Resolve LWG issue 2426

[EricWF]

Summary

This patch checks that expected is loaded before it is used. Libc++ already does this.

[ldionne]

If we want to mark the LWG issue as being resolved for libc++, I think the test should be moved to the libc++ test suite. Otherwise, we're only assessing that libc++ implements LWG2426 on Clang.

[EricWF]

In D69062#1711610, @ldionne wrote:

If we want to mark the LWG issue as being resolved for libc++, I think the test should be moved to the libc++ test suite. Otherwise, we're only assessing that libc++ implements LWG2426 on Clang.

I don't agree. Clang implements this behavior, the fact that C++ chose to put std::atomic in the library rather than the language has more to do with C++ and less to do with implementation responsibility.
If making changes to libc++ won't change the output of this test, then having the test doesn't matter.

I've marked the LWG issue as complete in a separate commit, and I'm going to close this now.

clang/test/CodeGen/atomic-ops.c

 }
 
 // CHECK-LABEL: @failureOrder
 void failureOrder(_Atomic(int) *ptr, int *ptr2) {
   __c11_atomic_compare_exchange_strong(ptr, ptr2, 43, memory_order_acquire, memory_order_relaxed);
   // CHECK: cmpxchg i32* {{%[0-9A-Za-z._]+}}, i32 {{%[0-9A-Za-z._]+}}, i32 {{%[0-9A-Za-z_.]+}} acquire monotonic
 
   __c11_atomic_compare_exchange_weak(ptr, ptr2, 43, memory_order_seq_cst, memory_order_acquire);
+  // CHECK: load i32, i32* {{%[0-9A-Za-z._]+}}, align 4
+  // CHECK: load i32, i32* {{%[0-9A-Za-z._]+}}, align 4
   // CHECK: cmpxchg weak i32* {{%[0-9A-Za-z._]+}}, i32 {{%[0-9A-Za-z._]+}}, i32 {{%[0-9A-Za-z_.]+}} seq_cst acquire
 
   // Unknown ordering: conservatively pick strongest valid option (for now!).
   __atomic_compare_exchange(ptr2, ptr2, ptr2, 0, memory_order_acq_rel, *ptr2);
   // CHECK: cmpxchg i32* {{%[0-9A-Za-z._]+}}, i32 {{%[0-9A-Za-z._]+}}, i32 {{%[0-9A-Za-z_.]+}} acq_rel acquire
 
   // Undefined behaviour: don't really care what that last ordering is so leave
   // it out:

libcxx/www/cxx1z_status.html

<!--   <I>Note: "NAD" means that the issue was deemed "Not a defect"</I> -->
    <tr><td></td><td></td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG2181">2181</a></td><td>Exceptions from seed sequence operations</td><td>Oulu</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2309">2309</a></td><td>mutex::lock() should not throw device_or_resource_busy</td><td>Oulu</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2310">2310</a></td><td>Public exposition only member in std::array</td><td>Oulu</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2312">2312</a></td><td>tuple's constructor constraints need to be phrased more precisely</td><td>Oulu</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2328">2328</a></td><td>Rvalue stream extraction should use perfect forwarding</td><td>Oulu</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2393">2393</a></td><td>std::function's Callable definition is broken</td><td>Oulu</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2422">2422</a></td><td>std::numeric_limits&lt;T&gt;::is_modulo description: "most machines" errata</td><td>Oulu</td><td>Complete</td></tr>
-  <tr><td><a href="https://wg21.link/LWG2426">2426</a></td><td>Issue about compare_exchange</td><td>Oulu</td><td></td></tr>
+  <tr><td><a href="https://wg21.link/LWG2426">2426</a></td><td>Issue about compare_exchange</td><td>Oulu</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2436">2436</a></td><td>Comparators for associative containers should always be CopyConstructible</td><td>Oulu</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2441">2441</a></td><td>Exact-width atomic typedefs should be provided</td><td>Oulu</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2451">2451</a></td><td>[fund.ts.v2] optional should 'forward' T's implicit conversions</td><td>Oulu</td><td>Nothing to do</td></tr>
   <tr><td><a href="https://wg21.link/LWG2509">2509</a></td><td>[fund.ts.v2] any_cast doesn't work with rvalue reference targets and cannot move with a value target</td><td>Oulu</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2516">2516</a></td><td>[fund.ts.v2] Public "exposition only" members in observer_ptr</td><td>Oulu</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG2542">2542</a></td><td>Missing const requirements for associative containers</td><td>Oulu</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG2549">2549</a></td><td>Tuple EXPLICIT constructor templates that take tuple parameters end up taking references to temporaries and will create dangling references</td><td>Oulu</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2550">2550</a></td><td>Wording of unordered container's clear() method complexity</td><td>Oulu</td><td>Complete</td></tr>