2 files changed, 20 insertions, 0 deletions
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 2ebdc2a62aa..60802da921b 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -2611,6 +2611,10 @@ static void ParsePreprocessorArgs(PreprocessorOptions &Opts, ArgList &Args,
   for (const Arg *A : Args.filtered(OPT_chain_include))
     Opts.ChainedIncludes.emplace_back(A->getValue());
 
+  // Add the ordered list of -fsystem-include-if-exists.
+  for (const Arg *A : Args.filtered(OPT_fsystem_include_if_exists))
+    Opts.FSystemIncludeIfExists.emplace_back(A->getValue());
+
   for (const Arg *A : Args.filtered(OPT_remap_file)) {
     std::pair<StringRef, StringRef> Split = StringRef(A->getValue()).split(';');
 
diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
index d3989049432..45760ae15e4 100644
--- a/clang/lib/Frontend/InitPreprocessor.cpp
+++ b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -70,6 +70,15 @@ static void AddImplicitInclude(MacroBuilder &Builder, StringRef File) {
   Builder.append(Twine("#include \"") + File + "\"");
 }
 
+/// AddImplicitSystemIncludeIfExists - Add an implicit system \#include of the 
+/// specified file to the predefines buffer: precheck with __has_include.
+static void AddImplicitSystemIncludeIfExists(MacroBuilder &Builder, 
+                                             StringRef File) {
+  Builder.append(Twine("#if __has_include( <") + File + ">)");
+  Builder.append(Twine("#include <") + File + ">");
+  Builder.append(Twine("#endif"));
+}
+
 static void AddImplicitIncludeMacros(MacroBuilder &Builder, StringRef File) {
   Builder.append(Twine("#__include_macros \"") + File + "\"");
   // Marker token to stop the __include_macros fetch loop.
@@ -1133,6 +1142,13 @@ void clang::InitializePreprocessor(
   if (!PP.getLangOpts().AsmPreprocessor)
     Builder.append("# 1 \"<built-in>\" 2");
 
+  // Process -fsystem-include-if-exists directives.
+  for (unsigned i = 0, 
+       e = InitOpts.FSystemIncludeIfExists.size(); i != e; ++i) {
+    const std::string &Path = InitOpts.FSystemIncludeIfExists[i];
+    AddImplicitSystemIncludeIfExists(Builder, Path);
+  }
+
   // If -imacros are specified, include them now.  These are processed before
   // any -include directives.
   for (unsigned i = 0, e = InitOpts.MacroIncludes.size(); i != e; ++i)