Mercurial > jdk8-mips64-public > hotspot / file revision

 /*

  * Copyright 2002-2008 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

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  *

  */

 #include "incls/_precompiled.incl"

 #include "incls/_buildOopMap.cpp.incl"

 // The functions in this file builds OopMaps after all scheduling is done.

 //

 // OopMaps contain a list of all registers and stack-slots containing oops (so

 // they can be updated by GC).  OopMaps also contain a list of derived-pointer

 // base-pointer pairs.  When the base is moved, the derived pointer moves to

 // follow it.  Finally, any registers holding callee-save values are also

 // recorded.  These might contain oops, but only the caller knows.

 //

 // BuildOopMaps implements a simple forward reaching-defs solution.  At each

 // GC point we'll have the reaching-def Nodes.  If the reaching Nodes are

 // typed as pointers (no offset), then they are oops.  Pointers+offsets are

 // derived pointers, and bases can be found from them.  Finally, we'll also

 // track reaching callee-save values.  Note that a copy of a callee-save value

 // "kills" it's source, so that only 1 copy of a callee-save value is alive at

 // a time.

 //

 // We run a simple bitvector liveness pass to help trim out dead oops.  Due to

 // irreducible loops, we can have a reaching def of an oop that only reaches

 // along one path and no way to know if it's valid or not on the other path.

 // The bitvectors are quite dense and the liveness pass is fast.

 //

 // At GC points, we consult this information to build OopMaps.  All reaching

 // defs typed as oops are added to the OopMap.  Only 1 instance of a

 // callee-save register can be recorded.  For derived pointers, we'll have to

 // find and record the register holding the base.

 //

 // The reaching def's is a simple 1-pass worklist approach.  I tried a clever

 // breadth-first approach but it was worse (showed O(n^2) in the

 // pick-next-block code).

 //

 // The relevant data is kept in a struct of arrays (it could just as well be

 // an array of structs, but the struct-of-arrays is generally a little more

 // efficient).  The arrays are indexed by register number (including

 // stack-slots as registers) and so is bounded by 200 to 300 elements in

 // practice.  One array will map to a reaching def Node (or NULL for

 // conflict/dead).  The other array will map to a callee-saved register or

 // OptoReg::Bad for not-callee-saved.

 //------------------------------OopFlow----------------------------------------

 // Structure to pass around

 struct OopFlow : public ResourceObj {

   short *_callees;              // Array mapping register to callee-saved

   Node **_defs;                 // array mapping register to reaching def

                                 // or NULL if dead/conflict

   // OopFlow structs, when not being actively modified, describe the _end_ of

   // this block.

   Block *_b;                    // Block for this struct

   OopFlow *_next;               // Next free OopFlow

   OopFlow( short *callees, Node **defs ) : _callees(callees), _defs(defs),

     _b(NULL), _next(NULL) { }

   // Given reaching-defs for this block start, compute it for this block end

   void compute_reach( PhaseRegAlloc *regalloc, int max_reg, Dict *safehash );

   // Merge these two OopFlows into the 'this' pointer.

   void merge( OopFlow *flow, int max_reg );

   // Copy a 'flow' over an existing flow

   void clone( OopFlow *flow, int max_size);

   // Make a new OopFlow from scratch

   static OopFlow *make( Arena *A, int max_size );

   // Build an oopmap from the current flow info

   OopMap *build_oop_map( Node *n, int max_reg, PhaseRegAlloc *regalloc, int* live );

 };

 //------------------------------compute_reach----------------------------------

 // Given reaching-defs for this block start, compute it for this block end

 void OopFlow::compute_reach( PhaseRegAlloc *regalloc, int max_reg, Dict *safehash ) {

   for( uint i=0; i<_b->_nodes.size(); i++ ) {

     Node *n = _b->_nodes[i];

     if( n->jvms() ) {           // Build an OopMap here?

