# HG changeset patch
# User redestad
# Date 1573741475 -3600
# Node ID 95a99e617f28c8b0271b599c7e4e1902542de9ec
# Parent  355f4f42dda5b1d0167e0f14d1a1cb6f0f070226
8234003: Improve IndexSet iteration
Reviewed-by: neliasso, thartmann

diff -r 355f4f42dda5 -r 95a99e617f28 src/hotspot/share/opto/chaitin.cpp
--- a/src/hotspot/share/opto/chaitin.cpp	Thu Nov 14 10:55:46 2019 +0100
+++ b/src/hotspot/share/opto/chaitin.cpp	Thu Nov 14 15:24:35 2019 +0100
@@ -1169,7 +1169,7 @@
       lrgs(lo)._next = _simplified;
       _simplified = lo;
       // If this guy is "at risk" then mark his current neighbors
-      if( lrgs(lo)._at_risk ) {
+      if (lrgs(lo)._at_risk && !_ifg->neighbors(lo)->is_empty()) {
         IndexSetIterator elements(_ifg->neighbors(lo));
         uint datum;
         while ((datum = elements.next()) != 0) {
@@ -1178,7 +1178,10 @@
       }
 
       // Yank this guy from the IFG.
-      IndexSet *adj = _ifg->remove_node( lo );
+      IndexSet *adj = _ifg->remove_node(lo);
+      if (adj->is_empty()) {
+        continue;
+      }
 
       // If any neighbors' degrees fall below their number of
       // allowed registers, then put that neighbor on the low degree
@@ -1202,8 +1205,11 @@
           // Pull from hi-degree list
           uint prev = n->_prev;
           uint next = n->_next;
-          if (prev) lrgs(prev)._next = next;
-          else _hi_degree = next;
+          if (prev) {
+            lrgs(prev)._next = next;
+          } else {
+            _hi_degree = next;
+          }
           lrgs(next)._prev = prev;
           n->_next = _lo_degree;
           _lo_degree = neighbor;
@@ -1314,7 +1320,7 @@
 
   // Check for "at_risk" LRG's
   uint risk_lrg = _lrg_map.find(lrg._risk_bias);
-  if( risk_lrg != 0 ) {
+  if (risk_lrg != 0 && !_ifg->neighbors(risk_lrg)->is_empty()) {
     // Walk the colored neighbors of the "at_risk" candidate
     // Choose a color which is both legal and already taken by a neighbor
     // of the "at_risk" candidate in order to improve the chances of the
@@ -1330,7 +1336,7 @@
   }
 
   uint copy_lrg = _lrg_map.find(lrg._copy_bias);
-  if( copy_lrg != 0 ) {
+  if (copy_lrg != 0) {
     // If he has a color,
     if(!_ifg->_yanked->test(copy_lrg)) {
       OptoReg::Name reg = lrgs(copy_lrg).reg();
@@ -1432,41 +1438,43 @@
 
     // Remove neighbor colors
     IndexSet *s = _ifg->neighbors(lidx);
+    debug_only(RegMask orig_mask = lrg->mask();)
 
-    debug_only(RegMask orig_mask = lrg->mask();)
-    IndexSetIterator elements(s);
-    uint neighbor;
-    while ((neighbor = elements.next()) != 0) {
-      // Note that neighbor might be a spill_reg.  In this case, exclusion
-      // of its color will be a no-op, since the spill_reg chunk is in outer
-      // space.  Also, if neighbor is in a different chunk, this exclusion
-      // will be a no-op.  (Later on, if lrg runs out of possible colors in
-      // its chunk, a new chunk of color may be tried, in which case
-      // examination of neighbors is started again, at retry_next_chunk.)
-      LRG &nlrg = lrgs(neighbor);
-      OptoReg::Name nreg = nlrg.reg();
-      // Only subtract masks in the same chunk
-      if( nreg >= chunk && nreg < chunk + RegMask::CHUNK_SIZE ) {
+    if (!s->is_empty()) {
+      IndexSetIterator elements(s);
+      uint neighbor;
+      while ((neighbor = elements.next()) != 0) {
+        // Note that neighbor might be a spill_reg.  In this case, exclusion
+        // of its color will be a no-op, since the spill_reg chunk is in outer
+        // space.  Also, if neighbor is in a different chunk, this exclusion
+        // will be a no-op.  (Later on, if lrg runs out of possible colors in
+        // its chunk, a new chunk of color may be tried, in which case
+        // examination of neighbors is started again, at retry_next_chunk.)
+        LRG &nlrg = lrgs(neighbor);
+        OptoReg::Name nreg = nlrg.reg();
+        // Only subtract masks in the same chunk
+        if (nreg >= chunk && nreg < chunk + RegMask::CHUNK_SIZE) {
 #ifndef PRODUCT
-        uint size = lrg->mask().Size();
-        RegMask rm = lrg->mask();
+          uint size = lrg->mask().Size();
+          RegMask rm = lrg->mask();
 #endif
-        lrg->SUBTRACT(nlrg.mask());
+          lrg->SUBTRACT(nlrg.mask());
 #ifndef PRODUCT
-        if (trace_spilling() && lrg->mask().Size() != size) {
-          ttyLocker ttyl;
-          tty->print("L%d ", lidx);
-          rm.dump();
-          tty->print(" intersected L%d ", neighbor);
-          nlrg.mask().dump();
-          tty->print(" removed ");
-          rm.SUBTRACT(lrg->mask());
-          rm.dump();
-          tty->print(" leaving ");
-          lrg->mask().dump();
-          tty->cr();
+          if (trace_spilling() && lrg->mask().Size() != size) {
+            ttyLocker ttyl;
+            tty->print("L%d ", lidx);
+            rm.dump();
+            tty->print(" intersected L%d ", neighbor);
+            nlrg.mask().dump();
+            tty->print(" removed ");
+            rm.SUBTRACT(lrg->mask());
+            rm.dump();
+            tty->print(" leaving ");
+            lrg->mask().dump();
+            tty->cr();
+          }
+#endif
         }
-#endif
       }
     }
     //assert(is_allstack == lrg->mask().is_AllStack(), "nbrs must not change AllStackedness");
@@ -1827,7 +1835,7 @@
 
