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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220340 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Bitcode/Writer/ValueEnumerator.cpp b/lib/Bitcode/Writer/ValueEnumerator.cpp
index f2be18e0378bbc3782d8278a74987c4166bf1897..c14591acc8adc3c03bd670a1569bef2e0a36e412 100644 (file)
--- a/lib/Bitcode/Writer/ValueEnumerator.cpp
+++ b/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