we expand cmpxchg as above, it would result =
in
spurious failure since the load is not atomic.=20

So for

 do
   pd->nextevent =3D __nptl_last_event;
 while (atomic_compare_and_exchange_bool_acq (&__nptl_last_event,
                                              pd, pd->nextevent));

who invokes atomic_compare_exchange_strong we may not simply adjust the
expander. It is better to know the call is in loop condition and relax it
accordingly.=