       JVMState *jvms = n->jvms();

       // no map needed for leaf calls

       if( n->is_MachSafePoint() && !n->is_MachCallLeaf() ) {

         int *live = (int*) (*safehash)[n];

         assert( live, "must find live" );

         n->as_MachSafePoint()->set_oop_map( build_oop_map(n,max_reg,regalloc, live) );

       }

     }

     // Assign new reaching def's.

     // Note that I padded the _defs and _callees arrays so it's legal

     // to index at _defs[OptoReg::Bad].

     OptoReg::Name first = regalloc->get_reg_first(n);

     OptoReg::Name second = regalloc->get_reg_second(n);

     _defs[first] = n;

     _defs[second] = n;

     // Pass callee-save info around copies

     int idx = n->is_Copy();

     if( idx ) {                 // Copies move callee-save info

       OptoReg::Name old_first = regalloc->get_reg_first(n->in(idx));

       OptoReg::Name old_second = regalloc->get_reg_second(n->in(idx));

       int tmp_first = _callees[old_first];

       int tmp_second = _callees[old_second];

       _callees[old_first] = OptoReg::Bad; // callee-save is moved, dead in old location

       _callees[old_second] = OptoReg::Bad;

       _callees[first] = tmp_first;

       _callees[second] = tmp_second;

     } else if( n->is_Phi() ) {  // Phis do not mod callee-saves

       assert( _callees[first] == _callees[regalloc->get_reg_first(n->in(1))], "" );

       assert( _callees[second] == _callees[regalloc->get_reg_second(n->in(1))], "" );

       assert( _callees[first] == _callees[regalloc->get_reg_first(n->in(n->req()-1))], "" );

       assert( _callees[second] == _callees[regalloc->get_reg_second(n->in(n->req()-1))], "" );

     } else {

       _callees[first] = OptoReg::Bad; // No longer holding a callee-save value

       _callees[second] = OptoReg::Bad;

       // Find base case for callee saves

       if( n->is_Proj() && n->in(0)->is_Start() ) {

         if( OptoReg::is_reg(first) &&

             regalloc->_matcher.is_save_on_entry(first) )

           _callees[first] = first;

         if( OptoReg::is_reg(second) &&

             regalloc->_matcher.is_save_on_entry(second) )

           _callees[second] = second;

       }

     }

   }

 }

 //------------------------------merge------------------------------------------

 // Merge the given flow into the 'this' flow

 void OopFlow::merge( OopFlow *flow, int max_reg ) {

   assert( _b == NULL, "merging into a happy flow" );

   assert( flow->_b, "this flow is still alive" );

   assert( flow != this, "no self flow" );

   // Do the merge.  If there are any differences, drop to 'bottom' which

   // is OptoReg::Bad or NULL depending.

   for( int i=0; i<max_reg; i++ ) {

     // Merge the callee-save's

     if( _callees[i] != flow->_callees[i] )

       _callees[i] = OptoReg::Bad;

     // Merge the reaching defs

     if( _defs[i] != flow->_defs[i] )

       _defs[i] = NULL;

   }

 }

 //------------------------------clone------------------------------------------

 void OopFlow::clone( OopFlow *flow, int max_size ) {

   _b = flow->_b;

   memcpy( _callees, flow->_callees, sizeof(short)*max_size);

   memcpy( _defs   , flow->_defs   , sizeof(Node*)*max_size);

 }

 //------------------------------make-------------------------------------------

 OopFlow *OopFlow::make( Arena *A, int max_size ) {

   short *callees = NEW_ARENA_ARRAY(A,short,max_size+1);

   Node **defs    = NEW_ARENA_ARRAY(A,Node*,max_size+1);

   debug_only( memset(defs,0,(max_size+1)*sizeof(Node*)) );

