Add AST_SIGNATURE record to unhashed control block of PCM files

[dang]

Summary

This record is constructed by hashing the bytes of the AST block in a similiar
fashion to the SIGNATURE record. This new signature only means anything if the
AST block is fully relocatable, i.e. it does not embed absolute offsets within
the PCM file. This change ensure this does not happen by repalcing these offsets
with offsets relative to the start of the AST block.

[dang]

Formatting fixes

[dang]

Accidently deleted a line in the previous update

[dang]

This should address the Windows test failure as well as some offline feedback

• Ensure that DeclOffset instances can only be read by providing an offset to the AST block
• Simplify ASTSignature test by using -fdisable-module-hash

[dexonsmith]

Thanks for working on this! I have a number of suggestions inline.

clang/include/clang/Serialization/ASTBitCodes.h

32–33

VERSION_MAJOR should be bumped.

244–245

Please update the first sentence to be accurate.

246

I don't think you'll need this comment anymore, after you fix the above. But if you keep it, please add a period at the end of the sentence.

397–403

These names and descriptions hard hard to differentiate. Is there another way of naming these that will be more clear?

(One idea I had is to create CONTROL_BLOCK_HASH and AST_BLOCK_HASH and then SIGNATURE could just be their hash-combine, but maybe you have another idea.)

clang/lib/Serialization/ASTReader.cpp

2931

Please submit this NFC change separately.

clang/lib/Serialization/ASTWriter.cpp

1047

Alternately, you could change ASTFileSignature to have three fields:

• ControlBlockHash
• ASTBlockHash
• Signature

(You could construct the signature by taking the SHA1 of the two block hashes.)

1048–1052

Rather than duplicating this code, would it be simpler to start with a patch that changes ASTFileSignature to not require this massaging? I suggest either adding a constructor that takes Hash directly, or just changing it to have the same type as Hash (and updating the serialization to expect that).

1064–1073

The default abbreviation (7-VBR) isn't great for hashes. Given that we're going to have two of these records, I suggest specifying a custom abbreviation.

If you were to split out a prep commit as suggested above to change SIGNATURE to be an array of 8-bit values (instead of the current array of 32-bit values), that would be a good opportunity to add an abbreviation.

2066

Can this be relative to the start of SOURCE_MANAGER_BLOCK_ID instead?

4718–4719

Is there ever a reason to use M.FileName here, or is that always redundant with what's in the control block? I wonder if we can just remove this complexity.

clang/lib/Serialization/ASTWriterDecl.cpp

2435

I suggest making WriteDecls relative to the start of the DECLTYPES_BLOCK_ID. That will make the block itself (more) self-contained, and also make the offsets smaller (and therefore cheaper to store in bitcode).

I also don't think you need to actually emit that integer, it's just an implicit contract between the reader and writer.

2440

Same here, this should be relative to the start of DECLTYPES_BLOCK_ID.

[dang]

I made the decision to make all relative offsets relative to the start of the AST block rather than their own sub-block or a neighboring block, in order to prevent adding complexity in ASTReader. My reasoning that it would be best if the reader didn't have to keep track of a bunch of offsets inside the bitcode at all times, but that it could just remember the offset of the AST block once and always use that for relative offsets. I agree that conceptually making the sub-blocks relocatable too would have been nice, but I figured it wasn't worth the extra complexity in the reader.

clang/lib/Serialization/ASTWriter.cpp

1064–1073

I think encoding the signature as a blob makes the most sense if we make it an array of 20 8-bit values, which is what the hasher gives us. However it is not quite so simple, as it would need a big change in the way direct imports are serialized. The way it is currently done embeds multiple strings and signatures within a single unabbreviated record. I can replace the unabbreviated record with a sub-block that keeps track of a string table and a record for each import containing offsets inside the string table and blobs for the signature.

4718–4719

The control block only tracks this information for direct imports although it can maybe be extended to do keep track of this for all dependencies. This bit of the table could then become and index inside the IMPORTS record in the control block.

clang/lib/Serialization/ASTWriterDecl.cpp

2435

Not sure what you mean by I don't actually need to emit that integer? If you mean ASTBlockRange.first then I don't emit it but rather pass it to the constructor of DeclOffset so that it performs the subtraction there. Since it is a self contained type, it forces the Reader to provide a value for it, making it harder to read the raw relative offset.

[dang]

Address some of the code review comments

[dang]

clang/include/clang/Serialization/ASTBitCodes.h

397–403

I kept the same hasher when computing both of these which mitigates the cost. I don't see the need for also emitting a hash for the control block, there are some optional records that are not in both the AST block and the control block anyway.

[dang]

clang/lib/Serialization/ASTWriter.cpp

1064–1073

But I do think it is outside the scope of this change to do this.

[dang]

clang/include/clang/Serialization/ASTBitCodes.h

397–403

I also think that the AST_BLOCK_HASH and the SIGNATURE are enough information already. In most cases you can deduce if the control block was different by just checking if the signatures were different and the ASTs the same.

[dexonsmith]

In D80383#2068596, @dang wrote:

I made the decision to make all relative offsets relative to the start of the AST block rather than their own sub-block or a neighboring block, in order to prevent adding complexity in ASTReader. My reasoning that it would be best if the reader didn't have to keep track of a bunch of offsets inside the bitcode at all times, but that it could just remember the offset of the AST block once and always use that for relative offsets. I agree that conceptually making the sub-blocks relocatable too would have been nice, but I figured it wasn't worth the extra complexity in the reader.

I think the extra complexity is small and worth it. The default abbreviation is 7-VBR, which is cheaper for small offsets. I doubt we'll bother to come back and fix this later, so might as well get it right now.

clang/include/clang/Basic/Module.h

60 ↗

(On Diff #267914)

Can you make this create? I think new code we prefer lowerCamelCase for method names.

66–72 ↗

(On Diff #267914)

This code looks correct, and it's better factored out like this, but I still don't understand why we convert to 32-bit numbers. Have you looked at the code simplification from storing this as an array of 8-bit numbers?

clang/include/clang/Serialization/ASTBitCodes.h

397–403

Thanks for updating to AST_BLOCK_HASH, I think that addressed the concern I had.

However, having thought further about it, I don't think it makes sense to create a new record type. I suggest just having a convention of writing the hashes out in a particular order.

