--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/share/vm/code/nmethod.hpp	Sat Dec 01 00:00:00 2007 +0000
@@ -0,0 +1,578 @@
+/*
+ * Copyright 1997-2007 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.
+ *
+ */
+
+// This class is used internally by nmethods, to cache
+// exception/pc/handler information.
+
+class ExceptionCache : public CHeapObj {
+  friend class VMStructs;
+ private:
+  static address _unwind_handler;
+  enum { cache_size = 16 };
+  klassOop _exception_type;
+  address  _pc[cache_size];
+  address  _handler[cache_size];
+  int      _count;
+  ExceptionCache* _next;
+
+  address pc_at(int index)                     { assert(index >= 0 && index < count(),""); return _pc[index]; }
+  void    set_pc_at(int index, address a)      { assert(index >= 0 && index < cache_size,""); _pc[index] = a; }
+  address handler_at(int index)                { assert(index >= 0 && index < count(),""); return _handler[index]; }
+  void    set_handler_at(int index, address a) { assert(index >= 0 && index < cache_size,""); _handler[index] = a; }
+  int     count()                              { return _count; }
+  void    increment_count()                    { _count++; }
+
+ public:
+
+  ExceptionCache(Handle exception, address pc, address handler);
+
+  klassOop  exception_type()                { return _exception_type; }
+  klassOop* exception_type_addr()           { return &_exception_type; }
+  ExceptionCache* next()                    { return _next; }
+  void      set_next(ExceptionCache *ec)    { _next = ec; }
+
+  address match(Handle exception, address pc);
+  bool    match_exception_with_space(Handle exception) ;
+  address test_address(address addr);
+  bool    add_address_and_handler(address addr, address handler) ;
+
+  static address unwind_handler() { return _unwind_handler; }
+};
+
+
+// cache pc descs found in earlier inquiries
+class PcDescCache VALUE_OBJ_CLASS_SPEC {
+  friend class VMStructs;
+ private:
+  enum { cache_size = 4 };
+  PcDesc* _last_pc_desc;         // most recent pc_desc found
+  PcDesc* _pc_descs[cache_size]; // last cache_size pc_descs found
+ public:
+  PcDescCache() { debug_only(_last_pc_desc = NULL); }
+  void    reset_to(PcDesc* initial_pc_desc);
+  PcDesc* find_pc_desc(int pc_offset, bool approximate);
+  void    add_pc_desc(PcDesc* pc_desc);
+  PcDesc* last_pc_desc() { return _last_pc_desc; }
+};
+
+
+// nmethods (native methods) are the compiled code versions of Java methods.
+
+struct nmFlags {
+  friend class VMStructs;
+  unsigned int version:8;                 // version number (0 = first version)
+  unsigned int level:4;                   // optimization level
+  unsigned int age:4;                     // age (in # of sweep steps)
+
+  unsigned int state:2;                   // {alive, zombie, unloaded)
+
+  unsigned int isUncommonRecompiled:1;    // recompiled because of uncommon trap?
+  unsigned int isToBeRecompiled:1;        // to be recompiled as soon as it matures
+  unsigned int hasFlushedDependencies:1;  // Used for maintenance of dependencies
+  unsigned int markedForReclamation:1;    // Used by NMethodSweeper
+
+  unsigned int has_unsafe_access:1;       // May fault due to unsafe access.
+
+  void clear();
+};
+
+
+// A nmethod contains:
+//  - header                 (the nmethod structure)
+//  [Relocation]
+//  - relocation information
+//  - constant part          (doubles, longs and floats used in nmethod)
+//  [Code]
+//  - code body
+//  - exception handler
+//  - stub code
+//  [Debugging information]
+//  - oop array
+//  - data array
+//  - pcs
+//  [Exception handler table]
+//  - handler entry point array
+//  [Implicit Null Pointer exception table]
+//  - implicit null table array
+
+class Dependencies;
+class ExceptionHandlerTable;
+class ImplicitExceptionTable;
+class AbstractCompiler;
+class xmlStream;
+
+class nmethod : public CodeBlob {
+  friend class VMStructs;
+  friend class NMethodSweeper;
+ private:
+  // Shared fields for all nmethod's
+  static int _zombie_instruction_size;
+
+  methodOop _method;
+  int       _entry_bci;        // != InvocationEntryBci if this nmethod is an on-stack replacement method
+
+  nmethod*  _link;             // To support simple linked-list chaining of nmethods
+
+  AbstractCompiler* _compiler; // The compiler which compiled this nmethod
+
+  // Offsets for different nmethod parts
+  int _exception_offset;
+  // All deoptee's will resume execution at this location described by this offset
+  int _deoptimize_offset;
+  int _stub_offset;
+  int _consts_offset;
+  int _scopes_data_offset;
+  int _scopes_pcs_offset;
+  int _dependencies_offset;
+  int _handler_table_offset;
+  int _nul_chk_table_offset;
+  int _nmethod_end_offset;
+
+  // location in frame (offset for sp) that deopt can store the original
+  // pc during a deopt.
