1 /*
  2  * Copyright (c) 2002, 2018, 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
 21  * questions.
 22  *
 23  */
 24 
 25 #include "precompiled.hpp"
 26 #include "code/vmreg.inline.hpp"
 27 #include "compiler/oopMap.hpp"
 28 #include "memory/resourceArea.hpp"
 29 #include "opto/addnode.hpp"
 30 #include "opto/callnode.hpp"
 31 #include "opto/compile.hpp"
 32 #include "opto/machnode.hpp"
 33 #include "opto/matcher.hpp"
 34 #include "opto/output.hpp"
 35 #include "opto/phase.hpp"
 36 #include "opto/regalloc.hpp"
 37 #include "opto/rootnode.hpp"
 38 #include "utilities/align.hpp"
 39 
 40 // The functions in this file builds OopMaps after all scheduling is done.
 41 //
 42 // OopMaps contain a list of all registers and stack-slots containing oops (so
 43 // they can be updated by GC).  OopMaps also contain a list of derived-pointer
 44 // base-pointer pairs.  When the base is moved, the derived pointer moves to
 45 // follow it.  Finally, any registers holding callee-save values are also
 46 // recorded.  These might contain oops, but only the caller knows.
 47 //
 48 // BuildOopMaps implements a simple forward reaching-defs solution.  At each
 49 // GC point we'll have the reaching-def Nodes.  If the reaching Nodes are
 50 // typed as pointers (no offset), then they are oops.  Pointers+offsets are
 51 // derived pointers, and bases can be found from them.  Finally, we'll also
 52 // track reaching callee-save values.  Note that a copy of a callee-save value
 53 // "kills" it's source, so that only 1 copy of a callee-save value is alive at
 54 // a time.
 55 //
 56 // We run a simple bitvector liveness pass to help trim out dead oops.  Due to
 57 // irreducible loops, we can have a reaching def of an oop that only reaches
 58 // along one path and no way to know if it's valid or not on the other path.
 59 // The bitvectors are quite dense and the liveness pass is fast.
 60 //
 61 // At GC points, we consult this information to build OopMaps.  All reaching
 62 // defs typed as oops are added to the OopMap.  Only 1 instance of a
 63 // callee-save register can be recorded.  For derived pointers, we'll have to
 64 // find and record the register holding the base.
 65 //
 66 // The reaching def's is a simple 1-pass worklist approach.  I tried a clever
 67 // breadth-first approach but it was worse (showed O(n^2) in the
 68 // pick-next-block code).
 69 //
 70 // The relevant data is kept in a struct of arrays (it could just as well be
 71 // an array of structs, but the struct-of-arrays is generally a little more
 72 // efficient).  The arrays are indexed by register number (including
 73 // stack-slots as registers) and so is bounded by 200 to 300 elements in
 74 // practice.  One array will map to a reaching def Node (or NULL for
 75 // conflict/dead).  The other array will map to a callee-saved register or
 76 // OptoReg::Bad for not-callee-saved.
 77 
 78 
 79 // Structure to pass around
 80 struct OopFlow : public ResourceObj {
 81   short *_callees;              // Array mapping register to callee-saved
 82   Node **_defs;                 // array mapping register to reaching def
 83                                 // or NULL if dead/conflict
 84   // OopFlow structs, when not being actively modified, describe the _end_ of
 85   // this block.
 86   Block *_b;                    // Block for this struct
 87   OopFlow *_next;               // Next free OopFlow
 88                                 // or NULL if dead/conflict
 89   Compile* C;
 90 
 91   OopFlow( short *callees, Node **defs, Compile* c ) : _callees(callees), _defs(defs),
 92     _b(NULL), _next(NULL), C(c) { }
 93 
 94   // Given reaching-defs for this block start, compute it for this block end
 95   void compute_reach( PhaseRegAlloc *regalloc, int max_reg, Dict *safehash );
 96 
 97   // Merge these two OopFlows into the 'this' pointer.
 98   void merge( OopFlow *flow, int max_reg );
 99 
100   // Copy a 'flow' over an existing flow
101   void clone( OopFlow *flow, int max_size);
102 
103   // Make a new OopFlow from scratch
104   static OopFlow *make( Arena *A, int max_size, Compile* C );
105 
106   // Build an oopmap from the current flow info
107   OopMap *build_oop_map( Node *n, int max_reg, PhaseRegAlloc *regalloc, int* live );
108 };
109 
110 // Given reaching-defs for this block start, compute it for this block end
111 void OopFlow::compute_reach( PhaseRegAlloc *regalloc, int max_reg, Dict *safehash ) {
112 
113   for( uint i=0; i<_b->number_of_nodes(); i++ ) {
114     Node *n = _b->get_node(i);
115 
116     if( n->jvms() ) {           // Build an OopMap here?
