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

 /*

  * Copyright 1997-2008 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

  * have any questions.

  *

  */

 // Optimization - Graph Style

 class Block;

 class CFGLoop;

 class MachCallNode;

 class Matcher;

 class RootNode;

 class VectorSet;

 struct Tarjan;

 //------------------------------Block_Array------------------------------------

 // Map dense integer indices to Blocks.  Uses classic doubling-array trick.

 // Abstractly provides an infinite array of Block*'s, initialized to NULL.

 // Note that the constructor just zeros things, and since I use Arena

 // allocation I do not need a destructor to reclaim storage.

 class Block_Array : public ResourceObj {

   uint _size;                   // allocated size, as opposed to formal limit

   debug_only(uint _limit;)      // limit to formal domain

 protected:

   Block **_blocks;

   void grow( uint i );          // Grow array node to fit

 public:

   Arena *_arena;                // Arena to allocate in

   Block_Array(Arena *a) : _arena(a), _size(OptoBlockListSize) {

     debug_only(_limit=0);

     _blocks = NEW_ARENA_ARRAY( a, Block *, OptoBlockListSize );

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

       _blocks[i] = NULL;

     }

   }

   Block *lookup( uint i ) const // Lookup, or NULL for not mapped

   { return (i<Max()) ? _blocks[i] : (Block*)NULL; }

   Block *operator[] ( uint i ) const // Lookup, or assert for not mapped

   { assert( i < Max(), "oob" ); return _blocks[i]; }

   // Extend the mapping: index i maps to Block *n.

   void map( uint i, Block *n ) { if( i>=Max() ) grow(i); _blocks[i] = n; }

   uint Max() const { debug_only(return _limit); return _size; }

 };

 class Block_List : public Block_Array {

 public:

   uint _cnt;

   Block_List() : Block_Array(Thread::current()->resource_area()), _cnt(0) {}

   void push( Block *b ) { map(_cnt++,b); }

   Block *pop() { return _blocks[--_cnt]; }

   Block *rpop() { Block *b = _blocks[0]; _blocks[0]=_blocks[--_cnt]; return b;}

   void remove( uint i );

   void insert( uint i, Block *n );

   uint size() const { return _cnt; }

   void reset() { _cnt = 0; }

   void print();

 };

 class CFGElement : public ResourceObj {

  public:

   float _freq; // Execution frequency (estimate)

   CFGElement() : _freq(0.0f) {}

   virtual bool is_block() { return false; }

   virtual bool is_loop()  { return false; }

   Block*   as_Block() { assert(is_block(), "must be block"); return (Block*)this; }

   CFGLoop* as_CFGLoop()  { assert(is_loop(),  "must be loop");  return (CFGLoop*)this;  }

 };

 //------------------------------Block------------------------------------------

 // This class defines a Basic Block.

 // Basic blocks are used during the output routines, and are not used during

 // any optimization pass.  They are created late in the game.

 class Block : public CFGElement {

  public:

   // Nodes in this block, in order

   Node_List _nodes;

   // Basic blocks have a Node which defines Control for all Nodes pinned in

   // this block.  This Node is a RegionNode.  Exception-causing Nodes

   // (division, subroutines) and Phi functions are always pinned.  Later,

   // every Node will get pinned to some block.

   Node *head() const { return _nodes[0]; }

   // CAUTION: num_preds() is ONE based, so that predecessor numbers match

   // input edges to Regions and Phis.

   uint num_preds() const { return head()->req(); }

   Node *pred(uint i) const { return head()->in(i); }

   // Array of successor blocks, same size as projs array

   Block_Array _succs;

   // Basic blocks have some number of Nodes which split control to all

   // following blocks.  These Nodes are always Projections.  The field in

   // the Projection and the block-ending Node determine which Block follows.

   uint _num_succs;

   // Basic blocks also carry all sorts of good old fashioned DFS information

   // used to find loops, loop nesting depth, dominators, etc.

   uint _pre_order;              // Pre-order DFS number

   // Dominator tree

   uint _dom_depth;              // Depth in dominator tree for fast LCA

   Block* _idom;                 // Immediate dominator block

   CFGLoop *_loop;               // Loop to which this block belongs

   uint _rpo;                    // Number in reverse post order walk

   virtual bool is_block() { return true; }

   float succ_prob(uint i);      // return probability of i'th successor

   int num_fall_throughs();      // How many fall-through candidate this block has

   void update_uncommon_branch(Block* un); // Lower branch prob to uncommon code

   bool succ_fall_through(uint i); // Is successor "i" is a fall-through candidate

   Block* lone_fall_through();   // Return lone fall-through Block or null

   Block* dom_lca(Block* that);  // Compute LCA in dominator tree.

