hotspot/src/share/vm/opto/memnode.hpp

/*
 * Copyright 1997-2009 Sun Microsystems, Inc.  All Rights Reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
 * CA 95054 USA or visit www.sun.com if you need additional information or
 * have any questions.
 *
 */

// Portions of code courtesy of Clifford Click

class MultiNode;
class PhaseCCP;
class PhaseTransform;

//------------------------------MemNode----------------------------------------
// Load or Store, possibly throwing a NULL pointer exception
class MemNode : public Node {
protected:
#ifdef ASSERT
  const TypePtr* _adr_type;     // What kind of memory is being addressed?
#endif
  virtual uint size_of() const; // Size is bigger (ASSERT only)
public:
  enum { Control,               // When is it safe to do this load?
         Memory,                // Chunk of memory is being loaded from
         Address,               // Actually address, derived from base
         ValueIn,               // Value to store
         OopStore               // Preceeding oop store, only in StoreCM
  };
protected:
  MemNode( Node *c0, Node *c1, Node *c2, const TypePtr* at )
    : Node(c0,c1,c2   ) {
    init_class_id(Class_Mem);
    debug_only(_adr_type=at; adr_type();)
  }
  MemNode( Node *c0, Node *c1, Node *c2, const TypePtr* at, Node *c3 )
    : Node(c0,c1,c2,c3) {
    init_class_id(Class_Mem);
    debug_only(_adr_type=at; adr_type();)
  }
  MemNode( Node *c0, Node *c1, Node *c2, const TypePtr* at, Node *c3, Node *c4)
    : Node(c0,c1,c2,c3,c4) {
    init_class_id(Class_Mem);
    debug_only(_adr_type=at; adr_type();)
  }

public:
  // Helpers for the optimizer.  Documented in memnode.cpp.
  static bool detect_ptr_independence(Node* p1, AllocateNode* a1,
                                      Node* p2, AllocateNode* a2,
                                      PhaseTransform* phase);
  static bool adr_phi_is_loop_invariant(Node* adr_phi, Node* cast);

  static Node *optimize_simple_memory_chain(Node *mchain, const TypePtr *t_adr, PhaseGVN *phase);
  static Node *optimize_memory_chain(Node *mchain, const TypePtr *t_adr, PhaseGVN *phase);
  // This one should probably be a phase-specific function:
  static bool all_controls_dominate(Node* dom, Node* sub);

  // Find any cast-away of null-ness and keep its control.
  static  Node *Ideal_common_DU_postCCP( PhaseCCP *ccp, Node* n, Node* adr );
  virtual Node *Ideal_DU_postCCP( PhaseCCP *ccp );

  virtual const class TypePtr *adr_type() const;  // returns bottom_type of address

  // Shared code for Ideal methods:
  Node *Ideal_common(PhaseGVN *phase, bool can_reshape);  // Return -1 for short-circuit NULL.

  // Helper function for adr_type() implementations.
  static const TypePtr* calculate_adr_type(const Type* t, const TypePtr* cross_check = NULL);

  // Raw access function, to allow copying of adr_type efficiently in
  // product builds and retain the debug info for debug builds.
  const TypePtr *raw_adr_type() const {
#ifdef ASSERT
    return _adr_type;
#else
    return 0;
#endif
  }

  // Map a load or store opcode to its corresponding store opcode.
  // (Return -1 if unknown.)
  virtual int store_Opcode() const { return -1; }

  // What is the type of the value in memory?  (T_VOID mean "unspecified".)
  virtual BasicType memory_type() const = 0;
  virtual int memory_size() const {
#ifdef ASSERT
    return type2aelembytes(memory_type(), true);
#else
    return type2aelembytes(memory_type());
#endif
  }

  // Search through memory states which precede this node (load or store).
  // Look for an exact match for the address, with no intervening
  // aliased stores.
  Node* find_previous_store(PhaseTransform* phase);

  // Can this node (load or store) accurately see a stored value in
  // the given memory state?  (The state may or may not be in(Memory).)
  Node* can_see_stored_value(Node* st, PhaseTransform* phase) const;

#ifndef PRODUCT
  static void dump_adr_type(const Node* mem, const TypePtr* adr_type, outputStream *st);
  virtual void dump_spec(outputStream *st) const;
#endif
};

//------------------------------LoadNode---------------------------------------
// Load value; requires Memory and Address
class LoadNode : public MemNode {
protected:
  virtual uint cmp( const Node &n ) const;
  virtual uint size_of() const; // Size is bigger
  const Type* const _type;      // What kind of value is loaded?
public:

  LoadNode( Node *c, Node *mem, Node *adr, const TypePtr* at, const Type *rt )
    : MemNode(c,mem,adr,at), _type(rt) {
    init_class_id(Class_Load);
  }

  // Polymorphic factory method:
  static Node* make( PhaseGVN& gvn, Node *c, Node *mem, Node *adr,
                     const TypePtr* at, const Type *rt, BasicType bt );

  virtual uint hash()   const;  // Check the type

  // Handle algebraic identities here.  If we have an identity, return the Node
  // we are equivalent to.  We look for Load of a Store.
  virtual Node *Identity( PhaseTransform *phase );

  // If the load is from Field memory and the pointer is non-null, we can
  // zero out the control input.
  virtual Node *Ideal(PhaseGVN *phase, bool can_reshape);

  // Split instance field load through Phi.
  Node* split_through_phi(PhaseGVN *phase);

  // Recover original value from boxed values
  Node *eliminate_autobox(PhaseGVN *phase);

  // Compute a new Type for this node.  Basically we just do the pre-check,
  // then call the virtual add() to set the type.
  virtual const Type *Value( PhaseTransform *phase ) const;

  // Common methods for LoadKlass and LoadNKlass nodes.
  const Type *klass_value_common( PhaseTransform *phase ) const;
  Node *klass_identity_common( PhaseTransform *phase );

  virtual uint ideal_reg() const;
  virtual const Type *bottom_type() const;
  // Following method is copied from TypeNode:
  void set_type(const Type* t) {
    assert(t != NULL, "sanity");
    debug_only(uint check_hash = (VerifyHashTableKeys && _hash_lock) ? hash() : NO_HASH);
    *(const Type**)&_type = t;   // cast away const-ness
    // If this node is in the hash table, make sure it doesn't need a rehash.
    assert(check_hash == NO_HASH || check_hash == hash(), "type change must preserve hash code");
  }
  const Type* type() const { assert(_type != NULL, "sanity"); return _type; };

  // Do not match memory edge
  virtual uint match_edge(uint idx) const;

