hotspot/src/share/vm/opto/split_if.cpp
changeset 1 489c9b5090e2
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0:fd16c54261b3 1:489c9b5090e2
       
     1 /*
       
     2  * Copyright 1999-2006 Sun Microsystems, Inc.  All Rights Reserved.
       
     3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
       
     4  *
       
     5  * This code is free software; you can redistribute it and/or modify it
       
     6  * under the terms of the GNU General Public License version 2 only, as
       
     7  * published by the Free Software Foundation.
       
     8  *
       
     9  * This code is distributed in the hope that it will be useful, but WITHOUT
       
    10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
       
    11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
       
    12  * version 2 for more details (a copy is included in the LICENSE file that
       
    13  * accompanied this code).
       
    14  *
       
    15  * You should have received a copy of the GNU General Public License version
       
    16  * 2 along with this work; if not, write to the Free Software Foundation,
       
    17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
       
    18  *
       
    19  * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
       
    20  * CA 95054 USA or visit www.sun.com if you need additional information or
       
    21  * have any questions.
       
    22  *
       
    23  */
       
    24 
       
    25 #include "incls/_precompiled.incl"
       
    26 #include "incls/_split_if.cpp.incl"
       
    27 
       
    28 
       
    29 //------------------------------split_thru_region------------------------------
       
    30 // Split Node 'n' through merge point.
       
    31 Node *PhaseIdealLoop::split_thru_region( Node *n, Node *region ) {
       
    32   uint wins = 0;
       
    33   assert( n->is_CFG(), "" );
       
    34   assert( region->is_Region(), "" );
       
    35   Node *r = new (C, region->req()) RegionNode( region->req() );
       
    36   IdealLoopTree *loop = get_loop( n );
       
    37   for( uint i = 1; i < region->req(); i++ ) {
       
    38     Node *x = n->clone();
       
    39     Node *in0 = n->in(0);
       
    40     if( in0->in(0) == region ) x->set_req( 0, in0->in(i) );
       
    41     for( uint j = 1; j < n->req(); j++ ) {
       
    42       Node *in = n->in(j);
       
    43       if( get_ctrl(in) == region )
       
    44         x->set_req( j, in->in(i) );
       
    45     }
       
    46     _igvn.register_new_node_with_optimizer(x);
       
    47     set_loop(x, loop);
       
    48     set_idom(x, x->in(0), dom_depth(x->in(0))+1);
       
    49     r->init_req(i, x);
       
    50   }
       
    51 
       
    52   // Record region
       
    53   r->set_req(0,region);         // Not a TRUE RegionNode
       
    54   _igvn.register_new_node_with_optimizer(r);
       
    55   set_loop(r, loop);
       
    56   if( !loop->_child )
       
    57     loop->_body.push(r);
       
    58   return r;
       
    59 }
       
    60 
       
    61 //------------------------------split_up---------------------------------------
       
    62 // Split block-local op up through the phis to empty the current block
       
    63 bool PhaseIdealLoop::split_up( Node *n, Node *blk1, Node *blk2 ) {
       
    64   if( n->is_CFG() ) {
       
    65     assert( n->in(0) != blk1, "Lousy candidate for split-if" );
       
    66     return false;
       
    67   }
       
    68   if( get_ctrl(n) != blk1 && get_ctrl(n) != blk2 )
       
    69     return false;               // Not block local
       
    70   if( n->is_Phi() ) return false; // Local PHIs are expected
       
    71 
       
    72   // Recursively split-up inputs
       
    73   for (uint i = 1; i < n->req(); i++) {
       
    74     if( split_up( n->in(i), blk1, blk2 ) ) {
       
    75       // Got split recursively and self went dead?
       
    76       if (n->outcnt() == 0)
       
    77         _igvn.remove_dead_node(n);
       
    78       return true;
       
    79     }
       
    80   }
       
    81 
       
    82   // Check for needing to clone-up a compare.  Can't do that, it forces
       
    83   // another (nested) split-if transform.  Instead, clone it "down".
       
    84   if( n->is_Cmp() ) {
       
    85     assert(get_ctrl(n) == blk2 || get_ctrl(n) == blk1, "must be in block with IF");
       
    86     // Check for simple Cmp/Bool/CMove which we can clone-up.  Cmp/Bool/CMove
       
    87     // sequence can have no other users and it must all reside in the split-if
       
    88     // block.  Non-simple Cmp/Bool/CMove sequences are 'cloned-down' below -
       
    89     // private, per-use versions of the Cmp and Bool are made.  These sink to
       
    90     // the CMove block.  If the CMove is in the split-if block, then in the
       
    91     // next iteration this will become a simple Cmp/Bool/CMove set to clone-up.
       