• First record is the overall signature.
• Second record is the AST block hash.
• In the future, we can add a third record for the control block hash.

In which case, the (single) record name should be something generic like SIGNATURE or SHA1_HASH.

Note: I doubt there would be additional cost to computing the AST and control block hashes separately, but I agree there isn't a compelling use for the control block hash currently (although it would be nice to confirm properties like if the CONTROL_BLOCK_HASH were typically stable when sources changed).

Note: I don't think the content of the "hashed" control block vs. unhashed control block have been carefully considered from first principles. At some point we might do that. I expect there are currently things that change the control block that would not invalidate it for importers.

clang/lib/Serialization/ASTWriter.cpp

1047

Have you considered changing ASTFileSignature to have multiple fields, one for the signature and another for the AST block hash? Working with pairs and tuples is always awkward.

1064–1073

However it is not quite so simple, as it would need a big change in the way direct imports are serialized. The way it is currently done embeds multiple strings and signatures within a single unabbreviated record.

I don't understand why switching to 8-bit values would affect this. You can just write each 8-bit value as a separate field in the record, just like it currently does for 32-bit values. I.e., the current code seems to work just as well:

for (auto I : M.Signature)
  Record.push_back(I);

That said, having a single IMPORTS record is kind of ridiculous. It would be nice to clean that up (either as a follow-up or prep commit). I think your suggestion of having an IMPORTS sub-block is reasonable. A simpler approach would be just to emit IMPORT records. In the reader you can figure out implicitly how many there are (is the next record another IMPORT? If so keep reading). If you did this, I would suggest abbreviating the IMPORT record since it'll be well-structured.

For SIGNATURE record abbreviation, I wasn't think a blob, just something simple like:

auto HashAbbrev = std::make_shared<BitCodeAbbrev>();
HashAbbrev->Add(BitCodeAbbrevOp(Signature));
for (int I = 0, E = 5; I != E; ++I)
  HashAbbrev->Add(BitCodeAbbrevOp(BitcodeAbbrevOp::Fixed, 32));

If you switch to 8-bit:

auto HashAbbrev = std::make_shared<BitCodeAbbrev>();
HashAbbrev->Add(BitCodeAbbrevOp(Signature));
for (int I = 0, E = 20; I != E; ++I)
  HashAbbrev->Add(BitCodeAbbrevOp(BitcodeAbbrevOp::Fixed, 8));

4718–4719

Okay. That might be a nice follow-up or prep commit. If you're not interested in that tangent though I suggest adding an accessor ModuleFile::isModule that you can use here.

clang/lib/Serialization/ASTWriterDecl.cpp

2435

Ah! I misread the code.

[dang]

Address some code review feedback

[dang]

See D81347 for the parent revision that makes ASTFileSignature a std::array<uint8_t, 20>. Adding the abbreviation doesn't make that much sense at the moment because of how the IMPORTS record is structured as discussed offline.

[dang]

Address @dexonsmith feedback about making the relative offsets in the AST block relative to the nearest relevant subblock.

clang/include/clang/Serialization/ASTBitCodes.h

397–403

I agree with the fact that it might be worth maintaining a control block hash, but I do think this is not the purpose of this change. I can do it as a follow up when I have some time. I am somewhat opposed to keeping them all in the same record for now, because it makes it harder to grep for one of the signatures in the output of llvm-bcanalyzer which is currently the only available tooling for looking inside the unhashed control block.

[dexonsmith]

LGTM, with one more comment inline.

clang/lib/Serialization/ASTWriter.cpp

2345

It looks to me like this field is ignored in the reader. If so we should just strip it. Can you confirm?

[dang]

Rebase on top of latest master that has needed NFC changes to calm down clang-format.

[dang]

clang/lib/Serialization/ASTReader.cpp

3706

@dexonsmith the field you asked about is used here and the MacroOffsetsBase is used in other places to compute offsets

clang/lib/Serialization/ASTWriter.cpp

2345

No, look at line 3706 of the Reader in the updated version. I added a comment on the relevant line.

clang/include/clang/Serialization/ASTBitCodes.h

 #include "clang/Basic/SourceLocation.h"
 #include "llvm/ADT/DenseMapInfo.h"
 #include "llvm/Bitstream/BitCodes.h"
 #include <cassert>
 #include <cstdint>
 
 namespace clang {
 namespace serialization {
 
 /// AST file major version number supported by this version of
 /// Clang.
 ///
 /// Whenever the AST file format changes in a way that makes it
 /// incompatible with previous versions (such that a reader
 /// designed for the previous version could not support reading
 /// the new version), this number should be increased.
 ///
 /// Version 4 of AST files also requires that the version control branch and
 /// revision match exactly, since there is no backward compatibility of
 /// AST files at this time.
-const unsigned VERSION_MAJOR = 10;
+const unsigned VERSION_MAJOR = 11;
 
 /// AST file minor version number supported by this version of
 /// Clang.
 ///
 /// Whenever the AST format changes in a way that is still
 /// compatible with previous versions (such that a reader designed
 /// for the previous version could still support reading the new
 /// version by ignoring new kinds of subblocks), this number
struct UnderalignedInt64 {
         return BitOffsetLow | (uint64_t(BitOffsetHigh) << 32);
       }
     };
 
     /// Source location and bit offset of a declaration.
     struct DeclOffset {
       /// Raw source location.
       unsigned Loc = 0;
 
-      /// Offset in the AST file. Keep structure alignment 32-bit and avoid
-      /// padding gap because undefined value in the padding affects AST hash.
+      /// Offset relative to the start of the DECLTYPES_BLOCK block. Keep
+      /// structure alignment 32-bit and avoid padding gap because undefined
+      /// value in the padding affects AST hash.
       UnderalignedInt64 BitOffset;
 
       DeclOffset() = default;
-      DeclOffset(SourceLocation Loc, uint64_t BitOffset) {
+      DeclOffset(SourceLocation Loc, uint64_t BitOffset,
+                 uint64_t DeclTypesBlockStartOffset) {
         setLocation(Loc);
-        setBitOffset(BitOffset);
+        setBitOffset(BitOffset, DeclTypesBlockStartOffset);
       }
 
       void setLocation(SourceLocation L) {
         Loc = L.getRawEncoding();
       }
 
       SourceLocation getLocation() const {
         return SourceLocation::getFromRawEncoding(Loc);
       }
 
