Initial loadjdk7-b24

1 files changed, 623 insertions, 0 deletions

diff --git a/src/share/vm/opto/buildOopMap.cpp b/src/share/vm/opto/buildOopMap.cpp
new file mode 100644
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--- /dev/null
+++ b/src/share/vm/opto/buildOopMap.cpp
@@ -0,0 +1,623 @@
+/*
+ * Copyright 2002-2007 Sun Microsystems, Inc.  All Rights Reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
+ * CA 95054 USA or visit www.sun.com if you need additional information or
+ * have any questions.
+ *
+ */
+
+#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 relevent 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( 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]);
+
+  }
+}