117       JVMState *jvms = n->jvms();
118       // no map needed for leaf calls
119       if( n->is_MachSafePoint() && !n->is_MachCallLeaf() ) {
120         int *live = (int*) (*safehash)[n];
121         assert( live, "must find live" );
122         n->as_MachSafePoint()->set_oop_map( build_oop_map(n,max_reg,regalloc, live) );
123       }
124     }
125 
126     // Assign new reaching def's.
127     // Note that I padded the _defs and _callees arrays so it's legal
128     // to index at _defs[OptoReg::Bad].
129     OptoReg::Name first = regalloc->get_reg_first(n);
130     OptoReg::Name second = regalloc->get_reg_second(n);
131     _defs[first] = n;
132     _defs[second] = n;
133 
134     // Pass callee-save info around copies
135     int idx = n->is_Copy();
136     if( idx ) {                 // Copies move callee-save info
137       OptoReg::Name old_first = regalloc->get_reg_first(n->in(idx));
138       OptoReg::Name old_second = regalloc->get_reg_second(n->in(idx));
139       int tmp_first = _callees[old_first];
140       int tmp_second = _callees[old_second];
141       _callees[old_first] = OptoReg::Bad; // callee-save is moved, dead in old location
142       _callees[old_second] = OptoReg::Bad;
143       _callees[first] = tmp_first;
144       _callees[second] = tmp_second;
145     } else if( n->is_Phi() ) {  // Phis do not mod callee-saves
146       assert( _callees[first] == _callees[regalloc->get_reg_first(n->in(1))], "" );
147       assert( _callees[second] == _callees[regalloc->get_reg_second(n->in(1))], "" );
148       assert( _callees[first] == _callees[regalloc->get_reg_first(n->in(n->req()-1))], "" );
149       assert( _callees[second] == _callees[regalloc->get_reg_second(n->in(n->req()-1))], "" );
150     } else {
151       _callees[first] = OptoReg::Bad; // No longer holding a callee-save value
152       _callees[second] = OptoReg::Bad;
153 
154       // Find base case for callee saves
155       if( n->is_Proj() && n->in(0)->is_Start() ) {
156         if( OptoReg::is_reg(first) &&
157             regalloc->_matcher.is_save_on_entry(first) )
158           _callees[first] = first;
159         if( OptoReg::is_reg(second) &&
160             regalloc->_matcher.is_save_on_entry(second) )
161           _callees[second] = second;
162       }
163     }
164   }
165 }
166 
167 // Merge the given flow into the 'this' flow
168 void OopFlow::merge( OopFlow *flow, int max_reg ) {
169   assert( _b == NULL, "merging into a happy flow" );
170   assert( flow->_b, "this flow is still alive" );
171   assert( flow != this, "no self flow" );
172 
173   // Do the merge.  If there are any differences, drop to 'bottom' which
174   // is OptoReg::Bad or NULL depending.
