[PATCH] libstdc++: Remove some more unconditional uses of atomics

[Jonathan Wakely via Gcc-patches]

Tested x86_64-linux and aarch64-linux. I intend to push this to trunk.

-- >8 --

These atomics cause linker errors on arm4t where __sync_synchronize is
not defined. For single-threaded targets we don't need the atomics.

libstdc++-v3/ChangeLog:

        * include/experimental/io_context (io_context) [!_GLIBCXX_HAS_GTHREADS]:
        Use a plain integer for _M_work_count for single-threaded
        targets.
        * src/c++17/memory_resource.cc [!_GLIBCXX_HAS_GTHREADS]
        (atomic_mem_res): Use unsynchronized type for single-threaded
        targets.
---
 libstdc++-v3/include/experimental/io_context |  4 ++
 libstdc++-v3/src/c++17/memory_resource.cc    | 49 ++++++++++----------
 2 files changed, 29 insertions(+), 24 deletions(-)

diff --git a/libstdc++-v3/include/experimental/io_context 
b/libstdc++-v3/include/experimental/io_context
index c59f8c8e73b..c878d5a7025 100644
--- a/libstdc++-v3/include/experimental/io_context
+++ b/libstdc++-v3/include/experimental/io_context
@@ -562,7 +562,11 @@ inline namespace v1
        }
       };
 
+#ifdef _GLIBCXX_HAS_GTHREADS
     atomic<count_type>         _M_work_count;
+#else
+    count_type                 _M_work_count;
+#endif
     mutable execution_context::mutex_type              _M_mtx;
     queue<function<void()>>    _M_op;
     bool                       _M_stopped = false;
diff --git a/libstdc++-v3/src/c++17/memory_resource.cc 
b/libstdc++-v3/src/c++17/memory_resource.cc
index c0c7cf0cf83..63856eadaf5 100644
--- a/libstdc++-v3/src/c++17/memory_resource.cc
+++ b/libstdc++-v3/src/c++17/memory_resource.cc
@@ -27,9 +27,9 @@
 #include <atomic>
 #include <bit>                         // has_single_bit, bit_ceil, bit_width
 #include <new>
+#include <bits/move.h>                 // std::__exchange
 #if ATOMIC_POINTER_LOCK_FREE != 2
 # include <bits/std_mutex.h>   // std::mutex, std::lock_guard
-# include <bits/move.h>                // std::__exchange
 #endif
 
 #if __has_cpp_attribute(clang::require_constant_initialization)
@@ -94,10 +94,31 @@ namespace pmr
 
     __constinit constant_init<newdel_res_t> newdel_res{};
     __constinit constant_init<null_res_t> null_res{};
-#if ATOMIC_POINTER_LOCK_FREE == 2
+
+#ifndef _GLIBCXX_HAS_GTHREADS
+# define _GLIBCXX_ATOMIC_MEM_RES_CAN_BE_CONSTANT_INITIALIZED
+    // Single-threaded, no need for synchronization
+    struct atomic_mem_res
+    {
+      constexpr
+      atomic_mem_res(memory_resource* r) : val(r) { }
+
+      memory_resource* val;
+
+      memory_resource* load(std::memory_order) const
+      {
+       return val;
+      }
+
+      memory_resource* exchange(memory_resource* r, std::memory_order)
+      {
+       return std::__exchange(val, r);
+      }
+    };
+#elif ATOMIC_POINTER_LOCK_FREE == 2
     using atomic_mem_res = atomic<memory_resource*>;
 # define _GLIBCXX_ATOMIC_MEM_RES_CAN_BE_CONSTANT_INITIALIZED
-#elif defined(_GLIBCXX_HAS_GTHREADS)
+#else
     // Can't use pointer-width atomics, define a type using a mutex instead:
     struct atomic_mem_res
     {
@@ -123,27 +144,7 @@ namespace pmr
        return std::__exchange(val, r);
       }
     };
-#else
-# define _GLIBCXX_ATOMIC_MEM_RES_CAN_BE_CONSTANT_INITIALIZED
-    // Single-threaded, no need for synchronization
-    struct atomic_mem_res
-    {
-      constexpr
-      atomic_mem_res(memory_resource* r) : val(r) { }
-
-      memory_resource* val;
-
-      memory_resource* load(std::memory_order) const
-      {
-       return val;
-      }
-
-      memory_resource* exchange(memory_resource* r, std::memory_order)
-      {
-       return std::__exchange(val, r);
-      }
-    };
-#endif // ATOMIC_POINTER_LOCK_FREE == 2
+#endif
 
 #ifdef _GLIBCXX_ATOMIC_MEM_RES_CAN_BE_CONSTANT_INITIALIZED
     __constinit constant_init<atomic_mem_res> default_res{&newdel_res.obj};
-- 
2.41.0