1 /*
   2  * Copyright (c) 2002, 2016, 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/phase.hpp"
  35 #include "opto/regalloc.hpp"
  36 #include "opto/rootnode.hpp"
  37 
  38 // The functions in this file builds OopMaps after all scheduling is done.
  39 //
  40 // OopMaps contain a list of all registers and stack-slots containing oops (so
  41 // they can be updated by GC).  OopMaps also contain a list of derived-pointer
  42 // base-pointer pairs.  When the base is moved, the derived pointer moves to
  43 // follow it.  Finally, any registers holding callee-save values are also
  44 // recorded.  These might contain oops, but only the caller knows.
  45 //
  46 // BuildOopMaps implements a simple forward reaching-defs solution.  At each
  47 // GC point we'll have the reaching-def Nodes.  If the reaching Nodes are
  48 // typed as pointers (no offset), then they are oops.  Pointers+offsets are
  49 // derived pointers, and bases can be found from them.  Finally, we'll also
  50 // track reaching callee-save values.  Note that a copy of a callee-save value
  51 // "kills" it's source, so that only 1 copy of a callee-save value is alive at
  52 // a time.
  53 //
  54 // We run a simple bitvector liveness pass to help trim out dead oops.  Due to
  55 // irreducible loops, we can have a reaching def of an oop that only reaches
  56 // along one path and no way to know if it's valid or not on the other path.
  57 // The bitvectors are quite dense and the liveness pass is fast.
  58 //
  59 // At GC points, we consult this information to build OopMaps.  All reaching
  60 // defs typed as oops are added to the OopMap.  Only 1 instance of a
  61 // callee-save register can be recorded.  For derived pointers, we'll have to
  62 // find and record the register holding the base.
  63 //
  64 // The reaching def's is a simple 1-pass worklist approach.  I tried a clever
  65 // breadth-first approach but it was worse (showed O(n^2) in the
  66 // pick-next-block code).
  67 //
  68 // The relevant data is kept in a struct of arrays (it could just as well be
  69 // an array of structs, but the struct-of-arrays is generally a little more
  70 // efficient).  The arrays are indexed by register number (including
  71 // stack-slots as registers) and so is bounded by 200 to 300 elements in
  72 // practice.  One array will map to a reaching def Node (or NULL for
  73 // conflict/dead).  The other array will map to a callee-saved register or
  74 // OptoReg::Bad for not-callee-saved.
  75 
  76 
  77 // Structure to pass around
  78 struct OopFlow : public ResourceObj {
  79   short *_callees;              // Array mapping register to callee-saved
  80   Node **_defs;                 // array mapping register to reaching def
  81                                 // or NULL if dead/conflict
  82   // OopFlow structs, when not being actively modified, describe the _end_ of
  83   // this block.
  84   Block *_b;                    // Block for this struct
  85   OopFlow *_next;               // Next free OopFlow
  86                                 // or NULL if dead/conflict
  87   Compile* C;
  88 
  89   OopFlow( short *callees, Node **defs, Compile* c ) : _callees(callees), _defs(defs),
  90     _b(NULL), _next(NULL), C(c) { }
  91 
  92   // Given reaching-defs for this block start, compute it for this block end
  93   void compute_reach( PhaseRegAlloc *regalloc, int max_reg, Dict *safehash );
  94 
  95   // Merge these two OopFlows into the 'this' pointer.
  96   void merge( OopFlow *flow, int max_reg );
  97 
  98   // Copy a 'flow' over an existing flow
  99   void clone( OopFlow *flow, int max_size);
 100 
 101   // Make a new OopFlow from scratch
 102   static OopFlow *make( Arena *A, int max_size, Compile* C );
 103 
 104   // Build an oopmap from the current flow info
 105   OopMap *build_oop_map( Node *n, int max_reg, PhaseRegAlloc *regalloc, int* live );
 106 };
 107 
 108 // Given reaching-defs for this block start, compute it for this block end
 109 void OopFlow::compute_reach( PhaseRegAlloc *regalloc, int max_reg, Dict *safehash ) {
 110 
 111   for( uint i=0; i<_b->number_of_nodes(); i++ ) {
 112     Node *n = _b->get_node(i);
 113 
 114     if( n->jvms() ) {           // Build an OopMap here?