175   for( int i=0; i<max_reg; i++ ) {
176     // Merge the callee-save's
177     if( _callees[i] != flow->_callees[i] )
178       _callees[i] = OptoReg::Bad;
179     // Merge the reaching defs
180     if( _defs[i] != flow->_defs[i] )
181       _defs[i] = NULL;
182   }
183 
184 }
185 
186 void OopFlow::clone( OopFlow *flow, int max_size ) {
187   _b = flow->_b;
188   memcpy( _callees, flow->_callees, sizeof(short)*max_size);
189   memcpy( _defs   , flow->_defs   , sizeof(Node*)*max_size);
190 }
191 
192 OopFlow *OopFlow::make( Arena *A, int max_size, Compile* C ) {
193   short *callees = NEW_ARENA_ARRAY(A,short,max_size+1);
194   Node **defs    = NEW_ARENA_ARRAY(A,Node*,max_size+1);
195   debug_only( memset(defs,0,(max_size+1)*sizeof(Node*)) );
196   OopFlow *flow = new (A) OopFlow(callees+1, defs+1, C);
197   assert( &flow->_callees[OptoReg::Bad] == callees, "Ok to index at OptoReg::Bad" );
198   assert( &flow->_defs   [OptoReg::Bad] == defs   , "Ok to index at OptoReg::Bad" );
199   return flow;
200 }
201 
202 static int get_live_bit( int *live, int reg ) {
203   return live[reg>>LogBitsPerInt] &   (1<<(reg&(BitsPerInt-1))); }
204 static void set_live_bit( int *live, int reg ) {
205          live[reg>>LogBitsPerInt] |=  (1<<(reg&(BitsPerInt-1))); }
206 static void clr_live_bit( int *live, int reg ) {
207          live[reg>>LogBitsPerInt] &= ~(1<<(reg&(BitsPerInt-1))); }
208 
209 // Build an oopmap from the current flow info
210 OopMap *OopFlow::build_oop_map( Node *n, int max_reg, PhaseRegAlloc *regalloc, int* live ) {
211   int framesize = regalloc->_framesize;
212   int max_inarg_slot = OptoReg::reg2stack(regalloc->_matcher._new_SP);
213   debug_only( char *dup_check = NEW_RESOURCE_ARRAY(char,OptoReg::stack0());
214               memset(dup_check,0,OptoReg::stack0()) );
215 
216   OopMap *omap = new OopMap( framesize,  max_inarg_slot );
217   MachCallNode *mcall = n->is_MachCall() ? n->as_MachCall() : NULL;
218   JVMState* jvms = n->jvms();
219 
220   // For all registers do...
221   for( int reg=0; reg<max_reg; reg++ ) {
222     if( get_live_bit(live,reg) == 0 )
223       continue;                 // Ignore if not live
224 
225     // %%% C2 can use 2 OptoRegs when the physical register is only one 64bit
226     // register in that case we'll get an non-concrete register for the second
227     // half. We only need to tell the map the register once!
228     //
229     // However for the moment we disable this change and leave things as they
230     // were.
231 
232     VMReg r = OptoReg::as_VMReg(OptoReg::Name(reg), framesize, max_inarg_slot);
233 
234     if (false && r->is_reg() && !r->is_concrete()) {
235       continue;
236     }
237 
238     // See if dead (no reaching def).
239     Node *def = _defs[reg];     // Get reaching def
240     assert( def, "since live better have reaching def" );
241 
242     // Classify the reaching def as oop, derived, callee-save, dead, or other
243     const Type *t = def->bottom_type();
244     if( t->isa_oop_ptr() ) {    // Oop or derived?
245       assert( !OptoReg::is_valid(_callees[reg]), "oop can't be callee save" );
246 #ifdef _LP64
247       // 64-bit pointers record oop-ishness on 2 aligned adjacent registers.
248       // Make sure both are record from the same reaching def, but do not
249       // put both into the oopmap.
250       if( (reg&1) == 1 ) {      // High half of oop-pair?
251         assert( _defs[reg-1] == _defs[reg], "both halves from same reaching def" );
252         continue;               // Do not record high parts in oopmap
253       }
254 #endif
255 
256       // Check for a legal reg name in the oopMap and bailout if it is not.
257       if (!omap->legal_vm_reg_name(r)) {
258         regalloc->C->record_method_not_compilable("illegal oopMap register name");
259         continue;
260       }
261       if( t->is_ptr()->_offset == 0 ) { // Not derived?
262         if( mcall ) {
263           // Outgoing argument GC mask responsibility belongs to the callee,
264           // not the caller.  Inspect the inputs to the call, to see if
265           // this live-range is one of them.
266           uint cnt = mcall->tf()->domain()->cnt();
267           uint j;
268           for( j = TypeFunc::Parms; j < cnt; j++)
269             if( mcall->in(j) == def )
270               break;            // reaching def is an argument oop
271           if( j < cnt )         // arg oops dont go in GC map
272             continue;           // Continue on to the next register
273         }
274         omap->set_oop(r);
275       } else {                  // Else it's derived.
276         // Find the base of the derived value.