-      void setBitOffset(uint64_t Offset) {
-        BitOffset.setBitOffset(Offset);
+      void setBitOffset(uint64_t Offset,
+                        const uint64_t DeclTypesBlockStartOffset) {
+        BitOffset.setBitOffset(Offset - DeclTypesBlockStartOffset);
       }
 
-      uint64_t getBitOffset() const {
-        return BitOffset.getBitOffset();
+      uint64_t getBitOffset(const uint64_t DeclTypesBlockStartOffset) const {
+        return BitOffset.getBitOffset() + DeclTypesBlockStartOffset;
       }
     };
 
     /// The number of predefined preprocessed entity IDs.
     const unsigned int NUM_PREDEF_PP_ENTITY_IDS = 1;
 
     /// Describes the various kinds of blocks that occur within
     /// an AST file.
enum OptionsRecordTypes {
       HEADER_SEARCH_OPTIONS,
 
       /// Record code for the preprocessor options table.
       PREPROCESSOR_OPTIONS,
     };
 
     /// Record codes for the unhashed control block.
     enum UnhashedControlBlockRecordTypes {
       /// Record code for the signature that identifiers this AST file.
       SIGNATURE = 1,
 
+      /// Record code for the content hash of the AST block.
+      AST_BLOCK_HASH,
+
       /// Record code for the diagnostic options table.
       DIAGNOSTIC_OPTIONS,
 
       /// Record code for \#pragma diagnostic mappings.
       DIAG_PRAGMA_MAPPINGS,
     };
 
     /// Record code for extension blocks.
     enum ExtensionBlockRecordTypes {

clang/include/clang/Serialization/ASTReader.h

public:
   /// This operation will read a new statement from the external
   /// source each time it is called, and is meant to be used via a
   /// LazyOffsetPtr (which is used by Decls for the body of functions, etc).
   Stmt *GetExternalDeclStmt(uint64_t Offset) override;
 
   /// ReadBlockAbbrevs - Enter a subblock of the specified BlockID with the
   /// specified cursor.  Read the abbreviations that are at the top of the block
   /// and then leave the cursor pointing into the block.
-  static bool ReadBlockAbbrevs(llvm::BitstreamCursor &Cursor, unsigned BlockID);
+  static bool ReadBlockAbbrevs(llvm::BitstreamCursor &Cursor, unsigned BlockID,
+                               uint64_t *StartOfBlockOffset = nullptr);
 
   /// Finds all the visible declarations with a given name.
   /// The current implementation of this method just loads the entire
   /// lookup table as unmaterialized references.
   bool FindExternalVisibleDeclsByName(const DeclContext *DC,
                                       DeclarationName Name) override;
 
   /// Read all of the declarations lexically stored in a

clang/include/clang/Serialization/ASTWriter.h

 #include "clang/Sema/SemaConsumer.h"
 #include "clang/Serialization/ASTBitCodes.h"
 #include "clang/Serialization/ASTDeserializationListener.h"
 #include "clang/Serialization/PCHContainerOperations.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/MapVector.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Bitstream/BitstreamWriter.h"
 #include <cassert>
 #include <cstddef>
 #include <cstdint>
 #include <ctime>
private:
   Preprocessor *PP = nullptr;
 
   /// The reader of existing AST files, if we're chaining.
   ASTReader *Chain = nullptr;
 
   /// The module we're currently writing, if any.
   Module *WritingModule = nullptr;
 
+  /// The offset of the first bit inside the AST_BLOCK.
+  uint64_t ASTBlockStartOffset = 0;
+
+  /// The range representing all the AST_BLOCK.
+  std::pair<uint64_t, uint64_t> ASTBlockRange;
+
   /// The base directory for any relative paths we emit.
   std::string BaseDirectory;
 
   /// Indicates whether timestamps should be written to the produced
   /// module file. This is the case for files implicitly written to the
   /// module cache, where we need the timestamps to determine if the module
   /// file is up to date, but not otherwise.
   bool IncludeTimestamps;
private:
   /// discovery) and start at 2. 1 is reserved for the translation
   /// unit, while 0 is reserved for NULL.
   llvm::DenseMap<const Decl *, serialization::DeclID> DeclIDs;
 
   /// Offset of each declaration in the bitstream, indexed by
   /// the declaration's ID.
   std::vector<serialization::DeclOffset> DeclOffsets;
 
+  /// The offset of the DECLTYPES_BLOCK. The offsets in DeclOffsets
+  /// are relative to this value.
+  uint64_t DeclTypesBlockStartOffset = 0;
+
   /// Sorted (by file offset) vector of pairs of file offset/DeclID.
   using LocDeclIDsTy =
       SmallVector<std::pair<unsigned, serialization::DeclID>, 64>;
   struct DeclIDInFileInfo {
     LocDeclIDsTy DeclIDs;
 
     /// Set when the DeclIDs vectors from all files are joined, this
     /// indicates the index that this particular vector has in the global one.
private:
   unsigned NumVisibleDeclContexts = 0;
 
   /// A mapping from each known submodule to its ID number, which will
   /// be a positive integer.
   llvm::DenseMap<Module *, unsigned> SubmoduleIDs;
 
   /// A list of the module file extension writers.
   std::vector<std::unique_ptr<ModuleFileExtensionWriter>>
       ModuleFileExtensionWriters;
 
   /// Retrieve or create a submodule ID for this module.
   unsigned getSubmoduleID(Module *Mod);
 
   /// Write the given subexpression to the bitstream.
   void WriteSubStmt(Stmt *S);
 
   void WriteBlockInfoBlock();
   void WriteControlBlock(Preprocessor &PP, ASTContext &Context,
                          StringRef isysroot, const std::string &OutputFile);
 
   /// Write out the signature and diagnostic options, and return the signature.
   ASTFileSignature writeUnhashedControlBlock(Preprocessor &PP,
                                              ASTContext &Context);
 
   /// Calculate hash of the pcm content.
-  static ASTFileSignature createSignature(StringRef Bytes);
+  static std::pair<ASTFileSignature, ASTFileSignature>
+  createSignature(StringRef AllBytes, StringRef ASTBlockBytes);
 
   void WriteInputFiles(SourceManager &SourceMgr, HeaderSearchOptions &HSOpts,
                        bool Modules);
   void WriteSourceManagerBlock(SourceManager &SourceMgr,
                                const Preprocessor &PP);
   void WritePreprocessor(const Preprocessor &PP, bool IsModule);
   void WriteHeaderSearch(const HeaderSearch &HS);
   void WritePreprocessorDetail(PreprocessingRecord &PPRec,

clang/include/clang/Serialization/ModuleFile.h

public:
 