   OopFlow *flow = new (A) OopFlow(callees+1, defs+1);

   assert( &flow->_callees[OptoReg::Bad] == callees, "Ok to index at OptoReg::Bad" );

   assert( &flow->_defs   [OptoReg::Bad] == defs   , "Ok to index at OptoReg::Bad" );

   return flow;

 }

 //------------------------------bit twiddlers----------------------------------

 static int get_live_bit( int *live, int reg ) {

   return live[reg>>LogBitsPerInt] &   (1<<(reg&(BitsPerInt-1))); }

 static void set_live_bit( int *live, int reg ) {

          live[reg>>LogBitsPerInt] |=  (1<<(reg&(BitsPerInt-1))); }

 static void clr_live_bit( int *live, int reg ) {

          live[reg>>LogBitsPerInt] &= ~(1<<(reg&(BitsPerInt-1))); }

 //------------------------------build_oop_map----------------------------------

 // Build an oopmap from the current flow info

 OopMap *OopFlow::build_oop_map( Node *n, int max_reg, PhaseRegAlloc *regalloc, int* live ) {

   int framesize = regalloc->_framesize;

   int max_inarg_slot = OptoReg::reg2stack(regalloc->_matcher._new_SP);

   debug_only( char *dup_check = NEW_RESOURCE_ARRAY(char,OptoReg::stack0());

               memset(dup_check,0,OptoReg::stack0()) );

   OopMap *omap = new OopMap( framesize,  max_inarg_slot );

   MachCallNode *mcall = n->is_MachCall() ? n->as_MachCall() : NULL;

   JVMState* jvms = n->jvms();

   // For all registers do...

   for( int reg=0; reg<max_reg; reg++ ) {

     if( get_live_bit(live,reg) == 0 )

       continue;                 // Ignore if not live

     // %%% C2 can use 2 OptoRegs when the physical register is only one 64bit

     // register in that case we'll get an non-concrete register for the second

     // half. We only need to tell the map the register once!

     //

     // However for the moment we disable this change and leave things as they

     // were.

     VMReg r = OptoReg::as_VMReg(OptoReg::Name(reg), framesize, max_inarg_slot);

     if (false && r->is_reg() && !r->is_concrete()) {

       continue;

     }

     // See if dead (no reaching def).

     Node *def = _defs[reg];     // Get reaching def

     assert( def, "since live better have reaching def" );

     // Classify the reaching def as oop, derived, callee-save, dead, or other

     const Type *t = def->bottom_type();

     if( t->isa_oop_ptr() ) {    // Oop or derived?

       assert( !OptoReg::is_valid(_callees[reg]), "oop can't be callee save" );

 #ifdef _LP64

       // 64-bit pointers record oop-ishness on 2 aligned adjacent registers.

       // Make sure both are record from the same reaching def, but do not

       // put both into the oopmap.

       if( (reg&1) == 1 ) {      // High half of oop-pair?

         assert( _defs[reg-1] == _defs[reg], "both halves from same reaching def" );

         continue;               // Do not record high parts in oopmap

       }

 #endif

       // Check for a legal reg name in the oopMap and bailout if it is not.

       if (!omap->legal_vm_reg_name(r)) {

         regalloc->C->record_method_not_compilable("illegal oopMap register name");

         continue;

       }

       if( t->is_ptr()->_offset == 0 ) { // Not derived?

         if( mcall ) {

           // Outgoing argument GC mask responsibility belongs to the callee,

           // not the caller.  Inspect the inputs to the call, to see if

           // this live-range is one of them.

           uint cnt = mcall->tf()->domain()->cnt();

           uint j;

           for( j = TypeFunc::Parms; j < cnt; j++)

             if( mcall->in(j) == def )

               break;            // reaching def is an argument oop

           if( j < cnt )         // arg oops dont go in GC map

             continue;           // Continue on to the next register

         }

         omap->set_oop(r);

       } else {                  // Else it's derived.

