1 /*
  2  * Copyright (c) 1997, 2021, Oracle and/or its affiliates. All rights reserved.
  3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  4  *
  5  * This code is free software; you can redistribute it and/or modify it
  6  * under the terms of the GNU General Public License version 2 only, as
  7  * published by the Free Software Foundation.
  8  *
  9  * This code is distributed in the hope that it will be useful, but WITHOUT
 10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 12  * version 2 for more details (a copy is included in the LICENSE file that
 13  * accompanied this code).
 14  *
 15  * You should have received a copy of the GNU General Public License version
 16  * 2 along with this work; if not, write to the Free Software Foundation,
 17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 18  *
 19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 20  * or visit www.oracle.com if you need additional information or have any
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 24 
 25 #ifndef SHARE_OOPS_GENERATEOOPMAP_HPP
 26 #define SHARE_OOPS_GENERATEOOPMAP_HPP
 27 
 28 #include "interpreter/bytecodeStream.hpp"
 29 #include "memory/allocation.hpp"
 30 #include "oops/method.hpp"
 31 #include "oops/oopsHierarchy.hpp"
 32 #include "runtime/signature.hpp"
 33 #include "utilities/bitMap.hpp"
 34 
 35 // Forward definition
 36 class GenerateOopMap;
 37 class BasicBlock;
 38 class CellTypeState;
 39 class StackMap;
 40 
 41 // These two should be removed. But requires som code to be cleaned up
 42 #define MAXARGSIZE      256      // This should be enough
 43 #define MAX_LOCAL_VARS  65536    // 16-bit entry
 44 
 45 typedef void (*jmpFct_t)(GenerateOopMap *c, int bcpDelta, int* data);
 46 
 47 
 48 //  RetTable
 49 //
 50 // Contains maping between jsr targets and there return addresses. One-to-many mapping
 51 //
 52 class RetTableEntry : public ResourceObj {
 53  private:
 54   static int _init_nof_jsrs;                      // Default size of jsrs list
 55   int _target_bci;                                // Target PC address of jump (bytecode index)
 56   GrowableArray<intptr_t> * _jsrs;                     // List of return addresses  (bytecode index)
 57   RetTableEntry *_next;                           // Link to next entry
 58  public:
 59    RetTableEntry(int target, RetTableEntry *next);
 60 
 61   // Query
 62   int target_bci() const                      { return _target_bci; }
 63   int nof_jsrs() const                        { return _jsrs->length(); }
 64   int jsrs(int i) const                       { assert(i>=0 && i<nof_jsrs(), "Index out of bounds"); return _jsrs->at(i); }
 65 
 66   // Update entry
 67   void add_jsr    (int return_bci)            { _jsrs->append(return_bci); }
 68   void add_delta  (int bci, int delta);
 69   RetTableEntry * next()  const               { return _next; }
 70 };
 71 
 72 
 73 class RetTable {
 74  private:
 75   RetTableEntry *_first;
 76   static int _init_nof_entries;
 77 
 78   void add_jsr(int return_bci, int target_bci);   // Adds entry to list
 79  public:
 80   RetTable()                                                  { _first = NULL; }
 81   void compute_ret_table(const methodHandle& method);
 82   void update_ret_table(int bci, int delta);
 83   RetTableEntry* find_jsrs_for_target(int targBci);
 84 };
 85 
 86 //
 87 // CellTypeState
 88 //
 89 class CellTypeState {
 90  private:
 91   unsigned int _state;
 92 
 93   // Masks for separating the BITS and INFO portions of a CellTypeState
 94   enum { info_mask            = right_n_bits(27),
 95          bits_mask            = (int)(~info_mask) };
 96 
 97   // These constant are used for manipulating the BITS portion of a
 98   // CellTypeState
 99   enum { uninit_bit           = (int)(nth_bit(31)),
100          ref_bit              = nth_bit(30),
101          val_bit              = nth_bit(29),
102          addr_bit             = nth_bit(28),
103          live_bits_mask       = (int)(bits_mask & ~uninit_bit) };
104 
105   // These constants are used for manipulating the INFO portion of a
106   // CellTypeState
107   enum { top_info_bit         = nth_bit(26),
108          not_bottom_info_bit  = nth_bit(25),
109          info_data_mask       = right_n_bits(25),
110          info_conflict        = info_mask };
111 
112   // Within the INFO data, these values are used to distinguish different
113   // kinds of references.