   /// The file entry for the module file.
   const FileEntry *File = nullptr;
 
   /// The signature of the module file, which may be used instead of the size
   /// and modification time to identify this particular file.
   ASTFileSignature Signature;
 
+  /// The signature of the AST block of the module file, this can be used to
+  /// unique module files based on AST contents.
+  ASTFileSignature ASTBlockHash;
+
   /// Whether this module has been directly imported by the
   /// user.
   bool DirectlyImported = false;
 
   /// The generation of which this module file is a part.
   unsigned Generation;
 
   /// The memory buffer that stores the data associated with
   /// this AST file, owned by the InMemoryModuleCache.
   llvm::MemoryBuffer *Buffer;
 
   /// The size of this file, in bits.
   uint64_t SizeInBits = 0;
 
   /// The global bit offset (or base) of this module
   uint64_t GlobalBitOffset = 0;
 
+  /// The bit offset of the AST block of this module.
+  uint64_t ASTBlockStartOffset = 0;
+
   /// The serialized bitstream data for this file.
   StringRef Data;
 
   /// The main bitstream cursor for the main block.
   llvm::BitstreamCursor Stream;
 
   /// The source location where the module was explicitly or implicitly
   /// imported in the local translation unit.
public:
   /// The time is specified in seconds since the start of the Epoch.
   uint64_t InputFilesValidationTimestamp = 0;
 
   // === Source Locations ===
 
   /// Cursor used to read source location entries.
   llvm::BitstreamCursor SLocEntryCursor;
 
+  /// The bit offset to the start of the SOURCE_MANAGER_BLOCK.
+  uint64_t SourceManagerBlockStartOffset = 0;
+
   /// The number of source location entries in this AST file.
   unsigned LocalNumSLocEntries = 0;
 
   /// The base ID in the source manager's view of this module.
   int SLocEntryBaseID = 0;
 
   /// The base offset in the source manager's view of this module.
   unsigned SLocEntryBaseOffset = 0;
public:
   /// ASTSelectorLookupTable.
   ///
   /// This hash table provides the IDs of all selectors, and the associated
   /// instance and factory methods.
   void *SelectorLookupTable = nullptr;
 
   // === Declarations ===
 
-  /// DeclsCursor - This is a cursor to the start of the DECLS_BLOCK block. It
-  /// has read all the abbreviations at the start of the block and is ready to
-  /// jump around with these in context.
+  /// DeclsCursor - This is a cursor to the start of the DECLTYPES_BLOCK block.
+  /// It has read all the abbreviations at the start of the block and is ready
+  /// to jump around with these in context.
   llvm::BitstreamCursor DeclsCursor;
 
+  /// The offset to the start of the DECLTYPES_BLOCK block.
+  uint64_t DeclsBlockStartOffset = 0;
+
   /// The number of declarations in this AST file.
   unsigned LocalNumDecls = 0;
 
   /// Offset of each declaration within the bitstream, indexed
   /// by the declaration ID (-1).
   const DeclOffset *DeclOffsets = nullptr;
 
   /// Base declaration ID for declarations local to this module.

clang/lib/Serialization/ASTReader.cpp

bool ASTReader::ReadSourceManagerBlock(ModuleFile &F) {
   }
 
   // Enter the source manager block.
   if (llvm::Error Err =
           SLocEntryCursor.EnterSubBlock(SOURCE_MANAGER_BLOCK_ID)) {
     Error(std::move(Err));
     return true;
   }
+  F.SourceManagerBlockStartOffset = SLocEntryCursor.GetCurrentBitNo();
 
   RecordData Record;
   while (true) {
     Expected<llvm::BitstreamEntry> MaybeE =
         SLocEntryCursor.advanceSkippingSubblocks();
     if (!MaybeE) {
       Error(MaybeE.takeError());
       return true;
if (F->ImportedBy.empty() || !F->ImportedBy[0]) {
     return SourceMgr.getLocForStartOfFile(SourceMgr.getMainFileID());
   }
   return F->ImportedBy[0]->FirstLoc;
 }
 
 /// Enter a subblock of the specified BlockID with the specified cursor. Read
 /// the abbreviations that are at the top of the block and then leave the cursor
 /// pointing into the block.
-bool ASTReader::ReadBlockAbbrevs(BitstreamCursor &Cursor, unsigned BlockID) {
+bool ASTReader::ReadBlockAbbrevs(BitstreamCursor &Cursor, unsigned BlockID,
+                                 uint64_t *StartOfBlockOffset) {
   if (llvm::Error Err = Cursor.EnterSubBlock(BlockID)) {
     // FIXME this drops errors on the floor.
     consumeError(std::move(Err));
     return true;
   }
 
+  if (StartOfBlockOffset)
+    *StartOfBlockOffset = Cursor.GetCurrentBitNo();
+
   while (true) {
     uint64_t Offset = Cursor.GetCurrentBitNo();
     Expected<unsigned> MaybeCode = Cursor.ReadCode();
     if (!MaybeCode) {
       // FIXME this drops errors on the floor.
       consumeError(MaybeCode.takeError());
       return true;
     }
case INPUT_FILE_OFFSETS:
       break;
     }
   }
 }
 
 ASTReader::ASTReadResult
 ASTReader::ReadASTBlock(ModuleFile &F, unsigned ClientLoadCapabilities) {
   BitstreamCursor &Stream = F.Stream;
 
   if (llvm::Error Err = Stream.EnterSubBlock(AST_BLOCK_ID)) {
     Error(std::move(Err));
     return Failure;
   }
+  F.ASTBlockStartOffset = Stream.GetCurrentBitNo();
 
