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

 /*

  * Copyright 1997-2009 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.

  *

  */

 // Optimization - Graph Style

 #include "incls/_precompiled.incl"

 #include "incls/_block.cpp.incl"

 //-----------------------------------------------------------------------------

 void Block_Array::grow( uint i ) {

   assert(i >= Max(), "must be an overflow");

   debug_only(_limit = i+1);

   if( i < _size )  return;

   if( !_size ) {

     _size = 1;

     _blocks = (Block**)_arena->Amalloc( _size * sizeof(Block*) );

     _blocks[0] = NULL;

   }

   uint old = _size;

   while( i >= _size ) _size <<= 1;      // Double to fit

   _blocks = (Block**)_arena->Arealloc( _blocks, old*sizeof(Block*),_size*sizeof(Block*));

   Copy::zero_to_bytes( &_blocks[old], (_size-old)*sizeof(Block*) );

 }

 //=============================================================================

 void Block_List::remove(uint i) {

   assert(i < _cnt, "index out of bounds");

   Copy::conjoint_words_to_lower((HeapWord*)&_blocks[i+1], (HeapWord*)&_blocks[i], ((_cnt-i-1)*sizeof(Block*)));

   pop(); // shrink list by one block

 }

 void Block_List::insert(uint i, Block *b) {

   push(b); // grow list by one block

   Copy::conjoint_words_to_higher((HeapWord*)&_blocks[i], (HeapWord*)&_blocks[i+1], ((_cnt-i-1)*sizeof(Block*)));

   _blocks[i] = b;

 }

 #ifndef PRODUCT

 void Block_List::print() {

   for (uint i=0; i < size(); i++) {

     tty->print("B%d ", _blocks[i]->_pre_order);

   }

   tty->print("size = %d\n", size());

 }

 #endif

 //=============================================================================

 uint Block::code_alignment() {

   // Check for Root block

   if( _pre_order == 0 ) return CodeEntryAlignment;

   // Check for Start block

   if( _pre_order == 1 ) return InteriorEntryAlignment;

   // Check for loop alignment

   if (has_loop_alignment())  return loop_alignment();

   return 1;                     // no particular alignment

 }

 uint Block::compute_loop_alignment() {

   Node *h = head();

   if( h->is_Loop() && h->as_Loop()->is_inner_loop() )  {

     // Pre- and post-loops have low trip count so do not bother with

     // NOPs for align loop head.  The constants are hidden from tuning

     // but only because my "divide by 4" heuristic surely gets nearly

     // all possible gain (a "do not align at all" heuristic has a

     // chance of getting a really tiny gain).

     if( h->is_CountedLoop() && (h->as_CountedLoop()->is_pre_loop() ||

                                 h->as_CountedLoop()->is_post_loop()) )

       return (OptoLoopAlignment > 4) ? (OptoLoopAlignment>>2) : 1;

     // Loops with low backedge frequency should not be aligned.

     Node *n = h->in(LoopNode::LoopBackControl)->in(0);

     if( n->is_MachIf() && n->as_MachIf()->_prob < 0.01 ) {

       return 1;             // Loop does not loop, more often than not!

     }

     return OptoLoopAlignment; // Otherwise align loop head

   }

   return 1;                     // no particular alignment

 }

 //-----------------------------------------------------------------------------

 // Compute the size of first 'inst_cnt' instructions in this block.

 // Return the number of instructions left to compute if the block has

 // less then 'inst_cnt' instructions. Stop, and return 0 if sum_size

 // exceeds OptoLoopAlignment.

 uint Block::compute_first_inst_size(uint& sum_size, uint inst_cnt,

                                     PhaseRegAlloc* ra) {

   uint last_inst = _nodes.size();

   for( uint j = 0; j < last_inst && inst_cnt > 0; j++ ) {

     uint inst_size = _nodes[j]->size(ra);

     if( inst_size > 0 ) {

       inst_cnt--;

       uint sz = sum_size + inst_size;

       if( sz <= (uint)OptoLoopAlignment ) {

         // Compute size of instructions which fit into fetch buffer only

         // since all inst_cnt instructions will not fit even if we align them.

         sum_size = sz;

       } else {

         return 0;

       }

     }

   }

   return inst_cnt;

 }

 //-----------------------------------------------------------------------------

 uint Block::find_node( const Node *n ) const {

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

     if( _nodes[i] == n )

       return i;

   }

   ShouldNotReachHere();

   return 0;

 }

 // Find and remove n from block list

 void Block::find_remove( const Node *n ) {

   _nodes.remove(find_node(n));

 }

 //------------------------------is_Empty---------------------------------------

 // Return empty status of a block.  Empty blocks contain only the head, other

 // ideal nodes, and an optional trailing goto.

 int Block::is_Empty() const {

   // Root or start block is not considered empty

   if (head()->is_Root() || head()->is_Start()) {

     return not_empty;

   }

   int success_result = completely_empty;

   int end_idx = _nodes.size()-1;

   // Check for ending goto

   if ((end_idx > 0) && (_nodes[end_idx]->is_Goto())) {

     success_result = empty_with_goto;

     end_idx--;

   }

   // Unreachable blocks are considered empty

   if (num_preds() <= 1) {

     return success_result;

   }

   // Ideal nodes are allowable in empty blocks: skip them  Only MachNodes

   // turn directly into code, because only MachNodes have non-trivial

   // emit() functions.

   while ((end_idx > 0) && !_nodes[end_idx]->is_Mach()) {

     end_idx--;

   }

   // No room for any interesting instructions?

   if (end_idx == 0) {

     return success_result;

   }

   return not_empty;

 }

 //------------------------------has_uncommon_code------------------------------

 // Return true if the block's code implies that it is likely to be

 // executed infrequently.  Check to see if the block ends in a Halt or

 // a low probability call.

 bool Block::has_uncommon_code() const {

   Node* en = end();

   if (en->is_Goto())

     en = en->in(0);

   if (en->is_Catch())

     en = en->in(0);

   if (en->is_Proj() && en->in(0)->is_MachCall()) {

     MachCallNode* call = en->in(0)->as_MachCall();

     if (call->cnt() != COUNT_UNKNOWN && call->cnt() <= PROB_UNLIKELY_MAG(4)) {

       // This is true for slow-path stubs like new_{instance,array},

       // slow_arraycopy, complete_monitor_locking, uncommon_trap.