 #ifdef ASSERT

   bool dominates(Block* that) {

     int dom_diff = this->_dom_depth - that->_dom_depth;

     if (dom_diff > 0)  return false;

     for (; dom_diff < 0; dom_diff++)  that = that->_idom;

     return this == that;

   }

 #endif

   // Report the alignment required by this block.  Must be a power of 2.

   // The previous block will insert nops to get this alignment.

   uint code_alignment();

   uint compute_loop_alignment();

   // BLOCK_FREQUENCY is a sentinel to mark uses of constant block frequencies.

   // It is currently also used to scale such frequencies relative to

   // FreqCountInvocations relative to the old value of 1500.

 #define BLOCK_FREQUENCY(f) ((f * (float) 1500) / FreqCountInvocations)

   // Register Pressure (estimate) for Splitting heuristic

   uint _reg_pressure;

   uint _ihrp_index;

   uint _freg_pressure;

   uint _fhrp_index;

   // Mark and visited bits for an LCA calculation in insert_anti_dependences.

   // Since they hold unique node indexes, they do not need reinitialization.

   node_idx_t _raise_LCA_mark;

   void    set_raise_LCA_mark(node_idx_t x)    { _raise_LCA_mark = x; }

   node_idx_t  raise_LCA_mark() const          { return _raise_LCA_mark; }

   node_idx_t _raise_LCA_visited;

   void    set_raise_LCA_visited(node_idx_t x) { _raise_LCA_visited = x; }

   node_idx_t  raise_LCA_visited() const       { return _raise_LCA_visited; }

   // Estimated size in bytes of first instructions in a loop.

   uint _first_inst_size;

   uint first_inst_size() const     { return _first_inst_size; }

   void set_first_inst_size(uint s) { _first_inst_size = s; }

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

   uint compute_first_inst_size(uint& sum_size, uint inst_cnt, PhaseRegAlloc* ra);

   // Compute alignment padding if the block needs it.

   // Align a loop if loop's padding is less or equal to padding limit

   // or the size of first instructions in the loop > padding.

   uint alignment_padding(int current_offset) {

     int block_alignment = code_alignment();

     int max_pad = block_alignment-relocInfo::addr_unit();

     if( max_pad > 0 ) {

       assert(is_power_of_2(max_pad+relocInfo::addr_unit()), "");

       int current_alignment = current_offset & max_pad;

       if( current_alignment != 0 ) {

         uint padding = (block_alignment-current_alignment) & max_pad;

         if( has_loop_alignment() &&

             padding > (uint)MaxLoopPad &&

             first_inst_size() <= padding ) {

           return 0;

         }

         return padding;

       }

     }

     return 0;

   }

   // Connector blocks. Connector blocks are basic blocks devoid of

   // instructions, but may have relevant non-instruction Nodes, such as

   // Phis or MergeMems. Such blocks are discovered and marked during the

   // RemoveEmpty phase, and elided during Output.

   bool _connector;

   void set_connector() { _connector = true; }

   bool is_connector() const { return _connector; };

   // Loop_alignment will be set for blocks which are at the top of loops.

   // The block layout pass may rotate loops such that the loop head may not

   // be the sequentially first block of the loop encountered in the linear

   // list of blocks.  If the layout pass is not run, loop alignment is set

   // for each block which is the head of a loop.

   uint _loop_alignment;

   void set_loop_alignment(Block *loop_top) {

     uint new_alignment = loop_top->compute_loop_alignment();

     if (new_alignment > _loop_alignment) {

       _loop_alignment = new_alignment;

     }

   }

   uint loop_alignment() const { return _loop_alignment; }

   bool has_loop_alignment() const { return loop_alignment() > 0; }

   // Create a new Block with given head Node.