   // Read all of the records and blocks for the AST file.
   RecordData Record;
   while (true) {
     Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advance();
     if (!MaybeEntry) {
       Error(MaybeEntry.takeError());
       return Failure;
case llvm::BitstreamEntry::SubBlock:
         // DeclsCursor cursor to point into it.  Clone our current bitcode
         // cursor to it, enter the block and read the abbrevs in that block.
         // With the main cursor, we just skip over it.
         F.DeclsCursor = Stream;
         if (llvm::Error Err = Stream.SkipBlock()) {
           Error(std::move(Err));
           return Failure;
         }
-        if (ReadBlockAbbrevs(F.DeclsCursor, DECLTYPES_BLOCK_ID)) {
+        if (ReadBlockAbbrevs(F.DeclsCursor, DECLTYPES_BLOCK_ID,
+                             &F.DeclsBlockStartOffset)) {
           Error("malformed block record in AST file");
           return Failure;
         }
         break;
 
       case PREPROCESSOR_BLOCK_ID:
         F.MacroCursor = Stream;
         if (!PP.getExternalSource())
case FILE_SORTED_DECLS:
       F.FileSortedDecls = (const DeclID *)Blob.data();
       F.NumFileSortedDecls = Record[0];
       break;
 
     case SOURCE_LOCATION_OFFSETS: {
       F.SLocEntryOffsets = (const uint32_t *)Blob.data();
       F.LocalNumSLocEntries = Record[0];
       unsigned SLocSpaceSize = Record[1];
-      F.SLocEntryOffsetsBase = Record[2];
+      F.SLocEntryOffsetsBase = Record[2] + F.SourceManagerBlockStartOffset;
       std::tie(F.SLocEntryBaseID, F.SLocEntryBaseOffset) =
           SourceMgr.AllocateLoadedSLocEntries(F.LocalNumSLocEntries,
                                               SLocSpaceSize);
       if (!F.SLocEntryBaseID) {
         Error("ran out of source locations");
         break;
       }
       // Make our entry in the range map. BaseID is negative and growing, so
while (true) {
     case MACRO_OFFSET: {
       if (F.LocalNumMacros != 0) {
         Error("duplicate MACRO_OFFSET record in AST file");
         return Failure;
       }
       F.MacroOffsets = (const uint32_t *)Blob.data();
       F.LocalNumMacros = Record[0];
       unsigned LocalBaseMacroID = Record[1];
-      F.MacroOffsetsBase = Record[2];
+      F.MacroOffsetsBase = Record[2] + F.ASTBlockStartOffset;
       F.BaseMacroID = getTotalNumMacros();
 
       if (F.LocalNumMacros > 0) {
         // Introduce the global -> local mapping for macros within this module.
         GlobalMacroMap.insert(std::make_pair(getTotalNumMacros() + 1, &F));
 
         // Introduce the local -> global mapping for macros within this module.
         F.MacroRemap.insertOrReplace(
void ASTReader::ReadModuleOffsetMap(ModuleFile &F) const {
   RemapBuilder PreprocessedEntityRemap(F.PreprocessedEntityRemap);
   RemapBuilder SubmoduleRemap(F.SubmoduleRemap);
   RemapBuilder SelectorRemap(F.SelectorRemap);
   RemapBuilder DeclRemap(F.DeclRemap);
   RemapBuilder TypeRemap(F.TypeRemap);
 
   while (Data < DataEnd) {
     // FIXME: Looking up dependency modules by filename is horrible. Let's
-    // start fixing this with prebuilt and explicit modules and see how it
-    // goes...
+    // start fixing this with prebuilt, explicit and implicit modules and see
+    // how it goes...
     using namespace llvm::support;
     ModuleKind Kind = static_cast<ModuleKind>(
       endian::readNext<uint8_t, little, unaligned>(Data));
     uint16_t Len = endian::readNext<uint16_t, little, unaligned>(Data);
     StringRef Name = StringRef((const char*)Data, Len);
     Data += Len;
-    ModuleFile *OM = (Kind == MK_PrebuiltModule || Kind == MK_ExplicitModule
+    ModuleFile *OM = (Kind == MK_PrebuiltModule || Kind == MK_ExplicitModule ||
+                              Kind == MK_ImplicitModule
                           ? ModuleMgr.lookupByModuleName(Name)
                           : ModuleMgr.lookupByFileName(Name));
     if (!OM) {
       std::string Msg =
           "SourceLocation remap refers to unknown module, cannot find ";
       Msg.append(std::string(Name));
       Error(Msg);
       return;
     }
 
if (!MaybeRecordType) {
       // FIXME this drops the error.
       return Failure;
     }
     switch ((UnhashedControlBlockRecordTypes)MaybeRecordType.get()) {
     case SIGNATURE:
       if (F)
         F->Signature = ASTFileSignature::create(Record.begin(), Record.end());
       break;
+    case AST_BLOCK_HASH:
+      if (F)
+        F->ASTBlockHash =
+            ASTFileSignature::create(Record.begin(), Record.end());
+      break;
     case DIAGNOSTIC_OPTIONS: {
       bool Complain = (ClientLoadCapabilities & ARR_OutOfDate) == 0;
       if (Listener && ValidateDiagnosticOptions &&
           !AllowCompatibleConfigurationMismatch &&
           ParseDiagnosticOptions(Record, Complain, *Listener))
         Result = OutOfDate; // Don't return early.  Read the signature.
       break;
     }

 }
 
 /// Get the correct cursor and offset for loading a type.
 ASTReader::RecordLocation ASTReader::TypeCursorForIndex(unsigned Index) {
   GlobalTypeMapType::iterator I = GlobalTypeMap.find(Index);
   assert(I != GlobalTypeMap.end() && "Corrupted global type map");
   ModuleFile *M = I->second;
   return RecordLocation(
-      M, M->TypeOffsets[Index - M->BaseTypeIndex].getBitOffset());
+      M, M->TypeOffsets[Index - M->BaseTypeIndex].getBitOffset() +
+             M->DeclsBlockStartOffset);
 }
 
 static llvm::Optional<Type::TypeClass> getTypeClassForCode(TypeCode code) {
   switch (code) {
 #define TYPE_BIT_CODE(CLASS_ID, CODE_ID, CODE_VALUE) \
   case TYPE_##CODE_ID: return Type::CLASS_ID;
 #include "clang/Serialization/TypeBitCodes.def"
   default: return llvm::None;

clang/lib/Serialization/ASTReaderDecl.cpp

 ASTReader::RecordLocation
 ASTReader::DeclCursorForID(DeclID ID, SourceLocation &Loc) {
   GlobalDeclMapType::iterator I = GlobalDeclMap.find(ID);
   assert(I != GlobalDeclMap.end() && "Corrupted global declaration map");
   ModuleFile *M = I->second;
   const DeclOffset &DOffs =
       M->DeclOffsets[ID - M->BaseDeclID - NUM_PREDEF_DECL_IDS];
   Loc = TranslateSourceLocation(*M, DOffs.getLocation());
-  return RecordLocation(M, DOffs.getBitOffset());
+  return RecordLocation(M, DOffs.getBitOffset(M->DeclsBlockStartOffset));
 }
 