 115       JVMState *jvms = n->jvms();
 116       // no map needed for leaf calls
 117       if( n->is_MachSafePoint() && !n->is_MachCallLeaf() ) {
 118         int *live = (int*) (*safehash)[n];
 119         assert( live, "must find live" );
 120         n->as_MachSafePoint()->set_oop_map( build_oop_map(n,max_reg,regalloc, live) );
 121       }
 122     }
 123 
 124     // Assign new reaching def's.
 125     // Note that I padded the _defs and _callees arrays so it's legal
 126     // to index at _defs[OptoReg::Bad].
 127     OptoReg::Name first = regalloc->get_reg_first(n);
 128     OptoReg::Name second = regalloc->get_reg_second(n);
 129     _defs[first] = n;
 130     _defs[second] = n;
 131 
 132     // Pass callee-save info around copies
 133     int idx = n->is_Copy();
 134     if( idx ) {                 // Copies move callee-save info
 135       OptoReg::Name old_first = regalloc->get_reg_first(n->in(idx));
 136       OptoReg::Name old_second = regalloc->get_reg_second(n->in(idx));
 137       int tmp_first = _callees[old_first];
 138       int tmp_second = _callees[old_second];
 139       _callees[old_first] = OptoReg::Bad; // callee-save is moved, dead in old location
 140       _callees[old_second] = OptoReg::Bad;
 141       _callees[first] = tmp_first;
 142       _callees[second] = tmp_second;
 143     } else if( n->is_Phi() ) {  // Phis do not mod callee-saves
 144       assert( _callees[first] == _callees[regalloc->get_reg_first(n->in(1))], "" );
 145       assert( _callees[second] == _callees[regalloc->get_reg_second(n->in(1))], "" );
 146       assert( _callees[first] == _callees[regalloc->get_reg_first(n->in(n->req()-1))], "" );
 147       assert( _callees[second] == _callees[regalloc->get_reg_second(n->in(n->req()-1))], "" );
 148     } else {
 149       _callees[first] = OptoReg::Bad; // No longer holding a callee-save value
 150       _callees[second] = OptoReg::Bad;
 151 
 152       // Find base case for callee saves
 153       if( n->is_Proj() && n->in(0)->is_Start() ) {
 154         if( OptoReg::is_reg(first) &&
 155             regalloc->_matcher.is_save_on_entry(first) )
 156           _callees[first] = first;
 157         if( OptoReg::is_reg(second) &&
 158             regalloc->_matcher.is_save_on_entry(second) )
 159           _callees[second] = second;
 160       }
 161     }
 162   }
 163 }
 164 
 165 // Merge the given flow into the 'this' flow
 166 void OopFlow::merge( OopFlow *flow, int max_reg ) {
 167   assert( _b == NULL, "merging into a happy flow" );
 168   assert( flow->_b, "this flow is still alive" );
 169   assert( flow != this, "no self flow" );
 170 
 171   // Do the merge.  If there are any differences, drop to 'bottom' which
 172   // is OptoReg::Bad or NULL depending.