       // Found a safepoint?
       JVMState *jvms = n->jvms();
-      if( jvms ) {
+      if (jvms && !liveout.is_empty()) {
         // Now scan for a live derived pointer
         IndexSetIterator elements(&liveout);
         uint neighbor;
diff -r 355f4f42dda5 -r 95a99e617f28 src/hotspot/share/opto/coalesce.cpp
--- a/src/hotspot/share/opto/coalesce.cpp	Thu Nov 14 10:55:46 2019 +0100
+++ b/src/hotspot/share/opto/coalesce.cpp	Thu Nov 14 15:24:35 2019 +0100
@@ -602,29 +602,42 @@
   // Some original neighbors of lr1 might have gone away
   // because the constrained register mask prevented them.
   // Remove lr1 from such neighbors.
-  IndexSetIterator one(n_lr1);
-  uint neighbor;
+  uint neighbor = 0;
   LRG &lrg1 = lrgs(lr1);
-  while ((neighbor = one.next()) != 0)
-    if( !_ulr.member(neighbor) )
-      if( _phc._ifg->neighbors(neighbor)->remove(lr1) )
-        lrgs(neighbor).inc_degree( -lrg1.compute_degree(lrgs(neighbor)) );
+  if (!n_lr1->is_empty()) {
+    IndexSetIterator one(n_lr1);
+    while ((neighbor = one.next()) != 0) {
+      if (!_ulr.member(neighbor)) {
+        if (_phc._ifg->neighbors(neighbor)->remove(lr1)) {
+          lrgs(neighbor).inc_degree(-lrg1.compute_degree(lrgs(neighbor)));
+        }
+      }
+    }
+  }
 
 
   // lr2 is now called (coalesced into) lr1.
   // Remove lr2 from the IFG.
-  IndexSetIterator two(n_lr2);
   LRG &lrg2 = lrgs(lr2);
-  while ((neighbor = two.next()) != 0)
-    if( _phc._ifg->neighbors(neighbor)->remove(lr2) )
-      lrgs(neighbor).inc_degree( -lrg2.compute_degree(lrgs(neighbor)) );
+  if (!n_lr2->is_empty()) {
+    IndexSetIterator two(n_lr2);
+    while ((neighbor = two.next()) != 0) {
+      if (_phc._ifg->neighbors(neighbor)->remove(lr2)) {
+        lrgs(neighbor).inc_degree(-lrg2.compute_degree(lrgs(neighbor)));
+      }
+    }
+  }
 
   // Some neighbors of intermediate copies now interfere with the
   // combined live range.
-  IndexSetIterator three(&_ulr);
-  while ((neighbor = three.next()) != 0)
-    if( _phc._ifg->neighbors(neighbor)->insert(lr1) )
-      lrgs(neighbor).inc_degree( lrg1.compute_degree(lrgs(neighbor)) );
+  if (!_ulr.is_empty()) {
+    IndexSetIterator three(&_ulr);
+    while ((neighbor = three.next()) != 0) {
+      if (_phc._ifg->neighbors(neighbor)->insert(lr1)) {
+        lrgs(neighbor).inc_degree(lrg1.compute_degree(lrgs(neighbor)));
+      }
+    }
+  }
 }
 
 static void record_bias( const PhaseIFG *ifg, int lr1, int lr2 ) {
diff -r 355f4f42dda5 -r 95a99e617f28 src/hotspot/share/opto/ifg.cpp
--- a/src/hotspot/share/opto/ifg.cpp	Thu Nov 14 10:55:46 2019 +0100
+++ b/src/hotspot/share/opto/ifg.cpp	Thu Nov 14 15:24:35 2019 +0100
@@ -80,11 +80,13 @@
   assert( !_is_square, "only on triangular" );
 
   // Simple transpose
-  for( uint i = 0; i < _maxlrg; i++ ) {
-    IndexSetIterator elements(&_adjs[i]);
-    uint datum;
-    while ((datum = elements.next()) != 0) {
-      _adjs[datum].insert( i );
+  for(uint i = 0; i < _maxlrg; i++ ) {
+    if (!_adjs[i].is_empty()) {
+      IndexSetIterator elements(&_adjs[i]);
+      uint datum;
+      while ((datum = elements.next()) != 0) {
+        _adjs[datum].insert(i);
+      }
     }
   }
   _is_square = true;
@@ -108,16 +110,18 @@
 }
 