       // The magic number corresponds to the probability of an uncommon_trap,

       // even though it is a count not a probability.

       return true;

     }

   }

   int op = en->is_Mach() ? en->as_Mach()->ideal_Opcode() : en->Opcode();

   return op == Op_Halt;

 }

 //------------------------------is_uncommon------------------------------------

 // True if block is low enough frequency or guarded by a test which

 // mostly does not go here.

 bool Block::is_uncommon( Block_Array &bbs ) const {

   // Initial blocks must never be moved, so are never uncommon.

   if (head()->is_Root() || head()->is_Start())  return false;

   // Check for way-low freq

   if( _freq < BLOCK_FREQUENCY(0.00001f) ) return true;

   // Look for code shape indicating uncommon_trap or slow path

   if (has_uncommon_code()) return true;

   const float epsilon = 0.05f;

   const float guard_factor = PROB_UNLIKELY_MAG(4) / (1.f - epsilon);

   uint uncommon_preds = 0;

   uint freq_preds = 0;

   uint uncommon_for_freq_preds = 0;

   for( uint i=1; i<num_preds(); i++ ) {

     Block* guard = bbs[pred(i)->_idx];

     // Check to see if this block follows its guard 1 time out of 10000

     // or less.

     //

     // See list of magnitude-4 unlikely probabilities in cfgnode.hpp which

     // we intend to be "uncommon", such as slow-path TLE allocation,

     // predicted call failure, and uncommon trap triggers.

     //

     // Use an epsilon value of 5% to allow for variability in frequency

     // predictions and floating point calculations. The net effect is

     // that guard_factor is set to 9500.

     //

     // Ignore low-frequency blocks.

     // The next check is (guard->_freq < 1.e-5 * 9500.).

     if(guard->_freq*BLOCK_FREQUENCY(guard_factor) < BLOCK_FREQUENCY(0.00001f)) {

       uncommon_preds++;

     } else {

       freq_preds++;

       if( _freq < guard->_freq * guard_factor ) {

         uncommon_for_freq_preds++;

       }

     }

   }

   if( num_preds() > 1 &&

       // The block is uncommon if all preds are uncommon or

       (uncommon_preds == (num_preds()-1) ||

       // it is uncommon for all frequent preds.

        uncommon_for_freq_preds == freq_preds) ) {

     return true;

   }

   return false;

 }

 //------------------------------dump-------------------------------------------

 #ifndef PRODUCT

 void Block::dump_bidx(const Block* orig) const {

   if (_pre_order) tty->print("B%d",_pre_order);

   else tty->print("N%d", head()->_idx);

   if (Verbose && orig != this) {

     // Dump the original block's idx

     tty->print(" (");

     orig->dump_bidx(orig);

     tty->print(")");

   }

 }

 void Block::dump_pred(const Block_Array *bbs, Block* orig) const {

   if (is_connector()) {

     for (uint i=1; i<num_preds(); i++) {

       Block *p = ((*bbs)[pred(i)->_idx]);

       p->dump_pred(bbs, orig);

     }

   } else {

     dump_bidx(orig);

     tty->print(" ");

   }

 }

 void Block::dump_head( const Block_Array *bbs ) const {

   // Print the basic block

   dump_bidx(this);

   tty->print(": #\t");

   // Print the incoming CFG edges and the outgoing CFG edges

   for( uint i=0; i<_num_succs; i++ ) {

     non_connector_successor(i)->dump_bidx(_succs[i]);

     tty->print(" ");

   }

   tty->print("<- ");

   if( head()->is_block_start() ) {

     for (uint i=1; i<num_preds(); i++) {

       Node *s = pred(i);

       if (bbs) {

         Block *p = (*bbs)[s->_idx];

         p->dump_pred(bbs, p);

       } else {

         while (!s->is_block_start())

           s = s->in(0);

         tty->print("N%d ", s->_idx );

       }

     }

   } else

     tty->print("BLOCK HEAD IS JUNK  ");

   // Print loop, if any

   const Block *bhead = this;    // Head of self-loop

   Node *bh = bhead->head();

   if( bbs && bh->is_Loop() && !head()->is_Root() ) {

     LoopNode *loop = bh->as_Loop();

     const Block *bx = (*bbs)[loop->in(LoopNode::LoopBackControl)->_idx];

     while (bx->is_connector()) {

       bx = (*bbs)[bx->pred(1)->_idx];

     }

     tty->print("\tLoop: B%d-B%d ", bhead->_pre_order, bx->_pre_order);

     // Dump any loop-specific bits, especially for CountedLoops.

     loop->dump_spec(tty);

   } else if (has_loop_alignment()) {

     tty->print(" top-of-loop");

   }

   tty->print(" Freq: %g",_freq);

   if( Verbose || WizardMode ) {

     tty->print(" IDom: %d/#%d", _idom ? _idom->_pre_order : 0, _dom_depth);

     tty->print(" RegPressure: %d",_reg_pressure);

     tty->print(" IHRP Index: %d",_ihrp_index);

     tty->print(" FRegPressure: %d",_freg_pressure);

     tty->print(" FHRP Index: %d",_fhrp_index);

   }

   tty->print_cr("");

 }

 void Block::dump() const { dump(0); }

 void Block::dump( const Block_Array *bbs ) const {

   dump_head(bbs);

   uint cnt = _nodes.size();

   for( uint i=0; i<cnt; i++ )

     _nodes[i]->dump();

   tty->print("\n");

 }

 #endif

 //=============================================================================

 //------------------------------PhaseCFG---------------------------------------

 PhaseCFG::PhaseCFG( Arena *a, RootNode *r, Matcher &m ) :

   Phase(CFG),

   _bbs(a),

   _root(r)

 #ifndef PRODUCT

   , _trace_opto_pipelining(TraceOptoPipelining || C->method_has_option("TraceOptoPipelining"))

 #endif

 #ifdef ASSERT

   , _raw_oops(a)

 #endif

 {

   ResourceMark rm;

   // I'll need a few machine-specific GotoNodes.  Make an Ideal GotoNode,

   // then Match it into a machine-specific Node.  Then clone the machine

   // Node on demand.

   Node *x = new (C, 1) GotoNode(NULL);

   x->init_req(0, x);

   _goto = m.match_tree(x);

   assert(_goto != NULL, "");

   _goto->set_req(0,_goto);

   // Build the CFG in Reverse Post Order

   _num_blocks = build_cfg();

   _broot = _bbs[_root->_idx];

 }

 //------------------------------build_cfg--------------------------------------

 // Build a proper looking CFG.  Make every block begin with either a StartNode

 // or a RegionNode.  Make every block end with either a Goto, If or Return.

 // The RootNode both starts and ends it's own block.  Do this with a recursive

 // backwards walk over the control edges.

 uint PhaseCFG::build_cfg() {

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

   VectorSet visited(a);

   // Allocate stack with enough space to avoid frequent realloc

   Node_Stack nstack(a, C->unique() >> 1);

   nstack.push(_root, 0);

   uint sum = 0;                 // Counter for blocks

   while (nstack.is_nonempty()) {

     // node and in's index from stack's top

     // 'np' is _root (see above) or RegionNode, StartNode: we push on stack

     // only nodes which point to the start of basic block (see below).

