[libcxx-commits] [libcxx] [libc++][test] Use loop with compare_exchange_weak calls (PR #185953)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libcxx

Author: Jake Egan (jakeegan)

<details>
<summary>Changes</summary>

On AIX, this test sometimes fails with error `Assertion failed: y == true`. The test assumes `compare_exchange_weak` should succeed on a single call, however according to the standard:

> A weak compare-and-exchange operation may fail spuriously. That is, even when the contents of memory referred to by expected and ptr are equal, it may return false and store back to expected the same memory contents that were originally there.
This spurious failure enables implementation of compare-and-exchange on a broader class of machines, e.g., load-locked store-conditional machines. A consequence of spurious failure is that nearly all uses of weak compare-and-exchange will be in a loop.

[atomics.ref.ops]/27

---
Full diff: https://github.com/llvm/llvm-project/pull/185953.diff


1 Files Affected:

- (modified) libcxx/test/std/atomics/atomics.ref/compare_exchange_weak.pass.cpp (+10-10) 


``````````diff
diff --git a/libcxx/test/std/atomics/atomics.ref/compare_exchange_weak.pass.cpp b/libcxx/test/std/atomics/atomics.ref/compare_exchange_weak.pass.cpp
index 58cf2e0fe338b..62bad17a21600 100644
--- a/libcxx/test/std/atomics/atomics.ref/compare_exchange_weak.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.ref/compare_exchange_weak.pass.cpp
@@ -33,11 +33,11 @@ struct TestCompareExchangeWeak {
       std::atomic_ref<T> const a(x);
 
       T t(T(1));
-      std::same_as<bool> decltype(auto) y = a.compare_exchange_weak(t, T(2));
-      assert(y == true);
+      while (!a.compare_exchange_weak(t, T(2))) {
+      }
       assert(a == T(2));
       assert(t == T(1));
-      y = a.compare_exchange_weak(t, T(3));
+      std::same_as<bool> decltype(auto) y = a.compare_exchange_weak(t, T(3));
       assert(y == false);
       assert(a == T(2));
       assert(t == T(2));
@@ -49,11 +49,11 @@ struct TestCompareExchangeWeak {
       std::atomic_ref<T> const a(x);
 
       T t(T(1));
-      std::same_as<bool> decltype(auto) y = a.compare_exchange_weak(t, T(2), std::memory_order_seq_cst);
-      assert(y == true);
+      while (!a.compare_exchange_weak(t, T(2), std::memory_order_seq_cst)) {
+      }
       assert(a == T(2));
       assert(t == T(1));
-      y = a.compare_exchange_weak(t, T(3), std::memory_order_seq_cst);
+      std::same_as<bool> decltype(auto) y = a.compare_exchange_weak(t, T(3), std::memory_order_seq_cst);
       assert(y == false);
       assert(a == T(2));
       assert(t == T(2));
@@ -65,12 +65,12 @@ struct TestCompareExchangeWeak {
       std::atomic_ref<T> const a(x);
 
       T t(T(1));
-      std::same_as<bool> decltype(auto) y =
-          a.compare_exchange_weak(t, T(2), std::memory_order_release, std::memory_order_relaxed);
-      assert(y == true);
+      while (!a.compare_exchange_weak(t, T(2), std::memory_order_release, std::memory_order_relaxed)) {
+      }
       assert(a == T(2));
       assert(t == T(1));
-      y = a.compare_exchange_weak(t, T(3), std::memory_order_release, std::memory_order_relaxed);
+      std::same_as<bool> decltype(auto) y =
+          a.compare_exchange_weak(t, T(3), std::memory_order_release, std::memory_order_relaxed);
       assert(y == false);
       assert(a == T(2));
       assert(t == T(2));

``````````

</details>


https://github.com/llvm/llvm-project/pull/185953