 // Union edges of B into A
-void PhaseIFG::Union( uint a, uint b ) {
+void PhaseIFG::Union(uint a, uint b) {
   assert( _is_square, "only on square" );
   IndexSet *A = &_adjs[a];
-  IndexSetIterator b_elements(&_adjs[b]);
-  uint datum;
-  while ((datum = b_elements.next()) != 0) {
-    if(A->insert(datum)) {
-      _adjs[datum].insert(a);
-      lrgs(a).invalid_degree();
-      lrgs(datum).invalid_degree();
+  if (!_adjs[b].is_empty()) {
+    IndexSetIterator b_elements(&_adjs[b]);
+    uint datum;
+    while ((datum = b_elements.next()) != 0) {
+      if (A->insert(datum)) {
+        _adjs[datum].insert(a);
+        lrgs(a).invalid_degree();
+        lrgs(datum).invalid_degree();
+      }
     }
   }
 }
@@ -130,12 +134,15 @@
   _yanked->set(a);
 
   // I remove the LRG from all neighbors.
-  IndexSetIterator elements(&_adjs[a]);
   LRG &lrg_a = lrgs(a);
-  uint datum;
-  while ((datum = elements.next()) != 0) {
-    _adjs[datum].remove(a);
-    lrgs(datum).inc_degree( -lrg_a.compute_degree(lrgs(datum)) );
+
+  if (!_adjs[a].is_empty()) {
+    IndexSetIterator elements(&_adjs[a]);
+    uint datum;
+    while ((datum = elements.next()) != 0) {
+      _adjs[datum].remove(a);
+      lrgs(datum).inc_degree(-lrg_a.compute_degree(lrgs(datum)));
+    }
   }
   return neighbors(a);
 }
@@ -146,6 +153,8 @@
   assert( _yanked->test(a), "" );
   _yanked->remove(a);
 
+  if (_adjs[a].is_empty()) return;
+
   IndexSetIterator elements(&_adjs[a]);
   uint datum;
   while ((datum = elements.next()) != 0) {
@@ -159,7 +168,7 @@
 // mis-aligned (or for Fat-Projections, not-adjacent) then we have to
 // MULTIPLY the sizes.  Inspect Brigg's thesis on register pairs to see why
 // this is so.
-int LRG::compute_degree( LRG &l ) const {
+int LRG::compute_degree(LRG &l) const {
   int tmp;
   int num_regs = _num_regs;
   int nregs = l.num_regs();
@@ -174,14 +183,15 @@
 // mis-aligned (or for Fat-Projections, not-adjacent) then we have to
 // MULTIPLY the sizes.  Inspect Brigg's thesis on register pairs to see why
 // this is so.
-int PhaseIFG::effective_degree( uint lidx ) const {
+int PhaseIFG::effective_degree(uint lidx) const {
+  IndexSet *s = neighbors(lidx);
+  if (s->is_empty()) return 0;
   int eff = 0;
   int num_regs = lrgs(lidx).num_regs();
   int fat_proj = lrgs(lidx)._fat_proj;
-  IndexSet *s = neighbors(lidx);
   IndexSetIterator elements(s);
   uint nidx;
-  while((nidx = elements.next()) != 0) {
+  while ((nidx = elements.next()) != 0) {
     LRG &lrgn = lrgs(nidx);
     int nregs = lrgn.num_regs();
     eff += (fat_proj || lrgn._fat_proj) // either is a fat-proj?
@@ -196,14 +206,16 @@
 void PhaseIFG::dump() const {
   tty->print_cr("-- Interference Graph --%s--",
                 _is_square ? "square" : "triangular" );
-  if( _is_square ) {
-    for( uint i = 0; i < _maxlrg; i++ ) {
+  if (_is_square) {
+    for (uint i = 0; i < _maxlrg; i++) {
       tty->print(_yanked->test(i) ? "XX " : "  ");
       tty->print("L%d: { ",i);
-      IndexSetIterator elements(&_adjs[i]);
-      uint datum;
-      while ((datum = elements.next()) != 0) {
-        tty->print("L%d ", datum);
+      if (!_adjs[i].is_empty()) {
+        IndexSetIterator elements(&_adjs[i]);
+        uint datum;
+        while ((datum = elements.next()) != 0) {
+          tty->print("L%d ", datum);
+        }
       }
       tty->print_cr("}");
 
@@ -221,10 +233,12 @@
         tty->print("L%d ",j - 1);
       }
     tty->print("| ");
-    IndexSetIterator elements(&_adjs[i]);
-    uint datum;
-    while ((datum = elements.next()) != 0) {
-      tty->print("L%d ", datum);
+    if (!_adjs[i].is_empty()) {
+      IndexSetIterator elements(&_adjs[i]);
+      uint datum;
+      while ((datum = elements.next()) != 0) {
+        tty->print("L%d ", datum);
+      }
     }
     tty->print("}\n");
   }
@@ -251,16 +265,18 @@
   for( uint i = 0; i < _maxlrg; i++ ) {
     assert(!_yanked->test(i) || !neighbor_cnt(i), "Is removed completely" );
     IndexSet *set = &_adjs[i];
-    IndexSetIterator elements(set);
-    uint idx;
-    uint last = 0;
-    while ((idx = elements.next()) != 0) {
-      assert(idx != i, "Must have empty diagonal");
-      assert(pc->_lrg_map.find_const(idx) == idx, "Must not need Find");
-      assert(_adjs[idx].member(i), "IFG not square");
-      assert(!_yanked->test(idx), "No yanked neighbors");
-      assert(last < idx, "not sorted increasing");
-      last = idx;
+    if (!set->is_empty()) {
+      IndexSetIterator elements(set);
+      uint idx;
+      uint last = 0;
+      while ((idx = elements.next()) != 0) {
+        assert(idx != i, "Must have empty diagonal");
+        assert(pc->_lrg_map.find_const(idx) == idx, "Must not need Find");
+        assert(_adjs[idx].member(i), "IFG not square");
+        assert(!_yanked->test(idx), "No yanked neighbors");
+        assert(last < idx, "not sorted increasing");
+        last = idx;
+      }
     }
     assert(!lrgs(i)._degree_valid || effective_degree(i) == lrgs(i).degree(), "degree is valid but wrong");
   }
@@ -273,16 +289,18 @@
  * Only interfere if acceptable register masks overlap.
  */
 void PhaseChaitin::interfere_with_live(uint lid, IndexSet* liveout) {
-  LRG& lrg = lrgs(lid);
-  const RegMask& rm = lrg.mask();
-  IndexSetIterator elements(liveout);
-  uint interfering_lid = elements.next();
-  while (interfering_lid != 0) {
-    LRG& interfering_lrg = lrgs(interfering_lid);
-    if (rm.overlap(interfering_lrg.mask())) {
-      _ifg->add_edge(lid, interfering_lid);
+  if (!liveout->is_empty()) {
+    LRG& lrg = lrgs(lid);
+    const RegMask &rm = lrg.mask();
+    IndexSetIterator elements(liveout);
+    uint interfering_lid = elements.next();
+    while (interfering_lid != 0) {
+      LRG& interfering_lrg = lrgs(interfering_lid);
+      if (rm.overlap(interfering_lrg.mask())) {
+        _ifg->add_edge(lid, interfering_lid);
+      }
+      interfering_lid = elements.next();
     }
-    interfering_lid = elements.next();
   }
 }
 