+  int _orig_pc_offset;
+
+  int _compile_id;                     // which compilation made this nmethod
+  int _comp_level;                     // compilation level
+
+  // offsets for entry points
+  address _entry_point;                // entry point with class check
+  address _verified_entry_point;       // entry point without class check
+  address _osr_entry_point;            // entry point for on stack replacement
+
+  nmFlags flags;           // various flags to keep track of nmethod state
+  bool _markedForDeoptimization;       // Used for stack deoptimization
+  enum { alive        = 0,
+         not_entrant  = 1, // uncommon trap has happend but activations may still exist
+         zombie       = 2,
+         unloaded     = 3 };
+
+  // used by jvmti to track if an unload event has been posted for this nmethod.
+  bool _unload_reported;
+
+  NOT_PRODUCT(bool _has_debug_info; )
+
+  // Nmethod Flushing lock (if non-zero, then the nmethod is not removed)
+  jint  _lock_count;
+
+  // not_entrant method removal. Each mark_sweep pass will update
+  // this mark to current sweep invocation count if it is seen on the
+  // stack.  An not_entrant method can be removed when there is no
+  // more activations, i.e., when the _stack_traversal_mark is less than
+  // current sweep traversal index.
+  long _stack_traversal_mark;
+
+  ExceptionCache *_exception_cache;
+  PcDescCache     _pc_desc_cache;
+
+  // These are only used for compiled synchronized native methods to
+  // locate the owner and stack slot for the BasicLock so that we can
+  // properly revoke the bias of the owner if necessary. They are
+  // needed because there is no debug information for compiled native
+  // wrappers and the oop maps are insufficient to allow
+  // frame::retrieve_receiver() to work. Currently they are expected
+  // to be byte offsets from the Java stack pointer for maximum code
+  // sharing between platforms. Note that currently biased locking
+  // will never cause Class instances to be biased but this code
+  // handles the static synchronized case as well.
+  ByteSize _compiled_synchronized_native_basic_lock_owner_sp_offset;
+  ByteSize _compiled_synchronized_native_basic_lock_sp_offset;
+
+  friend class nmethodLocker;
+
+  // For native wrappers
+  nmethod(methodOop method,
+          int nmethod_size,
+          CodeOffsets* offsets,
+          CodeBuffer *code_buffer,
+          int frame_size,
+          ByteSize basic_lock_owner_sp_offset, /* synchronized natives only */
+          ByteSize basic_lock_sp_offset,       /* synchronized natives only */
+          OopMapSet* oop_maps);
+
+  // Creation support
+  nmethod(methodOop method,
+          int nmethod_size,
+          int compile_id,
+          int entry_bci,
+          CodeOffsets* offsets,
+          int orig_pc_offset,
+          DebugInformationRecorder *recorder,
+          Dependencies* dependencies,
+          CodeBuffer *code_buffer,
+          int frame_size,
+          OopMapSet* oop_maps,
+          ExceptionHandlerTable* handler_table,
+          ImplicitExceptionTable* nul_chk_table,
+          AbstractCompiler* compiler,
+          int comp_level);
+
+  // helper methods
+  void* operator new(size_t size, int nmethod_size);
+  void check_store();
+
+  const char* reloc_string_for(u_char* begin, u_char* end);
+  void make_not_entrant_or_zombie(int state);
+  void inc_decompile_count();
+
+  // used to check that writes to nmFlags are done consistently.