 173   for( int i=0; i<max_reg; i++ ) {
 174     // Merge the callee-save's
 175     if( _callees[i] != flow->_callees[i] )
 176       _callees[i] = OptoReg::Bad;
 177     // Merge the reaching defs
 178     if( _defs[i] != flow->_defs[i] )
 179       _defs[i] = NULL;
 180   }
 181 
 182 }
 183 
 184 void OopFlow::clone( OopFlow *flow, int max_size ) {
 185   _b = flow->_b;
 186   memcpy( _callees, flow->_callees, sizeof(short)*max_size);
 187   memcpy( _defs   , flow->_defs   , sizeof(Node*)*max_size);
 188 }
 189 
 190 OopFlow *OopFlow::make( Arena *A, int max_size, Compile* C ) {
 191   short *callees = NEW_ARENA_ARRAY(A,short,max_size+1);
 192   Node **defs    = NEW_ARENA_ARRAY(A,Node*,max_size+1);
 193   debug_only( memset(defs,0,(max_size+1)*sizeof(Node*)) );
 194   OopFlow *flow = new (A) OopFlow(callees+1, defs+1, C);
 195   assert( &flow->_callees[OptoReg::Bad] == callees, "Ok to index at OptoReg::Bad" );
 196   assert( &flow->_defs   [OptoReg::Bad] == defs   , "Ok to index at OptoReg::Bad" );
 197   return flow;
 198 }
 199 
 200 static int get_live_bit( int *live, int reg ) {
 201   return live[reg>>LogBitsPerInt] &   (1<<(reg&(BitsPerInt-1))); }
 202 static void set_live_bit( int *live, int reg ) {
 203          live[reg>>LogBitsPerInt] |=  (1<<(reg&(BitsPerInt-1))); }
 204 static void clr_live_bit( int *live, int reg ) {
 205          live[reg>>LogBitsPerInt] &= ~(1<<(reg&(BitsPerInt-1))); }
 206 
 207 // Build an oopmap from the current flow info
 208 OopMap *OopFlow::build_oop_map( Node *n, int max_reg, PhaseRegAlloc *regalloc, int* live ) {
 209   int framesize = regalloc->_framesize;
 210   int max_inarg_slot = OptoReg::reg2stack(regalloc->_matcher._new_SP);
 211   debug_only( char *dup_check = NEW_RESOURCE_ARRAY(char,OptoReg::stack0());
 212               memset(dup_check,0,OptoReg::stack0()) );
 213 
 214   OopMap *omap = new OopMap( framesize,  max_inarg_slot );
 215   MachCallNode *mcall = n->is_MachCall() ? n->as_MachCall() : NULL;
 216   JVMState* jvms = n->jvms();
 217 
 218   // For all registers do...
 219   for( int reg=0; reg<max_reg; reg++ ) {
 220     if( get_live_bit(live,reg) == 0 )
 221       continue;                 // Ignore if not live
 222 
 223     // %%% C2 can use 2 OptoRegs when the physical register is only one 64bit
 224     // register in that case we'll get an non-concrete register for the second
 225     // half. We only need to tell the map the register once!
 226     //
 227     // However for the moment we disable this change and leave things as they
 228     // were.
 229 
 230     VMReg r = OptoReg::as_VMReg(OptoReg::Name(reg), framesize, max_inarg_slot);
 231 
 232     if (false && r->is_reg() && !r->is_concrete()) {
 233       continue;
 234     }
 235 
 236     // See if dead (no reaching def).
 237     Node *def = _defs[reg];     // Get reaching def
 238     assert( def, "since live better have reaching def" );
 239 
 240     // Classify the reaching def as oop, derived, callee-save, dead, or other
 241     const Type *t = def->bottom_type();
 242     if( t->isa_oop_ptr() ) {    // Oop or derived?
 243       assert( !OptoReg::is_valid(_callees[reg]), "oop can't be callee save" );
 244 #ifdef _LP64
 245       // 64-bit pointers record oop-ishness on 2 aligned adjacent registers.
 246       // Make sure both are record from the same reaching def, but do not
 247       // put both into the oopmap.
 248       if( (reg&1) == 1 ) {      // High half of oop-pair?
 249         assert( _defs[reg-1] == _defs[reg], "both halves from same reaching def" );
 250         continue;               // Do not record high parts in oopmap
 251       }
 252 #endif
 253 
 254       // Check for a legal reg name in the oopMap and bailout if it is not.