@@ -381,6 +399,9 @@
 
 #ifdef ASSERT
 uint PhaseChaitin::count_int_pressure(IndexSet* liveout) {
+  if (liveout->is_empty()) {
+    return 0;
+  }
   IndexSetIterator elements(liveout);
   uint lidx = elements.next();
   uint cnt = 0;
@@ -397,6 +418,9 @@
 }
 
 uint PhaseChaitin::count_float_pressure(IndexSet* liveout) {
+  if (liveout->is_empty()) {
+    return 0;
+  }
   IndexSetIterator elements(liveout);
   uint lidx = elements.next();
   uint cnt = 0;
@@ -494,13 +518,15 @@
  * the block from the area.
  */
 void PhaseChaitin::compute_initial_block_pressure(Block* b, IndexSet* liveout, Pressure& int_pressure, Pressure& float_pressure, double cost) {
-  IndexSetIterator elements(liveout);
-  uint lid = elements.next();
-  while (lid != 0) {
-    LRG& lrg = lrgs(lid);
-    lrg._area += cost;
-    raise_pressure(b, lrg, int_pressure, float_pressure);
-    lid = elements.next();
+  if (!liveout->is_empty()) {
+    IndexSetIterator elements(liveout);
+    uint lid = elements.next();
+    while (lid != 0) {
+      LRG &lrg = lrgs(lid);
+      lrg._area += cost;
+      raise_pressure(b, lrg, int_pressure, float_pressure);
+      lid = elements.next();
+    }
   }
   assert(int_pressure.current_pressure() == count_int_pressure(liveout), "the int pressure is incorrect");
   assert(float_pressure.current_pressure() == count_float_pressure(liveout), "the float pressure is incorrect");
@@ -512,13 +538,15 @@
 * and int/pointer registers.
 */
 void PhaseChaitin::compute_entry_block_pressure(Block* b) {
-  IndexSet* livein = _live->livein(b);
-  IndexSetIterator elements(livein);
-  uint lid = elements.next();
-  while (lid != 0) {
-    LRG& lrg = lrgs(lid);
-    raise_pressure(b, lrg, _sched_int_pressure, _sched_float_pressure);
-    lid = elements.next();
+  IndexSet *livein = _live->livein(b);
+  if (!livein->is_empty()) {
+    IndexSetIterator elements(livein);
+    uint lid = elements.next();
+    while (lid != 0) {
+      LRG &lrg = lrgs(lid);
+      raise_pressure(b, lrg, _sched_int_pressure, _sched_float_pressure);
+      lid = elements.next();
+    }
   }
   // Now check phis for locally defined inputs
   for (uint j = 0; j < b->number_of_nodes(); j++) {
@@ -546,15 +574,18 @@
 * and int/pointer registers.
 */
 void PhaseChaitin::compute_exit_block_pressure(Block* b) {
+
   IndexSet* livein = _live->live(b);
-  IndexSetIterator elements(livein);
   _sched_int_pressure.set_current_pressure(0);
   _sched_float_pressure.set_current_pressure(0);
-  uint lid = elements.next();
-  while (lid != 0) {
-    LRG& lrg = lrgs(lid);
-    raise_pressure(b, lrg, _sched_int_pressure, _sched_float_pressure);
-    lid = elements.next();
+  if (!livein->is_empty()) {
+    IndexSetIterator elements(livein);
+    uint lid = elements.next();
+    while (lid != 0) {
+      LRG &lrg = lrgs(lid);
+      raise_pressure(b, lrg, _sched_int_pressure, _sched_float_pressure);
+      lid = elements.next();
+    }
   }
 }
 
@@ -654,6 +685,7 @@
  * all conflicting parties and avoid the interference.
  */
 void PhaseChaitin::remove_bound_register_from_interfering_live_ranges(LRG& lrg, IndexSet* liveout, uint& must_spill) {
+  if (liveout->is_empty()) return;
   // Check for common case
   const RegMask& rm = lrg.mask();
   int r_size = lrg.num_regs();
@@ -833,7 +865,7 @@
       Node* n = block->get_node(location);
       uint lid = _lrg_map.live_range_id(n);
 