+  static void check_safepoint() PRODUCT_RETURN;
+
+  // Used to manipulate the exception cache
+  void add_exception_cache_entry(ExceptionCache* new_entry);
+  ExceptionCache* exception_cache_entry_for_exception(Handle exception);
+
+  // Inform external interfaces that a compiled method has been unloaded
+  inline void post_compiled_method_unload();
+
+ public:
+  // create nmethod with entry_bci
+  static nmethod* new_nmethod(methodHandle method,
+                              int compile_id,
+                              int entry_bci,
+                              CodeOffsets* offsets,
+                              int orig_pc_offset,
+                              DebugInformationRecorder* recorder,
+                              Dependencies* dependencies,
+                              CodeBuffer *code_buffer,
+                              int frame_size,
+                              OopMapSet* oop_maps,
+                              ExceptionHandlerTable* handler_table,
+                              ImplicitExceptionTable* nul_chk_table,
+                              AbstractCompiler* compiler,
+                              int comp_level);
+
+  static nmethod* new_native_nmethod(methodHandle method,
+                                     CodeBuffer *code_buffer,
+                                     int vep_offset,
+                                     int frame_complete,
+                                     int frame_size,
+                                     ByteSize receiver_sp_offset,
+                                     ByteSize basic_lock_sp_offset,
+                                     OopMapSet* oop_maps);
+
+  // accessors
+  methodOop method() const                        { return _method; }
+  AbstractCompiler* compiler() const              { return _compiler; }
+
+#ifndef PRODUCT
+  bool has_debug_info() const                     { return _has_debug_info; }
+  void set_has_debug_info(bool f)                 { _has_debug_info = false; }
+#endif // NOT PRODUCT
+
+  // type info
+  bool is_nmethod() const                         { return true; }
+  bool is_java_method() const                     { return !method()->is_native(); }
+  bool is_native_method() const                   { return method()->is_native(); }
+  bool is_osr_method() const                      { return _entry_bci != InvocationEntryBci; }
+  bool is_osr_only_method() const                 { return is_osr_method(); }
+
+  bool is_compiled_by_c1() const;
+  bool is_compiled_by_c2() const;
+
+  // boundaries for different parts
+  address code_begin         () const             { return _entry_point; }
+  address code_end           () const             { return           header_begin() + _stub_offset          ; }
+  address exception_begin    () const             { return           header_begin() + _exception_offset     ; }
+  address deopt_handler_begin() const             { return           header_begin() + _deoptimize_offset    ; }
+  address stub_begin         () const             { return           header_begin() + _stub_offset          ; }
+  address stub_end           () const             { return           header_begin() + _consts_offset        ; }
+  address consts_begin       () const             { return           header_begin() + _consts_offset        ; }
+  address consts_end         () const             { return           header_begin() + _scopes_data_offset   ; }
+  address scopes_data_begin  () const             { return           header_begin() + _scopes_data_offset   ; }
+  address scopes_data_end    () const             { return           header_begin() + _scopes_pcs_offset    ; }
+  PcDesc* scopes_pcs_begin   () const             { return (PcDesc*)(header_begin() + _scopes_pcs_offset   ); }
+  PcDesc* scopes_pcs_end     () const             { return (PcDesc*)(header_begin() + _dependencies_offset); }
+  address dependencies_begin () const             { return           header_begin() + _dependencies_offset ; }
+  address dependencies_end   () const             { return           header_begin() + _handler_table_offset ; }
+  address handler_table_begin() const             { return           header_begin() + _handler_table_offset ; }
+  address handler_table_end  () const             { return           header_begin() + _nul_chk_table_offset   ; }
+  address nul_chk_table_begin() const             { return           header_begin() + _nul_chk_table_offset ; }
+  address nul_chk_table_end  () const             { return           header_begin() + _nmethod_end_offset   ; }
+
+  int code_size         () const                  { return      code_end         () -      code_begin         (); }
+  int stub_size         () const                  { return      stub_end         () -      stub_begin         (); }
+  int consts_size       () const                  { return      consts_end       () -      consts_begin       (); }
+  int scopes_data_size  () const                  { return      scopes_data_end  () -      scopes_data_begin  (); }
+  int scopes_pcs_size   () const                  { return (intptr_t)scopes_pcs_end   () - (intptr_t)scopes_pcs_begin   (); }
+  int dependencies_size () const                  { return      dependencies_end () -      dependencies_begin (); }