    92     Node *bol, *cmov;
       
    93     if( !(n->outcnt() == 1 && n->unique_out()->is_Bool() &&
       
    94           (bol = n->unique_out()->as_Bool()) &&
       
    95           (get_ctrl(bol) == blk1 ||
       
    96            get_ctrl(bol) == blk2) &&
       
    97           bol->outcnt() == 1 &&
       
    98           bol->unique_out()->is_CMove() &&
       
    99           (cmov = bol->unique_out()->as_CMove()) &&
       
   100           (get_ctrl(cmov) == blk1 ||
       
   101            get_ctrl(cmov) == blk2) ) ) {
       
   102 
       
   103       // Must clone down
       
   104 #ifndef PRODUCT
       
   105       if( PrintOpto && VerifyLoopOptimizations ) {
       
   106         tty->print("Cloning down: ");
       
   107         n->dump();
       
   108       }
       
   109 #endif
       
   110       // Clone down any block-local BoolNode uses of this CmpNode
       
   111       for (DUIterator i = n->outs(); n->has_out(i); i++) {
       
   112         Node* bol = n->out(i);
       
   113         assert( bol->is_Bool(), "" );
       
   114         if (bol->outcnt() == 1) {
       
   115           Node* use = bol->unique_out();
       
   116           Node *use_c = use->is_If() ? use->in(0) : get_ctrl(use);
       
   117           if (use_c == blk1 || use_c == blk2) {
       
   118             continue;
       
   119           }
       
   120         }
       
   121         if (get_ctrl(bol) == blk1 || get_ctrl(bol) == blk2) {
       
   122           // Recursively sink any BoolNode
       
   123 #ifndef PRODUCT
       
   124           if( PrintOpto && VerifyLoopOptimizations ) {
       
   125             tty->print("Cloning down: ");
       
   126             bol->dump();
       
   127           }
       
   128 #endif
       
   129           for (DUIterator_Last jmin, j = bol->last_outs(jmin); j >= jmin; --j) {
       
   130             // Uses are either IfNodes or CMoves
       
   131             Node* iff = bol->last_out(j);
       
   132             assert( iff->in(1) == bol, "" );
       
   133             // Get control block of either the CMove or the If input
       
   134             Node *iff_ctrl = iff->is_If() ? iff->in(0) : get_ctrl(iff);
       
   135             Node *x = bol->clone();
       
   136             register_new_node(x, iff_ctrl);
       
   137             _igvn.hash_delete(iff);
       
   138             iff->set_req(1, x);
       
   139             _igvn._worklist.push(iff);
       
   140           }
       
   141           _igvn.remove_dead_node( bol );
       
   142           --i;
       
   143         }
       
   144       }
       
   145       // Clone down this CmpNode
       
   146       for (DUIterator_Last jmin, j = n->last_outs(jmin); j >= jmin; --j) {
       
   147         Node* bol = n->last_out(j);
       
   148         assert( bol->in(1) == n, "" );
       
   149         Node *x = n->clone();
       
   150         register_new_node(x, get_ctrl(bol));
       
   151         _igvn.hash_delete(bol);
       
   152         bol->set_req(1, x);
       
   153         _igvn._worklist.push(bol);
       
   154       }
       
   155       _igvn.remove_dead_node( n );
       
   156 
       
   157       return true;
       
   158     }
       
   159   }
       
   160 
       
   161   // See if splitting-up a Store.  Any anti-dep loads must go up as
       
   162   // well.  An anti-dep load might be in the wrong block, because in
       
   163   // this particular layout/schedule we ignored anti-deps and allow
       
   164   // memory to be alive twice.  This only works if we do the same
       
   165   // operations on anti-dep loads as we do their killing stores.
       
   166   if( n->is_Store() && n->in(MemNode::Memory)->in(0) == n->in(0) ) {
       
   167     // Get store's memory slice
       
   168     int alias_idx = C->get_alias_index(_igvn.type(n->in(MemNode::Address))->is_ptr());
       
   169 
       
   170     // Get memory-phi anti-dep loads will be using
       
   171     Node *memphi = n->in(MemNode::Memory);
       
   172     assert( memphi->is_Phi(), "" );
       
   173     // Hoist any anti-dep load to the splitting block;
       
   174     // it will then "split-up".
       
