[libcxx] Add atomic_support.h header to src that handles needed atomic operations.

Summary:
In some places in libc++ we need to use the `__atomic_*` builtins. This patch adds a header that provides access to those builtins in a uniform way from within the dylib source.

If the compiler building the dylib does not support these builtins then a warning is issued.

Only relaxed loads are needed within the headers. A singe function to do these relaxed loads has been added to `<memory>`.

This patch applies the new atomic builtins to `__shared_count` and `call_once`.

Reviewers: mclow.lists

Subscribers: majnemer, jroelofs, cfe-commits

Differential Revision: http://reviews.llvm.org/D10406

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@241532 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/utilities/memory/util.smartptr/race_condition.pass.cpp

@@ -0,0 +1,94 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// UNSUPPORTED: libcpp-has-no-threads
+//
+// <memory>
+//
+// class shared_ptr
+//
+// This test attempts to create a race condition surrounding use_count()
+// with the hope that TSAN will diagnose it.
+
+#include <memory>
+#include <atomic>
+#include <thread>
+#include <cassert>
+
+typedef std::shared_ptr<int> Ptr;
+typedef std::weak_ptr<int> WeakPtr;
+
+std::atomic_bool Start;
+std::atomic_bool KeepRunning;
+
+struct TestRunner {
+    TestRunner(Ptr xx) : x(xx) {}
+    void operator()() {
+        while (Start == false) {}
+        while (KeepRunning) {
+            // loop to prevent always checking the atomic.
+            for (int i=0; i < 100000; ++i) {
+                Ptr x2 = x; // increment shared count
+                WeakPtr x3 = x; // increment weak count
+                Ptr x4 = x3.lock(); // increment shared count via lock
+                WeakPtr x5 = x3; // increment weak count
+            }
+        }
+    }
+    Ptr x;
+};
+
+void run_test(Ptr p) {
+    Start = false;
+    KeepRunning = true;
+    assert(p.use_count() == 2);
+    TestRunner r(p);
+    assert(p.use_count() == 3);
+    std::thread t1(r); // Start the test thread.
+    assert(p.use_count() == 4);
+    Start = true;
+    // Run until we witness 25 use count changes via both
+    // shared and weak pointer methods.
+    WeakPtr w = p;
+    int shared_changes_count = 0;
+    int weak_changes_count = 0;
+    while (shared_changes_count < 25 && weak_changes_count < 25) {
+        // check use_count on the shared_ptr
+       int last = p.use_count();
+       int new_val = p.use_count();
+       assert(last >= 4);
+       assert(new_val >= 4);
+       if (last != new_val) ++shared_changes_count;
+       // Check use_count on the weak_ptr
+       last = w.use_count();
+       new_val = w.use_count();
+       assert(last >= 4);
+       assert(new_val >= 4);
+       if (last != new_val) ++weak_changes_count;
+    }
+    // kill the test thread.
+    KeepRunning = false;
+    t1.join();
+    assert(p.use_count() == 3);
+}
+
+int main() {
+  {
+    // Test with out-of-place shared_count.
+    Ptr p(new int(42));
+    run_test(p);
+    assert(p.use_count() == 1);
+  }
+  {
+    // Test with in-place shared_count.
+    Ptr p = std::make_shared<int>(42);
+    run_test(p);
+    assert(p.use_count() == 1);
+  }
+}