         // Find the base of the derived value.

         uint i;

         // Fast, common case, scan

         for( i = jvms->oopoff(); i < n->req(); i+=2 )

           if( n->in(i) == def ) break; // Common case

         if( i == n->req() ) {   // Missed, try a more generous scan

           // Scan again, but this time peek through copies

           for( i = jvms->oopoff(); i < n->req(); i+=2 ) {

             Node *m = n->in(i); // Get initial derived value

             while( 1 ) {

               Node *d = def;    // Get initial reaching def

               while( 1 ) {      // Follow copies of reaching def to end

                 if( m == d ) goto found; // breaks 3 loops

                 int idx = d->is_Copy();

                 if( !idx ) break;

                 d = d->in(idx);     // Link through copy

               }

               int idx = m->is_Copy();

               if( !idx ) break;

               m = m->in(idx);

             }

           }

          guarantee( 0, "must find derived/base pair" );

         }

       found: ;

         Node *base = n->in(i+1); // Base is other half of pair

         int breg = regalloc->get_reg_first(base);

         VMReg b = OptoReg::as_VMReg(OptoReg::Name(breg), framesize, max_inarg_slot);

         // I record liveness at safepoints BEFORE I make the inputs

         // live.  This is because argument oops are NOT live at a

         // safepoint (or at least they cannot appear in the oopmap).

         // Thus bases of base/derived pairs might not be in the

         // liveness data but they need to appear in the oopmap.

         if( get_live_bit(live,breg) == 0 ) {// Not live?

           // Flag it, so next derived pointer won't re-insert into oopmap

           set_live_bit(live,breg);

           // Already missed our turn?

           if( breg < reg ) {

             if (b->is_stack() || b->is_concrete() || true ) {

               omap->set_oop( b);

             }

           }

         }

         if (b->is_stack() || b->is_concrete() || true ) {

           omap->set_derived_oop( r, b);

         }

       }

     } else if( t->isa_narrowoop() ) {

       assert( !OptoReg::is_valid(_callees[reg]), "oop can't be callee save" );

       // Check for a legal reg name in the oopMap and bailout if it is not.

       if (!omap->legal_vm_reg_name(r)) {

         regalloc->C->record_method_not_compilable("illegal oopMap register name");

         continue;

       }

       if( mcall ) {

           // Outgoing argument GC mask responsibility belongs to the callee,

           // not the caller.  Inspect the inputs to the call, to see if

           // this live-range is one of them.

         uint cnt = mcall->tf()->domain()->cnt();

         uint j;

         for( j = TypeFunc::Parms; j < cnt; j++)

           if( mcall->in(j) == def )

             break;            // reaching def is an argument oop

         if( j < cnt )         // arg oops dont go in GC map

           continue;           // Continue on to the next register

       }

       omap->set_narrowoop(r);

     } else if( OptoReg::is_valid(_callees[reg])) { // callee-save?

       // It's a callee-save value

       assert( dup_check[_callees[reg]]==0, "trying to callee save same reg twice" );

       debug_only( dup_check[_callees[reg]]=1; )

       VMReg callee = OptoReg::as_VMReg(OptoReg::Name(_callees[reg]));

       if ( callee->is_concrete() || true ) {

         omap->set_callee_saved( r, callee);

       }

     } else {

       // Other - some reaching non-oop value

       omap->set_value( r);

     }

   }

 #ifdef ASSERT

   /* Nice, Intel-only assert

   int cnt_callee_saves=0;

   int reg2 = 0;

   while (OptoReg::is_reg(reg2)) {

     if( dup_check[reg2] != 0) cnt_callee_saves++;

     assert( cnt_callee_saves==3 || cnt_callee_saves==5, "missed some callee-save" );

     reg2++;

   }

   */

 #endif

   return omap;