   175     for (DUIterator_Fast imax,i = memphi->fast_outs(imax); i < imax; i++) {
       
   176       Node *load = memphi->fast_out(i);
       
   177       if( load->is_Load() && alias_idx == C->get_alias_index(_igvn.type(load->in(MemNode::Address))->is_ptr()) )
       
   178         set_ctrl(load,blk1);
       
   179     }
       
   180   }
       
   181 
       
   182   // Found some other Node; must clone it up
       
   183 #ifndef PRODUCT
       
   184   if( PrintOpto && VerifyLoopOptimizations ) {
       
   185     tty->print("Cloning up: ");
       
   186     n->dump();
       
   187   }
       
   188 #endif
       
   189 
       
   190   // Now actually split-up this guy.  One copy per control path merging.
       
   191   Node *phi = PhiNode::make_blank(blk1, n);
       
   192   for( uint j = 1; j < blk1->req(); j++ ) {
       
   193     Node *x = n->clone();
       
   194     if( n->in(0) && n->in(0) == blk1 )
       
   195       x->set_req( 0, blk1->in(j) );
       
   196     for( uint i = 1; i < n->req(); i++ ) {
       
   197       Node *m = n->in(i);
       
   198       if( get_ctrl(m) == blk1 ) {
       
   199         assert( m->in(0) == blk1, "" );
       
   200         x->set_req( i, m->in(j) );
       
   201       }
       
   202     }
       
   203     register_new_node( x, blk1->in(j) );
       
   204     phi->init_req( j, x );
       
   205   }
       
   206   // Announce phi to optimizer
       
   207   register_new_node(phi, blk1);
       
   208 
       
   209   // Remove cloned-up value from optimizer; use phi instead
       
   210   _igvn.hash_delete(n);
       
   211   _igvn.subsume_node( n, phi );
       
   212 
       
   213   // (There used to be a self-recursive call to split_up() here,
       
   214   // but it is not needed.  All necessary forward walking is done
       
   215   // by do_split_if() below.)
       
   216 
       
   217   return true;
       
   218 }
       
   219 
       
   220 //------------------------------register_new_node------------------------------
       
   221 void PhaseIdealLoop::register_new_node( Node *n, Node *blk ) {
       
   222   _igvn.register_new_node_with_optimizer(n);
       
   223   set_ctrl(n, blk);
       
   224   IdealLoopTree *loop = get_loop(blk);
       
   225   if( !loop->_child )
       
   226     loop->_body.push(n);
       
   227 }
       
   228 
       
   229 //------------------------------small_cache------------------------------------
       
   230 struct small_cache : public Dict {
       
   231 
       
   232   small_cache() : Dict( cmpkey, hashptr ) {}
       
   233   Node *probe( Node *use_blk ) { return (Node*)((*this)[use_blk]); }
       
   234   void lru_insert( Node *use_blk, Node *new_def ) { Insert(use_blk,new_def); }
       
   235 };
       
   236 
       
   237 //------------------------------spinup-----------------------------------------
       
   238 // "Spin up" the dominator tree, starting at the use site and stopping when we
       
   239 // find the post-dominating point.
       
   240 
       
   241 // We must be at the merge point which post-dominates 'new_false' and
       
   242 // 'new_true'.  Figure out which edges into the RegionNode eventually lead up
       
   243 // to false and which to true.  Put in a PhiNode to merge values; plug in
       
   244 // the appropriate false-arm or true-arm values.  If some path leads to the
       
   245 // original IF, then insert a Phi recursively.
       
   246 Node *PhaseIdealLoop::spinup( Node *iff_dom, Node *new_false, Node *new_true, Node *use_blk, Node *def, small_cache *cache ) {
       
   247   if (use_blk->is_top())        // Handle dead uses
       
   248     return use_blk;
       
   249   Node *prior_n = (Node*)0xdeadbeef;
       
   250   Node *n = use_blk;            // Get path input
       
   251   assert( use_blk != iff_dom, "" );
       
   252   // Here's the "spinup" the dominator tree loop.  Do a cache-check
       
   253   // along the way, in case we've come this way before.
       
   254   while( n != iff_dom ) {       // Found post-dominating point?
       
   255     prior_n = n;
       
   256     n = idom(n);                // Search higher
       
   257     Node *s = cache->probe( prior_n ); // Check cache
       
   258     if( s ) return s;           // Cache hit!
       