  // Map a load opcode to its corresponding store opcode.
  virtual int store_Opcode() const = 0;

  // Check if the load's memory input is a Phi node with the same control.
  bool is_instance_field_load_with_local_phi(Node* ctrl);

#ifndef PRODUCT
  virtual void dump_spec(outputStream *st) const;
#endif
protected:
  const Type* load_array_final_field(const TypeKlassPtr *tkls,
                                     ciKlass* klass) const;
};

//------------------------------LoadBNode--------------------------------------
// Load a byte (8bits signed) from memory
class LoadBNode : public LoadNode {
public:
  LoadBNode( Node *c, Node *mem, Node *adr, const TypePtr* at, const TypeInt *ti = TypeInt::BYTE )
    : LoadNode(c,mem,adr,at,ti) {}
  virtual int Opcode() const;
  virtual uint ideal_reg() const { return Op_RegI; }
  virtual Node *Ideal(PhaseGVN *phase, bool can_reshape);
  virtual int store_Opcode() const { return Op_StoreB; }
  virtual BasicType memory_type() const { return T_BYTE; }
};

//------------------------------LoadUBNode-------------------------------------
// Load a unsigned byte (8bits unsigned) from memory
class LoadUBNode : public LoadNode {
public:
  LoadUBNode(Node* c, Node* mem, Node* adr, const TypePtr* at, const TypeInt* ti = TypeInt::UBYTE )
    : LoadNode(c, mem, adr, at, ti) {}
  virtual int Opcode() const;
  virtual uint ideal_reg() const { return Op_RegI; }
  virtual Node* Ideal(PhaseGVN *phase, bool can_reshape);
  virtual int store_Opcode() const { return Op_StoreB; }
  virtual BasicType memory_type() const { return T_BYTE; }
};

//------------------------------LoadUSNode-------------------------------------
// Load an unsigned short/char (16bits unsigned) from memory
class LoadUSNode : public LoadNode {
public:
  LoadUSNode( Node *c, Node *mem, Node *adr, const TypePtr* at, const TypeInt *ti = TypeInt::CHAR )
    : LoadNode(c,mem,adr,at,ti) {}
  virtual int Opcode() const;
  virtual uint ideal_reg() const { return Op_RegI; }
  virtual Node *Ideal(PhaseGVN *phase, bool can_reshape);
  virtual int store_Opcode() const { return Op_StoreC; }
  virtual BasicType memory_type() const { return T_CHAR; }
};

//------------------------------LoadINode--------------------------------------
// Load an integer from memory
class LoadINode : public LoadNode {
public:
  LoadINode( Node *c, Node *mem, Node *adr, const TypePtr* at, const TypeInt *ti = TypeInt::INT )
    : LoadNode(c,mem,adr,at,ti) {}
  virtual int Opcode() const;
  virtual uint ideal_reg() const { return Op_RegI; }
  virtual int store_Opcode() const { return Op_StoreI; }
  virtual BasicType memory_type() const { return T_INT; }
};

//------------------------------LoadUI2LNode-----------------------------------
// Load an unsigned integer into long from memory
class LoadUI2LNode : public LoadNode {
public:
  LoadUI2LNode(Node* c, Node* mem, Node* adr, const TypePtr* at, const TypeLong* t = TypeLong::UINT)
    : LoadNode(c, mem, adr, at, t) {}
  virtual int Opcode() const;
  virtual uint ideal_reg() const { return Op_RegL; }
  virtual int store_Opcode() const { return Op_StoreL; }
  virtual BasicType memory_type() const { return T_LONG; }
};

//------------------------------LoadRangeNode----------------------------------
// Load an array length from the array
class LoadRangeNode : public LoadINode {
public:
  LoadRangeNode( Node *c, Node *mem, Node *adr, const TypeInt *ti = TypeInt::POS )
    : LoadINode(c,mem,adr,TypeAryPtr::RANGE,ti) {}
  virtual int Opcode() const;
  virtual const Type *Value( PhaseTransform *phase ) const;
  virtual Node *Identity( PhaseTransform *phase );
  virtual Node *Ideal(PhaseGVN *phase, bool can_reshape);
};

//------------------------------LoadLNode--------------------------------------
// Load a long from memory
class LoadLNode : public LoadNode {
  virtual uint hash() const { return LoadNode::hash() + _require_atomic_access; }
  virtual uint cmp( const Node &n ) const {
    return _require_atomic_access == ((LoadLNode&)n)._require_atomic_access
      && LoadNode::cmp(n);
  }
  virtual uint size_of() const { return sizeof(*this); }
  const bool _require_atomic_access;  // is piecewise load forbidden?

public:
  LoadLNode( Node *c, Node *mem, Node *adr, const TypePtr* at,
             const TypeLong *tl = TypeLong::LONG,
             bool require_atomic_access = false )
    : LoadNode(c,mem,adr,at,tl)
    , _require_atomic_access(require_atomic_access)
  {}
  virtual int Opcode() const;
  virtual uint ideal_reg() const { return Op_RegL; }
  virtual int store_Opcode() const { return Op_StoreL; }
  virtual BasicType memory_type() const { return T_LONG; }
  bool require_atomic_access() { return _require_atomic_access; }
  static LoadLNode* make_atomic(Compile *C, Node* ctl, Node* mem, Node* adr, const TypePtr* adr_type, const Type* rt);
#ifndef PRODUCT
  virtual void dump_spec(outputStream *st) const {
    LoadNode::dump_spec(st);
    if (_require_atomic_access)  st->print(" Atomic!");
  }
#endif
};

//------------------------------LoadL_unalignedNode----------------------------
// Load a long from unaligned memory
class LoadL_unalignedNode : public LoadLNode {
public:
  LoadL_unalignedNode( Node *c, Node *mem, Node *adr, const TypePtr* at )
    : LoadLNode(c,mem,adr,at) {}
  virtual int Opcode() const;
};

//------------------------------LoadFNode--------------------------------------
// Load a float (64 bits) from memory
class LoadFNode : public LoadNode {
public:
  LoadFNode( Node *c, Node *mem, Node *adr, const TypePtr* at, const Type *t = Type::FLOAT )
    : LoadNode(c,mem,adr,at,t) {}
  virtual int Opcode() const;
  virtual uint ideal_reg() const { return Op_RegF; }
  virtual int store_Opcode() const { return Op_StoreF; }
  virtual BasicType memory_type() const { return T_FLOAT; }
};