 255       if (!omap->legal_vm_reg_name(r)) {
 256         regalloc->C->record_method_not_compilable("illegal oopMap register name");
 257         continue;
 258       }
 259       if( t->is_ptr()->_offset == 0 ) { // Not derived?
 260         if( mcall ) {
 261           // Outgoing argument GC mask responsibility belongs to the callee,
 262           // not the caller.  Inspect the inputs to the call, to see if
 263           // this live-range is one of them.
 264           uint cnt = mcall->tf()->domain()->cnt();
 265           uint j;
 266           for( j = TypeFunc::Parms; j < cnt; j++)
 267             if( mcall->in(j) == def )
 268               break;            // reaching def is an argument oop
 269           if( j < cnt )         // arg oops dont go in GC map
 270             continue;           // Continue on to the next register
 271         }
 272         omap->set_oop(r);
 273       } else {                  // Else it's derived.
 274         // Find the base of the derived value.
 275         uint i;
 276         // Fast, common case, scan
 277         for( i = jvms->oopoff(); i < n->req(); i+=2 )
 278           if( n->in(i) == def ) break; // Common case
 279         if( i == n->req() ) {   // Missed, try a more generous scan
 280           // Scan again, but this time peek through copies
 281           for( i = jvms->oopoff(); i < n->req(); i+=2 ) {
 282             Node *m = n->in(i); // Get initial derived value
 283             while( 1 ) {
 284               Node *d = def;    // Get initial reaching def
 285               while( 1 ) {      // Follow copies of reaching def to end
 286                 if( m == d ) goto found; // breaks 3 loops
 287                 int idx = d->is_Copy();
 288                 if( !idx ) break;
 289                 d = d->in(idx);     // Link through copy
 290               }
 291               int idx = m->is_Copy();
 292               if( !idx ) break;
 293               m = m->in(idx);
 294             }
 295           }
 296           guarantee( 0, "must find derived/base pair" );
 297         }
 298       found: ;
 299         Node *base = n->in(i+1); // Base is other half of pair
 300         int breg = regalloc->get_reg_first(base);
 301         VMReg b = OptoReg::as_VMReg(OptoReg::Name(breg), framesize, max_inarg_slot);
 302 
 303         // I record liveness at safepoints BEFORE I make the inputs
 304         // live.  This is because argument oops are NOT live at a
 305         // safepoint (or at least they cannot appear in the oopmap).
 306         // Thus bases of base/derived pairs might not be in the
 307         // liveness data but they need to appear in the oopmap.
 308         if( get_live_bit(live,breg) == 0 ) {// Not live?
 309           // Flag it, so next derived pointer won't re-insert into oopmap
 310           set_live_bit(live,breg);
 311           // Already missed our turn?
 312           if( breg < reg ) {
 313             if (b->is_stack() || b->is_concrete() || true ) {
 314               omap->set_oop( b);
 315             }
 316           }
 317         }
 318         if (b->is_stack() || b->is_concrete() || true ) {
 319           omap->set_derived_oop( r, b);
 320         }
 321       }
 322 
 323     } else if( t->isa_narrowoop() ) {
 324       assert( !OptoReg::is_valid(_callees[reg]), "oop can't be callee save" );
 325       // Check for a legal reg name in the oopMap and bailout if it is not.
 326       if (!omap->legal_vm_reg_name(r)) {
 327         regalloc->C->record_method_not_compilable("illegal oopMap register name");
 328         continue;
 329       }
 330       if( mcall ) {
 331           // Outgoing argument GC mask responsibility belongs to the callee,
 332           // not the caller.  Inspect the inputs to the call, to see if
 333           // this live-range is one of them.