+  int handler_table_size() const                  { return      handler_table_end() -      handler_table_begin(); }
+  int nul_chk_table_size() const                  { return      nul_chk_table_end() -      nul_chk_table_begin(); }
+
+  int total_size        () const;
+
+  bool code_contains         (address addr) const { return code_begin         () <= addr && addr < code_end         (); }
+  bool stub_contains         (address addr) const { return stub_begin         () <= addr && addr < stub_end         (); }
+  bool consts_contains       (address addr) const { return consts_begin       () <= addr && addr < consts_end       (); }
+  bool scopes_data_contains  (address addr) const { return scopes_data_begin  () <= addr && addr < scopes_data_end  (); }
+  bool scopes_pcs_contains   (PcDesc* addr) const { return scopes_pcs_begin   () <= addr && addr < scopes_pcs_end   (); }
+  bool handler_table_contains(address addr) const { return handler_table_begin() <= addr && addr < handler_table_end(); }
+  bool nul_chk_table_contains(address addr) const { return nul_chk_table_begin() <= addr && addr < nul_chk_table_end(); }
+
+  // entry points
+  address entry_point() const                     { return _entry_point;             } // normal entry point
+  address verified_entry_point() const            { return _verified_entry_point;    } // if klass is correct
+
+  // flag accessing and manipulation
+  bool  is_in_use() const                         { return flags.state == alive; }
+  bool  is_alive() const                          { return flags.state == alive || flags.state == not_entrant; }
+  bool  is_not_entrant() const                    { return flags.state == not_entrant; }
+  bool  is_zombie() const                         { return flags.state == zombie; }
+  bool  is_unloaded() const                       { return flags.state == unloaded;   }
+
+  // Make the nmethod non entrant. The nmethod will continue to be alive.
+  // It is used when an uncommon trap happens.
+  void  make_not_entrant()                        { make_not_entrant_or_zombie(not_entrant); }
+  void  make_zombie()                             { make_not_entrant_or_zombie(zombie); }
+
+  // used by jvmti to track if the unload event has been reported
+  bool  unload_reported()                         { return _unload_reported; }
+  void  set_unload_reported()                     { _unload_reported = true; }
+
+  bool  is_marked_for_deoptimization() const      { return _markedForDeoptimization; }
+  void  mark_for_deoptimization()                 { _markedForDeoptimization = true; }
+
+  void  make_unloaded(BoolObjectClosure* is_alive, oop cause);
+
+  bool has_dependencies()                         { return dependencies_size() != 0; }
+  void flush_dependencies(BoolObjectClosure* is_alive);
+  bool  has_flushed_dependencies()                { return flags.hasFlushedDependencies; }
+  void  set_has_flushed_dependencies()            {
+    check_safepoint();
+    assert(!has_flushed_dependencies(), "should only happen once");
+    flags.hasFlushedDependencies = 1;
+  }
+
+  bool  is_marked_for_reclamation() const         { return flags.markedForReclamation; }
+  void  mark_for_reclamation()                    { check_safepoint(); flags.markedForReclamation = 1; }
+  void  unmark_for_reclamation()                  { check_safepoint(); flags.markedForReclamation = 0; }
+
+  bool  has_unsafe_access() const                 { return flags.has_unsafe_access; }
+  void  set_has_unsafe_access(bool z)             { flags.has_unsafe_access = z; }
+
+  int   level() const                             { return flags.level; }
+  void  set_level(int newLevel)                   { check_safepoint(); flags.level = newLevel; }
+
+  int   comp_level() const                        { return _comp_level; }
+
+  int   version() const                           { return flags.version; }
+  void  set_version(int v);
+
+  // Sweeper support
+  long  stack_traversal_mark()                    { return _stack_traversal_mark; }
+  void  set_stack_traversal_mark(long l)          { _stack_traversal_mark = l; }
+
+  // Exception cache support
+  ExceptionCache* exception_cache() const         { return _exception_cache; }
+  void set_exception_cache(ExceptionCache *ec)    { _exception_cache = ec; }
+  address handler_for_exception_and_pc(Handle exception, address pc);
+  void add_handler_for_exception_and_pc(Handle exception, address pc, address handler);
+  void remove_from_exception_cache(ExceptionCache* ec);
+
+  // implicit exceptions support
+  address continuation_for_implicit_exception(address pc);
+
+  // On-stack replacement support
+  int   osr_entry_bci() const                     { assert(_entry_bci != InvocationEntryBci, "wrong kind of nmethod"); return _entry_bci; }
+  address  osr_entry() const                      { assert(_entry_bci != InvocationEntryBci, "wrong kind of nmethod"); return _osr_entry_point; }
+  void  invalidate_osr_method();
+  nmethod* link() const                           { return _link; }
+  void     set_link(nmethod *n)                   { _link = n; }
+
+  // tells whether frames described by this nmethod can be deoptimized
+  // note: native wrappers cannot be deoptimized.