114   enum { ref_not_lock_bit     = nth_bit(24),  // 0 if this reference is locked as a monitor
115          ref_slot_bit         = nth_bit(23),  // 1 if this reference is a "slot" reference,
116                                               // 0 if it is a "line" reference.
117          ref_data_mask        = right_n_bits(23) };
118 
119   // Within the INFO data, these values are used to distinguish different
120   // kinds of value types.
121   enum { valuetype_slot_bit   = nth_bit(24),  // 1 if this reference is a "slot" value type,
122     // 0 if it is a "line" value type.
123     valuetype_data_mask  = right_n_bits(24) };
124 
125   // These values are used to initialize commonly used CellTypeState
126   // constants.
127   enum { bottom_value         = 0,
128          uninit_value         = (int)(uninit_bit | info_conflict),
129          ref_value            = ref_bit,
130          ref_conflict         = ref_bit | info_conflict,
131          val_value            = val_bit | info_conflict,
132          addr_value           = addr_bit,
133          addr_conflict        = addr_bit | info_conflict };
134 
135  public:
136 
137   // Since some C++ constructors generate poor code for declarations of the
138   // form...
139   //
140   //   CellTypeState vector[length];
141   //
142   // ...we avoid making a constructor for this class.  CellTypeState values
143   // should be constructed using one of the make_* methods:
144 
145   static CellTypeState make_any(int state) {
146     CellTypeState s;
147     s._state = state;
148     // Causes SS10 warning.
149     // assert(s.is_valid_state(), "check to see if CellTypeState is valid");
150     return s;
151   }
152 
153   static CellTypeState make_bottom() {
154     return make_any(0);
155   }
156 
157   static CellTypeState make_top() {
158     return make_any(AllBits);
159   }
160 
161   static CellTypeState make_addr(int bci) {
162     assert((bci >= 0) && (bci < info_data_mask), "check to see if ret addr is valid");
163     return make_any(addr_bit | not_bottom_info_bit | (bci & info_data_mask));
164   }
165 
166   static CellTypeState make_slot_ref(int slot_num) {
167     assert(slot_num >= 0 && slot_num < ref_data_mask, "slot out of range");
168     return make_any(ref_bit | not_bottom_info_bit | ref_not_lock_bit | ref_slot_bit |
169                     (slot_num & ref_data_mask));
170   }
171 
172   static CellTypeState make_line_ref(int bci) {
173     assert(bci >= 0 && bci < ref_data_mask, "line out of range");
174     return make_any(ref_bit | not_bottom_info_bit | ref_not_lock_bit |
175                     (bci & ref_data_mask));
176   }
177 
178   static CellTypeState make_lock_ref(int bci) {
179     assert(bci >= 0 && bci < ref_data_mask, "line out of range");
180     return make_any(ref_bit | not_bottom_info_bit | (bci & ref_data_mask));
181   }
182 
183   // Query methods:
184   bool is_bottom() const                { return _state == 0; }
185   bool is_live() const                  { return ((_state & live_bits_mask) != 0); }
186   bool is_valid_state() const {
187     // Uninitialized and value cells must contain no data in their info field:
188     if ((can_be_uninit() || can_be_value()) && !is_info_top()) {
189       return false;
190     }
191     // The top bit is only set when all info bits are set:
192     if (is_info_top() && ((_state & info_mask) != info_mask)) {
193       return false;
194     }
195     // The not_bottom_bit must be set when any other info bit is set:
196     if (is_info_bottom() && ((_state & info_mask) != 0)) {
197       return false;
198     }
199     return true;
200   }
201 
202   bool is_address() const               { return ((_state & bits_mask) == addr_bit); }
203   bool is_reference() const             { return ((_state & bits_mask) == ref_bit); }
204   bool is_inline_type() const           { return ((_state & bits_mask) == val_bit); }
205   bool is_uninit() const                { return ((_state & bits_mask) == (uint)uninit_bit); }
206 
207   bool can_be_address() const           { return ((_state & addr_bit) != 0); }
208   bool can_be_reference() const         { return ((_state & ref_bit) != 0); }
209   bool can_be_value() const             { return ((_state & val_bit) != 0); }
210   bool can_be_uninit() const            { return ((_state & uninit_bit) != 0); }
211 
212   bool is_info_bottom() const           { return ((_state & not_bottom_info_bit) == 0); }
213   bool is_info_top() const              { return ((_state & top_info_bit) != 0); }
214   int  get_info() const {