     Node *np = nstack.node();

     // idx > 0, except for the first node (_root) pushed on stack

     // at the beginning when idx == 0.

     // We will use the condition (idx == 0) later to end the build.

     uint idx = nstack.index();

     Node *proj = np->in(idx);

     const Node *x = proj->is_block_proj();

     // Does the block end with a proper block-ending Node?  One of Return,

     // If or Goto? (This check should be done for visited nodes also).

     if (x == NULL) {                    // Does not end right...

       Node *g = _goto->clone(); // Force it to end in a Goto

       g->set_req(0, proj);

       np->set_req(idx, g);

       x = proj = g;

     }

     if (!visited.test_set(x->_idx)) { // Visit this block once

       // Skip any control-pinned middle'in stuff

       Node *p = proj;

       do {

         proj = p;                   // Update pointer to last Control

         p = p->in(0);               // Move control forward

       } while( !p->is_block_proj() &&

                !p->is_block_start() );

       // Make the block begin with one of Region or StartNode.

       if( !p->is_block_start() ) {

         RegionNode *r = new (C, 2) RegionNode( 2 );

         r->init_req(1, p);         // Insert RegionNode in the way

         proj->set_req(0, r);        // Insert RegionNode in the way

         p = r;

       }

       // 'p' now points to the start of this basic block

       // Put self in array of basic blocks

       Block *bb = new (_bbs._arena) Block(_bbs._arena,p);

       _bbs.map(p->_idx,bb);

       _bbs.map(x->_idx,bb);

       if( x != p )                  // Only for root is x == p

         bb->_nodes.push((Node*)x);

       // Now handle predecessors

       ++sum;                        // Count 1 for self block

       uint cnt = bb->num_preds();

       for (int i = (cnt - 1); i > 0; i-- ) { // For all predecessors

         Node *prevproj = p->in(i);  // Get prior input

         assert( !prevproj->is_Con(), "dead input not removed" );

         // Check to see if p->in(i) is a "control-dependent" CFG edge -

         // i.e., it splits at the source (via an IF or SWITCH) and merges

         // at the destination (via a many-input Region).

         // This breaks critical edges.  The RegionNode to start the block

         // will be added when <p,i> is pulled off the node stack

         if ( cnt > 2 ) {             // Merging many things?

           assert( prevproj== bb->pred(i),"");

           if(prevproj->is_block_proj() != prevproj) { // Control-dependent edge?

             // Force a block on the control-dependent edge

             Node *g = _goto->clone();       // Force it to end in a Goto

             g->set_req(0,prevproj);

             p->set_req(i,g);

           }

         }

         nstack.push(p, i);  // 'p' is RegionNode or StartNode

       }

     } else { // Post-processing visited nodes

       nstack.pop();                 // remove node from stack

       // Check if it the fist node pushed on stack at the beginning.

       if (idx == 0) break;          // end of the build

       // Find predecessor basic block

       Block *pb = _bbs[x->_idx];

       // Insert into nodes array, if not already there

       if( !_bbs.lookup(proj->_idx) ) {

         assert( x != proj, "" );

         // Map basic block of projection

         _bbs.map(proj->_idx,pb);

         pb->_nodes.push(proj);

       }

       // Insert self as a child of my predecessor block

       pb->_succs.map(pb->_num_succs++, _bbs[np->_idx]);

       assert( pb->_nodes[ pb->_nodes.size() - pb->_num_succs ]->is_block_proj(),

               "too many control users, not a CFG?" );

     }

   }

   // Return number of basic blocks for all children and self

   return sum;

 }

 //------------------------------insert_goto_at---------------------------------

 // Inserts a goto & corresponding basic block between

 // block[block_no] and its succ_no'th successor block

 void PhaseCFG::insert_goto_at(uint block_no, uint succ_no) {

   // get block with block_no

   assert(block_no < _num_blocks, "illegal block number");

   Block* in  = _blocks[block_no];

   // get successor block succ_no

   assert(succ_no < in->_num_succs, "illegal successor number");

   Block* out = in->_succs[succ_no];

   // Compute frequency of the new block. Do this before inserting

   // new block in case succ_prob() needs to infer the probability from

   // surrounding blocks.

   float freq = in->_freq * in->succ_prob(succ_no);

   // get ProjNode corresponding to the succ_no'th successor of the in block

   ProjNode* proj = in->_nodes[in->_nodes.size() - in->_num_succs + succ_no]->as_Proj();

   // create region for basic block

   RegionNode* region = new (C, 2) RegionNode(2);

   region->init_req(1, proj);

   // setup corresponding basic block

   Block* block = new (_bbs._arena) Block(_bbs._arena, region);

   _bbs.map(region->_idx, block);

   C->regalloc()->set_bad(region->_idx);

   // add a goto node

   Node* gto = _goto->clone(); // get a new goto node

   gto->set_req(0, region);

   // add it to the basic block

   block->_nodes.push(gto);

   _bbs.map(gto->_idx, block);

   C->regalloc()->set_bad(gto->_idx);

   // hook up successor block

   block->_succs.map(block->_num_succs++, out);

   // remap successor's predecessors if necessary

   for (uint i = 1; i < out->num_preds(); i++) {

     if (out->pred(i) == proj) out->head()->set_req(i, gto);

   }

   // remap predecessor's successor to new block

   in->_succs.map(succ_no, block);

   // Set the frequency of the new block

   block->_freq = freq;

   // add new basic block to basic block list

   _blocks.insert(block_no + 1, block);

   _num_blocks++;

 }

 //------------------------------no_flip_branch---------------------------------

 // Does this block end in a multiway branch that cannot have the default case

 // flipped for another case?

 static bool no_flip_branch( Block *b ) {

   int branch_idx = b->_nodes.size() - b->_num_succs-1;

   if( branch_idx < 1 ) return false;

   Node *bra = b->_nodes[branch_idx];

   if( bra->is_Catch() )

     return true;

   if( bra->is_Mach() ) {

     if( bra->is_MachNullCheck() )

       return true;

     int iop = bra->as_Mach()->ideal_Opcode();

     if( iop == Op_FastLock || iop == Op_FastUnlock )

       return true;

   }

   return false;

 }

 //------------------------------convert_NeverBranch_to_Goto--------------------

 // Check for NeverBranch at block end.  This needs to become a GOTO to the

 // true target.  NeverBranch are treated as a conditional branch that always

 // goes the same direction for most of the optimizer and are used to give a

 // fake exit path to infinite loops.  At this late stage they need to turn

 // into Goto's so that when you enter the infinite loop you indeed hang.