277         uint i;
278         // Fast, common case, scan
279         for( i = jvms->oopoff(); i < n->req(); i+=2 )
280           if( n->in(i) == def ) break; // Common case
281         if( i == n->req() ) {   // Missed, try a more generous scan
282           // Scan again, but this time peek through copies
283           for( i = jvms->oopoff(); i < n->req(); i+=2 ) {
284             Node *m = n->in(i); // Get initial derived value
285             while( 1 ) {
286               Node *d = def;    // Get initial reaching def
287               while( 1 ) {      // Follow copies of reaching def to end
288                 if( m == d ) goto found; // breaks 3 loops
289                 int idx = d->is_Copy();
290                 if( !idx ) break;
291                 d = d->in(idx);     // Link through copy
292               }
293               int idx = m->is_Copy();
294               if( !idx ) break;
295               m = m->in(idx);
296             }
297           }
298           guarantee( 0, "must find derived/base pair" );
299         }
300       found: ;
301         Node *base = n->in(i+1); // Base is other half of pair
302         int breg = regalloc->get_reg_first(base);
303         VMReg b = OptoReg::as_VMReg(OptoReg::Name(breg), framesize, max_inarg_slot);
304 
305         // I record liveness at safepoints BEFORE I make the inputs
306         // live.  This is because argument oops are NOT live at a
307         // safepoint (or at least they cannot appear in the oopmap).
308         // Thus bases of base/derived pairs might not be in the
309         // liveness data but they need to appear in the oopmap.
310         if( get_live_bit(live,breg) == 0 ) {// Not live?
311           // Flag it, so next derived pointer won't re-insert into oopmap
312           set_live_bit(live,breg);
313           // Already missed our turn?
314           if( breg < reg ) {
315             if (b->is_stack() || b->is_concrete() || true ) {
316               omap->set_oop( b);
317             }
318           }
319         }
320         if (b->is_stack() || b->is_concrete() || true ) {
321           omap->set_derived_oop( r, b);
322         }
323       }
324 
325     } else if( t->isa_narrowoop() ) {
326       assert( !OptoReg::is_valid(_callees[reg]), "oop can't be callee save" );
327       // Check for a legal reg name in the oopMap and bailout if it is not.
328       if (!omap->legal_vm_reg_name(r)) {
329         regalloc->C->record_method_not_compilable("illegal oopMap register name");
330         continue;
331       }
332       if( mcall ) {
333           // Outgoing argument GC mask responsibility belongs to the callee,
334           // not the caller.  Inspect the inputs to the call, to see if
335           // this live-range is one of them.
336         uint cnt = mcall->tf()->domain()->cnt();
337         uint j;
338         for( j = TypeFunc::Parms; j < cnt; j++)
339           if( mcall->in(j) == def )
340             break;            // reaching def is an argument oop
341         if( j < cnt )         // arg oops dont go in GC map
342           continue;           // Continue on to the next register
343       }
344       omap->set_narrowoop(r);
345     } else if( OptoReg::is_valid(_callees[reg])) { // callee-save?
346       // It's a callee-save value
347       assert( dup_check[_callees[reg]]==0, "trying to callee save same reg twice" );
348       debug_only( dup_check[_callees[reg]]=1; )
349       VMReg callee = OptoReg::as_VMReg(OptoReg::Name(_callees[reg]));
350       if ( callee->is_concrete() || true ) {
351         omap->set_callee_saved( r, callee);
352       }
353 
354     } else {
355       // Other - some reaching non-oop value
356 #ifdef ASSERT
357       if( t->isa_rawptr() && C->cfg()->_raw_oops.member(def) ) {
358         def->dump();
359         n->dump();
360         assert(false, "there should be a oop in OopMap instead of a live raw oop at safepoint");
361       }
362 #endif
363     }
364 
365   }
366 
367 #ifdef ASSERT
368   /* Nice, Intel-only assert
369   int cnt_callee_saves=0;
370   int reg2 = 0;
371   while (OptoReg::is_reg(reg2)) {
372     if( dup_check[reg2] != 0) cnt_callee_saves++;
373     assert( cnt_callee_saves==3 || cnt_callee_saves==5, "missed some callee-save" );
374     reg2++;
375   }
376   */
377 #endif
378 
379 #ifdef ASSERT
380   for( OopMapStream oms1(omap); !oms1.is_done(); oms1.next()) {
381     OopMapValue omv1 = oms1.current();
382     if (omv1.type() != OopMapValue::derived_oop_value) {
383       continue;
384     }
385     bool found = false;
