[Remarks] Add an LLVM-bitstream-based remark serializer

Add a new serializer, using a binary format based on the LLVM bitstream
format.

This format provides a way to serialize the remarks in two modes:

1) Separate mode: the metadata is separate from the remark entries.
2) Standalone mode: the metadata and the remark entries are in the same
file.

The format contains:

* a meta block: container version, container type, string table,
external file path, remark version
* a remark block: type, remark name, pass name, function name, debug
file, debug line, debug column, hotness, arguments (key, value, debug
file, debug line, debug column)

A string table is required for this format, which will be dumped in the
meta block to be consumed before parsing the remark blocks.

On clang itself, we noticed a size reduction of 13.4x compared to YAML,
and a compile-time reduction of between 1.7% and 3.5% on CTMark.

Differential Revision: https://reviews.llvm.org/D63466

llvm-svn: 367364

llvm/lib/Bitcode/Reader/BitcodeAnalyzer.cpp

@@ -434,6 +434,13 @@ static Expected<CurStreamTypeType> ReadSignature(BitstreamCursor &Stream) {
       return std::move(Err);
     if (Signature[2] == 'A' && Signature[3] == 'G')
       return ClangSerializedDiagnosticsBitstream;
+  } else if (Signature[0] == 'R' && Signature[1] == 'M') {
+    if (Error Err = tryRead(Signature[2], 8))
+      return std::move(Err);
+    if (Error Err = tryRead(Signature[3], 8))
+      return std::move(Err);
+    if (Signature[2] == 'R' && Signature[3] == 'K')
+      return LLVMBitstreamRemarks;
   } else {
     if (Error Err = tryRead(Signature[2], 4))
       return std::move(Err);
@@ -627,6 +634,9 @@ void BitcodeAnalyzer::printStats(BCDumpOptions O,
   case ClangSerializedDiagnosticsBitstream:
     O.OS << "Clang Serialized Diagnostics\n";
     break;
+  case LLVMBitstreamRemarks:
+    O.OS << "LLVM Remarks\n";
+    break;
   }
   O.OS << "  # Toplevel Blocks: " << NumTopBlocks << "\n";
   O.OS << "\n";