//------------------------------LoadDNode--------------------------------------
// Load a double (64 bits) from memory
class LoadDNode : public LoadNode {
public:
  LoadDNode( Node *c, Node *mem, Node *adr, const TypePtr* at, const Type *t = Type::DOUBLE )
    : LoadNode(c,mem,adr,at,t) {}
  virtual int Opcode() const;
  virtual uint ideal_reg() const { return Op_RegD; }
  virtual int store_Opcode() const { return Op_StoreD; }
  virtual BasicType memory_type() const { return T_DOUBLE; }
};

//------------------------------LoadD_unalignedNode----------------------------
// Load a double from unaligned memory
class LoadD_unalignedNode : public LoadDNode {
public:
  LoadD_unalignedNode( Node *c, Node *mem, Node *adr, const TypePtr* at )
    : LoadDNode(c,mem,adr,at) {}
  virtual int Opcode() const;
};

//------------------------------LoadPNode--------------------------------------
// Load a pointer from memory (either object or array)
class LoadPNode : public LoadNode {
public:
  LoadPNode( Node *c, Node *mem, Node *adr, const TypePtr *at, const TypePtr* t )
    : LoadNode(c,mem,adr,at,t) {}
  virtual int Opcode() const;
  virtual uint ideal_reg() const { return Op_RegP; }
  virtual int store_Opcode() const { return Op_StoreP; }
  virtual BasicType memory_type() const { return T_ADDRESS; }
  // depends_only_on_test is almost always true, and needs to be almost always
  // true to enable key hoisting & commoning optimizations.  However, for the
  // special case of RawPtr loads from TLS top & end, the control edge carries
  // the dependence preventing hoisting past a Safepoint instead of the memory
  // edge.  (An unfortunate consequence of having Safepoints not set Raw
  // Memory; itself an unfortunate consequence of having Nodes which produce
  // results (new raw memory state) inside of loops preventing all manner of
  // other optimizations).  Basically, it's ugly but so is the alternative.
  // See comment in macro.cpp, around line 125 expand_allocate_common().
  virtual bool depends_only_on_test() const { return adr_type() != TypeRawPtr::BOTTOM; }
};


//------------------------------LoadNNode--------------------------------------
// Load a narrow oop from memory (either object or array)
class LoadNNode : public LoadNode {
public:
  LoadNNode( Node *c, Node *mem, Node *adr, const TypePtr *at, const Type* t )
    : LoadNode(c,mem,adr,at,t) {}
  virtual int Opcode() const;
  virtual uint ideal_reg() const { return Op_RegN; }
  virtual int store_Opcode() const { return Op_StoreN; }
  virtual BasicType memory_type() const { return T_NARROWOOP; }
  // depends_only_on_test is almost always true, and needs to be almost always
  // true to enable key hoisting & commoning optimizations.  However, for the
  // special case of RawPtr loads from TLS top & end, the control edge carries
  // the dependence preventing hoisting past a Safepoint instead of the memory
  // edge.  (An unfortunate consequence of having Safepoints not set Raw
  // Memory; itself an unfortunate consequence of having Nodes which produce
  // results (new raw memory state) inside of loops preventing all manner of
  // other optimizations).  Basically, it's ugly but so is the alternative.
  // See comment in macro.cpp, around line 125 expand_allocate_common().
  virtual bool depends_only_on_test() const { return adr_type() != TypeRawPtr::BOTTOM; }
};

//------------------------------LoadKlassNode----------------------------------
// Load a Klass from an object
class LoadKlassNode : public LoadPNode {
public:
  LoadKlassNode( Node *c, Node *mem, Node *adr, const TypePtr *at, const TypeKlassPtr *tk )
    : LoadPNode(c,mem,adr,at,tk) {}
  virtual int Opcode() const;
  virtual const Type *Value( PhaseTransform *phase ) const;
  virtual Node *Identity( PhaseTransform *phase );
  virtual bool depends_only_on_test() const { return true; }

  // Polymorphic factory method:
  static Node* make( PhaseGVN& gvn, Node *mem, Node *adr, const TypePtr* at,
                     const TypeKlassPtr *tk = TypeKlassPtr::OBJECT );
};

//------------------------------LoadNKlassNode---------------------------------
// Load a narrow Klass from an object.
class LoadNKlassNode : public LoadNNode {
public:
  LoadNKlassNode( Node *c, Node *mem, Node *adr, const TypePtr *at, const TypeNarrowOop *tk )
    : LoadNNode(c,mem,adr,at,tk) {}
  virtual int Opcode() const;
  virtual uint ideal_reg() const { return Op_RegN; }
  virtual int store_Opcode() const { return Op_StoreN; }
  virtual BasicType memory_type() const { return T_NARROWOOP; }

  virtual const Type *Value( PhaseTransform *phase ) const;
  virtual Node *Identity( PhaseTransform *phase );
  virtual bool depends_only_on_test() const { return true; }
};


//------------------------------LoadSNode--------------------------------------
// Load a short (16bits signed) from memory
class LoadSNode : public LoadNode {
public:
  LoadSNode( Node *c, Node *mem, Node *adr, const TypePtr* at, const TypeInt *ti = TypeInt::SHORT )
    : LoadNode(c,mem,adr,at,ti) {}
  virtual int Opcode() const;
  virtual uint ideal_reg() const { return Op_RegI; }
  virtual Node *Ideal(PhaseGVN *phase, bool can_reshape);
  virtual int store_Opcode() const { return Op_StoreC; }
  virtual BasicType memory_type() const { return T_SHORT; }
};

//------------------------------StoreNode--------------------------------------
// Store value; requires Store, Address and Value
class StoreNode : public MemNode {
protected:
  virtual uint cmp( const Node &n ) const;
  virtual bool depends_only_on_test() const { return false; }

  Node *Ideal_masked_input       (PhaseGVN *phase, uint mask);
  Node *Ideal_sign_extended_input(PhaseGVN *phase, int  num_bits);

public:
  StoreNode( Node *c, Node *mem, Node *adr, const TypePtr* at, Node *val )
    : MemNode(c,mem,adr,at,val) {
    init_class_id(Class_Store);
  }
  StoreNode( Node *c, Node *mem, Node *adr, const TypePtr* at, Node *val, Node *oop_store )
    : MemNode(c,mem,adr,at,val,oop_store) {
    init_class_id(Class_Store);
  }

  // Polymorphic factory method:
  static StoreNode* make( PhaseGVN& gvn, Node *c, Node *mem, Node *adr,
                          const TypePtr* at, Node *val, BasicType bt );

  virtual uint hash() const;    // Check the type

  // If the store is to Field memory and the pointer is non-null, we can
  // zero out the control input.
  virtual Node *Ideal(PhaseGVN *phase, bool can_reshape);