 ASTReader::RecordLocation ASTReader::getLocalBitOffset(uint64_t GlobalOffset) {
   auto I = GlobalBitOffsetsMap.find(GlobalOffset);
 
   assert(I != GlobalBitOffsetsMap.end() && "Corrupted global bit offsets map");
   return RecordLocation(I->second, GlobalOffset - I->second->GlobalBitOffset);
 }

clang/lib/Serialization/ASTWriter.cpp

 //===- ASTWriter.cpp - AST File Writer ------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 //  This file defines the ASTWriter class, which writes AST files.
 //
 //===----------------------------------------------------------------------===//
 
-#include "clang/AST/OpenMPClause.h"
-#include "clang/Serialization/ASTRecordWriter.h"
 #include "ASTCommon.h"
 #include "ASTReaderInternals.h"
 #include "MultiOnDiskHashTable.h"
-#include "clang/AST/AbstractTypeWriter.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTUnresolvedSet.h"
+#include "clang/AST/AbstractTypeWriter.h"
 #include "clang/AST/Attr.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclBase.h"
 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/DeclContextInternals.h"
 #include "clang/AST/DeclFriend.h"
 #include "clang/AST/DeclObjC.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/DeclarationName.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/LambdaCapture.h"
 #include "clang/AST/NestedNameSpecifier.h"
+#include "clang/AST/OpenMPClause.h"
 #include "clang/AST/RawCommentList.h"
 #include "clang/AST/TemplateName.h"
 #include "clang/AST/Type.h"
 #include "clang/AST/TypeLocVisitor.h"
 #include "clang/Basic/Diagnostic.h"
 #include "clang/Basic/DiagnosticOptions.h"
 #include "clang/Basic/FileManager.h"
 #include "clang/Basic/FileSystemOptions.h"

 #include "clang/Lex/PreprocessingRecord.h"
 #include "clang/Lex/Preprocessor.h"
 #include "clang/Lex/PreprocessorOptions.h"
 #include "clang/Lex/Token.h"
 #include "clang/Sema/IdentifierResolver.h"
 #include "clang/Sema/ObjCMethodList.h"
 #include "clang/Sema/Sema.h"
 #include "clang/Sema/Weak.h"
+#include "clang/Serialization/ASTBitCodes.h"
 #include "clang/Serialization/ASTReader.h"
+#include "clang/Serialization/ASTRecordWriter.h"
 #include "clang/Serialization/InMemoryModuleCache.h"
 #include "clang/Serialization/ModuleFile.h"
 #include "clang/Serialization/ModuleFileExtension.h"
 #include "clang/Serialization/SerializationDiagnostic.h"
 #include "llvm/ADT/APFloat.h"
 #include "llvm/ADT/APInt.h"
 #include "llvm/ADT/APSInt.h"
 #include "llvm/ADT/ArrayRef.h"
#define RECORD(X) EmitRecordID(X, #X, Stream, Record)
   RECORD(PPD_INCLUSION_DIRECTIVE);
 
   // Decls and Types block.
   BLOCK(EXTENSION_BLOCK);
   RECORD(EXTENSION_METADATA);
 
   BLOCK(UNHASHED_CONTROL_BLOCK);
   RECORD(SIGNATURE);
+  RECORD(AST_BLOCK_HASH);
   RECORD(DIAGNOSTIC_OPTIONS);
   RECORD(DIAG_PRAGMA_MAPPINGS);
 
 #undef RECORD
 #undef BLOCK
   Stream.ExitBlock();
 }
 
if (!llvm::sys::path::is_separator(Filename[Pos])) {
     // in a row. Use a better mechanism to indicate whether we have emitted an
     // absolute or relative path.
     ++Pos;
   }
 
   return Filename + Pos;
 }
 
-ASTFileSignature ASTWriter::createSignature(StringRef Bytes) {
-  // Calculate the hash till start of UNHASHED_CONTROL_BLOCK.
+std::pair<ASTFileSignature, ASTFileSignature>
+ASTWriter::createSignature(StringRef AllBytes, StringRef ASTBlockBytes) {
   llvm::SHA1 Hasher;
-  Hasher.update(ArrayRef<uint8_t>(Bytes.bytes_begin(), Bytes.size()));
+  Hasher.update(ASTBlockBytes);
   auto Hash = Hasher.result();
+  ASTFileSignature ASTBlockHash = ASTFileSignature::create(Hash);
 
-  return ASTFileSignature::create(Hash);
+  // Add the remaining bytes (i.e. bytes before the unhashed control block that
+  // are not part of the AST block).
+  Hasher.update(
+      AllBytes.take_front(ASTBlockBytes.bytes_end() - AllBytes.bytes_begin()));
+  Hasher.update(
+      AllBytes.take_back(AllBytes.bytes_end() - ASTBlockBytes.bytes_end()));
+  Hash = Hasher.result();
+  ASTFileSignature Signature = ASTFileSignature::create(Hash);
+
+  return std::make_pair(ASTBlockHash, Signature);
 }
 
 ASTFileSignature ASTWriter::writeUnhashedControlBlock(Preprocessor &PP,
                                                       ASTContext &Context) {
   // Flush first to prepare the PCM hash (signature).
   Stream.FlushToWord();
   auto StartOfUnhashedControl = Stream.GetCurrentBitNo() >> 3;
 
   // Enter the block and prepare to write records.
   RecordData Record;
   Stream.EnterSubblock(UNHASHED_CONTROL_BLOCK_ID, 5);
 
