[PR52549][clang-cl] Predefine _MSVC_EXECUTION_CHARACTER_SET

[zero9178]

Summary

Since VS 2022 17.1 MSVC predefines _MSVC_EXECUTION_CHARACTER_SET to inform the users of the execution character set defined at compile time. The value the macro expands to is a Windows Code Page Identifier which are documented here: https://docs.microsoft.com/en-us/windows/win32/intl/code-page-identifiers

As clang currently only supports UTF-8 it is defined as 65001. If clang-cl were to support a different execution character set in the future we'd have to change the value.

Fixes https://bugs.llvm.org/show_bug.cgi?id=52549

[mstorsjo]

Would it be possible to move setting of this flag to somewhere else (e.g. somewhere in Driver/ToolChains/MSVC.cpp), so that it would be picked up also when building with the gcc-style driver with --target=*-windows-msvc? Overall in earlier discussions, there's been mostly positive reception to making that mode more usable even if it isn't quite as supported as actually invoking clang-cl. (For some things, like the equivalent of the cl level -MD/-MT options, there's currently nothing available in the gcc style driver - but for a feature like this, I don't see any reason why it shouldn't be available everywhere.)

[zero9178]

Enable the macro for all MSVC targets, regardless of driver in use.

[mstorsjo]

Thanks! This LGTM - but maybe wait a while if someone else has something to say. (Although I guess americans are on holiday today, so there's probably not many that will see and care today.)

[CaseyCarter]

LGTM - thanks for the quick response!

[zero9178]

Seems like this is causing failures in a test for clang-tidy if the triple used is an MSVC one. Let me see what I can do about it

[zero9178]

Alternative implementation of the original patch.

Moves the defining macro from being added to the cc1 command line by the driver, to being a builtin macro. This should avoid the issues with clang-tidy.

[rnk]

In D114576#3153189, @mstorsjo wrote:

Would it be possible to move setting of this flag to somewhere else (e.g. somewhere in Driver/ToolChains/MSVC.cpp), so that it would be picked up also when building with the gcc-style driver with --target=*-windows-msvc? Overall in earlier discussions, there's been mostly positive reception to making that mode more usable even if it isn't quite as supported as actually invoking clang-cl. (For some things, like the equivalent of the cl level -MD/-MT options, there's currently nothing available in the gcc style driver - but for a feature like this, I don't see any reason why it shouldn't be available everywhere.)

+1, that was going to be my suggestion.

clang/lib/Basic/Targets/OSTargets.cpp

if (Opts.MicrosoftExt) {
     if (Opts.CPlusPlus11) {
       Builder.defineMacro("_RVALUE_REFERENCES_V2_SUPPORTED");
       Builder.defineMacro("_RVALUE_REFERENCES_SUPPORTED");
       Builder.defineMacro("_NATIVE_NULLPTR_SUPPORTED");
     }
   }
 
   Builder.defineMacro("_INTEGRAL_MAX_BITS", "64");
+
+  // 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:
+  // https://docs.microsoft.com/en-us/windows/win32/intl/code-page-identifiers
+  //
+  // Clang currently only supports UTF-8, so we'll use 65001
+  Builder.defineMacro("_MSVC_EXECUTION_CHARACTER_SET", "65001");
 }
 
 void addWindowsDefines(const llvm::Triple &Triple, const LangOptions &Opts,
                        MacroBuilder &Builder) {
   Builder.defineMacro("_WIN32");
   if (Triple.isArch64Bit())
     Builder.defineMacro("_WIN64");
   if (Triple.isWindowsGNUEnvironment())
     addMinGWDefines(Triple, Opts, Builder);
   else if (Triple.isKnownWindowsMSVCEnvironment() ||
            (Triple.isWindowsItaniumEnvironment() && Opts.MSVCCompat))
     addVisualCDefines(Opts, Builder);
 }
 
 } // namespace targets
 } // namespace clang

clang/test/Preprocessor/init.c

 //
 //
 // RUN: %clang_cc1 -fms-extensions -triple i686-pc-win32 -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix MSEXT %s
 //
 // MSEXT-NOT:#define __STDC__
 // MSEXT:#define _INTEGRAL_MAX_BITS 64
 // MSEXT-NOT:#define _NATIVE_WCHAR_T_DEFINED 1
 // MSEXT-NOT:#define _WCHAR_T_DEFINED 1
+// MSEXT:#define _MSVC_EXECUTION_CHARACTER_SET 65001
 //
 //
 // RUN: %clang_cc1 -x c++ -fms-extensions -triple i686-pc-win32 -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix MSEXT-CXX %s
 //
 // MSEXT-CXX:#define _NATIVE_WCHAR_T_DEFINED 1
 // MSEXT-CXX:#define _WCHAR_T_DEFINED 1
 // MSEXT-CXX:#define __BOOL_DEFINED 1
 //