 334         uint cnt = mcall->tf()->domain()->cnt();
 335         uint j;
 336         for( j = TypeFunc::Parms; j < cnt; j++)
 337           if( mcall->in(j) == def )
 338             break;            // reaching def is an argument oop
 339         if( j < cnt )         // arg oops dont go in GC map
 340           continue;           // Continue on to the next register
 341       }
 342       omap->set_narrowoop(r);
 343     } else if( OptoReg::is_valid(_callees[reg])) { // callee-save?
 344       // It's a callee-save value
 345       assert( dup_check[_callees[reg]]==0, "trying to callee save same reg twice" );
 346       debug_only( dup_check[_callees[reg]]=1; )
 347       VMReg callee = OptoReg::as_VMReg(OptoReg::Name(_callees[reg]));
 348       if ( callee->is_concrete() || true ) {
 349         omap->set_callee_saved( r, callee);
 350       }
 351 
 352     } else {
 353       // Other - some reaching non-oop value
 354       omap->set_value( r);
 355 #ifdef ASSERT
 356       if( t->isa_rawptr() && C->cfg()->_raw_oops.member(def) ) {
 357         def->dump();
 358         n->dump();
 359         assert(false, "there should be a oop in OopMap instead of a live raw oop at safepoint");
 360       }
 361 #endif
 362     }
 363 
 364   }
 365 
 366 #ifdef ASSERT
 367   /* Nice, Intel-only assert
 368   int cnt_callee_saves=0;
 369   int reg2 = 0;
 370   while (OptoReg::is_reg(reg2)) {
 371     if( dup_check[reg2] != 0) cnt_callee_saves++;
 372     assert( cnt_callee_saves==3 || cnt_callee_saves==5, "missed some callee-save" );
 373     reg2++;
 374   }
 375   */
 376 #endif
 377 
 378 #ifdef ASSERT
 379   for( OopMapStream oms1(omap, OopMapValue::derived_oop_value); !oms1.is_done(); oms1.next()) {
 380     OopMapValue omv1 = oms1.current();
 381     bool found = false;
 382     for( OopMapStream oms2(omap,OopMapValue::oop_value); !oms2.is_done(); oms2.next()) {
 383       if( omv1.content_reg() == oms2.current().reg() ) {
 384         found = true;
 385         break;
 386       }
 387     }
 388     assert( found, "derived with no base in oopmap" );
 389   }
 390 #endif
 391 
 392   return omap;
 393 }
 394 
 395 // Compute backwards liveness on registers
 396 static void do_liveness(PhaseRegAlloc* regalloc, PhaseCFG* cfg, Block_List* worklist, int max_reg_ints, Arena* A, Dict* safehash) {
 397   int* live = NEW_ARENA_ARRAY(A, int, (cfg->number_of_blocks() + 1) * max_reg_ints);
 398   int* tmp_live = &live[cfg->number_of_blocks() * max_reg_ints];
 399   Node* root = cfg->get_root_node();
 400   // On CISC platforms, get the node representing the stack pointer  that regalloc
 401   // used for spills
 402   Node *fp = NodeSentinel;
 403   if (UseCISCSpill && root->req() > 1) {
 404     fp = root->in(1)->in(TypeFunc::FramePtr);
 405   }
 406   memset(live, 0, cfg->number_of_blocks() * (max_reg_ints << LogBytesPerInt));
 407   // Push preds onto worklist
 408   for (uint i = 1; i < root->req(); i++) {
 409     Block* block = cfg->get_block_for_node(root->in(i));
 410     worklist->push(block);
 411   }
 412 
 413   // ZKM.jar includes tiny infinite loops which are unreached from below.
 414   // If we missed any blocks, we'll retry here after pushing all missed
 415   // blocks on the worklist.  Normally this outer loop never trips more
 416   // than once.
