IR: Reorder metadata bitcode serialization, NFC

Enumerate `MDNode`'s operands *before* the node itself, so that the
reader requires less RAUW.  Although this will cause different code
paths to be hit in the reader, this should effectively be no
functionality change.

llvm-svn: 220340

Author: dexonsmith

llvm/lib/Bitcode/Writer/ValueEnumerator.cpp

@@ -495,29 +495,31 @@ void ValueEnumerator::EnumerateMDNodeOperands(const MDNode *N) {
 void ValueEnumerator::EnumerateMetadata(const Value *MD) {
   assert((isa<MDNode>(MD) || isa<MDString>(MD)) && "Invalid metadata kind");
 
-  // Enumerate the type of this value.
-  EnumerateType(MD->getType());
-
+  // Skip function-local nodes themselves, but walk their operands.
   const MDNode *N = dyn_cast<MDNode>(MD);
-
-  // In the module-level pass, skip function-local nodes themselves, but
-  // do walk their operands.
   if (N && N->isFunctionLocal() && N->getFunction()) {
     EnumerateMDNodeOperands(N);
     return;
   }
 
-  // Check to see if it's already in!
-  unsigned &MDValueID = MDValueMap[MD];
-  if (MDValueID)
+  // Insert a dummy ID to block the co-recursive call to
+  // EnumerateMDNodeOperands() from re-visiting MD in a cyclic graph.
+  //
+  // Return early if there's already an ID.
+  if (!MDValueMap.insert(std::make_pair(MD, 0)).second)
     return;
 
-  MDValues.push_back(MD);
-  MDValueID = MDValues.size();
+  // Enumerate the type of this value.
+  EnumerateType(MD->getType());
 
-  // Enumerate all non-function-local operands.
+  // Visit operands first to minimize RAUW.
   if (N)
     EnumerateMDNodeOperands(N);
+
+  // Replace the dummy ID inserted above with the correct one.  MDValueMap may
+  // have changed by inserting operands, so we need a fresh lookup here.
+  MDValues.push_back(MD);
+  MDValueMap[MD] = MDValues.size();
 }
 
 /// EnumerateFunctionLocalMetadataa - Incorporate function-local metadata