 void PhaseCFG::convert_NeverBranch_to_Goto(Block *b) {

   // Find true target

   int end_idx = b->end_idx();

   int idx = b->_nodes[end_idx+1]->as_Proj()->_con;

   Block *succ = b->_succs[idx];

   Node* gto = _goto->clone(); // get a new goto node

   gto->set_req(0, b->head());

   Node *bp = b->_nodes[end_idx];

   b->_nodes.map(end_idx,gto); // Slam over NeverBranch

   _bbs.map(gto->_idx, b);

   C->regalloc()->set_bad(gto->_idx);

   b->_nodes.pop();              // Yank projections

   b->_nodes.pop();              // Yank projections

   b->_succs.map(0,succ);        // Map only successor

   b->_num_succs = 1;

   // remap successor's predecessors if necessary

   uint j;

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

     if( succ->pred(j)->in(0) == bp )

       succ->head()->set_req(j, gto);

   // Kill alternate exit path

   Block *dead = b->_succs[1-idx];

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

     if( dead->pred(j)->in(0) == bp )

       break;

   // Scan through block, yanking dead path from

   // all regions and phis.

   dead->head()->del_req(j);

   for( int k = 1; dead->_nodes[k]->is_Phi(); k++ )

     dead->_nodes[k]->del_req(j);

 }

 //------------------------------move_to_next-----------------------------------

 // Helper function to move block bx to the slot following b_index. Return

 // true if the move is successful, otherwise false

 bool PhaseCFG::move_to_next(Block* bx, uint b_index) {

   if (bx == NULL) return false;

   // Return false if bx is already scheduled.

   uint bx_index = bx->_pre_order;

   if ((bx_index <= b_index) && (_blocks[bx_index] == bx)) {

     return false;

   }

   // Find the current index of block bx on the block list

   bx_index = b_index + 1;

   while( bx_index < _num_blocks && _blocks[bx_index] != bx ) bx_index++;

   assert(_blocks[bx_index] == bx, "block not found");

   // If the previous block conditionally falls into bx, return false,

   // because moving bx will create an extra jump.

   for(uint k = 1; k < bx->num_preds(); k++ ) {

     Block* pred = _bbs[bx->pred(k)->_idx];

     if (pred == _blocks[bx_index-1]) {

       if (pred->_num_succs != 1) {

         return false;

       }

     }

   }

   // Reinsert bx just past block 'b'

   _blocks.remove(bx_index);

   _blocks.insert(b_index + 1, bx);

   return true;

 }

 //------------------------------move_to_end------------------------------------

 // Move empty and uncommon blocks to the end.

 void PhaseCFG::move_to_end(Block *b, uint i) {

   int e = b->is_Empty();

   if (e != Block::not_empty) {

     if (e == Block::empty_with_goto) {

       // Remove the goto, but leave the block.

       b->_nodes.pop();

     }

     // Mark this block as a connector block, which will cause it to be

     // ignored in certain functions such as non_connector_successor().

     b->set_connector();

   }

   // Move the empty block to the end, and don't recheck.

   _blocks.remove(i);

   _blocks.push(b);

 }

 //---------------------------set_loop_alignment--------------------------------

 // Set loop alignment for every block

 void PhaseCFG::set_loop_alignment() {

   uint last = _num_blocks;

   assert( _blocks[0] == _broot, "" );

   for (uint i = 1; i < last; i++ ) {

     Block *b = _blocks[i];

     if (b->head()->is_Loop()) {

       b->set_loop_alignment(b);

     }

   }

 }

 //-----------------------------remove_empty------------------------------------

 // Make empty basic blocks to be "connector" blocks, Move uncommon blocks

 // to the end.

 void PhaseCFG::remove_empty() {

   // Move uncommon blocks to the end

   uint last = _num_blocks;

   assert( _blocks[0] == _broot, "" );

   for (uint i = 1; i < last; i++) {

     Block *b = _blocks[i];

     if (b->is_connector()) break;

     // Check for NeverBranch at block end.  This needs to become a GOTO to the

     // true target.  NeverBranch are treated as a conditional branch that

     // always goes the same direction for most of the optimizer and are used

     // to give a fake exit path to infinite loops.  At this late stage they

     // need to turn into Goto's so that when you enter the infinite loop you

     // indeed hang.

     if( b->_nodes[b->end_idx()]->Opcode() == Op_NeverBranch )

       convert_NeverBranch_to_Goto(b);

     // Look for uncommon blocks and move to end.

     if (!C->do_freq_based_layout()) {

       if( b->is_uncommon(_bbs) ) {

         move_to_end(b, i);

         last--;                   // No longer check for being uncommon!

         if( no_flip_branch(b) ) { // Fall-thru case must follow?

           b = _blocks[i];         // Find the fall-thru block

           move_to_end(b, i);

           last--;

         }

         i--;                      // backup block counter post-increment

       }

     }

   }

   // Move empty blocks to the end

   last = _num_blocks;

   for (uint i = 1; i < last; i++) {

     Block *b = _blocks[i];

     if (b->is_Empty() != Block::not_empty) {

       move_to_end(b, i);

       last--;

       i--;

     }

   } // End of for all blocks

 }

 //-----------------------------fixup_flow--------------------------------------

 // Fix up the final control flow for basic blocks.

 void PhaseCFG::fixup_flow() {

   // Fixup final control flow for the blocks.  Remove jump-to-next

   // block.  If neither arm of a IF follows the conditional branch, we

   // have to add a second jump after the conditional.  We place the

   // TRUE branch target in succs[0] for both GOTOs and IFs.

   for (uint i=0; i < _num_blocks; i++) {

     Block *b = _blocks[i];

     b->_pre_order = i;          // turn pre-order into block-index

     // Connector blocks need no further processing.

     if (b->is_connector()) {

       assert((i+1) == _num_blocks || _blocks[i+1]->is_connector(),

              "All connector blocks should sink to the end");

       continue;

     }

     assert(b->is_Empty() != Block::completely_empty,

            "Empty blocks should be connectors");

     Block *bnext = (i < _num_blocks-1) ? _blocks[i+1] : NULL;

     Block *bs0 = b->non_connector_successor(0);

     // Check for multi-way branches where I cannot negate the test to

     // exchange the true and false targets.

     if( no_flip_branch( b ) ) {

       // Find fall through case - if must fall into its target

       int branch_idx = b->_nodes.size() - b->_num_succs;

       for (uint j2 = 0; j2 < b->_num_succs; j2++) {

         const ProjNode* p = b->_nodes[branch_idx + j2]->as_Proj();

         if (p->_con == 0) {

           // successor j2 is fall through case

           if (b->non_connector_successor(j2) != bnext) {

             // but it is not the next block => insert a goto

             insert_goto_at(i, j2);

           }

           // Put taken branch in slot 0

           if( j2 == 0 && b->_num_succs == 2) {

             // Flip targets in succs map

             Block *tbs0 = b->_succs[0];

             Block *tbs1 = b->_succs[1];

             b->_succs.map( 0, tbs1 );

             b->_succs.map( 1, tbs0 );

           }

           break;

         }

       }

       // Remove all CatchProjs

       for (uint j1 = 0; j1 < b->_num_succs; j1++) b->_nodes.pop();

     } else if (b->_num_succs == 1) {

       // Block ends in a Goto?

       if (bnext == bs0) {

         // We fall into next block; remove the Goto

         b->_nodes.pop();

       }

     } else if( b->_num_succs == 2 ) { // Block ends in a If?