   // For implicit modules, write the hash of the PCM as its signature.
   ASTFileSignature Signature;
   if (WritingModule &&
       PP.getHeaderSearchInfo().getHeaderSearchOpts().ModulesHashContent) {
-    Signature = createSignature(StringRef(Buffer.begin(), StartOfUnhashedControl));
+    ASTFileSignature ASTBlockHash;
+    auto ASTBlockStartByte = ASTBlockRange.first >> 3;
+    auto ASTBlockByteLength = (ASTBlockRange.second >> 3) - ASTBlockStartByte;
+    std::tie(ASTBlockHash, Signature) = createSignature(
+        StringRef(Buffer.begin(), StartOfUnhashedControl),
+        StringRef(Buffer.begin() + ASTBlockStartByte, ASTBlockByteLength));
+
+    Record.append(ASTBlockHash.begin(), ASTBlockHash.end());
+    Stream.EmitRecord(AST_BLOCK_HASH, Record);
+    Record.clear();
     Record.append(Signature.begin(), Signature.end());
     Stream.EmitRecord(SIGNATURE, Record);
     Record.clear();
   }
 
   // Diagnostic options.
   const auto &Diags = Context.getDiagnostics();
   const DiagnosticOptions &DiagOpts = Diags.getDiagnosticOptions();

 /// errors), we probably won't have to create file entries for any of
 /// the files in the AST.
 void ASTWriter::WriteSourceManagerBlock(SourceManager &SourceMgr,
                                         const Preprocessor &PP) {
   RecordData Record;
 
   // Enter the source manager block.
   Stream.EnterSubblock(SOURCE_MANAGER_BLOCK_ID, 4);
+  const uint64_t SourceManagerBlockOffset = Stream.GetCurrentBitNo();
 
   // Abbreviations for the various kinds of source-location entries.
   unsigned SLocFileAbbrv = CreateSLocFileAbbrev(Stream);
   unsigned SLocBufferAbbrv = CreateSLocBufferAbbrev(Stream);
   unsigned SLocBufferBlobAbbrv = CreateSLocBufferBlobAbbrev(Stream, false);
   unsigned SLocBufferBlobCompressedAbbrv =
       CreateSLocBufferBlobAbbrev(Stream, true);
   unsigned SLocExpansionAbbrv = CreateSLocExpansionAbbrev(Stream);
void ASTWriter::WriteSourceManagerBlock(SourceManager &SourceMgr,
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 16)); // total size
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 32)); // base offset
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Blob)); // offsets
   unsigned SLocOffsetsAbbrev = Stream.EmitAbbrev(std::move(Abbrev));
   {
     RecordData::value_type Record[] = {
         SOURCE_LOCATION_OFFSETS, SLocEntryOffsets.size(),
         SourceMgr.getNextLocalOffset() - 1 /* skip dummy */,
-        SLocEntryOffsetsBase};
+        SLocEntryOffsetsBase - SourceManagerBlockOffset};
     Stream.EmitRecordWithBlob(SLocOffsetsAbbrev, Record,
                               bytes(SLocEntryOffsets));
   }
   // Write the source location entry preloads array, telling the AST
   // reader which source locations entries it should load eagerly.
   Stream.EmitRecord(SOURCE_LOCATION_PRELOADS, PreloadSLocs);
 
   // Write the line table. It depends on remapping working, so it must come
void ASTWriter::WritePreprocessor(const Preprocessor &PP, bool IsModule) {
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 32)); // first ID
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 32));   // base offset
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Blob));
 
   unsigned MacroOffsetAbbrev = Stream.EmitAbbrev(std::move(Abbrev));
   {
     RecordData::value_type Record[] = {MACRO_OFFSET, MacroOffsets.size(),
                                        FirstMacroID - NUM_PREDEF_MACRO_IDS,
-                                       MacroOffsetsBase};
+                                       MacroOffsetsBase - ASTBlockStartOffset};
     Stream.EmitRecordWithBlob(MacroOffsetAbbrev, Record, bytes(MacroOffsets));
   }
 }
 
 void ASTWriter::WritePreprocessorDetail(PreprocessingRecord &PPRec,
                                         uint64_t MacroOffsetsBase) {
   if (PPRec.local_begin() == PPRec.local_end())
     return;
void ASTWriter::WriteType(QualType T) {
   TypeIdx &IdxRef = TypeIdxs[T];
   if (IdxRef.getIndex() == 0) // we haven't seen this type before.
     IdxRef = TypeIdx(NextTypeID++);
   TypeIdx Idx = IdxRef;
 
   assert(Idx.getIndex() >= FirstTypeID && "Re-writing a type from a prior AST");
 
   // Emit the type's representation.
-  uint64_t Offset = ASTTypeWriter(*this).write(T);
+  uint64_t Offset = ASTTypeWriter(*this).write(T) - DeclTypesBlockStartOffset;
 
   // Record the offset for this type.
   unsigned Index = Idx.getIndex() - FirstTypeID;
   if (TypeOffsets.size() == Index)
     TypeOffsets.emplace_back(Offset);
   else if (TypeOffsets.size() < Index) {
     TypeOffsets.resize(Index + 1);
     TypeOffsets[Index].setBitOffset(Offset);
for (const auto &DeleteExprsInfo :
       }
     }
   }
 
   // Write the control block
   WriteControlBlock(PP, Context, isysroot, OutputFile);
 
   // Write the remaining AST contents.
+  Stream.FlushToWord();
+  ASTBlockRange.first = Stream.GetCurrentBitNo();
   Stream.EnterSubblock(AST_BLOCK_ID, 5);
+  ASTBlockStartOffset = Stream.GetCurrentBitNo();
 
   // This is so that older clang versions, before the introduction
   // of the control block, can read and reject the newer PCH format.
   {
     RecordData Record = {VERSION_MAJOR};
     Stream.EmitRecord(METADATA_OLD_FORMAT, Record);
   }
 
if (Chain) {
     //   preprocessed-entity-id:i32
     //   macro-definition-id:i32
     //   submodule-id:i32
     //   selector-id:i32
     //   declaration-id:i32
     //   c++-base-specifiers-id:i32
     //   type-id:i32)
     //
-    // module-kind is the ModuleKind enum value. If it is MK_PrebuiltModule or
-    // MK_ExplicitModule, then the module-name is the module name. Otherwise,
-    // it is the module file name.
+    // module-kind is the ModuleKind enum value. If it is MK_PrebuiltModule,
+    // MK_ExplicitModule or MK_ImplicitModule, then the module-name is the
+    // module name. Otherwise, it is the module file name.
     auto Abbrev = std::make_shared<BitCodeAbbrev>();
     Abbrev->Add(BitCodeAbbrevOp(MODULE_OFFSET_MAP));
     Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Blob));
     unsigned ModuleOffsetMapAbbrev = Stream.EmitAbbrev(std::move(Abbrev));
     SmallString<2048> Buffer;
     {
       llvm::raw_svector_ostream Out(Buffer);
       for (ModuleFile &M : Chain->ModuleMgr) {
         using namespace llvm::support;
 