 417   while (1) {
 418 
 419     while( worklist->size() ) { // Standard worklist algorithm
 420       Block *b = worklist->rpop();
 421 
 422       // Copy first successor into my tmp_live space
 423       int s0num = b->_succs[0]->_pre_order;
 424       int *t = &live[s0num*max_reg_ints];
 425       for( int i=0; i<max_reg_ints; i++ )
 426         tmp_live[i] = t[i];
 427 
 428       // OR in the remaining live registers
 429       for( uint j=1; j<b->_num_succs; j++ ) {
 430         uint sjnum = b->_succs[j]->_pre_order;
 431         int *t = &live[sjnum*max_reg_ints];
 432         for( int i=0; i<max_reg_ints; i++ )
 433           tmp_live[i] |= t[i];
 434       }
 435 
 436       // Now walk tmp_live up the block backwards, computing live
 437       for( int k=b->number_of_nodes()-1; k>=0; k-- ) {
 438         Node *n = b->get_node(k);
 439         // KILL def'd bits
 440         int first = regalloc->get_reg_first(n);
 441         int second = regalloc->get_reg_second(n);
 442         if( OptoReg::is_valid(first) ) clr_live_bit(tmp_live,first);
 443         if( OptoReg::is_valid(second) ) clr_live_bit(tmp_live,second);
 444 
 445         MachNode *m = n->is_Mach() ? n->as_Mach() : NULL;
 446 
 447         // Check if m is potentially a CISC alternate instruction (i.e, possibly
 448         // synthesized by RegAlloc from a conventional instruction and a
 449         // spilled input)
 450         bool is_cisc_alternate = false;
 451         if (UseCISCSpill && m) {
 452           is_cisc_alternate = m->is_cisc_alternate();
 453         }
 454 
 455         // GEN use'd bits
 456         for( uint l=1; l<n->req(); l++ ) {
 457           Node *def = n->in(l);
 458           assert(def != 0, "input edge required");
 459           int first = regalloc->get_reg_first(def);
 460           int second = regalloc->get_reg_second(def);
 461           if( OptoReg::is_valid(first) ) set_live_bit(tmp_live,first);
 462           if( OptoReg::is_valid(second) ) set_live_bit(tmp_live,second);
 463           // If we use the stack pointer in a cisc-alternative instruction,
 464           // check for use as a memory operand.  Then reconstruct the RegName
 465           // for this stack location, and set the appropriate bit in the
 466           // live vector 4987749.
 467           if (is_cisc_alternate && def == fp) {
 468             const TypePtr *adr_type = NULL;
 469             intptr_t offset;
 470             const Node* base = m->get_base_and_disp(offset, adr_type);
 471             if (base == NodeSentinel) {
 472               // Machnode has multiple memory inputs. We are unable to reason
 473               // with these, but are presuming (with trepidation) that not any of
 474               // them are oops. This can be fixed by making get_base_and_disp()
 475               // look at a specific input instead of all inputs.
 476               assert(!def->bottom_type()->isa_oop_ptr(), "expecting non-oop mem input");
 477             } else if (base != fp || offset == Type::OffsetBot) {
 478               // Do nothing: the fp operand is either not from a memory use
 479               // (base == NULL) OR the fp is used in a non-memory context
 480               // (base is some other register) OR the offset is not constant,
 481               // so it is not a stack slot.
 482             } else {
 483               assert(offset >= 0, "unexpected negative offset");
 484               offset -= (offset % jintSize);  // count the whole word
 485               int stack_reg = regalloc->offset2reg(offset);
 486               if (OptoReg::is_stack(stack_reg)) {
 487                 set_live_bit(tmp_live, stack_reg);
 488               } else {
 489                 assert(false, "stack_reg not on stack?");
 490               }
 491             }
 492           }
 493         }
 494 
 495         if( n->jvms() ) {       // Record liveness at safepoint
 496 
 497           // This placement of this stanza means inputs to calls are
 498           // considered live at the callsite's OopMap.  Argument oops are
 499           // hence live, but NOT included in the oopmap.  See cutout in
 500           // build_oop_map.  Debug oops are live (and in OopMap).