       // Get opcode of 1st projection (matches _succs[0])

       // Note: Since this basic block has 2 exits, the last 2 nodes must

       //       be projections (in any order), the 3rd last node must be

       //       the IfNode (we have excluded other 2-way exits such as

       //       CatchNodes already).

       MachNode *iff   = b->_nodes[b->_nodes.size()-3]->as_Mach();

       ProjNode *proj0 = b->_nodes[b->_nodes.size()-2]->as_Proj();

       ProjNode *proj1 = b->_nodes[b->_nodes.size()-1]->as_Proj();

       // Assert that proj0 and succs[0] match up. Similarly for proj1 and succs[1].

       assert(proj0->raw_out(0) == b->_succs[0]->head(), "Mismatch successor 0");

       assert(proj1->raw_out(0) == b->_succs[1]->head(), "Mismatch successor 1");

       Block *bs1 = b->non_connector_successor(1);

       // Check for neither successor block following the current

       // block ending in a conditional. If so, move one of the

       // successors after the current one, provided that the

       // successor was previously unscheduled, but moveable

       // (i.e., all paths to it involve a branch).

       if( !C->do_freq_based_layout() && bnext != bs0 && bnext != bs1 ) {

         // Choose the more common successor based on the probability

         // of the conditional branch.

         Block *bx = bs0;

         Block *by = bs1;

         // _prob is the probability of taking the true path. Make

         // p the probability of taking successor #1.

         float p = iff->as_MachIf()->_prob;

         if( proj0->Opcode() == Op_IfTrue ) {

           p = 1.0 - p;

         }

         // Prefer successor #1 if p > 0.5

         if (p > PROB_FAIR) {

           bx = bs1;

           by = bs0;

         }

         // Attempt the more common successor first

         if (move_to_next(bx, i)) {

           bnext = bx;

         } else if (move_to_next(by, i)) {

           bnext = by;

         }

       }

       // Check for conditional branching the wrong way.  Negate

       // conditional, if needed, so it falls into the following block

       // and branches to the not-following block.

       // Check for the next block being in succs[0].  We are going to branch

       // to succs[0], so we want the fall-thru case as the next block in

       // succs[1].

       if (bnext == bs0) {

         // Fall-thru case in succs[0], so flip targets in succs map

         Block *tbs0 = b->_succs[0];

         Block *tbs1 = b->_succs[1];

         b->_succs.map( 0, tbs1 );

         b->_succs.map( 1, tbs0 );

         // Flip projection for each target

         { ProjNode *tmp = proj0; proj0 = proj1; proj1 = tmp; }

       } else if( bnext != bs1 ) {

         // Need a double-branch

         // The existing conditional branch need not change.

         // Add a unconditional branch to the false target.

         // Alas, it must appear in its own block and adding a

         // block this late in the game is complicated.  Sigh.

         insert_goto_at(i, 1);

       }

       // Make sure we TRUE branch to the target

       if( proj0->Opcode() == Op_IfFalse ) {

         iff->negate();

       }

       b->_nodes.pop();          // Remove IfFalse & IfTrue projections

       b->_nodes.pop();

     } else {

       // Multi-exit block, e.g. a switch statement

       // But we don't need to do anything here

     }

   } // End of for all blocks

 }

 //------------------------------dump-------------------------------------------

 #ifndef PRODUCT

 void PhaseCFG::_dump_cfg( const Node *end, VectorSet &visited  ) const {

   const Node *x = end->is_block_proj();

   assert( x, "not a CFG" );

   // Do not visit this block again

   if( visited.test_set(x->_idx) ) return;

   // Skip through this block

   const Node *p = x;

   do {

     p = p->in(0);               // Move control forward

     assert( !p->is_block_proj() || p->is_Root(), "not a CFG" );

   } while( !p->is_block_start() );

   // Recursively visit

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

     _dump_cfg(p->in(i),visited);

   // Dump the block

   _bbs[p->_idx]->dump(&_bbs);

 }

 void PhaseCFG::dump( ) const {

   tty->print("\n--- CFG --- %d BBs\n",_num_blocks);

   if( _blocks.size() ) {        // Did we do basic-block layout?

     for( uint i=0; i<_num_blocks; i++ )

       _blocks[i]->dump(&_bbs);

   } else {                      // Else do it with a DFS

     VectorSet visited(_bbs._arena);

     _dump_cfg(_root,visited);

   }

 }

 void PhaseCFG::dump_headers() {

   for( uint i = 0; i < _num_blocks; i++ ) {

     if( _blocks[i] == NULL ) continue;

     _blocks[i]->dump_head(&_bbs);

   }

 }

 void PhaseCFG::verify( ) const {

 #ifdef ASSERT

   // Verify sane CFG

   for( uint i = 0; i < _num_blocks; i++ ) {

     Block *b = _blocks[i];

     uint cnt = b->_nodes.size();

     uint j;

     for( j = 0; j < cnt; j++ ) {

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

       assert( _bbs[n->_idx] == b, "" );

       if( j >= 1 && n->is_Mach() &&

           n->as_Mach()->ideal_Opcode() == Op_CreateEx ) {

         assert( j == 1 || b->_nodes[j-1]->is_Phi(),

                 "CreateEx must be first instruction in block" );

       }

       for( uint k = 0; k < n->req(); k++ ) {

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

         if( def && def != n ) {

           assert( _bbs[def->_idx] || def->is_Con(),

                   "must have block; constants for debug info ok" );

           // Verify that instructions in the block is in correct order.

           // Uses must follow their definition if they are at the same block.