  // Compute a new Type for this node.  Basically we just do the pre-check,
  // then call the virtual add() to set the type.
  virtual const Type *Value( PhaseTransform *phase ) const;

  // Check for identity function on memory (Load then Store at same address)
  virtual Node *Identity( PhaseTransform *phase );

  // Do not match memory edge
  virtual uint match_edge(uint idx) const;

  virtual const Type *bottom_type() const;  // returns Type::MEMORY

  // Map a store opcode to its corresponding own opcode, trivially.
  virtual int store_Opcode() const { return Opcode(); }

  // have all possible loads of the value stored been optimized away?
  bool value_never_loaded(PhaseTransform *phase) const;
};

//------------------------------StoreBNode-------------------------------------
// Store byte to memory
class StoreBNode : public StoreNode {
public:
  StoreBNode( Node *c, Node *mem, Node *adr, const TypePtr* at, Node *val ) : StoreNode(c,mem,adr,at,val) {}
  virtual int Opcode() const;
  virtual Node *Ideal(PhaseGVN *phase, bool can_reshape);
  virtual BasicType memory_type() const { return T_BYTE; }
};

//------------------------------StoreCNode-------------------------------------
// Store char/short to memory
class StoreCNode : public StoreNode {
public:
  StoreCNode( Node *c, Node *mem, Node *adr, const TypePtr* at, Node *val ) : StoreNode(c,mem,adr,at,val) {}
  virtual int Opcode() const;
  virtual Node *Ideal(PhaseGVN *phase, bool can_reshape);
  virtual BasicType memory_type() const { return T_CHAR; }
};

//------------------------------StoreINode-------------------------------------
// Store int to memory
class StoreINode : public StoreNode {
public:
  StoreINode( Node *c, Node *mem, Node *adr, const TypePtr* at, Node *val ) : StoreNode(c,mem,adr,at,val) {}
  virtual int Opcode() const;
  virtual BasicType memory_type() const { return T_INT; }
};

//------------------------------StoreLNode-------------------------------------
// Store long to memory
class StoreLNode : public StoreNode {
  virtual uint hash() const { return StoreNode::hash() + _require_atomic_access; }
  virtual uint cmp( const Node &n ) const {
    return _require_atomic_access == ((StoreLNode&)n)._require_atomic_access
      && StoreNode::cmp(n);
  }
  virtual uint size_of() const { return sizeof(*this); }
  const bool _require_atomic_access;  // is piecewise store forbidden?

public:
  StoreLNode( Node *c, Node *mem, Node *adr, const TypePtr* at, Node *val,
              bool require_atomic_access = false )
    : StoreNode(c,mem,adr,at,val)
    , _require_atomic_access(require_atomic_access)
  {}
  virtual int Opcode() const;
  virtual BasicType memory_type() const { return T_LONG; }
  bool require_atomic_access() { return _require_atomic_access; }
  static StoreLNode* make_atomic(Compile *C, Node* ctl, Node* mem, Node* adr, const TypePtr* adr_type, Node* val);
#ifndef PRODUCT
  virtual void dump_spec(outputStream *st) const {
    StoreNode::dump_spec(st);
    if (_require_atomic_access)  st->print(" Atomic!");
  }
#endif
};

//------------------------------StoreFNode-------------------------------------
// Store float to memory
class StoreFNode : public StoreNode {
public:
  StoreFNode( Node *c, Node *mem, Node *adr, const TypePtr* at, Node *val ) : StoreNode(c,mem,adr,at,val) {}
  virtual int Opcode() const;
  virtual BasicType memory_type() const { return T_FLOAT; }
};

//------------------------------StoreDNode-------------------------------------
// Store double to memory
class StoreDNode : public StoreNode {
public:
  StoreDNode( Node *c, Node *mem, Node *adr, const TypePtr* at, Node *val ) : StoreNode(c,mem,adr,at,val) {}
  virtual int Opcode() const;
  virtual BasicType memory_type() const { return T_DOUBLE; }
};

//------------------------------StorePNode-------------------------------------
// Store pointer to memory
class StorePNode : public StoreNode {
public:
  StorePNode( Node *c, Node *mem, Node *adr, const TypePtr* at, Node *val ) : StoreNode(c,mem,adr,at,val) {}
  virtual int Opcode() const;
  virtual BasicType memory_type() const { return T_ADDRESS; }
};

//------------------------------StoreNNode-------------------------------------
// Store narrow oop to memory
class StoreNNode : public StoreNode {
public:
  StoreNNode( Node *c, Node *mem, Node *adr, const TypePtr* at, Node *val ) : StoreNode(c,mem,adr,at,val) {}
  virtual int Opcode() const;
  virtual BasicType memory_type() const { return T_NARROWOOP; }
};

//------------------------------StoreCMNode-----------------------------------
// Store card-mark byte to memory for CM
// The last StoreCM before a SafePoint must be preserved and occur after its "oop" store
// Preceeding equivalent StoreCMs may be eliminated.
class StoreCMNode : public StoreNode {
 private:
  int _oop_alias_idx;   // The alias_idx of OopStore
public:
  StoreCMNode( Node *c, Node *mem, Node *adr, const TypePtr* at, Node *val, Node *oop_store, int oop_alias_idx ) : StoreNode(c,mem,adr,at,val,oop_store), _oop_alias_idx(oop_alias_idx) {}
  virtual int Opcode() const;
  virtual Node *Identity( PhaseTransform *phase );
  virtual Node *Ideal(PhaseGVN *phase, bool can_reshape);
  virtual const Type *Value( PhaseTransform *phase ) const;
  virtual BasicType memory_type() const { return T_VOID; } // unspecific
  int oop_alias_idx() const { return _oop_alias_idx; }
};

//------------------------------LoadPLockedNode---------------------------------
// Load-locked a pointer from memory (either object or array).
// On Sparc & Intel this is implemented as a normal pointer load.
// On PowerPC and friends it's a real load-locked.
class LoadPLockedNode : public LoadPNode {
public:
  LoadPLockedNode( Node *c, Node *mem, Node *adr )
    : LoadPNode(c,mem,adr,TypeRawPtr::BOTTOM, TypeRawPtr::BOTTOM) {}
  virtual int Opcode() const;
  virtual int store_Opcode() const { return Op_StorePConditional; }
  virtual bool depends_only_on_test() const { return true; }
};

