[Preprocessor] Do not expand macros if the input is already preprocessed

Author: jmmartinez

clang/include/clang/Lex/Preprocessor.h

@@ -1831,6 +1831,11 @@ class Preprocessor {
     MacroExpansionInDirectivesOverride = true;
   }
 
+  void SetDisableMacroExpansion() {
+    DisableMacroExpansion = true;
+    MacroExpansionInDirectivesOverride = false;
+  }
+
   /// Peeks ahead N tokens and returns that token without consuming any
   /// tokens.
   ///

clang/lib/Frontend/InitPreprocessor.cpp

@@ -1556,6 +1556,13 @@ void clang::InitializePreprocessor(Preprocessor &PP,
                                    const PCHContainerReader &PCHContainerRdr,
                                    const FrontendOptions &FEOpts,
                                    const CodeGenOptions &CodeGenOpts) {
+
+  if (all_of(FEOpts.Inputs,
+             [](const FrontendInputFile &FI) { return FI.isPreprocessed(); })) {
+    PP.SetDisableMacroExpansion();
+    return;
+  }
+
   const LangOptions &LangOpts = PP.getLangOpts();
   std::string PredefineBuffer;
   PredefineBuffer.reserve(4080);

clang/test/Preprocessor/preprocess-cpp-output.c

@@ -1,7 +1,8 @@
 // RUN: %clang_cc1 -E -x c %s | FileCheck %s --check-prefixes=EXPANDED
-// RUN: %clang_cc1 -E -x cpp-output %s | FileCheck %s --check-prefixes=EXPANDED
+// RUN: %clang_cc1 -E -x cpp-output %s | FileCheck %s --check-prefixes=NOT-EXPANDED
 
 // EXPANDED: void __attribute__((__attribute__((always_inline)))) foo()
+// NOT-EXPANDED: void __attribute__((always_inline)) foo()
 
 #define always_inline __attribute__((always_inline))
 void __attribute__((always_inline)) foo() {