           // Mostly done to check that MachSpillCopy nodes are placed correctly

           // when CreateEx node is moved in build_ifg_physical().

           if( _bbs[def->_idx] == b &&

               !(b->head()->is_Loop() && n->is_Phi()) &&

               // See (+++) comment in reg_split.cpp

               !(n->jvms() != NULL && n->jvms()->is_monitor_use(k)) ) {

             bool is_loop = false;

             if (n->is_Phi()) {

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

                 if (n == def->in(l)) {

                   is_loop = true;

                   break; // Some kind of loop

                 }

               }

             }

             assert( is_loop || b->find_node(def) < j, "uses must follow definitions" );

           }

           if( def->is_SafePointScalarObject() ) {

             assert(_bbs[def->_idx] == b, "SafePointScalarObject Node should be at the same block as its SafePoint node");

             assert(_bbs[def->_idx] == _bbs[def->in(0)->_idx], "SafePointScalarObject Node should be at the same block as its control edge");

           }

         }

       }

     }

     j = b->end_idx();

     Node *bp = (Node*)b->_nodes[b->_nodes.size()-1]->is_block_proj();

     assert( bp, "last instruction must be a block proj" );

     assert( bp == b->_nodes[j], "wrong number of successors for this block" );

     if( bp->is_Catch() ) {

       while( b->_nodes[--j]->Opcode() == Op_MachProj ) ;

       assert( b->_nodes[j]->is_Call(), "CatchProj must follow call" );

     }

     else if( bp->is_Mach() && bp->as_Mach()->ideal_Opcode() == Op_If ) {

       assert( b->_num_succs == 2, "Conditional branch must have two targets");

     }

   }

 #endif

 }

 #endif

 //=============================================================================

 //------------------------------UnionFind--------------------------------------

 UnionFind::UnionFind( uint max ) : _cnt(max), _max(max), _indices(NEW_RESOURCE_ARRAY(uint,max)) {

   Copy::zero_to_bytes( _indices, sizeof(uint)*max );

 }

 void UnionFind::extend( uint from_idx, uint to_idx ) {

   _nesting.check();

   if( from_idx >= _max ) {

     uint size = 16;

     while( size <= from_idx ) size <<=1;

     _indices = REALLOC_RESOURCE_ARRAY( uint, _indices, _max, size );

     _max = size;

   }

   while( _cnt <= from_idx ) _indices[_cnt++] = 0;

   _indices[from_idx] = to_idx;

 }

 void UnionFind::reset( uint max ) {

   assert( max <= max_uint, "Must fit within uint" );

   // Force the Union-Find mapping to be at least this large

   extend(max,0);

   // Initialize to be the ID mapping.

   for( uint i=0; i<max; i++ ) map(i,i);

 }

 //------------------------------Find_compress----------------------------------

 // Straight out of Tarjan's union-find algorithm

 uint UnionFind::Find_compress( uint idx ) {

   uint cur  = idx;

   uint next = lookup(cur);

   while( next != cur ) {        // Scan chain of equivalences

     assert( next < cur, "always union smaller" );

     cur = next;                 // until find a fixed-point

     next = lookup(cur);

   }

   // Core of union-find algorithm: update chain of

   // equivalences to be equal to the root.

   while( idx != next ) {

     uint tmp = lookup(idx);

     map(idx, next);

     idx = tmp;

   }

   return idx;

 }

 //------------------------------Find_const-------------------------------------

 // Like Find above, but no path compress, so bad asymptotic behavior

 uint UnionFind::Find_const( uint idx ) const {

   if( idx == 0 ) return idx;    // Ignore the zero idx

   // Off the end?  This can happen during debugging dumps

   // when data structures have not finished being updated.

   if( idx >= _max ) return idx;

   uint next = lookup(idx);

   while( next != idx ) {        // Scan chain of equivalences

     idx = next;                 // until find a fixed-point

     next = lookup(idx);

   }

   return next;

 }

 //------------------------------Union------------------------------------------

 // union 2 sets together.

 void UnionFind::Union( uint idx1, uint idx2 ) {

   uint src = Find(idx1);

   uint dst = Find(idx2);

   assert( src, "" );

   assert( dst, "" );

   assert( src < _max, "oob" );

   assert( dst < _max, "oob" );

   assert( src < dst, "always union smaller" );

   map(dst,src);

 }

 #ifndef PRODUCT

 static void edge_dump(GrowableArray<CFGEdge *> *edges) {

   tty->print_cr("---- Edges ----");

   for (int i = 0; i < edges->length(); i++) {

     CFGEdge *e = edges->at(i);

     if (e != NULL) {

       edges->at(i)->dump();

     }

   }

 }

 static void trace_dump(Trace *traces[], int count) {

   tty->print_cr("---- Traces ----");

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

     Trace *tr = traces[i];

     if (tr != NULL) {

       tr->dump();

     }

   }

 }

 void Trace::dump( ) const {

   tty->print_cr("Trace (freq %f)", first_block()->_freq);

   for (Block *b = first_block(); b != NULL; b = next(b)) {

     tty->print("  B%d", b->_pre_order);

     if (b->head()->is_Loop()) {

       tty->print(" (L%d)", b->compute_loop_alignment());

     }

     if (b->has_loop_alignment()) {

       tty->print(" (T%d)", b->code_alignment());

     }

   }

   tty->cr();

 }

 void CFGEdge::dump( ) const {

   tty->print(" B%d  -->  B%d  Freq: %f  out:%3d%%  in:%3d%%  State: ",

              from()->_pre_order, to()->_pre_order, freq(), _from_pct, _to_pct);

   switch(state()) {

   case connected:

     tty->print("connected");

     break;

   case open:

     tty->print("open");

     break;

   case interior:

     tty->print("interior");

     break;

   }

   if (infrequent()) {

     tty->print("  infrequent");

   }

   tty->cr();

 }

 #endif

 //=============================================================================

 //------------------------------edge_order-------------------------------------

 // Comparison function for edges

 static int edge_order(CFGEdge **e0, CFGEdge **e1) {

   float freq0 = (*e0)->freq();

   float freq1 = (*e1)->freq();

   if (freq0 != freq1) {

     return freq0 > freq1 ? -1 : 1;

   }

   int dist0 = (*e0)->to()->_rpo - (*e0)->from()->_rpo;

   int dist1 = (*e1)->to()->_rpo - (*e1)->from()->_rpo;

   return dist1 - dist0;

 }

 //------------------------------trace_frequency_order--------------------------

 // Comparison function for edges

 static int trace_frequency_order(const void *p0, const void *p1) {

   Trace *tr0 = *(Trace **) p0;

   Trace *tr1 = *(Trace **) p1;

   Block *b0 = tr0->first_block();

   Block *b1 = tr1->first_block();

   // The trace of connector blocks goes at the end;