//------------------------------LoadLLockedNode---------------------------------
// Load-locked a pointer from memory (either object or array).
// On Sparc & Intel this is implemented as a normal long load.
class LoadLLockedNode : public LoadLNode {
public:
  LoadLLockedNode( Node *c, Node *mem, Node *adr )
    : LoadLNode(c,mem,adr,TypeRawPtr::BOTTOM, TypeLong::LONG) {}
  virtual int Opcode() const;
  virtual int store_Opcode() const { return Op_StoreLConditional; }
};

//------------------------------SCMemProjNode---------------------------------------
// This class defines a projection of the memory  state of a store conditional node.
// These nodes return a value, but also update memory.
class SCMemProjNode : public ProjNode {
public:
  enum {SCMEMPROJCON = (uint)-2};
  SCMemProjNode( Node *src) : ProjNode( src, SCMEMPROJCON) { }
  virtual int Opcode() const;
  virtual bool      is_CFG() const  { return false; }
  virtual const Type *bottom_type() const {return Type::MEMORY;}
  virtual const TypePtr *adr_type() const { return in(0)->in(MemNode::Memory)->adr_type();}
  virtual uint ideal_reg() const { return 0;} // memory projections don't have a register
  virtual const Type *Value( PhaseTransform *phase ) const;
#ifndef PRODUCT
  virtual void dump_spec(outputStream *st) const {};
#endif
};

//------------------------------LoadStoreNode---------------------------
// Note: is_Mem() method returns 'true' for this class.
class LoadStoreNode : public Node {
public:
  enum {
    ExpectedIn = MemNode::ValueIn+1 // One more input than MemNode
  };
  LoadStoreNode( Node *c, Node *mem, Node *adr, Node *val, Node *ex);
  virtual bool depends_only_on_test() const { return false; }
  virtual const Type *bottom_type() const { return TypeInt::BOOL; }
  virtual uint ideal_reg() const { return Op_RegI; }
  virtual uint match_edge(uint idx) const { return idx == MemNode::Address || idx == MemNode::ValueIn; }
};

//------------------------------StorePConditionalNode---------------------------
// Conditionally store pointer to memory, if no change since prior
// load-locked.  Sets flags for success or failure of the store.
class StorePConditionalNode : public LoadStoreNode {
public:
  StorePConditionalNode( Node *c, Node *mem, Node *adr, Node *val, Node *ll ) : LoadStoreNode(c, mem, adr, val, ll) { }
  virtual int Opcode() const;
  // Produces flags
  virtual uint ideal_reg() const { return Op_RegFlags; }
};

//------------------------------StoreIConditionalNode---------------------------
// Conditionally store int to memory, if no change since prior
// load-locked.  Sets flags for success or failure of the store.
class StoreIConditionalNode : public LoadStoreNode {
public:
  StoreIConditionalNode( Node *c, Node *mem, Node *adr, Node *val, Node *ii ) : LoadStoreNode(c, mem, adr, val, ii) { }
  virtual int Opcode() const;
  // Produces flags
  virtual uint ideal_reg() const { return Op_RegFlags; }
};

//------------------------------StoreLConditionalNode---------------------------
// Conditionally store long to memory, if no change since prior
// load-locked.  Sets flags for success or failure of the store.
class StoreLConditionalNode : public LoadStoreNode {
public:
  StoreLConditionalNode( Node *c, Node *mem, Node *adr, Node *val, Node *ll ) : LoadStoreNode(c, mem, adr, val, ll) { }
  virtual int Opcode() const;
  // Produces flags
  virtual uint ideal_reg() const { return Op_RegFlags; }
};


//------------------------------CompareAndSwapLNode---------------------------
class CompareAndSwapLNode : public LoadStoreNode {
public:
  CompareAndSwapLNode( Node *c, Node *mem, Node *adr, Node *val, Node *ex) : LoadStoreNode(c, mem, adr, val, ex) { }
  virtual int Opcode() const;
};


//------------------------------CompareAndSwapINode---------------------------
class CompareAndSwapINode : public LoadStoreNode {
public:
  CompareAndSwapINode( Node *c, Node *mem, Node *adr, Node *val, Node *ex) : LoadStoreNode(c, mem, adr, val, ex) { }
  virtual int Opcode() const;
};


//------------------------------CompareAndSwapPNode---------------------------
class CompareAndSwapPNode : public LoadStoreNode {
public:
  CompareAndSwapPNode( Node *c, Node *mem, Node *adr, Node *val, Node *ex) : LoadStoreNode(c, mem, adr, val, ex) { }
  virtual int Opcode() const;
};

//------------------------------CompareAndSwapNNode---------------------------
class CompareAndSwapNNode : public LoadStoreNode {
public:
  CompareAndSwapNNode( Node *c, Node *mem, Node *adr, Node *val, Node *ex) : LoadStoreNode(c, mem, adr, val, ex) { }
  virtual int Opcode() const;
};

//------------------------------ClearArray-------------------------------------
class ClearArrayNode: public Node {
public:
  ClearArrayNode( Node *ctrl, Node *arymem, Node *word_cnt, Node *base )
    : Node(ctrl,arymem,word_cnt,base) {
    init_class_id(Class_ClearArray);
  }
  virtual int         Opcode() const;
  virtual const Type *bottom_type() const { return Type::MEMORY; }
  // ClearArray modifies array elements, and so affects only the
  // array memory addressed by the bottom_type of its base address.
  virtual const class TypePtr *adr_type() const;
  virtual Node *Identity( PhaseTransform *phase );
  virtual Node *Ideal(PhaseGVN *phase, bool can_reshape);
  virtual uint match_edge(uint idx) const;

  // Clear the given area of an object or array.
  // The start offset must always be aligned mod BytesPerInt.
  // The end offset must always be aligned mod BytesPerLong.
  // Return the new memory.
  static Node* clear_memory(Node* control, Node* mem, Node* dest,
                            intptr_t start_offset,
                            intptr_t end_offset,
                            PhaseGVN* phase);
  static Node* clear_memory(Node* control, Node* mem, Node* dest,
                            intptr_t start_offset,
                            Node* end_offset,
                            PhaseGVN* phase);
  static Node* clear_memory(Node* control, Node* mem, Node* dest,
                            Node* start_offset,
                            Node* end_offset,
                            PhaseGVN* phase);
  // Return allocation input memory edge if it is different instance
  // or itself if it is the one we are looking for.
  static bool step_through(Node** np, uint instance_id, PhaseTransform* phase);
};