 501           int *n_live = NEW_ARENA_ARRAY(A, int, max_reg_ints);
 502           for( int l=0; l<max_reg_ints; l++ )
 503             n_live[l] = tmp_live[l];
 504           safehash->Insert(n,n_live);
 505         }
 506 
 507       }
 508 
 509       // Now at block top, see if we have any changes.  If so, propagate
 510       // to prior blocks.
 511       int *old_live = &live[b->_pre_order*max_reg_ints];
 512       int l;
 513       for( l=0; l<max_reg_ints; l++ )
 514         if( tmp_live[l] != old_live[l] )
 515           break;
 516       if( l<max_reg_ints ) {     // Change!
 517         // Copy in new value
 518         for( l=0; l<max_reg_ints; l++ )
 519           old_live[l] = tmp_live[l];
 520         // Push preds onto worklist
 521         for (l = 1; l < (int)b->num_preds(); l++) {
 522           Block* block = cfg->get_block_for_node(b->pred(l));
 523           worklist->push(block);
 524         }
 525       }
 526     }
 527 
 528     // Scan for any missing safepoints.  Happens to infinite loops
 529     // ala ZKM.jar
 530     uint i;
 531     for (i = 1; i < cfg->number_of_blocks(); i++) {
 532       Block* block = cfg->get_block(i);
 533       uint j;
 534       for (j = 1; j < block->number_of_nodes(); j++) {
 535         if (block->get_node(j)->jvms() && (*safehash)[block->get_node(j)] == NULL) {
 536            break;
 537         }
 538       }
 539       if (j < block->number_of_nodes()) {
 540         break;
 541       }
 542     }
 543     if (i == cfg->number_of_blocks()) {
 544       break;                    // Got 'em all
 545     }
 546 
 547     if (PrintOpto && Verbose) {
 548       tty->print_cr("retripping live calc");
 549     }
 550 
 551     // Force the issue (expensively): recheck everybody
 552     for (i = 1; i < cfg->number_of_blocks(); i++) {
 553       worklist->push(cfg->get_block(i));
 554     }
 555   }
 556 }
 557 
 558 // Collect GC mask info - where are all the OOPs?
 559 void Compile::BuildOopMaps() {
 560   TracePhase tp("bldOopMaps", &timers[_t_buildOopMaps]);
 561   // Can't resource-mark because I need to leave all those OopMaps around,
 562   // or else I need to resource-mark some arena other than the default.
 563   // ResourceMark rm;              // Reclaim all OopFlows when done
 564   int max_reg = _regalloc->_max_reg; // Current array extent
 565 
 566   Arena *A = Thread::current()->resource_area();
 567   Block_List worklist;          // Worklist of pending blocks
 568 
 569   int max_reg_ints = align_up(max_reg, BitsPerInt)>>LogBitsPerInt;
 570   Dict *safehash = NULL;        // Used for assert only
 571   // Compute a backwards liveness per register.  Needs a bitarray of
 572   // #blocks x (#registers, rounded up to ints)
 573   safehash = new Dict(cmpkey,hashkey,A);
 574   do_liveness( _regalloc, _cfg, &worklist, max_reg_ints, A, safehash );
 575   OopFlow *free_list = NULL;    // Free, unused
 576 
 577   // Array mapping blocks to completed oopflows
 578   OopFlow **flows = NEW_ARENA_ARRAY(A, OopFlow*, _cfg->number_of_blocks());
 579   memset( flows, 0, _cfg->number_of_blocks() * sizeof(OopFlow*) );
 580 
 581 
 582   // Do the first block 'by hand' to prime the worklist
 583   Block *entry = _cfg->get_block(1);
 584   OopFlow *rootflow = OopFlow::make(A,max_reg,this);
 585   // Initialize to 'bottom' (not 'top')
 586   memset( rootflow->_callees, OptoReg::Bad, max_reg*sizeof(short) );
 587   memset( rootflow->_defs   ,            0, max_reg*sizeof(Node*) );
 588   flows[entry->_pre_order] = rootflow;
 589 
 590   // Do the first block 'by hand' to prime the worklist
 591   rootflow->_b = entry;
 592   rootflow->compute_reach( _regalloc, max_reg, safehash );
 593   for( uint i=0; i<entry->_num_succs; i++ )
 594     worklist.push(entry->_succs[i]);
 595 
 596   // Now worklist contains blocks which have some, but perhaps not all,
 597   // predecessors visited.
