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

[Eli Friedman via libcxx-commits]

https://github.com/efriedma-quic created https://github.com/llvm/llvm-project/pull/208356

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.

>From 1c1c7bf8d2b3169713f18e46698f102c496f6042 Mon Sep 17 00:00:00 2001
From: Eli Friedman <efriedma at qti.qualcomm.com>
Date: Wed, 8 Jul 2026 16:57:44 -0700
Subject: [PATCH] Make ios_base::xalloc non-atomic with
 LIBCXX_ENABLE_THREADS=OFF.

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.
---
 libcxx/src/ios.cpp | 7 +++++++
 1 file changed, 7 insertions(+)

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++;
 }