//------------------------------StrComp-------------------------------------
class StrCompNode: public Node {
public:
  StrCompNode(Node* control, Node* char_array_mem,
              Node* s1, Node* c1,
              Node* s2, Node* c2): Node(control, char_array_mem,
                                        s1, c1,
                                        s2, c2) {};
  virtual int Opcode() const;
  virtual bool depends_only_on_test() const { return false; }
  virtual const Type* bottom_type() const { return TypeInt::INT; }
  virtual const TypePtr* adr_type() const { return TypeAryPtr::CHARS; }
  virtual uint match_edge(uint idx) const;
  virtual uint ideal_reg() const { return Op_RegI; }
  virtual Node *Ideal(PhaseGVN *phase, bool can_reshape);
};

//------------------------------StrEquals-------------------------------------
class StrEqualsNode: public Node {
public:
  StrEqualsNode(Node* control, Node* char_array_mem,
                Node* s1, Node* s2, Node* c): Node(control, char_array_mem,
                                                   s1, s2, c) {};
  virtual int Opcode() const;
  virtual bool depends_only_on_test() const { return false; }
  virtual const Type* bottom_type() const { return TypeInt::BOOL; }
  virtual const TypePtr* adr_type() const { return TypeAryPtr::CHARS; }
  virtual uint match_edge(uint idx) const;
  virtual uint ideal_reg() const { return Op_RegI; }
  virtual Node *Ideal(PhaseGVN *phase, bool can_reshape);
};

//------------------------------StrIndexOf-------------------------------------
class StrIndexOfNode: public Node {
public:
  StrIndexOfNode(Node* control, Node* char_array_mem,
                 Node* s1, Node* c1,
                 Node* s2, Node* c2): Node(control, char_array_mem,
                                           s1, c1,
                                           s2, c2) {};
  virtual int Opcode() const;
  virtual bool depends_only_on_test() const { return false; }
  virtual const Type* bottom_type() const { return TypeInt::INT; }
  virtual const TypePtr* adr_type() const { return TypeAryPtr::CHARS; }
  virtual uint match_edge(uint idx) const;
  virtual uint ideal_reg() const { return Op_RegI; }
  virtual Node *Ideal(PhaseGVN *phase, bool can_reshape);
};

//------------------------------AryEq---------------------------------------
class AryEqNode: public Node {
public:
  AryEqNode(Node* control, Node* char_array_mem,
            Node* s1, Node* s2): Node(control, char_array_mem, s1, s2) {};
  virtual int Opcode() const;
  virtual bool depends_only_on_test() const { return false; }
  virtual const Type* bottom_type() const { return TypeInt::BOOL; }
  virtual const TypePtr* adr_type() const { return TypeAryPtr::CHARS; }
  virtual uint match_edge(uint idx) const;
  virtual uint ideal_reg() const { return Op_RegI; }
  virtual Node *Ideal(PhaseGVN *phase, bool can_reshape);
};

//------------------------------MemBar-----------------------------------------
// There are different flavors of Memory Barriers to match the Java Memory
// Model.  Monitor-enter and volatile-load act as Aquires: no following ref
// can be moved to before them.  We insert a MemBar-Acquire after a FastLock or
// volatile-load.  Monitor-exit and volatile-store act as Release: no
// preceding ref can be moved to after them.  We insert a MemBar-Release
// before a FastUnlock or volatile-store.  All volatiles need to be
// serialized, so we follow all volatile-stores with a MemBar-Volatile to
// separate it from any following volatile-load.
class MemBarNode: public MultiNode {
  virtual uint hash() const ;                  // { return NO_HASH; }
  virtual uint cmp( const Node &n ) const ;    // Always fail, except on self

  virtual uint size_of() const { return sizeof(*this); }
  // Memory type this node is serializing.  Usually either rawptr or bottom.
  const TypePtr* _adr_type;

public:
  enum {
    Precedent = TypeFunc::Parms  // optional edge to force precedence
  };
  MemBarNode(Compile* C, int alias_idx, Node* precedent);
  virtual int Opcode() const = 0;
  virtual const class TypePtr *adr_type() const { return _adr_type; }
  virtual const Type *Value( PhaseTransform *phase ) const;
  virtual Node *Ideal(PhaseGVN *phase, bool can_reshape);
  virtual uint match_edge(uint idx) const { return 0; }
  virtual const Type *bottom_type() const { return TypeTuple::MEMBAR; }
  virtual Node *match( const ProjNode *proj, const Matcher *m );
  // Factory method.  Builds a wide or narrow membar.
  // Optional 'precedent' becomes an extra edge if not null.
  static MemBarNode* make(Compile* C, int opcode,
                          int alias_idx = Compile::AliasIdxBot,
                          Node* precedent = NULL);
};

// "Acquire" - no following ref can move before (but earlier refs can
// follow, like an early Load stalled in cache).  Requires multi-cpu
// visibility.  Inserted after a volatile load or FastLock.
class MemBarAcquireNode: public MemBarNode {
public:
  MemBarAcquireNode(Compile* C, int alias_idx, Node* precedent)
    : MemBarNode(C, alias_idx, precedent) {}
  virtual int Opcode() const;
};

// "Release" - no earlier ref can move after (but later refs can move
// up, like a speculative pipelined cache-hitting Load).  Requires
// multi-cpu visibility.  Inserted before a volatile store or FastUnLock.
class MemBarReleaseNode: public MemBarNode {
public:
  MemBarReleaseNode(Compile* C, int alias_idx, Node* precedent)
    : MemBarNode(C, alias_idx, precedent) {}
  virtual int Opcode() const;
};

// Ordering between a volatile store and a following volatile load.
// Requires multi-CPU visibility?
class MemBarVolatileNode: public MemBarNode {
public:
  MemBarVolatileNode(Compile* C, int alias_idx, Node* precedent)
    : MemBarNode(C, alias_idx, precedent) {}
  virtual int Opcode() const;
};

// Ordering within the same CPU.  Used to order unsafe memory references
// inside the compiler when we lack alias info.  Not needed "outside" the
// compiler because the CPU does all the ordering for us.
class MemBarCPUOrderNode: public MemBarNode {
public:
  MemBarCPUOrderNode(Compile* C, int alias_idx, Node* precedent)
    : MemBarNode(C, alias_idx, precedent) {}
  virtual int Opcode() const;
  virtual uint ideal_reg() const { return 0; } // not matched in the AD file
};