   // Creates the (empty) predecessor arrays.

   Block( Arena *a, Node *headnode )

     : CFGElement(),

       _nodes(a),

       _succs(a),

       _num_succs(0),

       _pre_order(0),

       _idom(0),

       _loop(NULL),

       _reg_pressure(0),

       _ihrp_index(1),

       _freg_pressure(0),

       _fhrp_index(1),

       _raise_LCA_mark(0),

       _raise_LCA_visited(0),

       _first_inst_size(999999),

       _connector(false),

       _loop_alignment(0) {

     _nodes.push(headnode);

   }

   // Index of 'end' Node

   uint end_idx() const {

     // %%%%% add a proj after every goto

     // so (last->is_block_proj() != last) always, then simplify this code

     // This will not give correct end_idx for block 0 when it only contains root.

     int last_idx = _nodes.size() - 1;

     Node *last  = _nodes[last_idx];

     assert(last->is_block_proj() == last || last->is_block_proj() == _nodes[last_idx - _num_succs], "");

     return (last->is_block_proj() == last) ? last_idx : (last_idx - _num_succs);

   }

   // Basic blocks have a Node which ends them.  This Node determines which

   // basic block follows this one in the program flow.  This Node is either an

   // IfNode, a GotoNode, a JmpNode, or a ReturnNode.

   Node *end() const { return _nodes[end_idx()]; }

   // Add an instruction to an existing block.  It must go after the head

   // instruction and before the end instruction.

   void add_inst( Node *n ) { _nodes.insert(end_idx(),n); }

   // Find node in block

   uint find_node( const Node *n ) const;

   // Find and remove n from block list

   void find_remove( const Node *n );

   // Schedule a call next in the block

   uint sched_call(Matcher &matcher, Block_Array &bbs, uint node_cnt, Node_List &worklist, int *ready_cnt, MachCallNode *mcall, VectorSet &next_call);

   // Perform basic-block local scheduling

   Node *select(PhaseCFG *cfg, Node_List &worklist, int *ready_cnt, VectorSet &next_call, uint sched_slot);

   void set_next_call( Node *n, VectorSet &next_call, Block_Array &bbs );

   void needed_for_next_call(Node *this_call, VectorSet &next_call, Block_Array &bbs);

   bool schedule_local(PhaseCFG *cfg, Matcher &m, int *ready_cnt, VectorSet &next_call);

   // Cleanup if any code lands between a Call and his Catch

   void call_catch_cleanup(Block_Array &bbs);

   // Detect implicit-null-check opportunities.  Basically, find NULL checks

   // with suitable memory ops nearby.  Use the memory op to do the NULL check.

   // I can generate a memory op if there is not one nearby.

   void implicit_null_check(PhaseCFG *cfg, Node *proj, Node *val, int allowed_reasons);

   // Return the empty status of a block

   enum { not_empty, empty_with_goto, completely_empty };

   int is_Empty() const;

   // Forward through connectors

   Block* non_connector() {

     Block* s = this;

     while (s->is_connector()) {

       s = s->_succs[0];

     }

     return s;

   }

   // Return true if b is a successor of this block

   bool has_successor(Block* b) const {

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

       if (non_connector_successor(i) == b) {

         return true;

       }

     }

     return false;

   }

   // Successor block, after forwarding through connectors

   Block* non_connector_successor(int i) const {

     return _succs[i]->non_connector();

   }

   // Examine block's code shape to predict if it is not commonly executed.

   bool has_uncommon_code() const;

   // Use frequency calculations and code shape to predict if the block

   // is uncommon.

   bool is_uncommon( Block_Array &bbs ) const;

 #ifndef PRODUCT

   // Debugging print of basic block

   void dump_bidx(const Block* orig) const;

   void dump_pred(const Block_Array *bbs, Block* orig) const;

   void dump_head( const Block_Array *bbs ) const;

   void dump( ) const;

   void dump( const Block_Array *bbs ) const;

 #endif

 };

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

 // Build an array of Basic Block pointers, one per Node.

 class PhaseCFG : public Phase {

  private:

   // Build a proper looking cfg.  Return count of basic blocks

   uint build_cfg();

   // Perform DFS search.