386     for( OopMapStream oms2(omap); !oms2.is_done(); oms2.next()) {
387       OopMapValue omv2 = oms2.current();
388       if (omv2.type() != OopMapValue::oop_value) {
389         continue;
390       }
391       if( omv1.content_reg() == omv2.reg() ) {
392         found = true;
393         break;
394       }
395     }
396     assert( found, "derived with no base in oopmap" );
397   }
398 #endif
399 
400   return omap;
401 }
402 
403 // Compute backwards liveness on registers
404 static void do_liveness(PhaseRegAlloc* regalloc, PhaseCFG* cfg, Block_List* worklist, int max_reg_ints, Arena* A, Dict* safehash) {
405   int* live = NEW_ARENA_ARRAY(A, int, (cfg->number_of_blocks() + 1) * max_reg_ints);
406   int* tmp_live = &live[cfg->number_of_blocks() * max_reg_ints];
407   Node* root = cfg->get_root_node();
408   // On CISC platforms, get the node representing the stack pointer  that regalloc
409   // used for spills
410   Node *fp = NodeSentinel;
411   if (UseCISCSpill && root->req() > 1) {
412     fp = root->in(1)->in(TypeFunc::FramePtr);
413   }
414   memset(live, 0, cfg->number_of_blocks() * (max_reg_ints << LogBytesPerInt));
415   // Push preds onto worklist
416   for (uint i = 1; i < root->req(); i++) {
417     Block* block = cfg->get_block_for_node(root->in(i));
418     worklist->push(block);
419   }
420 
421   // ZKM.jar includes tiny infinite loops which are unreached from below.
422   // If we missed any blocks, we'll retry here after pushing all missed
423   // blocks on the worklist.  Normally this outer loop never trips more
424   // than once.
425   while (1) {
426 
427     while( worklist->size() ) { // Standard worklist algorithm
428       Block *b = worklist->rpop();
429 
430       // Copy first successor into my tmp_live space
431       int s0num = b->_succs[0]->_pre_order;
432       int *t = &live[s0num*max_reg_ints];
433       for( int i=0; i<max_reg_ints; i++ )
434         tmp_live[i] = t[i];
435 
436       // OR in the remaining live registers
437       for( uint j=1; j<b->_num_succs; j++ ) {
438         uint sjnum = b->_succs[j]->_pre_order;
439         int *t = &live[sjnum*max_reg_ints];
440         for( int i=0; i<max_reg_ints; i++ )
441           tmp_live[i] |= t[i];
442       }
443 
444       // Now walk tmp_live up the block backwards, computing live
445       for( int k=b->number_of_nodes()-1; k>=0; k-- ) {
446         Node *n = b->get_node(k);
447         // KILL def'd bits
448         int first = regalloc->get_reg_first(n);
449         int second = regalloc->get_reg_second(n);
450         if( OptoReg::is_valid(first) ) clr_live_bit(tmp_live,first);
451         if( OptoReg::is_valid(second) ) clr_live_bit(tmp_live,second);
452 
453         MachNode *m = n->is_Mach() ? n->as_Mach() : NULL;
454 
455         // Check if m is potentially a CISC alternate instruction (i.e, possibly
456         // synthesized by RegAlloc from a conventional instruction and a
457         // spilled input)
458         bool is_cisc_alternate = false;
459         if (UseCISCSpill && m) {
460           is_cisc_alternate = m->is_cisc_alternate();
461         }
462 
463         // GEN use'd bits
464         for( uint l=1; l<n->req(); l++ ) {
465           Node *def = n->in(l);
466           assert(def != 0, "input edge required");
467           int first = regalloc->get_reg_first(def);
468           int second = regalloc->get_reg_second(def);
469           //If peephole had removed the node,do not set live bit for it.
470           if (!(def->is_Mach() && def->as_Mach()->get_removed())) {
471             if (OptoReg::is_valid(first)) set_live_bit(tmp_live,first);
472             if (OptoReg::is_valid(second)) set_live_bit(tmp_live,second);
473           }
474           // If we use the stack pointer in a cisc-alternative instruction,
475           // check for use as a memory operand.  Then reconstruct the RegName
476           // for this stack location, and set the appropriate bit in the
477           // live vector 4987749.
478           if (is_cisc_alternate && def == fp) {
479             const TypePtr *adr_type = NULL;
480             intptr_t offset;
481             const Node* base = m->get_base_and_disp(offset, adr_type);
482             if (base == NodeSentinel) {
483               // Machnode has multiple memory inputs. We are unable to reason
484               // with these, but are presuming (with trepidation) that not any of
485               // them are oops. This can be fixed by making get_base_and_disp()
486               // look at a specific input instead of all inputs.
