[clang] [clang] Add /Zc:__STDC__ flag to clang-cl (PR #68690)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Reagan (xbjfk)

<details>
<summary>Changes</summary>

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.
Link to MSVC documentation of this flag: https://learn.microsoft.com/en-us/cpp/build/reference/zc-stdc?view=msvc-170

---
Full diff: https://github.com/llvm/llvm-project/pull/68690.diff


5 Files Affected:

- (modified) clang/include/clang/Basic/LangOptions.def (+1-1) 
- (modified) clang/include/clang/Driver/Options.td (+7) 
- (modified) clang/lib/Driver/ToolChains/Clang.cpp (+10-1) 
- (modified) clang/lib/Frontend/InitPreprocessor.cpp (+1-1) 
- (modified) clang/test/Driver/cl-zc.cpp (+4) 


``````````diff
diff --git a/clang/include/clang/Basic/LangOptions.def b/clang/include/clang/Basic/LangOptions.def
index c0ea4ecb9806a5b..97101d0166a135a 100644
--- a/clang/include/clang/Basic/LangOptions.def
+++ b/clang/include/clang/Basic/LangOptions.def
@@ -282,7 +282,7 @@ LANGOPT(HIPUseNewLaunchAPI, 1, 0, "Use new kernel launching API for HIP")
 LANGOPT(OffloadUniformBlock, 1, 0, "Assume that kernels are launched with uniform block sizes (default true for CUDA/HIP and false otherwise)")
 LANGOPT(HIPStdPar, 1, 0, "Enable Standard Parallel Algorithm Acceleration for HIP (experimental)")
 LANGOPT(HIPStdParInterposeAlloc, 1, 0, "Replace allocations / deallocations with HIP RT calls when Standard Parallel Algorithm Acceleration for HIP is enabled (Experimental)")
-
+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")
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index c272a7f1c398aa6..fae1dcc2b5117f7 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -2821,6 +2821,10 @@ 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__ 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">,
@@ -7933,6 +7937,9 @@ 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>;
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index bfd6c5c2864abf7..b8f3ff6cd4c91e4 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -6744,8 +6744,13 @@ 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");
+    bool IsMSVCDefineStdc = Args.hasArg(options::OPT_fms_define_stdc);
+    if (IsMSVCDefineStdc)
+      CmdArgs.push_back("-fms-define-stdc");
+  }
+
 
   if (Triple.isWindowsMSVCEnvironment() && !D.IsCLMode() &&
       Args.hasArg(options::OPT_fms_runtime_lib_EQ))
@@ -7922,6 +7927,10 @@ void Clang::AddClangCLArgs(const ArgList &Args, types::ID InputType,
    CmdArgs.push_back("-fno-wchar");
  }
 
+ if (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 =
diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
index 846e5fce6de7b2c..46399f613257a74 100644
--- a/clang/lib/Frontend/InitPreprocessor.cpp
+++ b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -423,7 +423,7 @@ 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
diff --git a/clang/test/Driver/cl-zc.cpp b/clang/test/Driver/cl-zc.cpp
index c7cf5b1b6525bea..bb955222c647af3 100644
--- a/clang/test/Driver/cl-zc.cpp
+++ b/clang/test/Driver/cl-zc.cpp
@@ -123,6 +123,10 @@
 // CHECK-CHAR8_T_: "-fno-char8_t"
 
 
+// RUN: %clang_cl /dev/null /E -Xclang -dM 2> /dev/null | FileCheck -match-full-lines %s --check-prefix=STDCOFF
+// RUN: %clang_cl /dev/null /E -Xclang -dM /Zc:__STDC__ 2> /dev/null | FileCheck -match-full-lines %s --check-prefix=STDCON
+// STDCON: #define __STDC__ 1
+// STDCOFF-NOT: #define __STDC__ 1
 
 // These never warn, but don't have an effect yet.
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/68690