-      if(lid) {
+      if (lid) {
         LRG& lrg = lrgs(lid);
 
         // A DEF normally costs block frequency; rematerialized values are
diff -r 355f4f42dda5 -r 95a99e617f28 src/hotspot/share/opto/indexSet.cpp
--- a/src/hotspot/share/opto/indexSet.cpp	Thu Nov 14 10:55:46 2019 +0100
+++ b/src/hotspot/share/opto/indexSet.cpp	Thu Nov 14 15:24:35 2019 +0100
@@ -111,6 +111,9 @@
 IndexSet::BitBlock *IndexSet::alloc_block_containing(uint element) {
   BitBlock *block = alloc_block();
   uint bi = get_block_index(element);
+  if (bi >= _current_block_limit) {
+    _current_block_limit = bi + 1;
+  }
   _blocks[bi] = block;
   return block;
 }
@@ -125,7 +128,7 @@
   assert(block != &_empty_block, "cannot free the empty block");
   block->set_next((IndexSet::BitBlock*)Compile::current()->indexSet_free_block_list());
   Compile::current()->set_indexSet_free_block_list(block);
-  set_block(i,&_empty_block);
+  set_block(i, &_empty_block);
 }
 
 //------------------------------lrg_union--------------------------------------
@@ -168,38 +171,43 @@
   // other color.  (A variant of the Briggs assertion)
   uint reg_degree = 0;
 
-  uint element;
+  uint element = 0;
   // Load up the combined interference set with the neighbors of one
-  IndexSetIterator elements(one);
-  while ((element = elements.next()) != 0) {
-    LRG &lrg = ifg->lrgs(element);
-    if (mask.overlap(lrg.mask())) {
-      insert(element);
-      if( !lrg.mask().is_AllStack() ) {
-        reg_degree += lrg1.compute_degree(lrg);
-        if( reg_degree >= fail_degree ) return reg_degree;
-      } else {
-        // !!!!! Danger!  No update to reg_degree despite having a neighbor.
-        // A variant of the Briggs assertion.
-        // Not needed if I simplify during coalesce, ala George/Appel.
-        assert( lrg.lo_degree(), "" );
+  if (!one->is_empty()) {
+    IndexSetIterator elements(one);
+    while ((element = elements.next()) != 0) {
+      LRG &lrg = ifg->lrgs(element);
+      if (mask.overlap(lrg.mask())) {
+        insert(element);
+        if (!lrg.mask().is_AllStack()) {
+          reg_degree += lrg1.compute_degree(lrg);
+          if (reg_degree >= fail_degree) return reg_degree;
+        } else {
+          // !!!!! Danger!  No update to reg_degree despite having a neighbor.
+          // A variant of the Briggs assertion.
+          // Not needed if I simplify during coalesce, ala George/Appel.
+          assert(lrg.lo_degree(), "");
+        }
       }
     }
   }
   // Add neighbors of two as well
-  IndexSetIterator elements2(two);
-  while ((element = elements2.next()) != 0) {
-    LRG &lrg = ifg->lrgs(element);
-    if (mask.overlap(lrg.mask())) {
-      if (insert(element)) {
-        if( !lrg.mask().is_AllStack() ) {
-          reg_degree += lrg2.compute_degree(lrg);
-          if( reg_degree >= fail_degree ) return reg_degree;
-        } else {
-          // !!!!! Danger!  No update to reg_degree despite having a neighbor.
-          // A variant of the Briggs assertion.
-          // Not needed if I simplify during coalesce, ala George/Appel.
-          assert( lrg.lo_degree(), "" );
+
+  if (!two->is_empty()) {
+    IndexSetIterator elements2(two);
+    while ((element = elements2.next()) != 0) {
+      LRG &lrg = ifg->lrgs(element);
+      if (mask.overlap(lrg.mask())) {
+        if (insert(element)) {
+          if (!lrg.mask().is_AllStack()) {
+            reg_degree += lrg2.compute_degree(lrg);
+            if (reg_degree >= fail_degree) return reg_degree;
+          } else {
+            // !!!!! Danger!  No update to reg_degree despite having a neighbor.
+            // A variant of the Briggs assertion.
+            // Not needed if I simplify during coalesce, ala George/Appel.
+            assert(lrg.lo_degree(), "");
+          }
         }
       }
     }
@@ -219,6 +227,7 @@
   _max_elements = set->_max_elements;
 #endif
   _count = set->_count;
+  _current_block_limit = set->_current_block_limit;
   _max_blocks = set->_max_blocks;
   if (_max_blocks <= preallocated_block_list_size) {
     _blocks = _preallocated_block_list;
@@ -248,6 +257,7 @@
   _max_elements = max_elements;
 #endif
   _count = 0;
+  _current_block_limit = 0;
   _max_blocks = (max_elements + bits_per_block - 1) / bits_per_block;
 
   if (_max_blocks <= preallocated_block_list_size) {
@@ -272,6 +282,7 @@
   _max_elements = max_elements;
 #endif // ASSERT
   _count = 0;
+  _current_block_limit = 0;
   _max_blocks = (max_elements + bits_per_block - 1) / bits_per_block;
 
   if (_max_blocks <= preallocated_block_list_size) {
@@ -294,7 +305,8 @@
   set->check_watch("swap", _serial_number);
 #endif
 
-  for (uint i = 0; i < _max_blocks; i++) {
+  uint max = MAX2(_current_block_limit, set->_current_block_limit);
+  for (uint i = 0; i < max; i++) {
     BitBlock *temp = _blocks[i];
     set_block(i, set->_blocks[i]);
     set->set_block(i, temp);
@@ -302,6 +314,11 @@
   uint temp = _count;
   _count = set->_count;
   set->_count = temp;
+
+  temp = _current_block_limit;
+  _current_block_limit = set->_current_block_limit;
+  set->_current_block_limit = temp;
+
 }
 
 //---------------------------- IndexSet::dump() -----------------------------
@@ -383,78 +400,6 @@
 // Create an iterator for a set.  If empty blocks are detected when iterating
 // over the set, these blocks are replaced.
 
