[clang][modules] Adopt FileEntryRef in the HeaderFileInfo block in PCM files (#67383)

This patch adopts `FileEntryRef` in the `HeaderFileInfo`-writing part of
`ASTWriter`.

First, this patch removes the loop over
`FileManager::VirtualFileEntries`. It's redundant, since all virtual
file entries are also present in `SeenFileEntries` and thus already in
`UIDToFiles`.

Second, since we now no longer rely on
`FileEntry::getLastRef()`/`FileEntry::getName()`, this patch takes care
to establish which path gets used for each UID by picking the
`FileEntryRef` with the most "`<`" name (instead of just relying on the
`StringMap` iteration order).

Note that which `FileEntry`/`FileEntryRef` objects we pick for each UID
for serialization into the `llvm::OnDiskChainedHashTable` doesn't really
matter. The hash function only includes the file size and modification
time. The file name only plays role during resolution of hash
collisions, in which case it goes through `FileManager` and resolves to
a `FileEntry` that gets pointer-compared with the queried `FileEntry`.

(Reincarnation of [D143414](https://reviews.llvm.org/D143414) and
[D142780](https://reviews.llvm.org/D142780).)

Author: jansvoboda11

clang/include/clang/Basic/FileManager.h

@@ -311,9 +311,9 @@ class FileManager : public RefCountedBase<FileManager> {
   bool makeAbsolutePath(SmallVectorImpl<char> &Path) const;
 
   /// Produce an array mapping from the unique IDs assigned to each
-  /// file to the corresponding FileEntry pointer.
-  void GetUniqueIDMapping(
-                    SmallVectorImpl<const FileEntry *> &UIDToFiles) const;
+  /// file to the corresponding FileEntryRef.
+  void
+  GetUniqueIDMapping(SmallVectorImpl<OptionalFileEntryRef> &UIDToFiles) const;
 
   /// Retrieve the canonical name for a given directory.
   ///

clang/lib/Basic/FileManager.cpp

@@ -612,24 +612,21 @@ FileManager::getNoncachedStatValue(StringRef Path,
 }
 
 void FileManager::GetUniqueIDMapping(
-    SmallVectorImpl<const FileEntry *> &UIDToFiles) const {
+    SmallVectorImpl<OptionalFileEntryRef> &UIDToFiles) const {
   UIDToFiles.clear();
   UIDToFiles.resize(NextFileUID);
 
-  // Map file entries
-  for (llvm::StringMap<llvm::ErrorOr<FileEntryRef::MapValue>,
-                       llvm::BumpPtrAllocator>::const_iterator
-           FE = SeenFileEntries.begin(),
-           FEEnd = SeenFileEntries.end();
-       FE != FEEnd; ++FE)
-    if (llvm::ErrorOr<FileEntryRef::MapValue> Entry = FE->getValue()) {
-      if (const auto *FE = Entry->V.dyn_cast<FileEntry *>())
-        UIDToFiles[FE->getUID()] = FE;
-    }
-
-  // Map virtual file entries
-  for (const auto &VFE : VirtualFileEntries)
-    UIDToFiles[VFE->getUID()] = VFE;
+  for (const auto &Entry : SeenFileEntries) {
+    // Only return files that exist and are not redirected.
+    if (!Entry.getValue() || !Entry.getValue()->V.is<FileEntry *>())
+      continue;
+    FileEntryRef FE(Entry);
+    // Add this file if it's the first one with the UID, or if its name is
+    // better than the existing one.
+    OptionalFileEntryRef &ExistingFE = UIDToFiles[FE.getUID()];
+    if (!ExistingFE || FE.getName() < ExistingFE->getName())
+      ExistingFE = FE;
+  }
 }
 
 StringRef FileManager::getCanonicalName(DirectoryEntryRef Dir) {

clang/lib/Serialization/ASTWriter.cpp

@@ -167,23 +167,23 @@ std::set<const FileEntry *> GetAffectingModuleMaps(const Preprocessor &PP,
 
   const HeaderSearch &HS = PP.getHeaderSearchInfo();
 
-  SmallVector<const FileEntry *, 16> FilesByUID;
+  SmallVector<OptionalFileEntryRef, 16> FilesByUID;
   HS.getFileMgr().GetUniqueIDMapping(FilesByUID);
 
   if (FilesByUID.size() > HS.header_file_size())
     FilesByUID.resize(HS.header_file_size());
 
   for (unsigned UID = 0, LastUID = FilesByUID.size(); UID != LastUID; ++UID) {
-    const FileEntry *File = FilesByUID[UID];
+    OptionalFileEntryRef File = FilesByUID[UID];
     if (!File)
       continue;
 
     const HeaderFileInfo *HFI =
-        HS.getExistingFileInfo(File, /*WantExternal*/ false);
+        HS.getExistingFileInfo(*File, /*WantExternal*/ false);
     if (!HFI || (HFI->isModuleHeader && !HFI->isCompilingModuleHeader))
       continue;
 
-    for (const auto &KH : HS.findResolvedModulesForHeader(File)) {
+    for (const auto &KH : HS.findResolvedModulesForHeader(*File)) {
       if (!KH.getModule())
         continue;
       ModulesToProcess.push_back(KH.getModule());
@@ -2003,14 +2003,14 @@ void ASTWriter::WriteHeaderSearch(const HeaderSearch &HS) {
     }
   }
 
-  SmallVector<const FileEntry *, 16> FilesByUID;
+  SmallVector<OptionalFileEntryRef, 16> FilesByUID;
   HS.getFileMgr().GetUniqueIDMapping(FilesByUID);
 
   if (FilesByUID.size() > HS.header_file_size())
     FilesByUID.resize(HS.header_file_size());
 
   for (unsigned UID = 0, LastUID = FilesByUID.size(); UID != LastUID; ++UID) {
-    const FileEntry *File = FilesByUID[UID];
+    OptionalFileEntryRef File = FilesByUID[UID];
     if (!File)
       continue;
 
@@ -2021,7 +2021,7 @@ void ASTWriter::WriteHeaderSearch(const HeaderSearch &HS) {
     // from a different module; in that case, we rely on the module(s)
     // containing the header to provide this information.
     const HeaderFileInfo *HFI =
-        HS.getExistingFileInfo(File, /*WantExternal*/!Chain);
+        HS.getExistingFileInfo(*File, /*WantExternal*/!Chain);
     if (!HFI || (HFI->isModuleHeader && !HFI->isCompilingModuleHeader))
       continue;
 
@@ -2035,13 +2035,13 @@ void ASTWriter::WriteHeaderSearch(const HeaderSearch &HS) {
       SavedStrings.push_back(Filename.data());
     }
 
-    bool Included = PP->alreadyIncluded(File);
+    bool Included = PP->alreadyIncluded(*File);
 
     HeaderFileInfoTrait::key_type Key = {
-      Filename, File->getSize(), getTimestampForOutput(File)
+      Filename, File->getSize(), getTimestampForOutput(*File)
     };
     HeaderFileInfoTrait::data_type Data = {
-      *HFI, Included, HS.getModuleMap().findResolvedModulesForHeader(File), {}
+      *HFI, Included, HS.getModuleMap().findResolvedModulesForHeader(*File), {}
     };
     Generator.insert(Key, Data, GeneratorTrait);
     ++NumHeaderSearchEntries;