 598   while( worklist.size() ) {
 599     // Scan for a block with all predecessors visited, or any randoms slob
 600     // otherwise.  All-preds-visited order allows me to recycle OopFlow
 601     // structures rapidly and cut down on the memory footprint.
 602     // Note: not all predecessors might be visited yet (must happen for
 603     // irreducible loops).  This is OK, since every live value must have the
 604     // SAME reaching def for the block, so any reaching def is OK.
 605     uint i;
 606 
 607     Block *b = worklist.pop();
 608     // Ignore root block
 609     if (b == _cfg->get_root_block()) {
 610       continue;
 611     }
 612     // Block is already done?  Happens if block has several predecessors,
 613     // he can get on the worklist more than once.
 614     if( flows[b->_pre_order] ) continue;
 615 
 616     // If this block has a visited predecessor AND that predecessor has this
 617     // last block as his only undone child, we can move the OopFlow from the
 618     // pred to this block.  Otherwise we have to grab a new OopFlow.
 619     OopFlow *flow = NULL;       // Flag for finding optimized flow
 620     Block *pred = (Block*)0xdeadbeef;
 621     // Scan this block's preds to find a done predecessor
 622     for (uint j = 1; j < b->num_preds(); j++) {
 623       Block* p = _cfg->get_block_for_node(b->pred(j));
 624       OopFlow *p_flow = flows[p->_pre_order];
 625       if( p_flow ) {            // Predecessor is done
 626         assert( p_flow->_b == p, "cross check" );
 627         pred = p;               // Record some predecessor
 628         // If all successors of p are done except for 'b', then we can carry
 629         // p_flow forward to 'b' without copying, otherwise we have to draw
 630         // from the free_list and clone data.
 631         uint k;
 632         for( k=0; k<p->_num_succs; k++ )
 633           if( !flows[p->_succs[k]->_pre_order] &&
 634               p->_succs[k] != b )
 635             break;
 636 
 637         // Either carry-forward the now-unused OopFlow for b's use
 638         // or draw a new one from the free list
 639         if( k==p->_num_succs ) {
 640           flow = p_flow;
 641           break;                // Found an ideal pred, use him
 642         }
 643       }
 644     }
 645 
 646     if( flow ) {
 647       // We have an OopFlow that's the last-use of a predecessor.
 648       // Carry it forward.
 649     } else {                    // Draw a new OopFlow from the freelist
 650       if( !free_list )
 651         free_list = OopFlow::make(A,max_reg,C);
 652       flow = free_list;
 653       assert( flow->_b == NULL, "oopFlow is not free" );
 654       free_list = flow->_next;
 655       flow->_next = NULL;
 656 
 657       // Copy/clone over the data
 658       flow->clone(flows[pred->_pre_order], max_reg);
 659     }
 660 
 661     // Mark flow for block.  Blocks can only be flowed over once,
 662     // because after the first time they are guarded from entering
 663     // this code again.
 664     assert( flow->_b == pred, "have some prior flow" );
 665     flow->_b = NULL;
 666 
 667     // Now push flow forward
 668     flows[b->_pre_order] = flow;// Mark flow for this block
 669     flow->_b = b;
 670     flow->compute_reach( _regalloc, max_reg, safehash );
 671 
 672     // Now push children onto worklist
 673     for( i=0; i<b->_num_succs; i++ )
 674       worklist.push(b->_succs[i]);
 675 
 676   }
 677 }