-IndexSetIterator::IndexSetIterator(IndexSet *set) {
-#ifdef ASSERT
-  if (CollectIndexSetStatistics) {
-    set->tally_iteration_statistics();
-  }
-  set->check_watch("traversed", set->count());
-#endif
-  if (set->is_empty()) {
-    _current = 0;
-    _next_word = IndexSet::words_per_block;
-    _next_block = 1;
-    _max_blocks = 1;
-
-    // We don't need the following values when we iterate over an empty set.
-    // The commented out code is left here to document that the omission
-    // is intentional.
-    //
-    //_value = 0;
-    //_words = NULL;
-    //_blocks = NULL;
-    //_set = NULL;
-  } else {
-    _current = 0;
-    _value = 0;
-    _next_block = 0;
-    _next_word = IndexSet::words_per_block;
-
-    _max_blocks = set->_max_blocks;
-    _words = NULL;
-    _blocks = set->_blocks;
-    _set = set;
-  }
-}
-
-//---------------------------- IndexSetIterator(const) -----------------------------
-// Iterate over a constant IndexSet.
-
-IndexSetIterator::IndexSetIterator(const IndexSet *set) {
-#ifdef ASSERT
-  if (CollectIndexSetStatistics) {
-    set->tally_iteration_statistics();
-  }
-  // We don't call check_watch from here to avoid bad recursion.
-  //   set->check_watch("traversed const", set->count());
-#endif
-  if (set->is_empty()) {
-    _current = 0;
-    _next_word = IndexSet::words_per_block;
-    _next_block = 1;
-    _max_blocks = 1;
-
-    // We don't need the following values when we iterate over an empty set.
-    // The commented out code is left here to document that the omission
-    // is intentional.
-    //
-    //_value = 0;
-    //_words = NULL;
-    //_blocks = NULL;
-    //_set = NULL;
-  } else {
-    _current = 0;
-    _value = 0;
-    _next_block = 0;
-    _next_word = IndexSet::words_per_block;
-
-    _max_blocks = set->_max_blocks;
-    _words = NULL;
-    _blocks = set->_blocks;
-    _set = NULL;
-  }
-}
-
 //---------------------------- List16Iterator::advance_and_next() -----------------------------
 // Advance to the next non-empty word in the set being iterated over.  Return the next element
 // if there is one.  If we are done, return 0.  This method is called from the next() method
@@ -467,10 +412,8 @@
       // Found a non-empty word.
       _value = ((_next_block - 1) * IndexSet::bits_per_block) + (wi * IndexSet::bits_per_word);
       _current = _words[wi];
-
-      _next_word = wi+1;
-
-      return next();
+      _next_word = wi + 1;
+      return next_value();
     }
   }
 
@@ -488,8 +431,7 @@
 
           _next_block = bi+1;
           _next_word = wi+1;
-
-          return next();
+          return next_value();
         }
       }
 
@@ -501,11 +443,6 @@
     }
   }
 
-  // These assignments make redundant calls to next on a finished iterator
-  // faster.  Probably not necessary.
-  _next_block = _max_blocks;
-  _next_word = IndexSet::words_per_block;
-
   // No more words.
   return 0;
 }
diff -r 355f4f42dda5 -r 95a99e617f28 src/hotspot/share/opto/indexSet.hpp
--- a/src/hotspot/share/opto/indexSet.hpp	Thu Nov 14 10:55:46 2019 +0100
+++ b/src/hotspot/share/opto/indexSet.hpp	Thu Nov 14 15:24:35 2019 +0100
@@ -190,6 +190,9 @@
   // The number of elements in the set
   uint      _count;
 
+  // The current upper limit of blocks that has been allocated and might be in use
+  uint      _current_block_limit;
+
   // Our top level array of bitvector segments
   BitBlock **_blocks;
 
@@ -246,12 +249,13 @@
 
   void clear() {
     _count = 0;
-    for (uint i = 0; i < _max_blocks; i++) {
+    for (uint i = 0; i < _current_block_limit; i++) {
       BitBlock *block = _blocks[i];
       if (block != &_empty_block) {
         free_block(i);
       }
     }
+    _current_block_limit = 0;
   }
 
   uint count() const { return _count; }
@@ -380,18 +384,18 @@
   // The index of the next word we will inspect
   uint                  _next_word;
 
-  // A pointer to the contents of the current block
-  uint32_t             *_words;
-
   // The index of the next block we will inspect
   uint                  _next_block;
 
+  // The number of blocks in the set
+  uint                  _max_blocks;
+
+  // A pointer to the contents of the current block
+  uint32_t             *_words;
+
   // A pointer to the blocks in our set
   IndexSet::BitBlock **_blocks;
 
-  // The number of blocks in the set
-  uint                  _max_blocks;
-
   // If the iterator was created from a non-const set, we replace
   // non-canonical empty blocks with the _empty_block pointer.  If
   // _set is NULL, we do no replacement.
@@ -405,22 +409,64 @@
 