// Isolation of object setup after an AllocateNode and before next safepoint.
// (See comment in memnode.cpp near InitializeNode::InitializeNode for semantics.)
class InitializeNode: public MemBarNode {
  friend class AllocateNode;

  bool _is_complete;

public:
  enum {
    Control    = TypeFunc::Control,
    Memory     = TypeFunc::Memory,     // MergeMem for states affected by this op
    RawAddress = TypeFunc::Parms+0,    // the newly-allocated raw address
    RawStores  = TypeFunc::Parms+1     // zero or more stores (or TOP)
  };

  InitializeNode(Compile* C, int adr_type, Node* rawoop);
  virtual int Opcode() const;
  virtual uint size_of() const { return sizeof(*this); }
  virtual uint ideal_reg() const { return 0; } // not matched in the AD file
  virtual const RegMask &in_RegMask(uint) const;  // mask for RawAddress

  // Manage incoming memory edges via a MergeMem on in(Memory):
  Node* memory(uint alias_idx);

  // The raw memory edge coming directly from the Allocation.
  // The contents of this memory are *always* all-zero-bits.
  Node* zero_memory() { return memory(Compile::AliasIdxRaw); }

  // Return the corresponding allocation for this initialization (or null if none).
  // (Note: Both InitializeNode::allocation and AllocateNode::initialization
  // are defined in graphKit.cpp, which sets up the bidirectional relation.)
  AllocateNode* allocation();

  // Anything other than zeroing in this init?
  bool is_non_zero();

  // An InitializeNode must completed before macro expansion is done.
  // Completion requires that the AllocateNode must be followed by
  // initialization of the new memory to zero, then to any initializers.
  bool is_complete() { return _is_complete; }

  // Mark complete.  (Must not yet be complete.)
  void set_complete(PhaseGVN* phase);

#ifdef ASSERT
  // ensure all non-degenerate stores are ordered and non-overlapping
  bool stores_are_sane(PhaseTransform* phase);
#endif //ASSERT

  // See if this store can be captured; return offset where it initializes.
  // Return 0 if the store cannot be moved (any sort of problem).
  intptr_t can_capture_store(StoreNode* st, PhaseTransform* phase);

  // Capture another store; reformat it to write my internal raw memory.
  // Return the captured copy, else NULL if there is some sort of problem.
  Node* capture_store(StoreNode* st, intptr_t start, PhaseTransform* phase);

  // Find captured store which corresponds to the range [start..start+size).
  // Return my own memory projection (meaning the initial zero bits)
  // if there is no such store.  Return NULL if there is a problem.
  Node* find_captured_store(intptr_t start, int size_in_bytes, PhaseTransform* phase);

  // Called when the associated AllocateNode is expanded into CFG.
  Node* complete_stores(Node* rawctl, Node* rawmem, Node* rawptr,
                        intptr_t header_size, Node* size_in_bytes,
                        PhaseGVN* phase);

 private:
  void remove_extra_zeroes();

  // Find out where a captured store should be placed (or already is placed).
  int captured_store_insertion_point(intptr_t start, int size_in_bytes,
                                     PhaseTransform* phase);

  static intptr_t get_store_offset(Node* st, PhaseTransform* phase);

  Node* make_raw_address(intptr_t offset, PhaseTransform* phase);

  bool detect_init_independence(Node* n, bool st_is_pinned, int& count);

  void coalesce_subword_stores(intptr_t header_size, Node* size_in_bytes,
                               PhaseGVN* phase);

  intptr_t find_next_fullword_store(uint i, PhaseGVN* phase);
};

//------------------------------MergeMem---------------------------------------
// (See comment in memnode.cpp near MergeMemNode::MergeMemNode for semantics.)
class MergeMemNode: public Node {
  virtual uint hash() const ;                  // { return NO_HASH; }
  virtual uint cmp( const Node &n ) const ;    // Always fail, except on self
  friend class MergeMemStream;
  MergeMemNode(Node* def);  // clients use MergeMemNode::make

public:
  // If the input is a whole memory state, clone it with all its slices intact.
  // Otherwise, make a new memory state with just that base memory input.
  // In either case, the result is a newly created MergeMem.
  static MergeMemNode* make(Compile* C, Node* base_memory);

  virtual int Opcode() const;
  virtual Node *Identity( PhaseTransform *phase );
  virtual Node *Ideal(PhaseGVN *phase, bool can_reshape);
  virtual uint ideal_reg() const { return NotAMachineReg; }
  virtual uint match_edge(uint idx) const { return 0; }
  virtual const RegMask &out_RegMask() const;
  virtual const Type *bottom_type() const { return Type::MEMORY; }
  virtual const TypePtr *adr_type() const { return TypePtr::BOTTOM; }
  // sparse accessors
  // Fetch the previously stored "set_memory_at", or else the base memory.
  // (Caller should clone it if it is a phi-nest.)
  Node* memory_at(uint alias_idx) const;
  // set the memory, regardless of its previous value
  void set_memory_at(uint alias_idx, Node* n);
  // the "base" is the memory that provides the non-finite support
  Node* base_memory() const       { return in(Compile::AliasIdxBot); }
  // warning: setting the base can implicitly set any of the other slices too
  void set_base_memory(Node* def);
  // sentinel value which denotes a copy of the base memory:
  Node*   empty_memory() const    { return in(Compile::AliasIdxTop); }
  static Node* make_empty_memory(); // where the sentinel comes from
  bool is_empty_memory(Node* n) const { assert((n == empty_memory()) == n->is_top(), "sanity"); return n->is_top(); }
  // hook for the iterator, to perform any necessary setup
  void iteration_setup(const MergeMemNode* other = NULL);
  // push sentinels until I am at least as long as the other (semantic no-op)
  void grow_to_match(const MergeMemNode* other);
  bool verify_sparse() const PRODUCT_RETURN0;
#ifndef PRODUCT
  virtual void dump_spec(outputStream *st) const;
#endif
};

class MergeMemStream : public StackObj {
 private:
  MergeMemNode*       _mm;
  const MergeMemNode* _mm2;  // optional second guy, contributes non-empty iterations
  Node*               _mm_base;  // loop-invariant base memory of _mm
  int                 _idx;
  int                 _cnt;
  Node*               _mem;
  Node*               _mem2;
  int                 _cnt2;

  void init(MergeMemNode* mm, const MergeMemNode* mm2 = NULL) {
    // subsume_node will break sparseness at times, whenever a memory slice
    // folds down to a copy of the base ("fat") memory.  In such a case,
    // the raw edge will update to base, although it should be top.
    // This iterator will recognize either top or base_memory as an
    // "empty" slice.  See is_empty, is_empty2, and next below.
    //
    // The sparseness property is repaired in MergeMemNode::Ideal.
    // As long as access to a MergeMem goes through this iterator
    // or the memory_at accessor, flaws in the sparseness will
    // never be observed.
    //
    // Also, iteration_setup repairs sparseness.
    assert(mm->verify_sparse(), "please, no dups of base");
    assert(mm2==NULL || mm2->verify_sparse(), "please, no dups of base");