215     assert((!is_info_top() && !is_info_bottom()),
216            "check to make sure top/bottom info is not used");
217     return (_state & info_data_mask);
218   }
219 
220   bool is_good_address() const          { return is_address() && !is_info_top(); }
221   bool is_lock_reference() const {
222     return ((_state & (bits_mask | top_info_bit | ref_not_lock_bit)) == ref_bit);
223   }
224   bool is_nonlock_reference() const {
225     return ((_state & (bits_mask | top_info_bit | ref_not_lock_bit)) == (ref_bit | ref_not_lock_bit));
226   }
227 
228   bool equal(CellTypeState a) const     { return _state == a._state; }
229   bool equal_kind(CellTypeState a) const {
230     return (_state & bits_mask) == (a._state & bits_mask);
231   }
232 
233   char to_char() const;
234 
235   // Merge
236   CellTypeState merge (CellTypeState cts, int slot) const;
237 
238   // Debugging output
239   void print(outputStream *os);
240 
241   // Default values of common values
242   static CellTypeState bottom;
243   static CellTypeState uninit;
244   static CellTypeState ref;
245   static CellTypeState value;
246   static CellTypeState refUninit;
247   static CellTypeState varUninit;
248   static CellTypeState top;
249   static CellTypeState addr;
250 };
251 
252 
253 //
254 // BasicBlockStruct
255 //
256 class BasicBlock: ResourceObj {
257  private:
258   bool            _changed;                 // Reached a fixpoint or not
259  public:
260   enum Constants {
261     _dead_basic_block = -2,
262     _unreached        = -1                  // Alive but not yet reached by analysis
263     // >=0                                  // Alive and has a merged state
264   };
265 
266   int             _bci;                     // Start of basic block
267   int             _end_bci;                 // Bci of last instruction in basicblock
268   int             _max_locals;              // Determines split between vars and stack
269   int             _max_stack;               // Determines split between stack and monitors
270   CellTypeState*  _state;                   // State (vars, stack) at entry.
271   int             _stack_top;               // -1 indicates bottom stack value.
272   int             _monitor_top;             // -1 indicates bottom monitor stack value.
273 
274   CellTypeState* vars()                     { return _state; }
275   CellTypeState* stack()                    { return _state + _max_locals; }
276 
277   bool changed()                            { return _changed; }
278   void set_changed(bool s)                  { _changed = s; }
279 
280   bool is_reachable() const                 { return _stack_top >= 0; }  // Analysis has reached this basicblock
281 
282   // All basicblocks that are unreachable are going to have a _stack_top == _dead_basic_block.
283   // This info. is setup in a pre-parse before the real abstract interpretation starts.
284   bool is_dead() const                      { return _stack_top == _dead_basic_block; }
285   bool is_alive() const                     { return _stack_top != _dead_basic_block; }
286   void mark_as_alive()                      { assert(is_dead(), "must be dead"); _stack_top = _unreached; }
287 };
288 
289 
290 //
291 //  GenerateOopMap
292 //
293 // Main class used to compute the pointer-maps in a Method
294 //
295 class GenerateOopMap {
296  protected:
297 
298   // _monitor_top is set to this constant to indicate that a monitor matching
299   // problem was encountered prior to this point in control flow.
300   enum { bad_monitors = -1 };
301 
302   // Main variables
303   methodHandle _method;                     // The method we are examine
304   RetTable     _rt;                         // Contains the return address mappings
305   int          _max_locals;                 // Cached value of no. of locals
306   int          _max_stack;                  // Cached value of max. stack depth
307   int          _max_monitors;               // Cached value of max. monitor stack depth
308   int          _has_exceptions;             // True, if exceptions exist for method
309   bool         _got_error;                  // True, if an error occurred during interpretation.
310   Handle       _exception;                  // Exception if got_error is true.
311   bool         _did_rewriting;              // was bytecodes rewritten
312   bool         _did_relocation;             // was relocation neccessary
313   bool         _monitor_safe;               // The monitors in this method have been determined
314                                             // to be safe.
