[libcxx-commits] [libcxx] 3fea2f6 - [libc++][test] Use loop with compare_exchange_weak calls (#185953)

[via libcxx-commits]

Author: Jake Egan
Date: 2026-03-14T22:34:48-04:00
New Revision: 3fea2f61652e10c7f3abc17f0542e8bc0dd0ea25

URL: https://github.com/llvm/llvm-project/commit/3fea2f61652e10c7f3abc17f0542e8bc0dd0ea25
DIFF: https://github.com/llvm/llvm-project/commit/3fea2f61652e10c7f3abc17f0542e8bc0dd0ea25.diff

LOG: [libc++][test] Use loop with compare_exchange_weak calls (#185953)

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

Added: 
    

Modified: 
    libcxx/test/std/atomics/atomics.ref/compare_exchange_weak.pass.cpp

Removed: 
    


################################################################################
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));