487               assert(!def->bottom_type()->isa_oop_ptr(), "expecting non-oop mem input");
488             } else if (base != fp || offset == Type::OffsetBot) {
489               // Do nothing: the fp operand is either not from a memory use
490               // (base == NULL) OR the fp is used in a non-memory context
491               // (base is some other register) OR the offset is not constant,
492               // so it is not a stack slot.
493             } else {
494               assert(offset >= 0, "unexpected negative offset");
495               offset -= (offset % jintSize);  // count the whole word
496               int stack_reg = regalloc->offset2reg(offset);
497               if (OptoReg::is_stack(stack_reg)) {
498                 set_live_bit(tmp_live, stack_reg);
499               } else {
500                 assert(false, "stack_reg not on stack?");
501               }
502             }
503           }
504         }
505 
506         if( n->jvms() ) {       // Record liveness at safepoint
507 
508           // This placement of this stanza means inputs to calls are
509           // considered live at the callsite's OopMap.  Argument oops are
510           // hence live, but NOT included in the oopmap.  See cutout in
511           // build_oop_map.  Debug oops are live (and in OopMap).
512           int *n_live = NEW_ARENA_ARRAY(A, int, max_reg_ints);
513           for( int l=0; l<max_reg_ints; l++ )
514             n_live[l] = tmp_live[l];
515           safehash->Insert(n,n_live);
516         }
517 
518       }
519 
520       // Now at block top, see if we have any changes.  If so, propagate
521       // to prior blocks.
522       int *old_live = &live[b->_pre_order*max_reg_ints];
523       int l;
524       for( l=0; l<max_reg_ints; l++ )
525         if( tmp_live[l] != old_live[l] )
526           break;
527       if( l<max_reg_ints ) {     // Change!
528         // Copy in new value
529         for( l=0; l<max_reg_ints; l++ )
530           old_live[l] = tmp_live[l];
531         // Push preds onto worklist
532         for (l = 1; l < (int)b->num_preds(); l++) {
533           Block* block = cfg->get_block_for_node(b->pred(l));
534           worklist->push(block);
535         }
536       }
537     }
538 
539     // Scan for any missing safepoints.  Happens to infinite loops
540     // ala ZKM.jar
541     uint i;
542     for (i = 1; i < cfg->number_of_blocks(); i++) {
543       Block* block = cfg->get_block(i);
544       uint j;
545       for (j = 1; j < block->number_of_nodes(); j++) {
546         if (block->get_node(j)->jvms() && (*safehash)[block->get_node(j)] == NULL) {
547            break;
548         }
549       }
550       if (j < block->number_of_nodes()) {
551         break;
552       }
553     }
554     if (i == cfg->number_of_blocks()) {
555       break;                    // Got 'em all
556     }
557 
558     if (PrintOpto && Verbose) {
559       tty->print_cr("retripping live calc");
560     }
561 
562     // Force the issue (expensively): recheck everybody
563     for (i = 1; i < cfg->number_of_blocks(); i++) {
564       worklist->push(cfg->get_block(i));
565     }
566   }
567 }
568 
569 // Collect GC mask info - where are all the OOPs?
570 void PhaseOutput::BuildOopMaps() {
571   Compile::TracePhase tp("bldOopMaps", &timers[_t_buildOopMaps]);
572   // Can't resource-mark because I need to leave all those OopMaps around,
573   // or else I need to resource-mark some arena other than the default.