315 
316   // Working Cell type state
317   int            _state_len;                // Size of states
318   CellTypeState *_state;                    // list of states
319   char          *_state_vec_buf;            // Buffer used to print a readable version of a state
320   int            _stack_top;
321   int            _monitor_top;
322 
323   // Timing and statistics
324   static elapsedTimer _total_oopmap_time;   // Holds cumulative oopmap generation time
325   static uint64_t     _total_byte_count;    // Holds cumulative number of bytes inspected
326 
327   // Cell type methods
328   void            init_state();
329   void            make_context_uninitialized ();
330   int             methodsig_to_effect        (Symbol* signature, bool isStatic, CellTypeState* effect);
331   bool            merge_local_state_vectors  (CellTypeState* cts, CellTypeState* bbts);
332   bool            merge_monitor_state_vectors(CellTypeState* cts, CellTypeState* bbts);
333   void            copy_state                 (CellTypeState *dst, CellTypeState *src);
334   void            merge_state_into_bb        (BasicBlock *bb);
335   static void     merge_state                (GenerateOopMap *gom, int bcidelta, int* data);
336   void            set_var                    (int localNo, CellTypeState cts);
337   CellTypeState   get_var                    (int localNo);
338   CellTypeState   pop                        ();
339   void            push                       (CellTypeState cts);
340   CellTypeState   monitor_pop                ();
341   void            monitor_push               (CellTypeState cts);
342   CellTypeState * vars                       ()                                             { return _state; }
343   CellTypeState * stack                      ()                                             { return _state+_max_locals; }
344   CellTypeState * monitors                   ()                                             { return _state+_max_locals+_max_stack; }
345 
346   void            replace_all_CTS_matches    (CellTypeState match,
347                                               CellTypeState replace);
348   void            print_states               (outputStream *os, CellTypeState *vector, int num);
349   void            print_current_state        (outputStream   *os,
350                                               BytecodeStream *itr,
351                                               bool            detailed);
352   void            report_monitor_mismatch    (const char *msg);
353 
354   // Basicblock info
355   BasicBlock *    _basic_blocks;             // Array of basicblock info
356   int             _gc_points;
357   int             _bb_count;
358   ResourceBitMap  _bb_hdr_bits;
359 
360   // Basicblocks methods
361   void          initialize_bb               ();
362   void          mark_bbheaders_and_count_gc_points();
363   bool          is_bb_header                (int bci) const   {
364     return _bb_hdr_bits.at(bci);
365   }
366   int           gc_points                   () const                          { return _gc_points; }
367   int           bb_count                    () const                          { return _bb_count; }
368   void          set_bbmark_bit              (int bci);
369   BasicBlock *  get_basic_block_at          (int bci) const;
370   BasicBlock *  get_basic_block_containing  (int bci) const;
371   void          interp_bb                   (BasicBlock *bb);
372   void          restore_state               (BasicBlock *bb);
373   int           next_bb_start_pc            (BasicBlock *bb);
374   void          update_basic_blocks         (int bci, int delta, int new_method_size);
375   static void   bb_mark_fct                 (GenerateOopMap *c, int deltaBci, int *data);
376 
377   // Dead code detection
378   void          mark_reachable_code();
379   static void   reachable_basicblock        (GenerateOopMap *c, int deltaBci, int *data);
380 
381   // Interpretation methods (primary)
382   void  do_interpretation                   ();
383   void  init_basic_blocks                   ();
384   void  setup_method_entry_state            ();
385   void  interp_all                          ();
386 
387   // Interpretation methods (secondary)
388   void  interp1                             (BytecodeStream *itr);
389   void  do_exception_edge                   (BytecodeStream *itr);
390   void  check_type                          (CellTypeState expected, CellTypeState actual);
391   void  ppstore                             (CellTypeState *in,  int loc_no);
392   void  ppload                              (CellTypeState *out, int loc_no);
393   void  ppush1                              (CellTypeState in);
394   void  ppush                               (CellTypeState *in);
395   void  ppop1                               (CellTypeState out);
396   void  ppop                                (CellTypeState *out);
397   void  ppop_any                            (int poplen);
398   void  pp                                  (CellTypeState *in, CellTypeState *out);
399   void  pp_new_ref                          (CellTypeState *in, int bci);
400   void  ppdupswap                           (int poplen, const char *out);
401   void  do_ldc                              (int bci);
402   void  do_astore                           (int idx);
