[libcxx-commits] [libcxx] Make ios_base::xalloc non-atomic with LIBCXX_ENABLE_THREADS=OFF. (PR #208356)

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llvmorg-github-actions[bot] wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libcxx

Author: Eli Friedman (efriedma-quic)

<details>
<summary>Changes</summary>

762b77a moved the definition of "xindex" out of the header, and in the process dropped the _LIBCPP_HAS_THREADS check.  Re-add the check to maintain the status quo.

The discussion on https://github.com/llvm/llvm-project/pull/198994 indicates it's not clear whether LIBCXX_ENABLE_THREADS=OFF is actually supposed to mean single-threaded.  But it clearly does in practice: atomic_support.h uses non-atomic ops when threads are disabled, and a few other APIs have explicit non-atomic fallback paths.

My team ran into this trying to run libc++ tests for a RISC-V core without the "a" extension.

---
Full diff: https://github.com/llvm/llvm-project/pull/208356.diff


1 Files Affected:

- (modified) libcxx/src/ios.cpp (+7) 


``````````diff
diff --git a/libcxx/src/ios.cpp b/libcxx/src/ios.cpp
index 2e049098740dc..db6ca50f7f6ca 100644
--- a/libcxx/src/ios.cpp
+++ b/libcxx/src/ios.cpp
@@ -122,7 +122,14 @@ static size_t __ios_new_cap(size_t __req_size, size_t __current_cap) { // Precon
 }
 
 int ios_base::xalloc() {
+#if _LIBCPP_HAS_THREADS
   constinit static atomic<int> xindex = 0;
+#else
+  // If we don't have atomics, fall back to single-threaded implementation.
+  // FIXME: Should "single-threaded" be a separate option from
+  // _LIBCPP_HAS_THREADS?
+  static int xindex = 0;
+#endif // _LIBCPP_HAS_THREADS
   return xindex++;
 }
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/208356