   // Setup 'vertex' as DFS to vertex mapping.

   // Setup 'semi' as vertex to DFS mapping.

   // Set 'parent' to DFS parent.

   uint DFS( Tarjan *tarjan );

   // Helper function to insert a node into a block

   void schedule_node_into_block( Node *n, Block *b );

   void replace_block_proj_ctrl( Node *n );

   // Set the basic block for pinned Nodes

   void schedule_pinned_nodes( VectorSet &visited );

   // I'll need a few machine-specific GotoNodes.  Clone from this one.

   MachNode *_goto;

   Block* insert_anti_dependences(Block* LCA, Node* load, bool verify = false);

   void verify_anti_dependences(Block* LCA, Node* load) {

     assert(LCA == _bbs[load->_idx], "should already be scheduled");

     insert_anti_dependences(LCA, load, true);

   }

  public:

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

   uint _num_blocks;             // Count of basic blocks

   Block_List _blocks;           // List of basic blocks

   RootNode *_root;              // Root of whole program

   Block_Array _bbs;             // Map Nodes to owning Basic Block

   Block *_broot;                // Basic block of root

   uint _rpo_ctr;

   CFGLoop* _root_loop;

   // Per node latency estimation, valid only during GCM

   GrowableArray<uint> _node_latency;

 #ifndef PRODUCT

   bool _trace_opto_pipelining;  // tracing flag

 #endif

   // Build dominators

   void Dominators();

   // Estimate block frequencies based on IfNode probabilities

   void Estimate_Block_Frequency();

   // Global Code Motion.  See Click's PLDI95 paper.  Place Nodes in specific

   // basic blocks; i.e. _bbs now maps _idx for all Nodes to some Block.

   void GlobalCodeMotion( Matcher &m, uint unique, Node_List &proj_list );

   // Compute the (backwards) latency of a node from the uses

   void latency_from_uses(Node *n);

   // Compute the (backwards) latency of a node from a single use

   int latency_from_use(Node *n, const Node *def, Node *use);

   // Compute the (backwards) latency of a node from the uses of this instruction

   void partial_latency_of_defs(Node *n);

   // Schedule Nodes early in their basic blocks.

   bool schedule_early(VectorSet &visited, Node_List &roots);

   // For each node, find the latest block it can be scheduled into

   // and then select the cheapest block between the latest and earliest

   // block to place the node.

   void schedule_late(VectorSet &visited, Node_List &stack);

   // Pick a block between early and late that is a cheaper alternative

   // to late. Helper for schedule_late.

   Block* hoist_to_cheaper_block(Block* LCA, Block* early, Node* self);

   // Compute the instruction global latency with a backwards walk

   void ComputeLatenciesBackwards(VectorSet &visited, Node_List &stack);

   // Set loop alignment

   void set_loop_alignment();

   // Remove empty basic blocks

   void remove_empty();

   void fixup_flow();

   bool move_to_next(Block* bx, uint b_index);

   void move_to_end(Block* bx, uint b_index);

   void insert_goto_at(uint block_no, uint succ_no);

   // 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 convert_NeverBranch_to_Goto(Block *b);

   CFGLoop* create_loop_tree();

   // Insert a node into a block, and update the _bbs

   void insert( Block *b, uint idx, Node *n ) {

     b->_nodes.insert( idx, n );

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

   }

 #ifndef PRODUCT

   bool trace_opto_pipelining() const { return _trace_opto_pipelining; }

   // Debugging print of CFG

   void dump( ) const;           // CFG only

   void _dump_cfg( const Node *end, VectorSet &visited  ) const;

   void verify() const;

   void dump_headers();

 #else

   bool trace_opto_pipelining() const { return false; }

 #endif

 };

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

 // Map Block indices to a block-index for a cfg-cover.