   // we only expect one such trace

   if (b0->is_connector() != b1->is_connector()) {

     return b1->is_connector() ? -1 : 1;

   }

   // Pull more frequently executed blocks to the beginning

   float freq0 = b0->_freq;

   float freq1 = b1->_freq;

   if (freq0 != freq1) {

     return freq0 > freq1 ? -1 : 1;

   }

   int diff = tr0->first_block()->_rpo - tr1->first_block()->_rpo;

   return diff;

 }

 //------------------------------find_edges-------------------------------------

 // Find edges of interest, i.e, those which can fall through. Presumes that

 // edges which don't fall through are of low frequency and can be generally

 // ignored.  Initialize the list of traces.

 void PhaseBlockLayout::find_edges()

 {

   // Walk the blocks, creating edges and Traces

   uint i;

   Trace *tr = NULL;

   for (i = 0; i < _cfg._num_blocks; i++) {

     Block *b = _cfg._blocks[i];

     tr = new Trace(b, next, prev);

     traces[tr->id()] = tr;

     // All connector blocks should be at the end of the list

     if (b->is_connector()) break;

     // If this block and the next one have a one-to-one successor

     // predecessor relationship, simply append the next block

     int nfallthru = b->num_fall_throughs();

     while (nfallthru == 1 &&

            b->succ_fall_through(0)) {

       Block *n = b->_succs[0];

       // Skip over single-entry connector blocks, we don't want to

       // add them to the trace.

       while (n->is_connector() && n->num_preds() == 1) {

         n = n->_succs[0];

       }

       // We see a merge point, so stop search for the next block

       if (n->num_preds() != 1) break;

       i++;

       assert(n = _cfg._blocks[i], "expecting next block");

       tr->append(n);

       uf->map(n->_pre_order, tr->id());

       traces[n->_pre_order] = NULL;

       nfallthru = b->num_fall_throughs();

       b = n;

     }

     if (nfallthru > 0) {

       // Create a CFGEdge for each outgoing

       // edge that could be a fall-through.

       for (uint j = 0; j < b->_num_succs; j++ ) {

         if (b->succ_fall_through(j)) {

           Block *target = b->non_connector_successor(j);

           float freq = b->_freq * b->succ_prob(j);

           int from_pct = (int) ((100 * freq) / b->_freq);

           int to_pct = (int) ((100 * freq) / target->_freq);

           edges->append(new CFGEdge(b, target, freq, from_pct, to_pct));

         }

       }

     }

   }

   // Group connector blocks into one trace

   for (i++; i < _cfg._num_blocks; i++) {

     Block *b = _cfg._blocks[i];

     assert(b->is_connector(), "connector blocks at the end");

     tr->append(b);

     uf->map(b->_pre_order, tr->id());

     traces[b->_pre_order] = NULL;

   }

 }

 //------------------------------union_traces----------------------------------

 // Union two traces together in uf, and null out the trace in the list

 void PhaseBlockLayout::union_traces(Trace* updated_trace, Trace* old_trace)

 {

   uint old_id = old_trace->id();

   uint updated_id = updated_trace->id();

   uint lo_id = updated_id;

   uint hi_id = old_id;

   // If from is greater than to, swap values to meet

   // UnionFind guarantee.

   if (updated_id > old_id) {

     lo_id = old_id;

     hi_id = updated_id;

     // Fix up the trace ids

     traces[lo_id] = traces[updated_id];

     updated_trace->set_id(lo_id);

   }

   // Union the lower with the higher and remove the pointer

   // to the higher.

   uf->Union(lo_id, hi_id);

   traces[hi_id] = NULL;

 }

 //------------------------------grow_traces-------------------------------------

 // Append traces together via the most frequently executed edges

 void PhaseBlockLayout::grow_traces()

 {

   // Order the edges, and drive the growth of Traces via the most

   // frequently executed edges.

   edges->sort(edge_order);

   for (int i = 0; i < edges->length(); i++) {

     CFGEdge *e = edges->at(i);

     if (e->state() != CFGEdge::open) continue;

     Block *src_block = e->from();

     Block *targ_block = e->to();

     // Don't grow traces along backedges?

     if (!BlockLayoutRotateLoops) {

       if (targ_block->_rpo <= src_block->_rpo) {

         targ_block->set_loop_alignment(targ_block);

         continue;

       }

     }

     Trace *src_trace = trace(src_block);

     Trace *targ_trace = trace(targ_block);

     // If the edge in question can join two traces at their ends,

     // append one trace to the other.

    if (src_trace->last_block() == src_block) {

       if (src_trace == targ_trace) {

         e->set_state(CFGEdge::interior);

         if (targ_trace->backedge(e)) {

           // Reset i to catch any newly eligible edge

           // (Or we could remember the first "open" edge, and reset there)

           i = 0;

         }

       } else if (targ_trace->first_block() == targ_block) {

         e->set_state(CFGEdge::connected);

         src_trace->append(targ_trace);

         union_traces(src_trace, targ_trace);

       }

     }

   }

 }

 //------------------------------merge_traces-----------------------------------

 // Embed one trace into another, if the fork or join points are sufficiently

 // balanced.

 void PhaseBlockLayout::merge_traces(bool fall_thru_only)

 {

   // Walk the edge list a another time, looking at unprocessed edges.

   // Fold in diamonds

   for (int i = 0; i < edges->length(); i++) {

     CFGEdge *e = edges->at(i);

     if (e->state() != CFGEdge::open) continue;

     if (fall_thru_only) {

       if (e->infrequent()) continue;

     }

     Block *src_block = e->from();

     Trace *src_trace = trace(src_block);

     bool src_at_tail = src_trace->last_block() == src_block;

     Block *targ_block  = e->to();

     Trace *targ_trace  = trace(targ_block);

     bool targ_at_start = targ_trace->first_block() == targ_block;

     if (src_trace == targ_trace) {

       // This may be a loop, but we can't do much about it.

       e->set_state(CFGEdge::interior);

       continue;

     }

     if (fall_thru_only) {

       // If the edge links the middle of two traces, we can't do anything.

       // Mark the edge and continue.

       if (!src_at_tail & !targ_at_start) {

         continue;

       }

       // Don't grow traces along backedges?

       if (!BlockLayoutRotateLoops && (targ_block->_rpo <= src_block->_rpo)) {

           continue;

       }

       // If both ends of the edge are available, why didn't we handle it earlier?

       assert(src_at_tail ^ targ_at_start, "Should have caught this edge earlier.");

       if (targ_at_start) {

         // Insert the "targ" trace in the "src" trace if the insertion point

         // is a two way branch.

         // Better profitability check possible, but may not be worth it.