 }

 //------------------------------do_liveness------------------------------------

 // Compute backwards liveness on registers

 static void do_liveness( PhaseRegAlloc *regalloc, PhaseCFG *cfg, Block_List *worklist, int max_reg_ints, Arena *A, Dict *safehash ) {

   int *live = NEW_ARENA_ARRAY(A, int, (cfg->_num_blocks+1) * max_reg_ints);

   int *tmp_live = &live[cfg->_num_blocks * max_reg_ints];

   Node *root = cfg->C->root();

   // On CISC platforms, get the node representing the stack pointer  that regalloc

   // used for spills

   Node *fp = NodeSentinel;

   if (UseCISCSpill && root->req() > 1) {

     fp = root->in(1)->in(TypeFunc::FramePtr);

   }

   memset( live, 0, cfg->_num_blocks * (max_reg_ints<<LogBytesPerInt) );

   // Push preds onto worklist

   for( uint i=1; i<root->req(); i++ )

     worklist->push(cfg->_bbs[root->in(i)->_idx]);

   // ZKM.jar includes tiny infinite loops which are unreached from below.

   // If we missed any blocks, we'll retry here after pushing all missed

   // blocks on the worklist.  Normally this outer loop never trips more

   // than once.

   while( 1 ) {

     while( worklist->size() ) { // Standard worklist algorithm

       Block *b = worklist->rpop();

       // Copy first successor into my tmp_live space

       int s0num = b->_succs[0]->_pre_order;

       int *t = &live[s0num*max_reg_ints];

       for( int i=0; i<max_reg_ints; i++ )

         tmp_live[i] = t[i];

       // OR in the remaining live registers

       for( uint j=1; j<b->_num_succs; j++ ) {

         uint sjnum = b->_succs[j]->_pre_order;

         int *t = &live[sjnum*max_reg_ints];

         for( int i=0; i<max_reg_ints; i++ )

           tmp_live[i] |= t[i];

       }

       // Now walk tmp_live up the block backwards, computing live

       for( int k=b->_nodes.size()-1; k>=0; k-- ) {

         Node *n = b->_nodes[k];

         // KILL def'd bits

         int first = regalloc->get_reg_first(n);

         int second = regalloc->get_reg_second(n);

         if( OptoReg::is_valid(first) ) clr_live_bit(tmp_live,first);

         if( OptoReg::is_valid(second) ) clr_live_bit(tmp_live,second);

         MachNode *m = n->is_Mach() ? n->as_Mach() : NULL;

         // Check if m is potentially a CISC alternate instruction (i.e, possibly

         // synthesized by RegAlloc from a conventional instruction and a

         // spilled input)

         bool is_cisc_alternate = false;

         if (UseCISCSpill && m) {

           is_cisc_alternate = m->is_cisc_alternate();

         }

         // GEN use'd bits

         for( uint l=1; l<n->req(); l++ ) {

           Node *def = n->in(l);

           assert(def != 0, "input edge required");

           int first = regalloc->get_reg_first(def);

           int second = regalloc->get_reg_second(def);

           if( OptoReg::is_valid(first) ) set_live_bit(tmp_live,first);

           if( OptoReg::is_valid(second) ) set_live_bit(tmp_live,second);

           // If we use the stack pointer in a cisc-alternative instruction,

           // check for use as a memory operand.  Then reconstruct the RegName

           // for this stack location, and set the appropriate bit in the

           // live vector 4987749.

           if (is_cisc_alternate && def == fp) {

             const TypePtr *adr_type = NULL;

             intptr_t offset;

             const Node* base = m->get_base_and_disp(offset, adr_type);

             if (base == NodeSentinel) {

               // Machnode has multiple memory inputs. We are unable to reason

               // with these, but are presuming (with trepidation) that not any of

               // them are oops. This can be fixed by making get_base_and_disp()

               // look at a specific input instead of all inputs.

               assert(!def->bottom_type()->isa_oop_ptr(), "expecting non-oop mem input");

             } else if (base != fp || offset == Type::OffsetBot) {

               // Do nothing: the fp operand is either not from a memory use

               // (base == NULL) OR the fp is used in a non-memory context

               // (base is some other register) OR the offset is not constant,

               // so it is not a stack slot.

