[PATCH v2] libstdc++: Fix incorrect return values and comments on atomic timed waits

[Jonathan Wakely]

The __detail::__wait_until function has a comment that should have been
removed when r16-1000-g225622398a9631 changed the return type from a
std::pair to a struct with three members.

Fix _M_spin_until_impl so that the _M_has_val member of the result is
accurate.  If the deadline has passed then it never enters the loop and
so never loads a fresh value, so _M_has_val should be false.

There's also a redundant clock::now() call in __spin_until_impl which
can be removed, we can reuse the call immediately before it.

libstdc++-v3/ChangeLog:

        * include/bits/atomic_timed_wait.h (__detail::__wait_until):
        Remove incorrect comment.
        * src/c++20/atomic.cc (__spin_until_impl): Fix incorrect
        return value. Reuse result of first call to clock.
---

Patch v2 adjusts the commit summary to not say it only changes comments.
Also removed the change to the __atomic_wait_address_for_v return
statement, which logically belongs in the other patch touching that
funciton.

Tested x86_64-linux.

 libstdc++-v3/include/bits/atomic_timed_wait.h |  1 -
 libstdc++-v3/src/c++20/atomic.cc              | 24 ++++++++++++-------
 2 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/libstdc++-v3/include/bits/atomic_timed_wait.h 
b/libstdc++-v3/include/bits/atomic_timed_wait.h
index 230afbc96e7d..71638fd59a05 100644
--- a/libstdc++-v3/include/bits/atomic_timed_wait.h
+++ b/libstdc++-v3/include/bits/atomic_timed_wait.h
@@ -87,7 +87,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     __wait_until_impl(const void* __addr, __wait_args_base& __args,
                      const __wait_clock_t::duration& __atime);
 
-    // Returns {true, val} if wait ended before a timeout.
     template<typename _Clock, typename _Dur>
       __wait_result_type
       __wait_until(const void* __addr, __wait_args_base& __args,
diff --git a/libstdc++-v3/src/c++20/atomic.cc b/libstdc++-v3/src/c++20/atomic.cc
index a3ec92a10d56..4120e1a0817f 100644
--- a/libstdc++-v3/src/c++20/atomic.cc
+++ b/libstdc++-v3/src/c++20/atomic.cc
@@ -397,17 +397,18 @@ __cond_wait_until(__condvar& __cv, mutex& __mx,
 }
 #endif // ! HAVE_PLATFORM_TIMED_WAIT
 
-// Like __spin_impl, always returns _M_has_val == true.
+// Unlike __spin_impl, does not always return _M_has_val == true.
+// If the deadline has already passed then no fresh value is loaded.
 __wait_result_type
 __spin_until_impl(const __platform_wait_t* __addr,
                  const __wait_args_base& __args,
                  const __wait_clock_t::time_point& __deadline)
 {
-  auto __t0 = __wait_clock_t::now();
   using namespace literals::chrono_literals;
 
-  __platform_wait_t __val{};
-  auto __now = __wait_clock_t::now();
+  __wait_result_type __res{};
+  auto __t0 = __wait_clock_t::now();
+  auto __now = __t0;
   for (; __now < __deadline; __now = __wait_clock_t::now())
     {
       auto __elapsed = __now - __t0;
@@ -422,16 +423,21 @@ __spin_until_impl(const __platform_wait_t* __addr,
        __thread_yield();
       else
        {
-         auto __res = __detail::__spin_impl(__addr, __args);
+         __res = __detail::__spin_impl(__addr, __args);
          if (!__res._M_timeout)
            return __res;
        }
 
-      __atomic_load(__addr, &__val, __args._M_order);
-      if (__val != __args._M_old)
-       return { ._M_val = __val, ._M_has_val = true, ._M_timeout = false };
+      __atomic_load(__addr, &__res._M_val, __args._M_order);
+      __res._M_has_val = true;
+      if (__res._M_val != __args._M_old)
+       {
+         __res._M_timeout = false;
+         return __res;
+       }
     }
-  return { ._M_val = __val, ._M_has_val = true, ._M_timeout = true };
+  __res._M_timeout = true;
+  return __res;
 }
 } // namespace
 
-- 
2.49.0