[DebugInfo] Unify ChecksumKind and Checksum value in DIFile

Rather than encode the absence of a checksum with a Kind variant, instead put
both the kind and value in a struct and wrap it in an Optional.

Differential Revision: http://reviews.llvm.org/D43043

llvm-svn: 324928

lib/Bitcode/Reader/MetadataLoader.cpp

@@ -1354,13 +1354,20 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
       return error("Invalid record");
 
     IsDistinct = Record[0];
+    Optional<DIFile::ChecksumInfo<MDString *>> Checksum;
+    // The BitcodeWriter writes null bytes into Record[3:4] when the Checksum
+    // is not present. This matches up with the old internal representation,
+    // and the old encoding for CSK_None in the ChecksumKind. The new
+    // representation reserves the value 0 in the ChecksumKind to continue to
+    // encode None in a backwards-compatible way.
+    if (Record.size() == 5 && Record[3] && Record[4])
+      Checksum.emplace(static_cast<DIFile::ChecksumKind>(Record[3]),
+                       getMDString(Record[4]));
     MetadataList.assignValue(
         GET_OR_DISTINCT(
             DIFile,
             (Context, getMDString(Record[1]), getMDString(Record[2]),
-             Record.size() == 3 ? DIFile::CSK_None
-                                : static_cast<DIFile::ChecksumKind>(Record[3]),
-             Record.size() == 3 ? nullptr : getMDString(Record[4]))),
+             Checksum)),
         NextMetadataNo);
     NextMetadataNo++;
     break;