         endian::Writer LE(Out, little);
         LE.write<uint8_t>(static_cast<uint8_t>(M.Kind));
-        StringRef Name =
-          M.Kind == MK_PrebuiltModule || M.Kind == MK_ExplicitModule
-          ? M.ModuleName
-          : M.FileName;
+        StringRef Name = M.isModule() ? M.ModuleName : M.FileName;
         LE.write<uint16_t>(Name.size());
         Out.write(Name.data(), Name.size());
 
         // Note: if a base ID was uint max, it would not be possible to load
         // another module after it or have more than one entity inside it.
         uint32_t None = std::numeric_limits<uint32_t>::max();
 
         auto writeBaseIDOrNone = [&](uint32_t BaseID, bool ShouldWrite) {
           assert(BaseID < std::numeric_limits<uint32_t>::max() && "base id too high");
ASTFileSignature ASTWriter::WriteASTCore(Sema &SemaRef, StringRef isysroot,
   for (auto *D : SemaRef.DeclsToCheckForDeferredDiags)
     AddDeclRef(D, DeclsToCheckForDeferredDiags);
 
   RecordData DeclUpdatesOffsetsRecord;
 
   // Keep writing types, declarations, and declaration update records
   // until we've emitted all of them.
   Stream.EnterSubblock(DECLTYPES_BLOCK_ID, /*bits for abbreviations*/5);
+  DeclTypesBlockStartOffset = Stream.GetCurrentBitNo();
   WriteTypeAbbrevs();
   WriteDeclAbbrevs();
   do {
     WriteDeclUpdatesBlocks(DeclUpdatesOffsetsRecord);
     while (!DeclTypesToEmit.empty()) {
       DeclOrType DOT = DeclTypesToEmit.front();
       DeclTypesToEmit.pop();
       if (DOT.isType())
ASTFileSignature ASTWriter::WriteASTCore(Sema &SemaRef, StringRef isysroot,
   WritePackPragmaOptions(SemaRef);
   WriteFloatControlPragmaOptions(SemaRef);
 
   // Some simple statistics
   RecordData::value_type Record[] = {
       NumStatements, NumMacros, NumLexicalDeclContexts, NumVisibleDeclContexts};
   Stream.EmitRecord(STATISTICS, Record);
   Stream.ExitBlock();
+  Stream.FlushToWord();
+  ASTBlockRange.second = Stream.GetCurrentBitNo();
 
   // Write the module file extension blocks.
   for (const auto &ExtWriter : ModuleFileExtensionWriters)
     WriteModuleFileExtension(SemaRef, *ExtWriter);
 
   return writeUnhashedControlBlock(PP, Context);
 }
 

clang/lib/Serialization/ASTWriterDecl.cpp

void ASTWriter::WriteDecl(ASTContext &Context, Decl *D) {
 
   // Emit this declaration to the bitstream.
   uint64_t Offset = W.Emit(D);
 
   // Record the offset for this declaration
   SourceLocation Loc = D->getLocation();
   unsigned Index = ID - FirstDeclID;
   if (DeclOffsets.size() == Index)
-    DeclOffsets.emplace_back(Loc, Offset);
+    DeclOffsets.emplace_back(Loc, Offset, DeclTypesBlockStartOffset);
   else if (DeclOffsets.size() < Index) {
     // FIXME: Can/should this happen?
     DeclOffsets.resize(Index+1);
     DeclOffsets[Index].setLocation(Loc);
-    DeclOffsets[Index].setBitOffset(Offset);
+    DeclOffsets[Index].setBitOffset(Offset, DeclTypesBlockStartOffset);
   } else {
     llvm_unreachable("declarations should be emitted in ID order");
   }
 
   SourceManager &SM = Context.getSourceManager();
   if (Loc.isValid() && SM.isLocalSourceLocation(Loc))
     associateDeclWithFile(D, ID);
 

clang/test/Modules/ASTSignature.c

+// RUN: rm -rf %t
+// RUN: %clang_cc1 -iquote %S/Inputs/ASTHash/ -fsyntax-only -fmodules \
+// RUN:   -fimplicit-module-maps -fmodules-strict-context-hash \
+// RUN:   -fmodules-cache-path=%t -fdisable-module-hash %s
+// RUN: cp %t/MyHeader2.pcm %t1.pcm
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -iquote "/dev/null" -iquote %S/Inputs/ASTHash/ -fsyntax-only \
+// RUN:   -fmodules -fimplicit-module-maps -fmodules-strict-context-hash \
+// RUN:   -fmodules-cache-path=%t -fdisable-module-hash %s
+// RUN: cp %t/MyHeader2.pcm %t2.pcm
+// RUN: llvm-bcanalyzer --dump --disable-histogram %t1.pcm > %t1.dump
+// RUN: llvm-bcanalyzer --dump --disable-histogram %t2.pcm > %t2.dump
+// RUN: cat %t1.dump %t2.dump | FileCheck %s
+
+#include "my_header_2.h"
+
+my_int var = 42;
+
+// CHECK: [[AST_BLOCK_HASH:<AST_BLOCK_HASH .*>]]
+// CHECK: [[SIGNATURE:<SIGNATURE .*>]]
+// CHECK: [[AST_BLOCK_HASH]]
+// CHECK-NOT: [[SIGNATURE]]
+// The modules built by this test are designed to yield the same AST. If this
+// test fails, it means that the AST block is has become non-relocatable.

clang/test/Modules/Inputs/ASTHash/module.modulemap

+module MyHeader1 {
+  header "my_header_1.h"
+}
+
+module MyHeader2 {
+  header "my_header_2.h"
+  export *
+}

clang/test/Modules/Inputs/ASTHash/my_header_1.h

+typedef int my_int;

clang/test/Modules/Inputs/ASTHash/my_header_2.h

+#include "my_header_1.h"
+
+extern my_int var;