             } else {

               assert(offset >= 0, "unexpected negative offset");

               offset -= (offset % jintSize);  // count the whole word

               int stack_reg = regalloc->offset2reg(offset);

               if (OptoReg::is_stack(stack_reg)) {

                 set_live_bit(tmp_live, stack_reg);

               } else {

                 assert(false, "stack_reg not on stack?");

               }

             }

           }

         }

         if( n->jvms() ) {       // Record liveness at safepoint

           // This placement of this stanza means inputs to calls are

           // considered live at the callsite's OopMap.  Argument oops are

           // hence live, but NOT included in the oopmap.  See cutout in

           // build_oop_map.  Debug oops are live (and in OopMap).

           int *n_live = NEW_ARENA_ARRAY(A, int, max_reg_ints);

           for( int l=0; l<max_reg_ints; l++ )

             n_live[l] = tmp_live[l];

           safehash->Insert(n,n_live);

         }

       }

       // Now at block top, see if we have any changes.  If so, propagate

       // to prior blocks.

       int *old_live = &live[b->_pre_order*max_reg_ints];

       int l;

       for( l=0; l<max_reg_ints; l++ )

         if( tmp_live[l] != old_live[l] )

           break;

       if( l<max_reg_ints ) {     // Change!

         // Copy in new value

         for( l=0; l<max_reg_ints; l++ )

           old_live[l] = tmp_live[l];

         // Push preds onto worklist

         for( l=1; l<(int)b->num_preds(); l++ )

           worklist->push(cfg->_bbs[b->pred(l)->_idx]);

       }

     }

     // Scan for any missing safepoints.  Happens to infinite loops

     // ala ZKM.jar

     uint i;

     for( i=1; i<cfg->_num_blocks; i++ ) {

       Block *b = cfg->_blocks[i];

       uint j;

       for( j=1; j<b->_nodes.size(); j++ )

         if( b->_nodes[j]->jvms() &&

             (*safehash)[b->_nodes[j]] == NULL )

            break;

       if( j<b->_nodes.size() ) break;

     }

     if( i == cfg->_num_blocks )

       break;                    // Got 'em all

 #ifndef PRODUCT

     if( PrintOpto && Verbose )

       tty->print_cr("retripping live calc");

 #endif

     // Force the issue (expensively): recheck everybody

     for( i=1; i<cfg->_num_blocks; i++ )

       worklist->push(cfg->_blocks[i]);

   }

 }

 //------------------------------BuildOopMaps-----------------------------------

 // Collect GC mask info - where are all the OOPs?

 void Compile::BuildOopMaps() {

   NOT_PRODUCT( TracePhase t3("bldOopMaps", &_t_buildOopMaps, TimeCompiler); )

   // Can't resource-mark because I need to leave all those OopMaps around,

   // or else I need to resource-mark some arena other than the default.

   // ResourceMark rm;              // Reclaim all OopFlows when done

   int max_reg = _regalloc->_max_reg; // Current array extent

   Arena *A = Thread::current()->resource_area();

   Block_List worklist;          // Worklist of pending blocks

   int max_reg_ints = round_to(max_reg, BitsPerInt)>>LogBitsPerInt;

   Dict *safehash = NULL;        // Used for assert only

   // Compute a backwards liveness per register.  Needs a bitarray of

   // #blocks x (#registers, rounded up to ints)

   safehash = new Dict(cmpkey,hashkey,A);

   do_liveness( _regalloc, _cfg, &worklist, max_reg_ints, A, safehash );

   OopFlow *free_list = NULL;    // Free, unused

   // Array mapping blocks to completed oopflows

   OopFlow **flows = NEW_ARENA_ARRAY(A, OopFlow*, _cfg->_num_blocks);

   memset( flows, 0, _cfg->_num_blocks*sizeof(OopFlow*) );