   259   }
       
   260 
       
   261   Node *phi_post;
       
   262   if( prior_n == new_false || prior_n == new_true ) {
       
   263     phi_post = def->clone();
       
   264     phi_post->set_req(0, prior_n );
       
   265     register_new_node(phi_post, prior_n);
       
   266   } else {
       
   267     // This method handles both control uses (looking for Regions) or data
       
   268     // uses (looking for Phis).  If looking for a control use, then we need
       
   269     // to insert a Region instead of a Phi; however Regions always exist
       
   270     // previously (the hash_find_insert below would always hit) so we can
       
   271     // return the existing Region.
       
   272     if( def->is_CFG() ) {
       
   273       phi_post = prior_n;       // If looking for CFG, return prior
       
   274     } else {
       
   275       assert( def->is_Phi(), "" );
       
   276       assert( prior_n->is_Region(), "must be a post-dominating merge point" );
       
   277 
       
   278       // Need a Phi here
       
   279       phi_post = PhiNode::make_blank(prior_n, def);
       
   280       // Search for both true and false on all paths till find one.
       
   281       for( uint i = 1; i < phi_post->req(); i++ ) // For all paths
       
   282         phi_post->init_req( i, spinup( iff_dom, new_false, new_true, prior_n->in(i), def, cache ) );
       
   283       Node *t = _igvn.hash_find_insert(phi_post);
       
   284       if( t ) {                 // See if we already have this one
       
   285         // phi_post will not be used, so kill it
       
   286         _igvn.remove_dead_node(phi_post);
       
   287         phi_post->destruct();
       
   288         phi_post = t;
       
   289       } else {
       
   290         register_new_node( phi_post, prior_n );
       
   291       }
       
   292     }
       
   293   }
       
   294 
       
   295   // Update cache everywhere
       
   296   prior_n = (Node*)0xdeadbeef;  // Reset IDOM walk
       
   297   n = use_blk;                  // Get path input
       
   298   // Spin-up the idom tree again, basically doing path-compression.
       
   299   // Insert cache entries along the way, so that if we ever hit this
       
   300   // point in the IDOM tree again we'll stop immediately on a cache hit.
       
   301   while( n != iff_dom ) {       // Found post-dominating point?
       
   302     prior_n = n;
       
   303     n = idom(n);                // Search higher
       
   304     cache->lru_insert( prior_n, phi_post ); // Fill cache
       
   305   } // End of while not gone high enough
       
   306 
       
   307   return phi_post;
       
   308 }
       
   309 
       
   310 //------------------------------find_use_block---------------------------------
       
   311 // Find the block a USE is in.  Normally USE's are in the same block as the
       
   312 // using instruction.  For Phi-USE's, the USE is in the predecessor block
       
   313 // along the corresponding path.
       
   314 Node *PhaseIdealLoop::find_use_block( Node *use, Node *def, Node *old_false, Node *new_false, Node *old_true, Node *new_true ) {
       
   315   // CFG uses are their own block
       
   316   if( use->is_CFG() )
       
   317     return use;
       
   318 
       
   319   if( use->is_Phi() ) {         // Phi uses in prior block
       
   320     // Grab the first Phi use; there may be many.
       
   321     // Each will be handled as a seperate iteration of
       
   322     // the "while( phi->outcnt() )" loop.
       
   323     uint j;
       
   324     for( j = 1; j < use->req(); j++ )
       
   325       if( use->in(j) == def )
       
   326         break;
       
   327     assert( j < use->req(), "def should be among use's inputs" );
       
   328     return use->in(0)->in(j);
       
   329   }
       
   330   // Normal (non-phi) use
       
   331   Node *use_blk = get_ctrl(use);
       
   332   // Some uses are directly attached to the old (and going away)
       
   333   // false and true branches.
       
   334   if( use_blk == old_false ) {
       
   335     use_blk = new_false;
       
   336     set_ctrl(use, new_false);
       
   337   }
       
   338   if( use_blk == old_true ) {
       
   339     use_blk = new_true;
       
   340     set_ctrl(use, new_true);
       
   341   }
       
   342 
       
   343   if (use_blk == NULL) {        // He's dead, Jim
       
   344     _igvn.hash_delete(use);
       
   345     _igvn.subsume_node(use, C->top());
       
   346   }
       
   347 
       
   348   return use_blk;
       
   349 }
       
   350 
       
   351 //------------------------------handle_use-------------------------------------
       
   352 // Handle uses of the merge point.  Basically, split-if makes the merge point
       
   353 // go away so all uses of the merge point must go away as well.  Most block
       
   354 // local uses have already been split-up, through the merge point.  Uses from
       
   355 // far below the merge point can't always be split up (e.g., phi-uses are
       
   356 // pinned) and it makes too much stuff live.  Instead we use a path-based
       
   357 // solution to move uses down.
       