   // If an iterator is built from a constant set then empty blocks
   // are not canonicalized.
-  IndexSetIterator(IndexSet *set);
-  IndexSetIterator(const IndexSet *set);
+  IndexSetIterator(IndexSet *set) :
+    _current(0),
+    _value(0),
+    _next_word(IndexSet::words_per_block),
+    _next_block(0),
+    _max_blocks(set->is_empty() ? 0 : set->_current_block_limit),
+    _words(NULL),
+    _blocks(set->_blocks),
+    _set(set) {
+  #ifdef ASSERT
+    if (CollectIndexSetStatistics) {
+      set->tally_iteration_statistics();
+    }
+    set->check_watch("traversed", set->count());
+  #endif
+  }
+
+  IndexSetIterator(const IndexSet *set) :
+    _current(0),
+    _value(0),
+    _next_word(IndexSet::words_per_block),
+    _next_block(0),
+    _max_blocks(set->is_empty() ? 0 : set->_current_block_limit),
+    _words(NULL),
+    _blocks(set->_blocks),
+    _set(NULL)
+  {
+  #ifdef ASSERT
+    if (CollectIndexSetStatistics) {
+      set->tally_iteration_statistics();
+    }
+    // We don't call check_watch from here to avoid bad recursion.
+    //   set->check_watch("traversed const", set->count());
+  #endif
+  }
+
+  // Return the next element of the set.
+  uint next_value() {
+    uint current = _current;
+    assert(current != 0, "sanity");
+    uint advance = count_trailing_zeros(current);
+    assert(((current >> advance) & 0x1) == 1, "sanity");
+    _current = (current >> advance) - 1;
+    _value += advance;
+    return _value;
+  }
 
   // Return the next element of the set.  Return 0 when done.
   uint next() {
-    uint current = _current;
-    if (current != 0) {
-      uint advance = count_trailing_zeros(current);
-      assert(((current >> advance) & 0x1) == 1, "sanity");
-      _current = (current >> advance) - 1;
-      _value += advance;
-      return _value;
+    if (_current != 0) {
+      return next_value();
+    } else if (_next_word < IndexSet::words_per_block || _next_block < _max_blocks) {
+      return advance_and_next();
     } else {
-      return advance_and_next();
+      return 0;
     }
   }
+
 };
 
 #endif // SHARE_OPTO_INDEXSET_HPP
diff -r 355f4f42dda5 -r 95a99e617f28 src/hotspot/share/opto/live.cpp
--- a/src/hotspot/share/opto/live.cpp	Thu Nov 14 10:55:46 2019 +0100
+++ b/src/hotspot/share/opto/live.cpp	Thu Nov 14 15:24:35 2019 +0100
@@ -84,7 +84,7 @@
 
   // Array of delta-set pointers, indexed by block pre_order-1.
   _deltas = NEW_RESOURCE_ARRAY(IndexSet*,_cfg.number_of_blocks());
-  memset( _deltas, 0, sizeof(IndexSet*)* _cfg.number_of_blocks());
+  memset(_deltas, 0, sizeof(IndexSet*)* _cfg.number_of_blocks());
 
   _free_IndexSet = NULL;
 
@@ -108,8 +108,8 @@
 
       uint r = _names.at(n->_idx);
       assert(!def_outside->member(r), "Use of external LRG overlaps the same LRG defined in this block");
-      def->insert( r );
-      use->remove( r );
+      def->insert(r);
+      use->remove(r);
       uint cnt = n->req();
       for (uint k = 1; k < cnt; k++) {
         Node *nk = n->in(k);
@@ -152,7 +152,7 @@
     while (_worklist->size()) {
       Block* block = _worklist->pop();
       IndexSet *delta = getset(block);
-      assert( delta->count(), "missing delta set" );
+      assert(delta->count(), "missing delta set");
 
       // Add new-live-in to predecessors live-out sets
       for (uint l = 1; l < block->num_preds(); l++) {
@@ -191,36 +191,34 @@
 
 // Get an IndexSet for a block.  Return existing one, if any.  Make a new
 // empty one if a prior one does not exist.
-IndexSet *PhaseLive::getset( Block *p ) {
+IndexSet *PhaseLive::getset(Block *p) {
   IndexSet *delta = _deltas[p->_pre_order-1];
-  if( !delta )                  // Not on worklist?
+  if (!delta) {                 // Not on worklist?
     // Get a free set; flag as being on worklist
     delta = _deltas[p->_pre_order-1] = getfreeset();
+  }
   return delta;                 // Return set of new live-out items
 }
 
 // Pull from free list, or allocate.  Internal allocation on the returned set
 // is always from thread local storage.
-IndexSet *PhaseLive::getfreeset( ) {
+IndexSet *PhaseLive::getfreeset() {
   IndexSet *f = _free_IndexSet;
-  if( !f ) {
+  if (!f) {
     f = new IndexSet;
-//    f->set_arena(Thread::current()->resource_area());
     f->initialize(_maxlrg, Thread::current()->resource_area());
   } else {
     // Pull from free list
     _free_IndexSet = f->next();
-  //f->_cnt = 0;                        // Reset to empty
-//    f->set_arena(Thread::current()->resource_area());
     f->initialize(_maxlrg, Thread::current()->resource_area());
   }
   return f;
 }
 
 // Free an IndexSet from a block.
-void PhaseLive::freeset( Block *p ) {
+void PhaseLive::freeset(Block *p) {
   IndexSet *f = _deltas[p->_pre_order-1];
-  if ( _keep_deltas ) {
+  if (_keep_deltas) {
     add_livein(p, f);
   }
   f->set_next(_free_IndexSet);
@@ -230,40 +228,45 @@
 
