[clang] __STDC_NO_THREADS__ is no longer necessary for VS 2022 1939 and above

[dipeshs809]

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

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[llvmbot]

@llvm/pr-subscribers-clang

Author: Dipesh Sharma (dipeshs809)

Changes

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

---

Full diff: https://github.com/llvm/llvm-project/pull/117149.diff

2 Files Affected:

• (modified) clang/include/clang/Basic/LangOptions.h (+1)
• (modified) clang/lib/Basic/Targets/OSTargets.cpp (+13-2)

diff --git a/clang/include/clang/Basic/LangOptions.h b/clang/include/clang/Basic/LangOptions.h
index 949c8f5d448bcf..114a5d34a008bd 100644
--- a/clang/include/clang/Basic/LangOptions.h
+++ b/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 {
diff --git a/clang/lib/Basic/Targets/OSTargets.cpp b/clang/lib/Basic/Targets/OSTargets.cpp
index 88c054150ab224..f8d974e9979e4d 100644
--- a/clang/lib/Basic/Targets/OSTargets.cpp
+++ b/clang/lib/Basic/Targets/OSTargets.cpp
@@ -248,8 +248,19 @@ 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)) {
+    Builder.defineMacro("__STDC_NO_THREADS__");
+  } else {
+#if defined(__has_include) && !__has_include(<threads.h>)
+    Builder.defineMacro("__STDC_NO_THREADS__");
+#endif
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ < 201112L
+    Builder.defineMacro("__STDC_NO_THREADS__");
+#endif
+  }
+  // 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:

[AaronBallman]

Thank you for working on this! Please be sure to add some test coverage for the changes to clang/test/Headers and a release note in clang/docs/ReleaseNotes.rst.

[AaronBallman]

These changes are looking at the host compiler's behavior when building Clang, not the state of the system on which Clang is running, so they won't do what you expect.

Trying to determine if the system has a threads.h header on the search path is possible, but would be really tricky to do from here because this function doesn't have access to what's needed to do that work.

As for checking for C11, that can be done via !Opts.C11

[dipeshs809]

Thank you for working on this! Please be sure to add some test coverage for the changes to clang/test/Headers and a release note in clang/docs/ReleaseNotes.rst.

Thanks for the review. I'll incorporate the changes suggested and come back to you.

[AaronBallman]

I found a few minor nits, but I think we're pretty close!

[dipeshs809]

@AaronBallman Thanks for reviewing the PR, have incorporated the changes.

[AaronBallman]

LGTM thank you for the fix!

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