574   // ResourceMark rm;              // Reclaim all OopFlows when done
575   int max_reg = C->regalloc()->_max_reg; // Current array extent
576 
577   Arena *A = Thread::current()->resource_area();
578   Block_List worklist;          // Worklist of pending blocks
579 
580   int max_reg_ints = align_up(max_reg, BitsPerInt)>>LogBitsPerInt;
581   Dict *safehash = NULL;        // Used for assert only
582   // Compute a backwards liveness per register.  Needs a bitarray of
583   // #blocks x (#registers, rounded up to ints)
584   safehash = new Dict(cmpkey,hashkey,A);
585   do_liveness( C->regalloc(), C->cfg(), &worklist, max_reg_ints, A, safehash );
586   OopFlow *free_list = NULL;    // Free, unused
587 
588   // Array mapping blocks to completed oopflows
589   OopFlow **flows = NEW_ARENA_ARRAY(A, OopFlow*, C->cfg()->number_of_blocks());
590   memset( flows, 0, C->cfg()->number_of_blocks() * sizeof(OopFlow*) );
591 
592 
593   // Do the first block 'by hand' to prime the worklist
594   Block *entry = C->cfg()->get_block(1);
595   OopFlow *rootflow = OopFlow::make(A,max_reg,C);
596   // Initialize to 'bottom' (not 'top')
597   memset( rootflow->_callees, OptoReg::Bad, max_reg*sizeof(short) );
598   memset( rootflow->_defs   ,            0, max_reg*sizeof(Node*) );
599   flows[entry->_pre_order] = rootflow;
600 
601   // Do the first block 'by hand' to prime the worklist
602   rootflow->_b = entry;
603   rootflow->compute_reach( C->regalloc(), max_reg, safehash );
604   for( uint i=0; i<entry->_num_succs; i++ )
605     worklist.push(entry->_succs[i]);
606 
607   // Now worklist contains blocks which have some, but perhaps not all,
608   // predecessors visited.
609   while( worklist.size() ) {
610     // Scan for a block with all predecessors visited, or any randoms slob
611     // otherwise.  All-preds-visited order allows me to recycle OopFlow
612     // structures rapidly and cut down on the memory footprint.
613     // Note: not all predecessors might be visited yet (must happen for
614     // irreducible loops).  This is OK, since every live value must have the
615     // SAME reaching def for the block, so any reaching def is OK.
616     uint i;
617 
618     Block *b = worklist.pop();
619     // Ignore root block
620     if (b == C->cfg()->get_root_block()) {
621       continue;
622     }
623     // Block is already done?  Happens if block has several predecessors,
624     // he can get on the worklist more than once.
625     if( flows[b->_pre_order] ) continue;
626 
627     // If this block has a visited predecessor AND that predecessor has this
628     // last block as his only undone child, we can move the OopFlow from the
629     // pred to this block.  Otherwise we have to grab a new OopFlow.
630     OopFlow *flow = NULL;       // Flag for finding optimized flow
631     Block *pred = (Block*)((intptr_t)0xdeadbeef);
632     // Scan this block's preds to find a done predecessor
633     for (uint j = 1; j < b->num_preds(); j++) {
634       Block* p = C->cfg()->get_block_for_node(b->pred(j));
635       OopFlow *p_flow = flows[p->_pre_order];
636       if( p_flow ) {            // Predecessor is done
637         assert( p_flow->_b == p, "cross check" );
638         pred = p;               // Record some predecessor
639         // If all successors of p are done except for 'b', then we can carry
640         // p_flow forward to 'b' without copying, otherwise we have to draw
641         // from the free_list and clone data.
642         uint k;
643         for( k=0; k<p->_num_succs; k++ )
644           if( !flows[p->_succs[k]->_pre_order] &&
645               p->_succs[k] != b )
646             break;
647 
648         // Either carry-forward the now-unused OopFlow for b's use
649         // or draw a new one from the free list
650         if( k==p->_num_succs ) {
651           flow = p_flow;
652           break;                // Found an ideal pred, use him
653         }
654       }
655     }
656 
657     if( flow ) {
658       // We have an OopFlow that's the last-use of a predecessor.
659       // Carry it forward.
660     } else {                    // Draw a new OopFlow from the freelist
661       if( !free_list )
662         free_list = OopFlow::make(A,max_reg,C);
663       flow = free_list;
664       assert( flow->_b == NULL, "oopFlow is not free" );
665       free_list = flow->_next;
666       flow->_next = NULL;
667 
668       // Copy/clone over the data
669       flow->clone(flows[pred->_pre_order], max_reg);
670     }
671 
672     // Mark flow for block.  Blocks can only be flowed over once,
673     // because after the first time they are guarded from entering
674     // this code again.
675     assert( flow->_b == pred, "have some prior flow" );
676     flow->_b = NULL;
677 
678     // Now push flow forward
679     flows[b->_pre_order] = flow;// Mark flow for this block
680     flow->_b = b;
681     flow->compute_reach( C->regalloc(), max_reg, safehash );
682 
683     // Now push children onto worklist
684     for( i=0; i<b->_num_succs; i++ )
685       worklist.push(b->_succs[i]);
686 
687   }
688 }