[HeaderSearch] Use isImport only for imported headers and not for #pragma once.

There is a separate field `isPragmaOnce` and when `isImport` combines
both, it complicates HeaderFileInfo serialization as `#pragma once` is
the inherent property of the header while `isImport` reflects how other
headers use it. The usage of the header can be different in different
contexts, that's why `isImport` requires tracking separate from `#pragma once`.

Differential Revision: https://reviews.llvm.org/D104351

Author: vsapsai

clang/include/clang/Lex/HeaderSearch.h

@@ -51,7 +51,7 @@ class TargetInfo;
 /// The preprocessor keeps track of this information for each
 /// file that is \#included.
 struct HeaderFileInfo {
-  /// True if this is a \#import'd or \#pragma once file.
+  /// True if this is a \#import'd file.
   unsigned isImport : 1;
 
   /// True if this is a \#pragma once file.
@@ -450,11 +450,10 @@ class HeaderSearch {
     return (SrcMgr::CharacteristicKind)getFileInfo(File).DirInfo;
   }
 
-  /// Mark the specified file as a "once only" file, e.g. due to
+  /// Mark the specified file as a "once only" file due to
   /// \#pragma once.
   void MarkFileIncludeOnce(const FileEntry *File) {
     HeaderFileInfo &FI = getFileInfo(File);
-    FI.isImport = true;
     FI.isPragmaOnce = true;
   }
 
@@ -500,8 +499,7 @@ class HeaderSearch {
   /// This routine does not consider the effect of \#import
   bool isFileMultipleIncludeGuarded(const FileEntry *File);
 
-  /// Determine whether the given file is known to have ever been \#imported
-  /// (or if it has been \#included and we've encountered a \#pragma once).
+  /// Determine whether the given file is known to have ever been \#imported.
   bool hasFileBeenImported(const FileEntry *File) {
     const HeaderFileInfo *FI = getExistingFileInfo(File);
     return FI && FI->isImport;

clang/lib/Lex/HeaderSearch.cpp

@@ -91,7 +91,7 @@ void HeaderSearch::PrintStats() {
                << FileInfo.size() << " files tracked.\n";
   unsigned NumOnceOnlyFiles = 0, MaxNumIncludes = 0, NumSingleIncludedFiles = 0;
   for (unsigned i = 0, e = FileInfo.size(); i != e; ++i) {
-    NumOnceOnlyFiles += FileInfo[i].isImport;
+    NumOnceOnlyFiles += (FileInfo[i].isPragmaOnce || FileInfo[i].isImport);
     if (MaxNumIncludes < FileInfo[i].NumIncludes)
       MaxNumIncludes = FileInfo[i].NumIncludes;
     NumSingleIncludedFiles += FileInfo[i].NumIncludes == 1;
@@ -1325,7 +1325,7 @@ bool HeaderSearch::ShouldEnterIncludeFile(Preprocessor &PP,
   } else {
     // Otherwise, if this is a #include of a file that was previously #import'd
     // or if this is the second #include of a #pragma once file, ignore it.
-    if (FileInfo.isImport && !TryEnterImported())
+    if ((FileInfo.isPragmaOnce || FileInfo.isImport) && !TryEnterImported())
       return false;
   }