         // Someday, see if the this "fork" has an associated "join";

         // then make a policy on merging this trace at the fork or join.

         // For example, other things being equal, it may be better to place this

         // trace at the join point if the "src" trace ends in a two-way, but

         // the insertion point is one-way.

         assert(src_block->num_fall_throughs() == 2, "unexpected diamond");

         e->set_state(CFGEdge::connected);

         src_trace->insert_after(src_block, targ_trace);

         union_traces(src_trace, targ_trace);

       } else if (src_at_tail) {

         if (src_trace != trace(_cfg._broot)) {

           e->set_state(CFGEdge::connected);

           targ_trace->insert_before(targ_block, src_trace);

           union_traces(targ_trace, src_trace);

         }

       }

     } else if (e->state() == CFGEdge::open) {

       // Append traces, even without a fall-thru connection.

       // But leave root entry at the beginning of the block list.

       if (targ_trace != trace(_cfg._broot)) {

         e->set_state(CFGEdge::connected);

         src_trace->append(targ_trace);

         union_traces(src_trace, targ_trace);

       }

     }

   }

 }

 //----------------------------reorder_traces-----------------------------------

 // Order the sequence of the traces in some desirable way, and fixup the

 // jumps at the end of each block.

 void PhaseBlockLayout::reorder_traces(int count)

 {

   ResourceArea *area = Thread::current()->resource_area();

   Trace ** new_traces = NEW_ARENA_ARRAY(area, Trace *, count);

   Block_List worklist;

   int new_count = 0;

   // Compact the traces.

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

     Trace *tr = traces[i];

     if (tr != NULL) {

       new_traces[new_count++] = tr;

     }

   }

   // The entry block should be first on the new trace list.

   Trace *tr = trace(_cfg._broot);

   assert(tr == new_traces[0], "entry trace misplaced");

   // Sort the new trace list by frequency

   qsort(new_traces + 1, new_count - 1, sizeof(new_traces[0]), trace_frequency_order);

   // Patch up the successor blocks

   _cfg._blocks.reset();

   _cfg._num_blocks = 0;

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

     Trace *tr = new_traces[i];

     if (tr != NULL) {

       tr->fixup_blocks(_cfg);

     }

   }

 }

 //------------------------------PhaseBlockLayout-------------------------------

 // Order basic blocks based on frequency

 PhaseBlockLayout::PhaseBlockLayout(PhaseCFG &cfg) :

   Phase(BlockLayout),

   _cfg(cfg)

 {

   ResourceMark rm;

   ResourceArea *area = Thread::current()->resource_area();

   // List of traces

   int size = _cfg._num_blocks + 1;

   traces = NEW_ARENA_ARRAY(area, Trace *, size);

   memset(traces, 0, size*sizeof(Trace*));

   next = NEW_ARENA_ARRAY(area, Block *, size);

   memset(next,   0, size*sizeof(Block *));

   prev = NEW_ARENA_ARRAY(area, Block *, size);

   memset(prev  , 0, size*sizeof(Block *));

   // List of edges

   edges = new GrowableArray<CFGEdge*>;

   // Mapping block index --> block_trace

   uf = new UnionFind(size);

   uf->reset(size);

   // Find edges and create traces.

   find_edges();

   // Grow traces at their ends via most frequent edges.

   grow_traces();

   // Merge one trace into another, but only at fall-through points.

   // This may make diamonds and other related shapes in a trace.

   merge_traces(true);

   // Run merge again, allowing two traces to be catenated, even if

   // one does not fall through into the other. This appends loosely

   // related traces to be near each other.

   merge_traces(false);

   // Re-order all the remaining traces by frequency

   reorder_traces(size);

   assert(_cfg._num_blocks >= (uint) (size - 1), "number of blocks can not shrink");

 }

 //------------------------------backedge---------------------------------------

 // Edge e completes a loop in a trace. If the target block is head of the

 // loop, rotate the loop block so that the loop ends in a conditional branch.

 bool Trace::backedge(CFGEdge *e) {

   bool loop_rotated = false;

   Block *src_block  = e->from();

   Block *targ_block    = e->to();

   assert(last_block() == src_block, "loop discovery at back branch");

   if (first_block() == targ_block) {

     if (BlockLayoutRotateLoops && last_block()->num_fall_throughs() < 2) {

       // Find the last block in the trace that has a conditional

       // branch.

       Block *b;

       for (b = last_block(); b != NULL; b = prev(b)) {

         if (b->num_fall_throughs() == 2) {

           break;

         }

       }

       if (b != last_block() && b != NULL) {

         loop_rotated = true;

         // Rotate the loop by doing two-part linked-list surgery.

         append(first_block());

         break_loop_after(b);

       }

     }

     // Backbranch to the top of a trace

     // Scroll forward through the trace from the targ_block. If we find

     // a loop head before another loop top, use the the loop head alignment.

     for (Block *b = targ_block; b != NULL; b = next(b)) {

       if (b->has_loop_alignment()) {

         break;

       }

       if (b->head()->is_Loop()) {

         targ_block = b;

         break;

       }

     }

     first_block()->set_loop_alignment(targ_block);

   } else {

     // Backbranch into the middle of a trace

     targ_block->set_loop_alignment(targ_block);

   }

   return loop_rotated;

 }

 //------------------------------fixup_blocks-----------------------------------

 // push blocks onto the CFG list

 // ensure that blocks have the correct two-way branch sense

 void Trace::fixup_blocks(PhaseCFG &cfg) {

   Block *last = last_block();

   for (Block *b = first_block(); b != NULL; b = next(b)) {

     cfg._blocks.push(b);

     cfg._num_blocks++;

     if (!b->is_connector()) {

       int nfallthru = b->num_fall_throughs();

       if (b != last) {

         if (nfallthru == 2) {

           // Ensure that the sense of the branch is correct

           Block *bnext = next(b);

           Block *bs0 = b->non_connector_successor(0);

           MachNode *iff = b->_nodes[b->_nodes.size()-3]->as_Mach();

           ProjNode *proj0 = b->_nodes[b->_nodes.size()-2]->as_Proj();

           ProjNode *proj1 = b->_nodes[b->_nodes.size()-1]->as_Proj();

           if (bnext == bs0) {

             // Fall-thru case in succs[0], should be in succs[1]

             // Flip targets in _succs map

             Block *tbs0 = b->_succs[0];

             Block *tbs1 = b->_succs[1];

             b->_succs.map( 0, tbs1 );

             b->_succs.map( 1, tbs0 );

             // Flip projections to match targets

             b->_nodes.map(b->_nodes.size()-2, proj1);

             b->_nodes.map(b->_nodes.size()-1, proj0);

           }

         }

       }

     }

   }

 }