+  bool can_be_deoptimized() const { return is_java_method(); }
+
+  // Inline cache support
+  void clear_inline_caches();
+  void cleanup_inline_caches();
+  bool inlinecache_check_contains(address addr) const {
+    return (addr >= instructions_begin() && addr < verified_entry_point());
+  }
+
+  // unlink and deallocate this nmethod
+  // Only NMethodSweeper class is expected to use this. NMethodSweeper is not
+  // expected to use any other private methods/data in this class.
+
+ protected:
+  void flush();
+
+ public:
+  // If returning true, it is unsafe to remove this nmethod even though it is a zombie
+  // nmethod, since the VM might have a reference to it. Should only be called from a  safepoint.
+  bool is_locked_by_vm() const                    { return _lock_count >0; }
+
+  // See comment at definition of _last_seen_on_stack
+  void mark_as_seen_on_stack();
+  bool can_not_entrant_be_converted();
+
+  // Evolution support. We make old (discarded) compiled methods point to new methodOops.
+  void set_method(methodOop method) { _method = method; }
+
+  // GC support
+  void do_unloading(BoolObjectClosure* is_alive, OopClosure* keep_alive,
+                    bool unloading_occurred);
+  bool can_unload(BoolObjectClosure* is_alive, OopClosure* keep_alive,
+                  oop* root, bool unloading_occurred);
+
+  void preserve_callee_argument_oops(frame fr, const RegisterMap *reg_map,
+                                     OopClosure* f);
+  void oops_do(OopClosure* f);
+
+  // ScopeDesc for an instruction
+  ScopeDesc* scope_desc_at(address pc);
+
+ private:
+  ScopeDesc* scope_desc_in(address begin, address end);
+
+  address* orig_pc_addr(const frame* fr ) { return (address*) ((address)fr->unextended_sp() + _orig_pc_offset); }
+
+  PcDesc* find_pc_desc_internal(address pc, bool approximate);
+
+  PcDesc* find_pc_desc(address pc, bool approximate) {
+    PcDesc* desc = _pc_desc_cache.last_pc_desc();
+    if (desc != NULL && desc->pc_offset() == pc - instructions_begin()) {
+      return desc;
+    }
+    return find_pc_desc_internal(pc, approximate);
+  }
+
+ public:
+  // ScopeDesc retrieval operation
+  PcDesc* pc_desc_at(address pc)   { return find_pc_desc(pc, false); }
+  // pc_desc_near returns the first PcDesc at or after the givne pc.
+  PcDesc* pc_desc_near(address pc) { return find_pc_desc(pc, true); }
+
+ public:
+  // copying of debugging information
+  void copy_scopes_pcs(PcDesc* pcs, int count);
+  void copy_scopes_data(address buffer, int size);
+
+  // deopt
+  // return true is the pc is one would expect if the frame is being deopted.
+  bool is_deopt_pc(address pc);
+  // Accessor/mutator for the original pc of a frame before a frame was deopted.
