<locale>: Double-checked locking for locale::classic

benchmarks/CMakeLists.txt

@@ -105,5 +105,6 @@ function(add_benchmark name)
     target_compile_definitions(benchmark-${name} PRIVATE BENCHMARK_STATIC_DEFINE)
 endfunction()
 
-add_benchmark(std_copy src/std_copy.cpp)
+add_benchmark(locale_classic src/locale_classic.cpp)
 add_benchmark(random_integer_generation src/random_integer_generation.cpp)
+add_benchmark(std_copy src/std_copy.cpp)

benchmarks/src/locale_classic.cpp

@@ -0,0 +1,16 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <benchmark/benchmark.h>
+#include <locale>
+using namespace std;
+
+// GH-3048 <locale>: Double-checked locking for locale::classic
+static void BM_locale_classic(benchmark::State& state) {
+    for (auto _ : state) {
+        benchmark::DoNotOptimize(locale::classic());
+    }
+}
+BENCHMARK(BM_locale_classic);
+
+BENCHMARK_MAIN();

stl/src/locale0.cpp

@@ -5,6 +5,7 @@
 
 #include <crtdbg.h>
 #include <internal_shared.h>
+#include <xatomic.h>
 #include <xfacet>
 
 // This must be as small as possible, because its contents are
@@ -136,7 +137,22 @@ __PURE_APPDOMAIN_GLOBAL locale::id ctype<unsigned short>::id(0);
 __PURE_APPDOMAIN_GLOBAL locale::id codecvt<unsigned short, char, mbstate_t>::id(0);
 
 _MRTIMP2_PURE const locale& __CLRCALL_PURE_OR_CDECL locale::classic() { // get reference to "C" locale
-    _Init();
+#if !defined(_M_CEE_PURE)
+    const auto mem = reinterpret_cast<const intptr_t*>(&locale::_Locimp::_Clocptr);
+    intptr_t as_bytes;
+#ifdef _WIN64
+    as_bytes = __iso_volatile_load64(mem);
+#else // ^^^ 64-bit / 32-bit vvv
+    as_bytes = __iso_volatile_load32(mem);
+#endif // ^^^ 32-bit ^^^
+    _Compiler_or_memory_barrier();
+    const auto ptr = reinterpret_cast<locale::_Locimp*>(as_bytes);
+    if (ptr == nullptr)
+#endif // !defined(_M_CEE_PURE)
+    {
+        _Init();
+    }
+
     return classic_locale;
 }
 
@@ -157,9 +173,21 @@ _MRTIMP2_PURE locale::_Locimp* __CLRCALL_PURE_OR_CDECL locale::_Init(bool _Do_in
         ptr->_Catmask = all; // set current locale to "C"
         ptr->_Name    = "C";
 
-        _Locimp::_Clocptr = ptr; // set classic to match
-        _Locimp::_Clocptr->_Incref();
-        ::new (&classic_locale) locale(_Locimp::_Clocptr);
+        // set classic to match
+        ptr->_Incref();
+        ::new (&classic_locale) locale(ptr);
+#if defined(_M_CEE_PURE)
+        locale::_Locimp::_Clocptr = ptr;
+#else // ^^^ _M_CEE_PURE / !_M_CEE_PURE vvv
+        const auto mem      = reinterpret_cast<volatile intptr_t*>(&locale::_Locimp::_Clocptr);
+        const auto as_bytes = reinterpret_cast<intptr_t>(ptr);
+        _Compiler_or_memory_barrier();
+#ifdef _WIN64
+        __iso_volatile_store64(mem, as_bytes);
+#else // ^^^ 64-bit / 32-bit vvv
+        __iso_volatile_store32(mem, as_bytes);
+#endif // ^^^ 32-bit ^^^
+#endif // ^^^ !defined(_M_CEE_PURE) ^^^
     }
 
     if (_Do_incref) {