    _mm  = mm;
    _mm_base = mm->base_memory();
    _mm2 = mm2;
    _cnt = mm->req();
    _idx = Compile::AliasIdxBot-1; // start at the base memory
    _mem = NULL;
    _mem2 = NULL;
  }

#ifdef ASSERT
  Node* check_memory() const {
    if (at_base_memory())
      return _mm->base_memory();
    else if ((uint)_idx < _mm->req() && !_mm->in(_idx)->is_top())
      return _mm->memory_at(_idx);
    else
      return _mm_base;
  }
  Node* check_memory2() const {
    return at_base_memory()? _mm2->base_memory(): _mm2->memory_at(_idx);
  }
#endif

  static bool match_memory(Node* mem, const MergeMemNode* mm, int idx) PRODUCT_RETURN0;
  void assert_synch() const {
    assert(!_mem || _idx >= _cnt || match_memory(_mem, _mm, _idx),
           "no side-effects except through the stream");
  }

 public:

  // expected usages:
  // for (MergeMemStream mms(mem->is_MergeMem()); next_non_empty(); ) { ... }
  // for (MergeMemStream mms(mem1, mem2); next_non_empty2(); ) { ... }

  // iterate over one merge
  MergeMemStream(MergeMemNode* mm) {
    mm->iteration_setup();
    init(mm);
    debug_only(_cnt2 = 999);
  }
  // iterate in parallel over two merges
  // only iterates through non-empty elements of mm2
  MergeMemStream(MergeMemNode* mm, const MergeMemNode* mm2) {
    assert(mm2, "second argument must be a MergeMem also");
    ((MergeMemNode*)mm2)->iteration_setup();  // update hidden state
    mm->iteration_setup(mm2);
    init(mm, mm2);
    _cnt2 = mm2->req();
  }
#ifdef ASSERT
  ~MergeMemStream() {
    assert_synch();
  }
#endif

  MergeMemNode* all_memory() const {
    return _mm;
  }
  Node* base_memory() const {
    assert(_mm_base == _mm->base_memory(), "no update to base memory, please");
    return _mm_base;
  }
  const MergeMemNode* all_memory2() const {
    assert(_mm2 != NULL, "");
    return _mm2;
  }
  bool at_base_memory() const {
    return _idx == Compile::AliasIdxBot;
  }
  int alias_idx() const {
    assert(_mem, "must call next 1st");
    return _idx;
  }

  const TypePtr* adr_type() const {
    return Compile::current()->get_adr_type(alias_idx());
  }

  const TypePtr* adr_type(Compile* C) const {
    return C->get_adr_type(alias_idx());
  }
  bool is_empty() const {
    assert(_mem, "must call next 1st");
    assert(_mem->is_top() == (_mem==_mm->empty_memory()), "correct sentinel");
    return _mem->is_top();
  }
  bool is_empty2() const {
    assert(_mem2, "must call next 1st");
    assert(_mem2->is_top() == (_mem2==_mm2->empty_memory()), "correct sentinel");
    return _mem2->is_top();
  }
  Node* memory() const {
    assert(!is_empty(), "must not be empty");
    assert_synch();
    return _mem;
  }
  // get the current memory, regardless of empty or non-empty status
  Node* force_memory() const {
    assert(!is_empty() || !at_base_memory(), "");
    // Use _mm_base to defend against updates to _mem->base_memory().
    Node *mem = _mem->is_top() ? _mm_base : _mem;
    assert(mem == check_memory(), "");
    return mem;
  }
  Node* memory2() const {
    assert(_mem2 == check_memory2(), "");
    return _mem2;
  }
  void set_memory(Node* mem) {
    if (at_base_memory()) {
      // Note that this does not change the invariant _mm_base.
      _mm->set_base_memory(mem);
    } else {
      _mm->set_memory_at(_idx, mem);
    }
    _mem = mem;
    assert_synch();
  }

  // Recover from a side effect to the MergeMemNode.
  void set_memory() {
    _mem = _mm->in(_idx);
  }

  bool next()  { return next(false); }
  bool next2() { return next(true); }

  bool next_non_empty()  { return next_non_empty(false); }
  bool next_non_empty2() { return next_non_empty(true); }
  // next_non_empty2 can yield states where is_empty() is true

 private:
  // find the next item, which might be empty
  bool next(bool have_mm2) {
    assert((_mm2 != NULL) == have_mm2, "use other next");
    assert_synch();
    if (++_idx < _cnt) {
      // Note:  This iterator allows _mm to be non-sparse.
      // It behaves the same whether _mem is top or base_memory.
      _mem = _mm->in(_idx);
      if (have_mm2)
        _mem2 = _mm2->in((_idx < _cnt2) ? _idx : Compile::AliasIdxTop);
      return true;
    }
    return false;
  }

  // find the next non-empty item
  bool next_non_empty(bool have_mm2) {
    while (next(have_mm2)) {
      if (!is_empty()) {
        // make sure _mem2 is filled in sensibly
        if (have_mm2 && _mem2->is_top())  _mem2 = _mm2->base_memory();
        return true;
      } else if (have_mm2 && !is_empty2()) {
        return true;   // is_empty() == true
      }
    }
    return false;
  }
};

//------------------------------Prefetch---------------------------------------

// Non-faulting prefetch load.  Prefetch for many reads.
class PrefetchReadNode : public Node {
public:
  PrefetchReadNode(Node *abio, Node *adr) : Node(0,abio,adr) {}
  virtual int Opcode() const;
  virtual uint ideal_reg() const { return NotAMachineReg; }
  virtual uint match_edge(uint idx) const { return idx==2; }
  virtual const Type *bottom_type() const { return Type::ABIO; }
};

// Non-faulting prefetch load.  Prefetch for many reads & many writes.
class PrefetchWriteNode : public Node {
public:
  PrefetchWriteNode(Node *abio, Node *adr) : Node(0,abio,adr) {}
  virtual int Opcode() const;
  virtual uint ideal_reg() const { return NotAMachineReg; }
  virtual uint match_edge(uint idx) const { return idx==2; }
  virtual const Type *bottom_type() const { return Type::ABIO; }
};