+  address get_original_pc(const frame* fr) { return *orig_pc_addr(fr); }
+  void    set_original_pc(const frame* fr, address pc) { *orig_pc_addr(fr) = pc; }
+
+  // jvmti support:
+  void post_compiled_method_load_event();
+
+  // verify operations
+  void verify();
+  void verify_scopes();
+  void verify_interrupt_point(address interrupt_point);
+
+  // printing support
+  void print()                          const     PRODUCT_RETURN;
+  void print_code()                               PRODUCT_RETURN;
+  void print_relocations()                        PRODUCT_RETURN;
+  void print_pcs()                                PRODUCT_RETURN;
+  void print_scopes()                             PRODUCT_RETURN;
+  void print_dependencies()                       PRODUCT_RETURN;
+  void print_value_on(outputStream* st) const     PRODUCT_RETURN;
+  void print_calls(outputStream* st)              PRODUCT_RETURN;
+  void print_handler_table()                      PRODUCT_RETURN;
+  void print_nul_chk_table()                      PRODUCT_RETURN;
+  void print_nmethod(bool print_code)             PRODUCT_RETURN;
+
+  void print_on(outputStream* st, const char* title) const;
+
+  // Logging
+  void log_identity(xmlStream* log) const;
+  void log_new_nmethod() const;
+  void log_state_change(int state) const;
+
+  // Prints a comment for one native instruction (reloc info, pc desc)
+  void print_code_comment_on(outputStream* st, int column, address begin, address end) PRODUCT_RETURN;
+  static void print_statistics()                  PRODUCT_RETURN;
+
+  // Compiler task identification.  Note that all OSR methods
+  // are numbered in an independent sequence if CICountOSR is true,
+  // and native method wrappers are also numbered independently if
+  // CICountNative is true.
+  int  compile_id() const                         { return _compile_id; }
+  const char* compile_kind() const;
+
+  // For debugging
+  // CompiledIC*    IC_at(char* p) const;
+  // PrimitiveIC*   primitiveIC_at(char* p) const;
+  oop embeddedOop_at(address p);
+
+  // tells if any of this method's dependencies have been invalidated
+  // (this is expensive!)
+  bool check_all_dependencies();
+
+  // tells if this compiled method is dependent on the given changes,
+  // and the changes have invalidated it
+  bool check_dependency_on(DepChange& changes);
+
+  // Evolution support. Tells if this compiled method is dependent on any of
+  // methods m() of class dependee, such that if m() in dependee is replaced,
+  // this compiled method will have to be deoptimized.
+  bool is_evol_dependent_on(klassOop dependee);
+
+  // Fast breakpoint support. Tells if this compiled method is
+  // dependent on the given method. Returns true if this nmethod
+  // corresponds to the given method as well.
+  bool is_dependent_on_method(methodOop dependee);
+
+  // is it ok to patch at address?
+  bool is_patchable_at(address instr_address);
+
+  // UseBiasedLocking support
+  ByteSize compiled_synchronized_native_basic_lock_owner_sp_offset() {
+    return _compiled_synchronized_native_basic_lock_owner_sp_offset;
+  }
+  ByteSize compiled_synchronized_native_basic_lock_sp_offset() {
+    return _compiled_synchronized_native_basic_lock_sp_offset;
+  }
+
+  // support for code generation
+  static int verified_entry_point_offset()        { return offset_of(nmethod, _verified_entry_point); }
+  static int osr_entry_point_offset()             { return offset_of(nmethod, _osr_entry_point); }
+  static int entry_bci_offset()                   { return offset_of(nmethod, _entry_bci); }
+
+};
+
+// Locks an nmethod so its code will not get removed, even if it is a zombie/not_entrant method
+class nmethodLocker : public StackObj {
+  nmethod* _nm;
+
+  static void lock_nmethod(nmethod* nm);   // note: nm can be NULL
+  static void unlock_nmethod(nmethod* nm); // (ditto)
+
+ public:
+  nmethodLocker(address pc); // derive nm from pc
+  nmethodLocker(nmethod *nm) { _nm = nm; lock_nmethod(_nm); }
+  nmethodLocker() { _nm = NULL; }
+  ~nmethodLocker() { unlock_nmethod(_nm); }
+
+  nmethod* code() { return _nm; }
+  void set_code(nmethod* new_nm) {
+    unlock_nmethod(_nm);   // note:  This works even if _nm==new_nm.
+    _nm = new_nm;
+    lock_nmethod(_nm);
+  }
+};