Sink unnecessary steps for Windows in CompilerInvocation::computeCXXStdlibOptions

Author: hjyamauchi

lib/Frontend/CompilerInvocation.cpp

@@ -364,26 +364,6 @@ setBridgingHeaderFromFrontendOptions(ClangImporterOptions &ImporterOpts,
 }
 
 void CompilerInvocation::computeCXXStdlibOptions() {
-  auto [clangDriver, clangDiagEngine] =
-      ClangImporter::createClangDriver(LangOpts, ClangImporterOpts);
-  auto clangDriverArgs = ClangImporter::createClangArgs(
-      ClangImporterOpts, SearchPathOpts, clangDriver);
-  auto &clangToolchain =
-      clangDriver.getToolChain(clangDriverArgs, LangOpts.Target);
-  auto cxxStdlibKind = clangToolchain.GetCXXStdlibType(clangDriverArgs);
-  auto cxxDefaultStdlibKind = clangToolchain.GetDefaultCXXStdlibType();
-
-  auto toCXXStdlibKind =
-      [](clang::driver::ToolChain::CXXStdlibType clangCXXStdlibType)
-      -> CXXStdlibKind {
-    switch (clangCXXStdlibType) {
-    case clang::driver::ToolChain::CST_Libcxx:
-      return CXXStdlibKind::Libcxx;
-    case clang::driver::ToolChain::CST_Libstdcxx:
-      return CXXStdlibKind::Libstdcxx;
-    }
-  };
-
   // The MSVC driver in Clang is not aware of the C++ stdlib, and currently
   // always assumes libstdc++, which is incorrect: the Microsoft stdlib is
   // normally used.
@@ -393,8 +373,27 @@ void CompilerInvocation::computeCXXStdlibOptions() {
     // (see https://reviews.llvm.org/D101479).
     LangOpts.CXXStdlib = CXXStdlibKind::Msvcprt;
     LangOpts.PlatformDefaultCXXStdlib = CXXStdlibKind::Msvcprt;
-  }
-  if (LangOpts.Target.isOSLinux() || LangOpts.Target.isOSDarwin()) {
+  } else if (LangOpts.Target.isOSLinux() || LangOpts.Target.isOSDarwin()) {
+    auto [clangDriver, clangDiagEngine] =
+        ClangImporter::createClangDriver(LangOpts, ClangImporterOpts);
+    auto clangDriverArgs = ClangImporter::createClangArgs(
+        ClangImporterOpts, SearchPathOpts, clangDriver);
+    auto &clangToolchain =
+        clangDriver.getToolChain(clangDriverArgs, LangOpts.Target);
+    auto cxxStdlibKind = clangToolchain.GetCXXStdlibType(clangDriverArgs);
+    auto cxxDefaultStdlibKind = clangToolchain.GetDefaultCXXStdlibType();
+
+    auto toCXXStdlibKind =
+        [](clang::driver::ToolChain::CXXStdlibType clangCXXStdlibType)
+        -> CXXStdlibKind {
+      switch (clangCXXStdlibType) {
+      case clang::driver::ToolChain::CST_Libcxx:
+        return CXXStdlibKind::Libcxx;
+      case clang::driver::ToolChain::CST_Libstdcxx:
+        return CXXStdlibKind::Libstdcxx;
+      }
+    };
+
     LangOpts.CXXStdlib = toCXXStdlibKind(cxxStdlibKind);
     LangOpts.PlatformDefaultCXXStdlib = toCXXStdlibKind(cxxDefaultStdlibKind);
   }