Reorder Value and User fields to save 8 bytes of padding on 64-bit

[• rafael]

The sizes that I am seeing are

patch
64: value=40 use=48
// 32: value=20 use=28

master
64: value=40 use=56
// 32: value=20 use=28

So it is a saving of 8 bytes in Use. Value has the same size. Awesome!

[rnk]

Closed by commit rL210501 (authored by @rnk).

include/llvm/IR/User.h

 
 class User : public Value {
   User(const User &) LLVM_DELETED_FUNCTION;
   void *operator new(size_t) LLVM_DELETED_FUNCTION;
   template <unsigned>
   friend struct HungoffOperandTraits;
   virtual void anchor();
 protected:
+  /// NumOperands - The number of values used by this User.
+  ///
+  unsigned NumOperands;
+
   /// OperandList - This is a pointer to the array of Uses for this User.
   /// For nodes of fixed arity (e.g. a binary operator) this array will live
   /// prefixed to some derived class instance.  For nodes of resizable variable
   /// arity (e.g. PHINodes, SwitchInst etc.), this memory will be dynamically
   /// allocated and should be destroyed by the classes' virtual dtor.
   Use *OperandList;
 
-  /// NumOperands - The number of values used by this User.
-  ///
-  unsigned NumOperands;
-
   void *operator new(size_t s, unsigned Us);
   User(Type *ty, unsigned vty, Use *OpList, unsigned NumOps)
-    : Value(ty, vty), OperandList(OpList), NumOperands(NumOps) {}
+    : Value(ty, vty), NumOperands(NumOps), OperandList(OpList) {}
   Use *allocHungoffUses(unsigned) const;
   void dropHungoffUses() {
     Use::zap(OperandList, OperandList + NumOperands, true);
     OperandList = nullptr;
     // Reset NumOperands so User::operator delete() does the right thing.
     NumOperands = 0;
   }
 public:

include/llvm/IR/Value.h

 ///
 /// Every value has a "use list" that keeps track of which other Values are
 /// using this Value.  A Value can also have an arbitrary number of ValueHandle
 /// objects that watch it and listen to RAUW and Destroy events.  See
 /// llvm/IR/ValueHandle.h for details.
 ///
 /// @brief LLVM Value Representation
 class Value {
+  Type *VTy;
+  Use *UseList;
+
+  friend class ValueSymbolTable; // Allow ValueSymbolTable to directly mod Name.
+  friend class ValueHandleBase;
+  ValueName *Name;
+
   const unsigned char SubclassID;   // Subclass identifier (for isa/dyn_cast)
   unsigned char HasValueHandle : 1; // Has a ValueHandle pointing to this?
 protected:
   /// SubclassOptionalData - This member is similar to SubclassData, however it
   /// is for holding information which may be used to aid optimization, but
   /// which may be cleared to zero without affecting conservative
   /// interpretation.
   unsigned char SubclassOptionalData : 7;
 
 private:
+  /// SubclassData - This member is defined by this class, but is not used for
+  /// anything.  Subclasses can use it to hold whatever state they find useful.
+  /// This field is initialized to zero by the ctor.
+  unsigned short SubclassData;
+
   template <typename UseT> // UseT == 'Use' or 'const Use'
   class use_iterator_impl
       : public std::iterator<std::forward_iterator_tag, UseT *, ptrdiff_t> {
     typedef std::iterator<std::forward_iterator_tag, UseT *, ptrdiff_t> super;
 
     UseT *U;
     explicit use_iterator_impl(UseT *u) : U(u) {}
     friend class Value;
public:
 
     Use &getUse() const { return *UI; }
 
     /// \brief Return the operand # of this use in its User.
     /// FIXME: Replace all callers with a direct call to Use::getOperandNo.
     unsigned getOperandNo() const { return UI->getOperandNo(); }
   };
 
-  /// SubclassData - This member is defined by this class, but is not used for
-  /// anything.  Subclasses can use it to hold whatever state they find useful.
-  /// This field is initialized to zero by the ctor.
-  unsigned short SubclassData;
-
-  Type *VTy;
-  Use *UseList;
-
-  friend class ValueSymbolTable; // Allow ValueSymbolTable to directly mod Name.
-  friend class ValueHandleBase;
-  ValueName *Name;
-
   void operator=(const Value &) LLVM_DELETED_FUNCTION;
   Value(const Value &) LLVM_DELETED_FUNCTION;
 
 protected:
   Value(Type *Ty, unsigned scid);
 public:
   virtual ~Value();
 

lib/IR/Value.cpp

 using namespace llvm;
 
 //===----------------------------------------------------------------------===//
 //                                Value Class
 //===----------------------------------------------------------------------===//
 
 static inline Type *checkType(Type *Ty) {
   assert(Ty && "Value defined with a null type: Error!");
-  return const_cast<Type*>(Ty);
+  return Ty;
 }
 
 Value::Value(Type *ty, unsigned scid)
-  : SubclassID(scid), HasValueHandle(0),
-    SubclassOptionalData(0), SubclassData(0), VTy((Type*)checkType(ty)),
-    UseList(nullptr), Name(nullptr) {
+    : VTy(checkType(ty)), UseList(nullptr), Name(nullptr), SubclassID(scid),
+      HasValueHandle(0), SubclassOptionalData(0), SubclassData(0) {
   // FIXME: Why isn't this in the subclass gunk??
   // Note, we cannot call isa<CallInst> before the CallInst has been
   // constructed.
   if (SubclassID == Instruction::Call || SubclassID == Instruction::Invoke)
     assert((VTy->isFirstClassType() || VTy->isVoidTy() || VTy->isStructTy()) &&
            "invalid CallInst type!");
   else if (SubclassID != BasicBlockVal &&
            (SubclassID < ConstantFirstVal || SubclassID > ConstantLastVal))