[Verifier] Complain if a metadata attachment is null

[loladiro]

Summary

The bitcode writer asserts if a metadata attachment to an instruction is null, but the Verifier didn't catch it before. Additionally, also make the IR writer more accepting when it encounters null metadata attachments, since it is used by the Verifier to print its errors.

[dexonsmith]

LGTM.

[dexonsmith]

In D16167#329709, @dexonsmith wrote:

LGTM.

Not sure if this is still relevant, but I'm clicking "accept" to take this off my queue.

lib/IR/AsmWriter.cpp

   SmallVector<std::pair<unsigned, MDNode *>, 4> MDs;
   I.getAllMetadata(MDs);
   for (auto &MD : MDs)
-    CreateMetadataSlot(MD.second);
+    if (MD.second)
+      CreateMetadataSlot(MD.second);
 }
 
 /// Clean up after incorporating a function. This is the only way to get out of

                                    TypePrinting *TypePrinter,
                                    SlotTracker *Machine, const Module *Context,
                                    bool FromValue) {
+  if (!MD) {
+    Out << "<null>";
+    return;
+  }
+
   if (const MDNode *N = dyn_cast<MDNode>(MD)) {
     std::unique_ptr<SlotTracker> MachineStorage;
     if (!Machine) {

   if (MDs.empty())
     return;
 
-  if (MDNames.empty())
+  // Second should not be null in well formed, IR, but this is also
+  // called from the Verifier, so be permissive.
+  if (MDNames.empty() && MDs[0].second)
     MDs[0].second->getContext().getMDKindNames(MDNames);
 
   for (const auto &I : MDs) {

lib/IR/TypeFinder.cpp

         // Incorporate types hiding in metadata.
         I.getAllMetadataOtherThanDebugLoc(MDForInst);
         for (unsigned i = 0, e = MDForInst.size(); i != e; ++i)
-          incorporateMDNode(MDForInst[i].second);
+          if (MDForInst[i].second)
+            incorporateMDNode(MDForInst[i].second);
 
         MDForInst.clear();
       }

lib/IR/Verifier.cpp

     }
   }
 
+  SmallVector<std::pair<unsigned, MDNode *>, 4> InstMD;
+  I.getAllMetadata(InstMD);
+  for (auto MD : InstMD) {
+    Assert(MD.second, "Metadata attachment may not be null!", &I);
+  }
+
   if (MDNode *MD = I.getMetadata(LLVMContext::MD_fpmath)) {
     Assert(I.getType()->isFPOrFPVectorTy(),
            "fpmath requires a floating point result!", &I);