1 files changed, 24 insertions, 9 deletions

diff --git a/clang/lib/Frontend/PrintPreprocessedOutput.cpp b/clang/lib/Frontend/PrintPreprocessedOutput.cpp
index e2fc862..7737d04 100644
--- a/clang/lib/Frontend/PrintPreprocessedOutput.cpp
+++ b/clang/lib/Frontend/PrintPreprocessedOutput.cpp
@@ -96,6 +96,7 @@ private:
   bool UseLineDirectives;
   bool IsFirstFileEntered;
   bool MinimizeWhitespace;
+  bool DirectivesOnly;
 
   Token PrevTok;
   Token PrevPrevTok;
@@ -103,12 +104,13 @@ private:
 public:
   PrintPPOutputPPCallbacks(Preprocessor &pp, raw_ostream &os, bool lineMarkers,
                            bool defines, bool DumpIncludeDirectives,
-                           bool UseLineDirectives, bool MinimizeWhitespace)
+                           bool UseLineDirectives, bool MinimizeWhitespace,
+                           bool DirectivesOnly)
       : PP(pp), SM(PP.getSourceManager()), ConcatInfo(PP), OS(os),
         DisableLineMarkers(lineMarkers), DumpDefines(defines),
         DumpIncludeDirectives(DumpIncludeDirectives),
         UseLineDirectives(UseLineDirectives),
-        MinimizeWhitespace(MinimizeWhitespace) {
+        MinimizeWhitespace(MinimizeWhitespace), DirectivesOnly(DirectivesOnly) {
     CurLine = 0;
     CurFilename += "<uninit>";
     EmittedTokensOnThisLine = false;
@@ -467,12 +469,21 @@ void PrintPPOutputPPCallbacks::Ident(SourceLocation Loc, StringRef S) {
 void PrintPPOutputPPCallbacks::MacroDefined(const Token &MacroNameTok,
                                             const MacroDirective *MD) {
   const MacroInfo *MI = MD->getMacroInfo();
-  // Only print out macro definitions in -dD mode.
-  if (!DumpDefines ||
+  // Print out macro definitions in -dD mode and when we have -fdirectives-only
+  // for C++20 header units.
+  if ((!DumpDefines && !DirectivesOnly) ||
       // Ignore __FILE__ etc.
-      MI->isBuiltinMacro()) return;
+      MI->isBuiltinMacro())
+    return;
 
-  MoveToLine(MI->getDefinitionLoc(), /*RequireStartOfLine=*/true);
+  SourceLocation DefLoc = MI->getDefinitionLoc();
+  if (DirectivesOnly && !MI->isUsed()) {
+    SourceManager &SM = PP.getSourceManager();
+    if (SM.isWrittenInBuiltinFile(DefLoc) ||
+        SM.isWrittenInCommandLineFile(DefLoc))
+      return;
+  }
+  MoveToLine(DefLoc, /*RequireStartOfLine=*/true);
   PrintMacroDefinition(*MacroNameTok.getIdentifierInfo(), *MI, PP, OS);
   setEmittedDirectiveOnThisLine();
 }
@@ -480,8 +491,10 @@ void PrintPPOutputPPCallbacks::MacroDefined(const Token &MacroNameTok,
 void PrintPPOutputPPCallbacks::MacroUndefined(const Token &MacroNameTok,
                                               const MacroDefinition &MD,
                                               const MacroDirective *Undef) {
-  // Only print out macro definitions in -dD mode.
-  if (!DumpDefines) return;
+  // Print out macro definitions in -dD mode and when we have -fdirectives-only
+  // for C++20 header units.
+  if (!DumpDefines && !DirectivesOnly)
+    return;
 
   MoveToLine(MacroNameTok.getLocation(), /*RequireStartOfLine=*/true);
   OS << "#undef " << MacroNameTok.getIdentifierInfo()->getName();
@@ -959,7 +972,7 @@ void clang::DoPrintPreprocessedInput(Preprocessor &PP, raw_ostream *OS,
   PrintPPOutputPPCallbacks *Callbacks = new PrintPPOutputPPCallbacks(
       PP, *OS, !Opts.ShowLineMarkers, Opts.ShowMacros,
       Opts.ShowIncludeDirectives, Opts.UseLineDirectives,
-      Opts.MinimizeWhitespace);
+      Opts.MinimizeWhitespace, Opts.DirectivesOnly);
 
   // Expand macros in pragmas with -fms-extensions.  The assumption is that
   // the majority of pragmas in such a file will be Microsoft pragmas.
@@ -995,6 +1008,8 @@ void clang::DoPrintPreprocessedInput(Preprocessor &PP, raw_ostream *OS,
 
   // After we have configured the preprocessor, enter the main file.
   PP.EnterMainSourceFile();
+  if (Opts.DirectivesOnly)
+    PP.SetMacroExpansionOnlyInDirectives();
 
   // Consume all of the tokens that come from the predefines buffer.  Those
   // should not be emitted into the output and are guaranteed to be at the