[clang] Add /Zc:__STDC__ flag to clang-cl

This commit adds the /Zc:__STDC__ argument from MSVC, which defines __STDC__.
This means, alongside stronger feature parity with MSVC,
that things that rely on __STDC__, such as autoconf, can work.

Author: xbjfk

clang/docs/ReleaseNotes.rst

@@ -177,6 +177,9 @@ New Compiler Flags
 - ``-Wmissing-designated-field-initializers``, grouped under ``-Wmissing-field-initializers``.
   This diagnostic can be disabled to make ``-Wmissing-field-initializers`` behave
   like it did before Clang 18.x. Fixes (`#56628 <https://github.com/llvm/llvm-project/issues/68933>`_)
+- ``-fms-define-stdc`` and its clang-cl counterpart ``/Zc:__STDC__``.
+  Matches MSVC behaviour by defining ``__STDC__`` to ``1`` when
+  MSVC compatibility mode is used. It has no effect for C++ code.
 
 Deprecated Compiler Flags
 -------------------------

clang/include/clang/Basic/LangOptions.def

@@ -291,6 +291,7 @@ LANGOPT(HIPStdParInterposeAlloc, 1, 0, "Replace allocations / deallocations with
 
 LANGOPT(OpenACC           , 1, 0, "OpenACC Enabled")
 
+LANGOPT(MSVCEnableStdcMacro , 1, 0, "Define __STDC__ with '-fms-compatability'")
 LANGOPT(SizedDeallocation , 1, 0, "sized deallocation")
 LANGOPT(AlignedAllocation , 1, 0, "aligned allocation")
 LANGOPT(AlignedAllocationUnavailable, 1, 0, "aligned allocation functions are unavailable")

clang/include/clang/Driver/Options.td

@@ -2934,6 +2934,10 @@ def fms_compatibility : Flag<["-"], "fms-compatibility">, Group<f_Group>,
   Visibility<[ClangOption, CC1Option, CLOption]>,
   HelpText<"Enable full Microsoft Visual C++ compatibility">,
   MarshallingInfoFlag<LangOpts<"MSVCCompat">>;
+def fms_define_stdc : Flag<["-"], "fms-define-stdc">, Group<f_Group>,
+  Visibility<[ClangOption, CC1Option]>,
+  HelpText<"Define '__STDC__' to '1' in MSVC Compatibility mode">,
+  MarshallingInfoFlag<LangOpts<"MSVCEnableStdcMacro">>;
 def fms_extensions : Flag<["-"], "fms-extensions">, Group<f_Group>,
   Visibility<[ClangOption, CC1Option, CLOption]>,
   HelpText<"Accept some non-standard constructs supported by the Microsoft compiler">,
@@ -8172,6 +8176,9 @@ def _SLASH_vd : CLJoined<"vd">, HelpText<"Control vtordisp placement">,
   Alias<vtordisp_mode_EQ>;
 def _SLASH_X : CLFlag<"X">,
   HelpText<"Do not add %INCLUDE% to include search path">, Alias<nostdlibinc>;
+def _SLASH_Zc___STDC__ : CLFlag<"Zc:__STDC__">,
+  HelpText<"Define __STDC__">,
+  Alias<fms_define_stdc>;
 def _SLASH_Zc_sizedDealloc : CLFlag<"Zc:sizedDealloc">,
   HelpText<"Enable C++14 sized global deallocation functions">,
   Alias<fsized_deallocation>;

clang/lib/Driver/ToolChains/Clang.cpp

@@ -6888,8 +6888,12 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
       options::OPT_fms_compatibility, options::OPT_fno_ms_compatibility,
       (IsWindowsMSVC && Args.hasFlag(options::OPT_fms_extensions,
                                      options::OPT_fno_ms_extensions, true)));
-  if (IsMSVCCompat)
+  if (IsMSVCCompat) {
     CmdArgs.push_back("-fms-compatibility");
+    if (!types::isCXX(Input.getType()) &&
+        Args.hasArg(options::OPT_fms_define_stdc))
+      CmdArgs.push_back("-fms-define-stdc");
+  }
 
   if (Triple.isWindowsMSVCEnvironment() && !D.IsCLMode() &&
       Args.hasArg(options::OPT_fms_runtime_lib_EQ))
@@ -8065,6 +8069,9 @@ void Clang::AddClangCLArgs(const ArgList &Args, types::ID InputType,
    CmdArgs.push_back("-fno-wchar");
  }
 
+ if (!types::isCXX(InputType) && Args.hasArg(options::OPT_fms_define_stdc))
+   CmdArgs.push_back("-fms-define-stdc");
+
  if (Args.hasArg(options::OPT__SLASH_kernel)) {
    llvm::Triple::ArchType Arch = getToolChain().getArch();
    std::vector<std::string> Values =

clang/lib/Frontend/InitPreprocessor.cpp

@@ -433,7 +433,8 @@ static void InitializeStandardPredefinedMacros(const TargetInfo &TI,
   //      [C++] Whether __STDC__ is predefined and if so, what its value is,
   //      are implementation-defined.
   // (Removed in C++20.)
-  if (!LangOpts.MSVCCompat && !LangOpts.TraditionalCPP)
+  if ((!LangOpts.MSVCCompat || LangOpts.MSVCEnableStdcMacro) &&
+      !LangOpts.TraditionalCPP)
     Builder.defineMacro("__STDC__");
   //   -- __STDC_HOSTED__
   //      The integer literal 1 if the implementation is a hosted

clang/test/Driver/ms-define-stdc.c

@@ -0,0 +1,11 @@
+// Note: %s must be preceded by --, otherwise it may be interpreted as a
+// command-line option, e.g. on Mac where %s is commonly under /Users.
+//
+// Note: see also cl-zc.cpp
+
+// RUN: %clang_cl /TC /dev/null /E -Xclang -dM /Zc:__STDC__- 2>&1 | FileCheck %s --check-prefix=ZCSTDCIGNORED
+// ZCSTDCIGNORED-NOT: #define __STDC__ 1
+// ZCSTDCIGNORED: argument unused during compilation
+
+// RUN: not %clang -Xclang -fno-ms-define-stdc %s 2>&1 | FileCheck %s --check-prefix="NOARG"
+// NOARG: error: unknown argument: '-fno-ms-define-stdc'

clang/test/Preprocessor/stdc-ms-extension.cpp

@@ -0,0 +1,9 @@
+// RUN: %clang_cl /TC /dev/null /E -Xclang -dM 2> /dev/null | FileCheck -match-full-lines %s --check-prefix=NOSTDC
+// RUN: %clang_cl /TC /dev/null /E -Xclang -dM /Zc:__STDC__ 2> /dev/null | FileCheck -match-full-lines %s --check-prefix=YESSTDC
+// __STDC__ should never be defined in C++ mode with fms-compatibility.
+// RUN: %clang_cl /dev/null /E -Xclang -dM 2>&1 | FileCheck %s --check-prefix=NOSTDC
+// RUN: %clang_cl /dev/null /E -Xclang -dM /Zc:__STDC__ 2>&1 | FileCheck %s --check-prefix=ZCSTDCIGNORED
+// YESSTDC: #define __STDC__ 1
+// NOSTDC-NOT: #define __STDC__ 1
+// ZCSTDCIGNORED-NOT: #define __STDC__ 1
+// ZCSTDCIGNORED: argument unused during compilation