do not bother reuniquing mdnodes whose operands drop to null. Doing

so can be a huge performance issue when tearing down modules and mdnodes
are not guaranteed to be unique anyway.  This speeds up:
$ time ~/llvm/Release/bin/clang gcc.c -w -S -g

from 72 to 35s, where gcc.c is from:
http://people.csail.mit.edu/smcc/projects/single-file-programs/



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@92315 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Metadata.h

@@ -85,8 +85,6 @@ class MDNodeElement;
   
 //===----------------------------------------------------------------------===//
 /// MDNode - a tuple of other values.
-/// These contain a list of the values that represent the metadata. 
-/// MDNode is always unnamed.
 class MDNode : public MetadataBase, public FoldingSetNode {
   MDNode(const MDNode &);                // DO NOT IMPLEMENT
   void operator=(const MDNode &);        // DO NOT IMPLEMENT
@@ -97,7 +95,14 @@ class MDNode : public MetadataBase, public FoldingSetNode {
   
   // Subclass data enums.
   enum {
-    FunctionLocalBit = 1
+    /// FunctionLocalBit - This bit is set if this MDNode is function local.
+    /// This is true when it (potentially transitively) contains a reference to
+    /// something in a function, like an argument, basicblock, or instruction.
+    FunctionLocalBit = 1 << 0,
+    
+    /// NotUniquedBit - This is set on MDNodes that are not uniqued because they
+    /// have a null perand.
+    NotUniquedBit    = 1 << 1
   };
   
   // Replace each instance of F from the element list of this node with T.
@@ -138,6 +143,13 @@ public:
     return V->getValueID() == MDNodeVal;
   }
 private:
+  bool isNotUniqued() const { 
+    return (getSubclassDataFromValue() & NotUniquedBit) != 0;
+  }
+  void setIsNotUniqued() {
+    setValueSubclassData(getSubclassDataFromValue() | NotUniquedBit);
+  }
+  
   // Shadow Value::setValueSubclassData with a private forwarding method so that
   // any future subclasses cannot accidentally use it.
   void setValueSubclassData(unsigned short D) {

lib/VMCore/Metadata.cpp

@@ -87,8 +87,10 @@ void MDNodeElement::allUsesReplacedWith(Value *NV) {
 
 /// ~MDNode - Destroy MDNode.
 MDNode::~MDNode() {
-  LLVMContextImpl *pImpl = getType()->getContext().pImpl;
-  pImpl->MDNodeSet.RemoveNode(this);
+  if (!isNotUniqued()) {
+    LLVMContextImpl *pImpl = getType()->getContext().pImpl;
+    pImpl->MDNodeSet.RemoveNode(this);
+  }
   delete [] Operands;
   Operands = NULL;
 }
@@ -97,6 +99,7 @@ MDNode::MDNode(LLVMContext &C, Value *const *Vals, unsigned NumVals,
                bool isFunctionLocal)
   : MetadataBase(Type::getMetadataTy(C), Value::MDNodeVal) {
   NumOperands = NumVals;
+  // FIXME: Coallocate the operand list.  These have fixed arity.
   Operands = new MDNodeElement[NumOperands];
     
   for (unsigned i = 0; i != NumVals; ++i) 
@@ -146,25 +149,40 @@ void MDNode::replaceElement(MDNodeElement *Op, Value *To) {
   if (From == To)
     return;
 
+  // Update the operand.
+  Op->set(To, this);
+
+  // If this node is already not being uniqued (because one of the operands
+  // already went to null), then there is nothing else to do here.
+  if (isNotUniqued()) return;
+  
   LLVMContextImpl *pImpl = getType()->getContext().pImpl;
 
   // Remove "this" from the context map.  FoldingSet doesn't have to reprofile
   // this node to remove it, so we don't care what state the operands are in.
   pImpl->MDNodeSet.RemoveNode(this);
 
-  // Update the operand.
-  Op->set(To, this);
-
-  // Insert updated "this" into the context's folding node set.
-  // If a node with same element list already exist then before inserting 
-  // updated "this" into the folding node set, replace all uses of existing 
-  // node with updated "this" node.
+  // If we are dropping an argument to null, we choose to not unique the MDNode
+  // anymore.  This commonly occurs during destruction, and uniquing these
+  // brings little reuse.
+  if (To == 0) {
+    setIsNotUniqued();
+    return;
+  }
+  
+  // Now that the node is out of the folding set, get ready to reinsert it.
+  // First, check to see if another node with the same operands already exists
+  // in the set.  If it doesn't exist, this returns the position to insert it.
   FoldingSetNodeID ID;
   Profile(ID);
   void *InsertPoint;
   MDNode *N = pImpl->MDNodeSet.FindNodeOrInsertPos(ID, InsertPoint);
 
   if (N) {
+    // FIXME:
+    // If it already exists in the set, we don't reinsert it, we just claim it
+    // isn't uniqued.
+    
     N->replaceAllUsesWith(this);
     delete N;
     N = pImpl->MDNodeSet.FindNodeOrInsertPos(ID, InsertPoint);