403   void  do_jsr                              (int delta);
404   void  do_field                            (int is_get, int is_static, int idx, int bci);
405   void  do_method                           (int is_static, int idx, int bci);
406   void  do_withfield                       (int idx, int bci);
407   void  do_multianewarray                   (int dims, int bci);
408   void  do_monitorenter                     (int bci);
409   void  do_monitorexit                      (int bci);
410   void  do_return_monitor_check             ();
411   void  do_checkcast                        ();
412   CellTypeState *signature_to_effect        (const Symbol* sig, int bci, CellTypeState *out);
413   int copy_cts                              (CellTypeState *dst, CellTypeState *src);
414 
415   // Error handling
416   void  error_work                          (const char *format, va_list ap) ATTRIBUTE_PRINTF(2, 0);
417   void  report_error                        (const char *format, ...) ATTRIBUTE_PRINTF(2, 3);
418   void  verify_error                        (const char *format, ...) ATTRIBUTE_PRINTF(2, 3);
419   bool  got_error()                         { return _got_error; }
420 
421   // Create result set
422   bool  _report_result;
423   bool  _report_result_for_send;            // Unfortunatly, stackmaps for sends are special, so we need some extra
424   BytecodeStream *_itr_send;                // variables to handle them properly.
425 
426   void  report_result                       ();
427 
428   // Initvars
429   GrowableArray<intptr_t> * _init_vars;
430 
431   void  initialize_vars                     ();
432   void  add_to_ref_init_set                 (int localNo);
433 
434   // Conflicts rewrite logic
435   bool      _conflict;                      // True, if a conflict occurred during interpretation
436   int       _nof_refval_conflicts;          // No. of conflicts that require rewrites
437   int *     _new_var_map;
438 
439   void record_refval_conflict               (int varNo);
440   void rewrite_refval_conflicts             ();
441   void rewrite_refval_conflict              (int from, int to);
442   bool rewrite_refval_conflict_inst         (BytecodeStream *i, int from, int to);
443   bool rewrite_load_or_store                (BytecodeStream *i, Bytecodes::Code bc, Bytecodes::Code bc0, unsigned int varNo);
444 
445   void expand_current_instr                 (int bci, int ilen, int newIlen, u_char inst_buffer[]);
446   bool is_astore                            (BytecodeStream *itr, int *index);
447   bool is_aload                             (BytecodeStream *itr, int *index);
448 
449   // List of bci's where a return address is on top of the stack
450   GrowableArray<intptr_t> *_ret_adr_tos;
451 
452   bool stack_top_holds_ret_addr             (int bci);
453   void compute_ret_adr_at_TOS               ();
454   void update_ret_adr_at_TOS                (int bci, int delta);
455 
456   int  binsToHold                           (int no)                      { return  ((no+(BitsPerWord-1))/BitsPerWord); }
457   char *state_vec_to_string                 (CellTypeState* vec, int len);
458 
459   // Helper method. Can be used in subclasses to fx. calculate gc_points. If the current instuction
460   // is a control transfer, then calls the jmpFct all possible destinations.
461   void  ret_jump_targets_do                 (BytecodeStream *bcs, jmpFct_t jmpFct, int varNo,int *data);
462   bool  jump_targets_do                     (BytecodeStream *bcs, jmpFct_t jmpFct, int *data);
463 
464   friend class RelocCallback;
465  public:
466   GenerateOopMap(const methodHandle& method);
467 
468   // Compute the map - returns true on success and false on error.
469   bool compute_map(Thread* current);
470   // Returns the exception related to any error, if the map was computed by a suitable JavaThread.
471   Handle exception() { return _exception; }
472 
473   void result_for_basicblock(int bci);    // Do a callback on fill_stackmap_for_opcodes for basicblock containing bci
474 
475   // Query
476   int max_locals() const                           { return _max_locals; }
477   Method* method() const                           { return _method(); }
478   methodHandle method_as_handle() const            { return _method; }
479 
480   bool did_rewriting()                             { return _did_rewriting; }
481   bool did_relocation()                            { return _did_relocation; }
482 
483   static void print_time();
484 
485   // Monitor query
486   bool monitor_safe()                              { return _monitor_safe; }
487 
488   // Specialization methods. Intended use:
489   // - possible_gc_point must return true for every bci for which the stackmaps must be returned
490   // - fill_stackmap_prolog is called just before the result is reported. The arguments tells the estimated
491   //   number of gc points
492   // - fill_stackmap_for_opcodes is called once for each bytecode index in order (0...code_length-1)
493   // - fill_stackmap_epilog is called after all results has been reported. Note: Since the algorithm does not report
494   //   stackmaps for deadcode, fewer gc_points might have been encounted than assumed during the epilog. It is the
495   //   responsibility of the subclass to count the correct number.
