[libcxx-commits] [libcxx] [libcxx][test] Prevent compiler optimising out uses of 64-bit atomics (PR #172517)

[via libcxx-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libcxx

Author: David Spickett (DavidSpickett)

<details>
<summary>Changes</summary>

I found this while trying different optimisation options to fix a test failure on #<!-- -->169901.

Normally the tests are built for Cortex-M with no options, aka -O0. These tests fail to builds because we do not have 64-bit atomics on that platform:
ld.lld: error: undefined symbol: __atomic_load_8

This is noted by an XFAIL annotation in the test, which is correct when you're using -O0.

When you optimise this code, the call to the atomic is actually removed and the test does compile and does run. Turning it into an unexpected pass.

To fix this I've added DoNotOptimise in various places. To make this truly robust, it probably needs to be in many more, but I've stuck to what turned the tests back into XFAILs for Cortex-M.

---
Full diff: https://github.com/llvm/llvm-project/pull/172517.diff


5 Files Affected:

- (modified) libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/assign.pass.cpp (+1) 
- (modified) libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/load.pass.cpp (+1) 
- (modified) libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.float.pass.cpp (+1) 
- (modified) libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/store.pass.cpp (+1) 
- (modified) libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/wait.pass.cpp (+1) 


``````````diff
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/assign.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/assign.pass.cpp
index 687b324f3f19b..473e511703284 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/assign.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/assign.pass.cpp
@@ -32,6 +32,7 @@ void test_impl() {
   // assignment
   {
     MaybeVolatile<std::atomic<T>> a(T(3.1));
+    DoNotOptimize(a);
     std::same_as<T> decltype(auto) r = (a = T(1.2));
     assert(a.load() == T(1.2));
     assert(r == T(1.2));
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/load.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/load.pass.cpp
index 695fd593b51e2..acc75d9acb15e 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/load.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/load.pass.cpp
@@ -40,6 +40,7 @@ void test_impl() {
   // load
   {
     MaybeVolatile<std::atomic<T>> a(T(3.1));
+    DoNotOptimize(a);
     a.store(T(1.2));
     std::same_as<T> decltype(auto) r = a.load(std::memory_order::relaxed);
     assert(r == T(1.2));
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.float.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.float.pass.cpp
index 1f08906065742..2e64764f3bce2 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.float.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.float.pass.cpp
@@ -29,6 +29,7 @@ void test_impl() {
   // operator float
   {
     MaybeVolatile<std::atomic<T>> a(T(3.1));
+    DoNotOptimize(a);
     T r = a;
     assert(r == T(3.1));
   }
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/store.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/store.pass.cpp
index 34bd09e945f94..24cadab0c1559 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/store.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/store.pass.cpp
@@ -39,6 +39,7 @@ void test_impl() {
   // store
   {
     MaybeVolatile<std::atomic<T>> a(T(3.1));
+    DoNotOptimize(a);
     a.store(T(1.2), std::memory_order::relaxed);
     assert(a.load() == T(1.2));
   }
diff --git a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/wait.pass.cpp b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/wait.pass.cpp
index 846d276c0ede8..fba523d8c2b55 100644
--- a/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/wait.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/wait.pass.cpp
@@ -38,6 +38,7 @@ void test_impl() {
   // wait with different value
   {
     MaybeVolatile<std::atomic<T>> a(T(3.1));
+    DoNotOptimize(a);
     a.wait(T(1.1), std::memory_order::relaxed);
   }
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/172517