[Serialized diagnostics] Use a 5-bit VBR encoding for file IDs.

File IDs have been expressed using a 10-bit fixed field. This limits us
to 1023 different files in which we can emit diagnostics, where
overflowing would simply crash. With the introduction of macros and
their generated source buffers, we're much more likely to overflow.

Switch to a 5-bit VBR field, which is slightly smaller for the common
case where there are few files involving diagnositcs, and which also
allows us to have up to 2^32-1 file IDs.

Author: DougGregor

lib/Frontend/SerializedDiagnosticConsumer.cpp

@@ -404,7 +404,7 @@ static void emitRecordID(unsigned ID, const char *Name,
 static void
 addSourceLocationAbbrev(std::shared_ptr<llvm::BitCodeAbbrev> Abbrev) {
   using namespace llvm;
-  Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 10)); // File ID.
+  Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 5));    // File ID.
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 32)); // Line.
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 32)); // Column.
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 32)); // Offset;
@@ -488,7 +488,7 @@ void SerializedDiagnosticConsumer::emitBlockInfoBlock() {
   // Emit the abbreviation for RECORD_FILENAME.
   Abbrev = std::make_shared<BitCodeAbbrev>();
   Abbrev->Add(BitCodeAbbrevOp(RECORD_FILENAME));
-  Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 10)); // Mapped file ID.
+  Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 5)); // Mapped file ID.
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 32)); // Size.
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 32)); // Modification time.
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 16)); // Text size.
@@ -508,9 +508,9 @@ void SerializedDiagnosticConsumer::emitBlockInfoBlock() {
   // Emit the abbreviation for RECORD_SOURCE_FILE_CONTENTS.
   Abbrev = std::make_shared<BitCodeAbbrev>();
   Abbrev->Add(BitCodeAbbrevOp(RECORD_SOURCE_FILE_CONTENTS));
-  Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 10)); // File ID.
+  Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 5)); // File ID.
   addRangeLocationAbbrev(Abbrev);
-  Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 16)); // File size.
+  Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 16)); // File size.
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Blob)); // File contents.
   Abbrevs.set(RECORD_SOURCE_FILE_CONTENTS,
               Stream.EmitBlockInfoAbbrev(BLOCK_DIAG, Abbrev));