496   // - fill_init_vars are called once with the result of the init_vars computation
497   //
498   // All these methods are used during a call to: compute_map. Note: Non of the return results are valid
499   // after compute_map returns, since all values are allocated as resource objects.
500   //
501   // All virtual method must be implemented in subclasses
502   virtual bool allow_rewrites             () const                        { return false; }
503   virtual bool report_results             () const                        { return true;  }
504   virtual bool report_init_vars           () const                        { return true;  }
505   virtual bool possible_gc_point          (BytecodeStream *bcs)           { ShouldNotReachHere(); return false; }
506   virtual void fill_stackmap_prolog       (int nof_gc_points)             { ShouldNotReachHere(); }
507   virtual void fill_stackmap_epilog       ()                              { ShouldNotReachHere(); }
508   virtual void fill_stackmap_for_opcodes  (BytecodeStream *bcs,
509                                            CellTypeState* vars,
510                                            CellTypeState* stack,
511                                            int stackTop)                  { ShouldNotReachHere(); }
512   virtual void fill_init_vars             (GrowableArray<intptr_t> *init_vars) { ShouldNotReachHere();; }
513 };
514 
515 //
516 // Subclass of the GenerateOopMap Class that just do rewrites of the method, if needed.
517 // It does not store any oopmaps.
518 //
519 class ResolveOopMapConflicts: public GenerateOopMap {
520  private:
521 
522   bool _must_clear_locals;
523 
524   virtual bool report_results() const     { return false; }
525   virtual bool report_init_vars() const   { return true;  }
526   virtual bool allow_rewrites() const     { return true;  }
527   virtual bool possible_gc_point          (BytecodeStream *bcs)           { return false; }
528   virtual void fill_stackmap_prolog       (int nof_gc_points)             {}
529   virtual void fill_stackmap_epilog       ()                              {}
530   virtual void fill_stackmap_for_opcodes  (BytecodeStream *bcs,
531                                            CellTypeState* vars,
532                                            CellTypeState* stack,
533                                            int stack_top)                 {}
534   virtual void fill_init_vars             (GrowableArray<intptr_t> *init_vars) { _must_clear_locals = init_vars->length() > 0; }
535 
536 #ifndef PRODUCT
537   // Statistics
538   static int _nof_invocations;
539   static int _nof_rewrites;
540   static int _nof_relocations;
541 #endif
542 
543  public:
544   ResolveOopMapConflicts(const methodHandle& method) : GenerateOopMap(method) { _must_clear_locals = false; };
545 
546   methodHandle do_potential_rewrite(TRAPS);
547   bool must_clear_locals() const { return _must_clear_locals; }
548 };
549 
550 
551 //
552 // Subclass used by the compiler to generate pairing infomation
553 //
554 class GeneratePairingInfo: public GenerateOopMap {
555  private:
556 
557   virtual bool report_results() const     { return false; }
558   virtual bool report_init_vars() const   { return false; }
559   virtual bool allow_rewrites() const     { return false;  }
560   virtual bool possible_gc_point          (BytecodeStream *bcs)           { return false; }
561   virtual void fill_stackmap_prolog       (int nof_gc_points)             {}
562   virtual void fill_stackmap_epilog       ()                              {}
563   virtual void fill_stackmap_for_opcodes  (BytecodeStream *bcs,
564                                            CellTypeState* vars,
565                                            CellTypeState* stack,
566                                            int stack_top)                 {}
567   virtual void fill_init_vars             (GrowableArray<intptr_t> *init_vars) {}
568  public:
569   GeneratePairingInfo(const methodHandle& method) : GenerateOopMap(method)       {};
570 
571   // Call compute_map() to generate info.
572 };
573 
574 #endif // SHARE_OOPS_GENERATEOOPMAP_HPP