   // Do the first block 'by hand' to prime the worklist

   Block *entry = _cfg->_blocks[1];

   OopFlow *rootflow = OopFlow::make(A,max_reg);

   // Initialize to 'bottom' (not 'top')

   memset( rootflow->_callees, OptoReg::Bad, max_reg*sizeof(short) );

   memset( rootflow->_defs   ,            0, max_reg*sizeof(Node*) );

   flows[entry->_pre_order] = rootflow;

   // Do the first block 'by hand' to prime the worklist

   rootflow->_b = entry;

   rootflow->compute_reach( _regalloc, max_reg, safehash );

   for( uint i=0; i<entry->_num_succs; i++ )

     worklist.push(entry->_succs[i]);

   // Now worklist contains blocks which have some, but perhaps not all,

   // predecessors visited.

   while( worklist.size() ) {

     // Scan for a block with all predecessors visited, or any randoms slob

     // otherwise.  All-preds-visited order allows me to recycle OopFlow

     // structures rapidly and cut down on the memory footprint.

     // Note: not all predecessors might be visited yet (must happen for

     // irreducible loops).  This is OK, since every live value must have the

     // SAME reaching def for the block, so any reaching def is OK.

     uint i;

     Block *b = worklist.pop();

     // Ignore root block

     if( b == _cfg->_broot ) continue;

     // Block is already done?  Happens if block has several predecessors,

     // he can get on the worklist more than once.

     if( flows[b->_pre_order] ) continue;

     // If this block has a visited predecessor AND that predecessor has this

     // last block as his only undone child, we can move the OopFlow from the

     // pred to this block.  Otherwise we have to grab a new OopFlow.

     OopFlow *flow = NULL;       // Flag for finding optimized flow

     Block *pred = (Block*)0xdeadbeef;

     uint j;

     // Scan this block's preds to find a done predecessor

     for( j=1; j<b->num_preds(); j++ ) {

       Block *p = _cfg->_bbs[b->pred(j)->_idx];

       OopFlow *p_flow = flows[p->_pre_order];

       if( p_flow ) {            // Predecessor is done

         assert( p_flow->_b == p, "cross check" );

         pred = p;               // Record some predecessor

         // If all successors of p are done except for 'b', then we can carry

         // p_flow forward to 'b' without copying, otherwise we have to draw

         // from the free_list and clone data.

         uint k;

         for( k=0; k<p->_num_succs; k++ )

           if( !flows[p->_succs[k]->_pre_order] &&

               p->_succs[k] != b )

             break;

         // Either carry-forward the now-unused OopFlow for b's use

         // or draw a new one from the free list

         if( k==p->_num_succs ) {

           flow = p_flow;

           break;                // Found an ideal pred, use him

         }

       }

     }

     if( flow ) {

       // We have an OopFlow that's the last-use of a predecessor.

       // Carry it forward.

     } else {                    // Draw a new OopFlow from the freelist

       if( !free_list )

         free_list = OopFlow::make(A,max_reg);

       flow = free_list;

       assert( flow->_b == NULL, "oopFlow is not free" );

       free_list = flow->_next;

       flow->_next = NULL;

       // Copy/clone over the data

       flow->clone(flows[pred->_pre_order], max_reg);

     }

     // Mark flow for block.  Blocks can only be flowed over once,

     // because after the first time they are guarded from entering

     // this code again.

     assert( flow->_b == pred, "have some prior flow" );

     flow->_b = NULL;

     // Now push flow forward

     flows[b->_pre_order] = flow;// Mark flow for this block

     flow->_b = b;

     flow->compute_reach( _regalloc, max_reg, safehash );

     // Now push children onto worklist

     for( i=0; i<b->_num_succs; i++ )

       worklist.push(b->_succs[i]);

   }

 }