[clang][modules] Use relative offsets for input files

This patch replaces absolute offsets into the input files block with offsets relative to the block start. This makes the whole section "relocatable". I confirmed all other uses of `GetCurrentBitNo()` are turned into relative offsets before being serialized into the AST file.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D158572

Author: jansvoboda11

clang/include/clang/Serialization/ModuleFile.h

@@ -245,7 +245,10 @@ class ModuleFile {
   /// The cursor to the start of the input-files block.
   llvm::BitstreamCursor InputFilesCursor;
 
-  /// Offsets for all of the input file entries in the AST file.
+  /// Absolute offset of the start of the input-files block.
+  uint64_t InputFilesOffsetBase = 0;
+
+  /// Relative offsets for all of the input file entries in the AST file.
   const llvm::support::unaligned_uint64_t *InputFileOffsets = nullptr;
 
   /// The input files that have been loaded from this AST file.

clang/lib/Serialization/ASTReader.cpp

@@ -2326,7 +2326,8 @@ InputFileInfo ASTReader::getInputFileInfo(ModuleFile &F, unsigned ID) {
   // Go find this input file.
   BitstreamCursor &Cursor = F.InputFilesCursor;
   SavedStreamPosition SavedPosition(Cursor);
-  if (llvm::Error Err = Cursor.JumpToBit(F.InputFileOffsets[ID - 1])) {
+  if (llvm::Error Err = Cursor.JumpToBit(F.InputFilesOffsetBase +
+                                         F.InputFileOffsets[ID - 1])) {
     // FIXME this drops errors on the floor.
     consumeError(std::move(Err));
   }
@@ -2410,7 +2411,8 @@ InputFile ASTReader::getInputFile(ModuleFile &F, unsigned ID, bool Complain) {
   // Go find this input file.
   BitstreamCursor &Cursor = F.InputFilesCursor;
   SavedStreamPosition SavedPosition(Cursor);
-  if (llvm::Error Err = Cursor.JumpToBit(F.InputFileOffsets[ID - 1])) {
+  if (llvm::Error Err = Cursor.JumpToBit(F.InputFilesOffsetBase +
+                                         F.InputFileOffsets[ID - 1])) {
     // FIXME this drops errors on the floor.
     consumeError(std::move(Err));
   }
@@ -2788,6 +2790,7 @@ ASTReader::ReadControlBlock(ModuleFile &F,
           Error("malformed block record in AST file");
           return Failure;
         }
+        F.InputFilesOffsetBase = F.InputFilesCursor.GetCurrentBitNo();
         continue;
 
       case OPTIONS_BLOCK_ID:
@@ -5328,6 +5331,7 @@ bool ASTReader::readASTFileControlBlock(
   bool NeedsSystemInputFiles = Listener.needsSystemInputFileVisitation();
   bool NeedsImports = Listener.needsImportVisitation();
   BitstreamCursor InputFilesCursor;
+  uint64_t InputFilesOffsetBase = 0;
 
   RecordData Record;
   std::string ModuleDir;
@@ -5363,6 +5367,7 @@ bool ASTReader::readASTFileControlBlock(
         if (NeedsInputFiles &&
             ReadBlockAbbrevs(InputFilesCursor, INPUT_FILES_BLOCK_ID))
           return true;
+        InputFilesOffsetBase = InputFilesCursor.GetCurrentBitNo();
         break;
 
       default:
@@ -5435,7 +5440,8 @@ bool ASTReader::readASTFileControlBlock(
 
         BitstreamCursor &Cursor = InputFilesCursor;
         SavedStreamPosition SavedPosition(Cursor);
-        if (llvm::Error Err = Cursor.JumpToBit(InputFileOffs[I])) {
+        if (llvm::Error Err =
+                Cursor.JumpToBit(InputFilesOffsetBase + InputFileOffs[I])) {
           // FIXME this drops errors on the floor.
           consumeError(std::move(Err));
         }

clang/lib/Serialization/ASTWriter.cpp

@@ -1570,6 +1570,8 @@ void ASTWriter::WriteInputFiles(SourceManager &SourceMgr,
   IFHAbbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 32));
   unsigned IFHAbbrevCode = Stream.EmitAbbrev(std::move(IFHAbbrev));
 
+  uint64_t InputFilesOffsetBase = Stream.GetCurrentBitNo();
+
   // Get all ContentCache objects for files.
   std::vector<InputFileEntry> UserFiles;
   std::vector<InputFileEntry> SystemFiles;
@@ -1633,7 +1635,7 @@ void ASTWriter::WriteInputFiles(SourceManager &SourceMgr,
       continue; // already recorded this file.
 
     // Record this entry's offset.
-    InputFileOffsets.push_back(Stream.GetCurrentBitNo());
+    InputFileOffsets.push_back(Stream.GetCurrentBitNo() - InputFilesOffsetBase);
 
     InputFileID = InputFileOffsets.size();