   358 //
       
   359 // If the use is along the pre-split-CFG true branch, then the new use will
       
   360 // be from the post-split-CFG true merge point.  Vice-versa for the false
       
   361 // path.  Some uses will be along both paths; then we sink the use to the
       
   362 // post-dominating location; we may need to insert a Phi there.
       
   363 void PhaseIdealLoop::handle_use( Node *use, Node *def, small_cache *cache, Node *region_dom, Node *new_false, Node *new_true, Node *old_false, Node *old_true ) {
       
   364 
       
   365   Node *use_blk = find_use_block(use,def,old_false,new_false,old_true,new_true);
       
   366   if( !use_blk ) return;        // He's dead, Jim
       
   367 
       
   368   // Walk up the dominator tree until I hit either the old IfFalse, the old
       
   369   // IfTrue or the old If.  Insert Phis where needed.
       
   370   Node *new_def = spinup( region_dom, new_false, new_true, use_blk, def, cache );
       
   371 
       
   372   // Found where this USE goes.  Re-point him.
       
   373   uint i;
       
   374   for( i = 0; i < use->req(); i++ )
       
   375     if( use->in(i) == def )
       
   376       break;
       
   377   assert( i < use->req(), "def should be among use's inputs" );
       
   378   _igvn.hash_delete(use);
       
   379   use->set_req(i, new_def);
       
   380   _igvn._worklist.push(use);
       
   381 }
       
   382 
       
   383 //------------------------------do_split_if------------------------------------
       
   384 // Found an If getting its condition-code input from a Phi in the same block.
       
   385 // Split thru the Region.
       
   386 void PhaseIdealLoop::do_split_if( Node *iff ) {
       
   387 #ifndef PRODUCT
       
   388   if( PrintOpto && VerifyLoopOptimizations )
       
   389     tty->print_cr("Split-if");
       
   390 #endif
       
   391   C->set_major_progress();
       
   392   Node *region = iff->in(0);
       
   393   Node *region_dom = idom(region);
       
   394 
       
   395   // We are going to clone this test (and the control flow with it) up through
       
   396   // the incoming merge point.  We need to empty the current basic block.
       
   397   // Clone any instructions which must be in this block up through the merge
       
   398   // point.
       
   399   DUIterator i, j;
       
   400   bool progress = true;
       
   401   while (progress) {
       
   402     progress = false;
       
   403     for (i = region->outs(); region->has_out(i); i++) {
       
   404       Node* n = region->out(i);
       
   405       if( n == region ) continue;
       
   406       // The IF to be split is OK.
       
   407       if( n == iff ) continue;
       
   408       if( !n->is_Phi() ) {      // Found pinned memory op or such
       
   409         if (split_up(n, region, iff)) {
       
   410           i = region->refresh_out_pos(i);
       
   411           progress = true;
       
   412         }
       
   413         continue;
       
   414       }
       
   415       assert( n->in(0) == region, "" );
       
   416 
       
   417       // Recursively split up all users of a Phi
       
   418       for (j = n->outs(); n->has_out(j); j++) {
       
   419         Node* m = n->out(j);
       
   420         // If m is dead, throw it away, and declare progress
       
   421         if (_nodes[m->_idx] == NULL) {
       
   422           _igvn.remove_dead_node(m);
       
   423           // fall through
       
   424         }
       
   425         else if (m != iff && split_up(m, region, iff)) {
       
   426           // fall through
       
   427         } else {
       
   428           continue;
       
   429         }
       
   430         // Something unpredictable changed.
       
   431         // Tell the iterators to refresh themselves, and rerun the loop.
       
   432         i = region->refresh_out_pos(i);
       
   433         j = region->refresh_out_pos(j);
       
   434         progress = true;
       
   435       }
       
   436     }
       
   437   }
       
   438 
       
   439   // Now we have no instructions in the block containing the IF.
       
   440   // Split the IF.
       
   441   Node *new_iff = split_thru_region( iff, region );
       
   442 
       
   443   // Replace both uses of 'new_iff' with Regions merging True/False
       
   444   // paths.  This makes 'new_iff' go dead.
       
