[Assignment Tracking][9/*] Don't drop DIAssignID in dropUnknownNonDebugMetadata

[Orlando]

Summary

Make it difficult to accidently drop DIAssignID metadata attachments.

[jmorse]

LGTM

llvm/lib/IR/Metadata.cpp

1312–1313

AS "NonDebug" is in the name of the function, we can probably get away with saying "DIAssignID is debug metadata, don't drop it".

llvm/lib/IR/Metadata.cpp

 MDNode *Instruction::getMetadataImpl(StringRef Kind) const {
   return getMetadataImpl(getContext().getMDKindID(Kind));
 }
 
 void Instruction::dropUnknownNonDebugMetadata(ArrayRef<unsigned> KnownIDs) {
   if (!Value::hasMetadata())
     return; // Nothing to remove!
 
-  if (KnownIDs.empty()) {
-    // Just drop our entry at the store.
-    clearMetadata();
-    return;
-  }
-
   SmallSet<unsigned, 4> KnownSet;
   KnownSet.insert(KnownIDs.begin(), KnownIDs.end());
 
+  // A DIAssignID attachment is debug metadata, don't drop it.
+  KnownSet.insert(LLVMContext::MD_DIAssignID);
+
   auto &MetadataStore = getContext().pImpl->ValueMetadata;
   auto &Info = MetadataStore[this];
   assert(!Info.empty() && "bit out of sync with hash table");
   Info.remove_if([&KnownSet](const MDAttachments::Attachment &I) {
     return !KnownSet.count(I.MDKind);
   });
 
   if (Info.empty()) {

llvm/unittests/IR/DebugInfoTest.cpp

// store1           store1 !x
     Stores[1]->mergeDIAssignID(Stores[0]);
     // Check that the stores share the same ID (note store1 starts with none).
     Metadata *NewID0 = Stores[0]->getMetadata(LLVMContext::MD_DIAssignID);
     Metadata *NewID1 = Stores[1]->getMetadata(LLVMContext::MD_DIAssignID);
     EXPECT_NE(NewID0, nullptr);
     EXPECT_EQ(NewID0, NewID1);
     EXPECT_EQ(Markers[0]->getAssignID(), NewID0);
   }
+
+  // Test 4 - dropUnknownNonDebugMetadata.
+  //
+  // Input            store0->dropUnknownNonDebugMetadata()
+  // -----            -------------------------
+  // store0 !x        store0 !x
+  {
+    Stores[0]->dropUnknownNonDebugMetadata();
+    Metadata *NewID0 = Stores[0]->getMetadata(LLVMContext::MD_DIAssignID);
+    EXPECT_NE(NewID0, nullptr);
+  }
 }
 
 } // end namespace