 // Array lookup in the optimized case.

 class UnionFind : public ResourceObj {

   uint _cnt, _max;

   uint* _indices;

   ReallocMark _nesting;  // assertion check for reallocations

 public:

   UnionFind( uint max );

   void reset( uint max );  // Reset to identity map for [0..max]

   uint lookup( uint nidx ) const {

     return _indices[nidx];

   }

   uint operator[] (uint nidx) const { return lookup(nidx); }

   void map( uint from_idx, uint to_idx ) {

     assert( from_idx < _cnt, "oob" );

     _indices[from_idx] = to_idx;

   }

   void extend( uint from_idx, uint to_idx );

   uint Size() const { return _cnt; }

   uint Find( uint idx ) {

     assert( idx < 65536, "Must fit into uint");

     uint uf_idx = lookup(idx);

     return (uf_idx == idx) ? uf_idx : Find_compress(idx);

   }

   uint Find_compress( uint idx );

   uint Find_const( uint idx ) const;

   void Union( uint idx1, uint idx2 );

 };

 //----------------------------BlockProbPair---------------------------

 // Ordered pair of Node*.

 class BlockProbPair VALUE_OBJ_CLASS_SPEC {

 protected:

   Block* _target;      // block target

   float  _prob;        // probability of edge to block

 public:

   BlockProbPair() : _target(NULL), _prob(0.0) {}

   BlockProbPair(Block* b, float p) : _target(b), _prob(p) {}

   Block* get_target() const { return _target; }

   float get_prob() const { return _prob; }

 };

 //------------------------------CFGLoop-------------------------------------------

 class CFGLoop : public CFGElement {

   int _id;

   int _depth;

   CFGLoop *_parent;      // root of loop tree is the method level "pseudo" loop, it's parent is null

   CFGLoop *_sibling;     // null terminated list

   CFGLoop *_child;       // first child, use child's sibling to visit all immediately nested loops

   GrowableArray<CFGElement*> _members; // list of members of loop

   GrowableArray<BlockProbPair> _exits; // list of successor blocks and their probabilities

   float _exit_prob;       // probability any loop exit is taken on a single loop iteration

   void update_succ_freq(Block* b, float freq);

  public:

   CFGLoop(int id) :

     CFGElement(),

     _id(id),

     _depth(0),

     _parent(NULL),

     _sibling(NULL),

     _child(NULL),

     _exit_prob(1.0f) {}

   CFGLoop* parent() { return _parent; }

   void push_pred(Block* blk, int i, Block_List& worklist, Block_Array& node_to_blk);

   void add_member(CFGElement *s) { _members.push(s); }

   void add_nested_loop(CFGLoop* cl);

   Block* head() {

     assert(_members.at(0)->is_block(), "head must be a block");

     Block* hd = _members.at(0)->as_Block();

     assert(hd->_loop == this, "just checking");

     assert(hd->head()->is_Loop(), "must begin with loop head node");

     return hd;

   }

   Block* backedge_block(); // Return the block on the backedge of the loop (else NULL)

   void compute_loop_depth(int depth);

   void compute_freq(); // compute frequency with loop assuming head freq 1.0f

   void scale_freq();   // scale frequency by loop trip count (including outer loops)

   bool in_loop_nest(Block* b);

   float trip_count() const { return 1.0f / _exit_prob; }

   virtual bool is_loop()  { return true; }

   int id() { return _id; }

 #ifndef PRODUCT

   void dump( ) const;

   void dump_tree() const;

 #endif

 };

 //----------------------------------CFGEdge------------------------------------

 // A edge between two basic blocks that will be embodied by a branch or a

 // fall-through.

 class CFGEdge : public ResourceObj {

  private:

   Block * _from;        // Source basic block

   Block * _to;          // Destination basic block

   float _freq;          // Execution frequency (estimate)

   int   _state;

   bool  _infrequent;

   int   _from_pct;

   int   _to_pct;

   // Private accessors

   int  from_pct() const { return _from_pct; }

   int  to_pct()   const { return _to_pct;   }

   int  from_infrequent() const { return from_pct() < BlockLayoutMinDiamondPercentage; }

   int  to_infrequent()   const { return to_pct()   < BlockLayoutMinDiamondPercentage; }

  public:

   enum {

     open,               // initial edge state; unprocessed

     connected,          // edge used to connect two traces together

     interior            // edge is interior to trace (could be backedge)

