Index: include/llvm/IR/User.h
===================================================================
--- include/llvm/IR/User.h
+++ include/llvm/IR/User.h
@@ -46,10 +46,12 @@
   /// of operands, ie, 'hung off uses'.
   void *operator new(size_t Size);
 
-  /// Allocate a User with the operands co-allocated.
+  /// Allocate a User with the operands co-allocated.  If DescBytes is non-zero
+  /// then allocate an additional DescBytes bytes before the operands. These
+  /// bytes can be accessed by calling getDescriptor.
   ///
   /// This is used for subclasses which have a fixed number of operands.
-  void *operator new(size_t Size, unsigned Us);
+  void *operator new(size_t Size, unsigned Us, unsigned DescBytes = 0);
 
   User(Type *ty, unsigned vty, Use *OpList, unsigned NumOps)
       : Value(ty, vty) {
@@ -137,6 +139,14 @@
 
   unsigned getNumOperands() const { return NumUserOperands; }
 
+  /// Returns the descriptor co-allocated with this User instance as a range of
+  /// bytes [Begin, End).
+  std::pair<const uint8_t *, const uint8_t *> getDescriptor() const;
+
+  /// Returns the descriptor co-allocated with this User instance as a range of
+  /// bytes [Begin, End).
+  std::pair<uint8_t *, uint8_t *> getDescriptor();
+
   /// Set the number of operands on a GlobalVariable.
   ///
   /// GlobalVariable always allocates space for a single operands, but
Index: include/llvm/IR/Value.h
===================================================================
--- include/llvm/IR/Value.h
+++ include/llvm/IR/Value.h
@@ -104,12 +104,13 @@
   ///
   /// Note, this should *NOT* be used directly by any class other than User.
   /// User uses this value to find the Use list.
-  enum : unsigned { NumUserOperandsBits = 29 };
+  enum : unsigned { NumUserOperandsBits = 28 };
   unsigned NumUserOperands : NumUserOperandsBits;
 
   bool IsUsedByMD : 1;
   bool HasName : 1;
   bool HasHungOffUses : 1;
+  bool HasDescriptor : 1;
 
 private:
   template <typename UseT> // UseT == 'Use' or 'const Use'
Index: lib/IR/User.cpp
===================================================================
--- lib/IR/User.cpp
+++ lib/IR/User.cpp
@@ -87,19 +87,49 @@
   Use::zap(OldOps, OldOps + OldNumUses, true);
 }
 
+
+// This is a private struct used by `User` to track the co-allocated descriptor
+// section.
+struct DescriptorInfo {
+  uint32_t Size;
+};
+
+std::pair<const uint8_t *, const uint8_t *> User::getDescriptor() const {
+  assert(HasDescriptor && "Don't call otherwise!");
+  auto *DI = reinterpret_cast<const DescriptorInfo *>(&getOperandUse(0)) - 1;
+  return std::make_pair(reinterpret_cast<const uint8_t *>(DI) - DI->Size,
+                        reinterpret_cast<const uint8_t *>(DI));
+}
+
+std::pair<uint8_t *, uint8_t *> User::getDescriptor() {
+  assert(HasDescriptor && "Don't call otherwise!");
+  auto *DI = reinterpret_cast<DescriptorInfo *>(&getOperandUse(0)) - 1;
+  return std::make_pair(reinterpret_cast<uint8_t *>(DI) - DI->Size,
+                        reinterpret_cast<uint8_t *>(DI));
+}
+
 //===----------------------------------------------------------------------===//
 //                         User operator new Implementations
 //===----------------------------------------------------------------------===//
 
-void *User::operator new(size_t Size, unsigned Us) {
+void *User::operator new(size_t Size, unsigned Us, unsigned DescBytes) {
   assert(Us < (1u << NumUserOperandsBits) && "Too many operands");
-  void *Storage = ::operator new(Size + sizeof(Use) * Us);
-  Use *Start = static_cast<Use*>(Storage);
+
+  unsigned DescBytesToAllocate =
+      DescBytes == 0 ? 0 : (DescBytes + sizeof(DescriptorInfo));
+  uint8_t *Storage = static_cast<uint8_t *>(
+      ::operator new(Size + sizeof(Use) * Us + DescBytesToAllocate));
+  Use *Start = reinterpret_cast<Use *>(Storage + DescBytesToAllocate);
   Use *End = Start + Us;
   User *Obj = reinterpret_cast<User*>(End);
   Obj->NumUserOperands = Us;
   Obj->HasHungOffUses = false;
+  Obj->HasDescriptor = DescBytes != 0;
   Use::initTags(Start, End);
+
+  if (DescBytes != 0)
+    reinterpret_cast<DescriptorInfo *>(Storage + DescBytes)->Size = DescBytes;
+
   return Obj;
 }
 
@@ -110,6 +140,7 @@
   User *Obj = reinterpret_cast<User *>(HungOffOperandList + 1);
   Obj->NumUserOperands = 0;
   Obj->HasHungOffUses = true;
+  Obj->HasDescriptor = false;
   *HungOffOperandList = nullptr;
   return Obj;
 }
@@ -123,11 +154,20 @@
   // use a Use[] allocated prior to the user.
   User *Obj = static_cast<User *>(Usr);
   if (Obj->HasHungOffUses) {
+    assert(!Obj->HasDescriptor && "not supported!");
+
     Use **HungOffOperandList = static_cast<Use **>(Usr) - 1;
     // drop the hung off uses.
     Use::zap(*HungOffOperandList, *HungOffOperandList + Obj->NumUserOperands,
              /* Delete */ true);
     ::operator delete(HungOffOperandList);
+  } else if (Obj->HasDescriptor) {
+    Use *UseBegin = static_cast<Use *>(Usr) - Obj->NumUserOperands;
+    Use::zap(UseBegin, UseBegin + Obj->NumUserOperands, /* Delete */ false);
+
+    auto *DI = reinterpret_cast<DescriptorInfo *>(UseBegin) - 1;
+    uint8_t *Storage = reinterpret_cast<uint8_t *>(DI) - DI->Size;
+    ::operator delete(Storage);
   } else {
     Use *Storage = static_cast<Use *>(Usr) - Obj->NumUserOperands;
     Use::zap(Storage, Storage + Obj->NumUserOperands,