   445   Node *old_false, *old_true;
       
   446   Node *new_false, *new_true;
       
   447   for (DUIterator_Last j2min, j2 = iff->last_outs(j2min); j2 >= j2min; --j2) {
       
   448     Node *ifp = iff->last_out(j2);
       
   449     assert( ifp->Opcode() == Op_IfFalse || ifp->Opcode() == Op_IfTrue, "" );
       
   450     ifp->set_req(0, new_iff);
       
   451     Node *ifpx = split_thru_region( ifp, region );
       
   452 
       
   453     // Replace 'If' projection of a Region with a Region of
       
   454     // 'If' projections.
       
   455     ifpx->set_req(0, ifpx);       // A TRUE RegionNode
       
   456 
       
   457     // Setup dominator info
       
   458     set_idom(ifpx, region_dom, dom_depth(region_dom) + 1);
       
   459 
       
   460     // Check for splitting loop tails
       
   461     if( get_loop(iff)->tail() == ifp )
       
   462       get_loop(iff)->_tail = ifpx;
       
   463 
       
   464     // Replace in the graph with lazy-update mechanism
       
   465     new_iff->set_req(0, new_iff); // hook self so it does not go dead
       
   466     lazy_replace_proj( ifp, ifpx );
       
   467     new_iff->set_req(0, region);
       
   468 
       
   469     // Record bits for later xforms
       
   470     if( ifp->Opcode() == Op_IfFalse ) {
       
   471       old_false = ifp;
       
   472       new_false = ifpx;
       
   473     } else {
       
   474       old_true = ifp;
       
   475       new_true = ifpx;
       
   476     }
       
   477   }
       
   478   _igvn.remove_dead_node(new_iff);
       
   479   // Lazy replace IDOM info with the region's dominator
       
   480   lazy_replace( iff, region_dom );
       
   481 
       
   482   // Now make the original merge point go dead, by handling all its uses.
       
   483   small_cache region_cache;
       
   484   // Preload some control flow in region-cache
       
   485   region_cache.lru_insert( new_false, new_false );
       
   486   region_cache.lru_insert( new_true , new_true  );
       
   487   // Now handle all uses of the splitting block
       
   488   for (DUIterator_Last kmin, k = region->last_outs(kmin); k >= kmin; --k) {
       
   489     Node* phi = region->last_out(k);
       
   490     if( !phi->in(0) ) {         // Dead phi?  Remove it
       
   491       _igvn.remove_dead_node(phi);
       
   492       continue;
       
   493     }
       
   494     assert( phi->in(0) == region, "" );
       
   495     if( phi == region ) {       // Found the self-reference
       
   496       phi->set_req(0, NULL);
       
   497       continue;                 // Break the self-cycle
       
   498     }
       
   499     // Expected common case: Phi hanging off of Region
       
   500     if( phi->is_Phi() ) {
       
   501       // Need a per-def cache.  Phi represents a def, so make a cache
       
   502       small_cache phi_cache;
       
   503 
       
   504       // Inspect all Phi uses to make the Phi go dead
       
   505       for (DUIterator_Last lmin, l = phi->last_outs(lmin); l >= lmin; --l) {
       
   506         Node* use = phi->last_out(l);
       
   507         // Compute the new DEF for this USE.  New DEF depends on the path
       
   508         // taken from the original DEF to the USE.  The new DEF may be some
       
   509         // collection of PHI's merging values from different paths.  The Phis
       
   510         // inserted depend only on the location of the USE.  We use a
       
   511         // 2-element cache to handle multiple uses from the same block.
       
   512         handle_use( use, phi, &phi_cache, region_dom, new_false, new_true, old_false, old_true );
       
   513       } // End of while phi has uses
       
   514 
       
   515       // Because handle_use might relocate region->_out,
       
   516       // we must refresh the iterator.
       
   517       k = region->last_outs(kmin);
       
   518 
       
   519       // Remove the dead Phi
       
   520       _igvn.remove_dead_node( phi );
       
   521 
       
   522     } else {
       
   523       // Random memory op guarded by Region.  Compute new DEF for USE.
       
   524       handle_use( phi, region, &region_cache, region_dom, new_false, new_true, old_false, old_true );
       
   525     }
       
   526 
       
   527   } // End of while merge point has phis
       
   528 
       
   529   // Any leftover bits in the splitting block must not have depended on local
       
   530   // Phi inputs (these have already been split-up).  Hence it's safe to hoist
       
   531   // these guys to the dominating point.
       
   532   lazy_replace( region, region_dom );
       
   533 #ifndef PRODUCT
       
   534   if( VerifyLoopOptimizations ) verify();
       
   535 #endif
       
   536 }