   };

   CFGEdge(Block *from, Block *to, float freq, int from_pct, int to_pct) :

     _from(from), _to(to), _freq(freq),

     _from_pct(from_pct), _to_pct(to_pct), _state(open) {

     _infrequent = from_infrequent() || to_infrequent();

   }

   float  freq() const { return _freq; }

   Block* from() const { return _from; }

   Block* to  () const { return _to;   }

   int  infrequent() const { return _infrequent; }

   int state() const { return _state; }

   void set_state(int state) { _state = state; }

 #ifndef PRODUCT

   void dump( ) const;

 #endif

 };

 //-----------------------------------Trace-------------------------------------

 // An ordered list of basic blocks.

 class Trace : public ResourceObj {

  private:

   uint _id;             // Unique Trace id (derived from initial block)

   Block ** _next_list;  // Array mapping index to next block

   Block ** _prev_list;  // Array mapping index to previous block

   Block * _first;       // First block in the trace

   Block * _last;        // Last block in the trace

   // Return the block that follows "b" in the trace.

   Block * next(Block *b) const { return _next_list[b->_pre_order]; }

   void set_next(Block *b, Block *n) const { _next_list[b->_pre_order] = n; }

   // Return the block that precedes "b" in the trace.

   Block * prev(Block *b) const { return _prev_list[b->_pre_order]; }

   void set_prev(Block *b, Block *p) const { _prev_list[b->_pre_order] = p; }

   // We've discovered a loop in this trace. Reset last to be "b", and first as

   // the block following "b

   void break_loop_after(Block *b) {

     _last = b;

     _first = next(b);

     set_prev(_first, NULL);

     set_next(_last, NULL);

   }

  public:

   Trace(Block *b, Block **next_list, Block **prev_list) :

     _first(b),

     _last(b),

     _next_list(next_list),

     _prev_list(prev_list),

     _id(b->_pre_order) {

     set_next(b, NULL);

     set_prev(b, NULL);

   };

   // Return the id number

   uint id() const { return _id; }

   void set_id(uint id) { _id = id; }

   // Return the first block in the trace

   Block * first_block() const { return _first; }

   // Return the last block in the trace

   Block * last_block() const { return _last; }

   // Insert a trace in the middle of this one after b

   void insert_after(Block *b, Trace *tr) {

     set_next(tr->last_block(), next(b));

     if (next(b) != NULL) {

       set_prev(next(b), tr->last_block());

     }

     set_next(b, tr->first_block());

     set_prev(tr->first_block(), b);

     if (b == _last) {

       _last = tr->last_block();

     }

   }

   void insert_before(Block *b, Trace *tr) {

     Block *p = prev(b);

     assert(p != NULL, "use append instead");

     insert_after(p, tr);

   }

   // Append another trace to this one.

   void append(Trace *tr) {

     insert_after(_last, tr);

   }

   // Append a block at the end of this trace

   void append(Block *b) {

     set_next(_last, b);

     set_prev(b, _last);

     _last = b;

   }

   // Adjust the the blocks in this trace

   void fixup_blocks(PhaseCFG &cfg);

   bool backedge(CFGEdge *e);

 #ifndef PRODUCT

   void dump( ) const;

 #endif

 };

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

 // Rearrange blocks into some canonical order, based on edges and their frequencies

 class PhaseBlockLayout : public Phase {

   PhaseCFG &_cfg;               // Control flow graph

   GrowableArray<CFGEdge *> *edges;

   Trace **traces;

   Block **next;

   Block **prev;

   UnionFind *uf;

   // Given a block, find its encompassing Trace

   Trace * trace(Block *b) {

     return traces[uf->Find_compress(b->_pre_order)];

   }

  public:

   PhaseBlockLayout(PhaseCFG &cfg);

   void find_edges();

   void grow_traces();

   void merge_traces(bool loose_connections);

   void reorder_traces(int count);

   void union_traces(Trace* from, Trace* to);

 };