[clang] __STDC_NO_THREADS__ is no longer necessary for VS 2022 1939 and above (#117149)

Since `__STDC_NO_THREADS__` is a reserved identifier,
- If `MSVC version < 17.9`
- C version < C11(201112L)
- When `<threads.h>` is unavailable `!__has_include(<threads.h>)` is
`__has_include` is defined.

Closes #115529

Author: dipeshs809

clang/docs/ReleaseNotes.rst

@@ -387,6 +387,7 @@ C Language Changes
 ------------------
 
 - Extend clang's ``<limits.h>`` to define ``LONG_LONG_*`` macros for Android's bionic.
+- Macro ``__STDC_NO_THREADS__`` is no longer necessary for MSVC 2022 1939 and later.
 
 C2y Feature Support
 ^^^^^^^^^^^^^^^^^^^

clang/include/clang/Basic/LangOptions.h

@@ -144,6 +144,7 @@ class LangOptionsBase {
     MSVC2019_5 = 1925,
     MSVC2019_8 = 1928,
     MSVC2022_3 = 1933,
+    MSVC2022_9 = 1939,
   };
 
   enum SYCLMajorVersion {

clang/lib/Basic/Targets/OSTargets.cpp

@@ -259,8 +259,10 @@ static void addVisualCDefines(const LangOptions &Opts, MacroBuilder &Builder) {
     Builder.defineMacro("_KERNEL_MODE");
 
   Builder.defineMacro("_INTEGRAL_MAX_BITS", "64");
-  Builder.defineMacro("__STDC_NO_THREADS__");
-
+  // Define __STDC_NO_THREADS__ based on MSVC version, threads.h availability,
+  // and language standard.
+  if (!(Opts.isCompatibleWithMSVC(LangOptions::MSVC2022_9) && Opts.C11))
+    Builder.defineMacro("__STDC_NO_THREADS__");
   // Starting with VS 2022 17.1, MSVC predefines the below macro to inform
   // users of the execution character set defined at compile time.
   // The value given is the Windows Code Page Identifier:

clang/test/Preprocessor/deprecate-threads-macro-definition-msvc1939.c

@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -E -dM -triple=arm64ec-windows-msvc -std=c89 -fms-compatibility-version=19.33 -ffreestanding < /dev/null | FileCheck -check-prefix=C89_MSVC33 %s
+// RUN: %clang_cc1 -E -dM -triple=arm64ec-windows-msvc -std=c99 -fms-compatibility-version=19.33 -ffreestanding < /dev/null | FileCheck -check-prefix=C99_MSVC33 %s
+// RUN: %clang_cc1 -E -dM -triple=arm64ec-windows-msvc -std=c11 -fms-compatibility-version=19.33 -ffreestanding < /dev/null | FileCheck -check-prefix=C11_MSVC33 %s
+// RUN: %clang_cc1 -E -dM -triple=arm64ec-windows-msvc -std=c89 -fms-compatibility-version=19.39 -ffreestanding < /dev/null | FileCheck -check-prefix=C89_MSVC39 %s
+// RUN: %clang_cc1 -E -dM -triple=arm64ec-windows-msvc -std=c99 -fms-compatibility-version=19.39 -ffreestanding < /dev/null | FileCheck -check-prefix=C99_MSVC39 %s
+// RUN: %clang_cc1 -E -dM -triple=arm64ec-windows-msvc -std=c11 -fms-compatibility-version=19.39 -ffreestanding < /dev/null | FileCheck -check-prefix=C11_MSVC39 %s
+// RUN: %clang_cc1 -E -dM -triple=arm64ec-windows-msvc -std=c11 -fms-compatibility-version=19.40 -ffreestanding < /dev/null | FileCheck -check-prefix=C11_MSVC40 %s
+
+// C89_MSVC33: #define __STDC_NO_THREADS__ 1
+// C99_MSVC33: #define __STDC_NO_THREADS__ 1
+// C11_MSVC33: #define __STDC_NO_THREADS__ 1
+// C89_MSVC39: #define __STDC_NO_THREADS__ 1
+// C99_MSVC39: #define __STDC_NO_THREADS__ 1
+// C11_MSVC39-NOT: #define __STDC_NO_THREADS__
+// C11_MSVC40-NOT: #define __STDC_NO_THREADS__

clang/test/Preprocessor/init-aarch64.c

@@ -772,7 +772,7 @@
 // AARCH64-MSVC: #define __WINT_WIDTH__ 16
 // AARCH64-MSVC: #define __aarch64__ 1
 
-// RUN: %clang_cc1 -E -dM -ffreestanding -triple=arm64ec-windows-msvc < /dev/null | FileCheck -match-full-lines -check-prefix ARM64EC-MSVC %s
+// RUN: %clang_cc1 -E -dM -fms-compatibility-version=19.33 -ffreestanding -triple=arm64ec-windows-msvc < /dev/null | FileCheck -match-full-lines -check-prefix ARM64EC-MSVC %s
 
 // ARM64EC-MSVC: #define _INTEGRAL_MAX_BITS 64
 // ARM64EC-MSVC: #define _M_AMD64 100