 // Add a live-out value to a given blocks live-out set.  If it is new, then
 // also add it to the delta set and stick the block on the worklist.
-void PhaseLive::add_liveout( Block *p, uint r, VectorSet &first_pass ) {
+void PhaseLive::add_liveout(Block *p, uint r, VectorSet &first_pass) {
   IndexSet *live = &_live[p->_pre_order-1];
-  if( live->insert(r) ) {       // If actually inserted...
+  if (live->insert(r)) {        // If actually inserted...
     // We extended the live-out set.  See if the value is generated locally.
     // If it is not, then we must extend the live-in set.
-    if( !_defs[p->_pre_order-1].member( r ) ) {
-      if( !_deltas[p->_pre_order-1] && // Not on worklist?
-          first_pass.test(p->_pre_order) )
+    if (!_defs[p->_pre_order-1].member(r)) {
+      if (!_deltas[p->_pre_order-1] && // Not on worklist?
+          first_pass.test(p->_pre_order)) {
         _worklist->push(p);     // Actually go on worklist if already 1st pass
+      }
       getset(p)->insert(r);
     }
   }
 }
 
 // Add a vector of live-out values to a given blocks live-out set.
-void PhaseLive::add_liveout( Block *p, IndexSet *lo, VectorSet &first_pass ) {
+void PhaseLive::add_liveout(Block *p, IndexSet *lo, VectorSet &first_pass) {
   IndexSet *live = &_live[p->_pre_order-1];
   IndexSet *defs = &_defs[p->_pre_order-1];
   IndexSet *on_worklist = _deltas[p->_pre_order-1];
   IndexSet *delta = on_worklist ? on_worklist : getfreeset();
 
-  IndexSetIterator elements(lo);
-  uint r;
-  while ((r = elements.next()) != 0) {
-    if( live->insert(r) &&      // If actually inserted...
-        !defs->member( r ) )    // and not defined locally
-      delta->insert(r);         // Then add to live-in set
+  if (!lo->is_empty()) {
+    IndexSetIterator elements(lo);
+    uint r;
+    while ((r = elements.next()) != 0) {
+      if (live->insert(r) &&      // If actually inserted...
+          !defs->member(r)) {     // and not defined locally
+        delta->insert(r);         // Then add to live-in set
+      }
+    }
   }
 
-  if( delta->count() ) {                // If actually added things
+  if (delta->count()) {                // If actually added things
     _deltas[p->_pre_order-1] = delta; // Flag as on worklist now
-    if( !on_worklist &&         // Not on worklist?
-        first_pass.test(p->_pre_order) )
+    if (!on_worklist &&         // Not on worklist?
+        first_pass.test(p->_pre_order)) {
       _worklist->push(p);       // Actually go on worklist if already 1st pass
+    }
   } else {                      // Nothing there; just free it
     delta->set_next(_free_IndexSet);
     _free_IndexSet = delta;     // Drop onto free list
@@ -273,23 +276,25 @@
 // Add a vector of live-in values to a given blocks live-in set.
 void PhaseLive::add_livein(Block *p, IndexSet *lo) {
   IndexSet *livein = &_livein[p->_pre_order-1];
-  IndexSetIterator elements(lo);
-  uint r;
-  while ((r = elements.next()) != 0) {
-    livein->insert(r);         // Then add to live-in set
+  if (!livein->is_empty()) {
+    IndexSetIterator elements(lo);
+    uint r;
+    while ((r = elements.next()) != 0) {
+      livein->insert(r);         // Then add to live-in set
+    }
   }
 }
 
 #ifndef PRODUCT
 // Dump the live-out set for a block
-void PhaseLive::dump( const Block *b ) const {
+void PhaseLive::dump(const Block *b) const {
   tty->print("Block %d: ",b->_pre_order);
-  if ( _keep_deltas ) {
+  if (_keep_deltas) {
     tty->print("LiveIn: ");  _livein[b->_pre_order-1].dump();
   }
   tty->print("LiveOut: ");  _live[b->_pre_order-1].dump();
   uint cnt = b->number_of_nodes();
-  for( uint i=0; i<cnt; i++ ) {
+  for (uint i = 0; i < cnt; i++) {
     tty->print("L%d/", _names.at(b->get_node(i)->_idx));
     b->get_node(i)->dump();
   }
@@ -297,7 +302,7 @@
 }
 
 // Verify that base pointers and derived pointers are still sane.
-void PhaseChaitin::verify_base_ptrs( ResourceArea *a ) const {
+void PhaseChaitin::verify_base_ptrs(ResourceArea *a) const {
 #ifdef ASSERT
   Unique_Node_List worklist(a);
   for (uint i = 0; i < _cfg.number_of_blocks(); i++) {
@@ -322,17 +327,17 @@
               worklist.clear();
               worklist.push(check);
               uint k = 0;
-              while( k < worklist.size() ) {
+              while (k < worklist.size()) {
                 check = worklist.at(k);
                 assert(check,"Bad base or derived pointer");
                 // See PhaseChaitin::find_base_for_derived() for all cases.
                 int isc = check->is_Copy();
-                if( isc ) {
+                if (isc) {
                   worklist.push(check->in(isc));
-                } else if( check->is_Phi() ) {
+                } else if (check->is_Phi()) {
                   for (uint m = 1; m < check->req(); m++)
                     worklist.push(check->in(m));
-                } else if( check->is_Con() ) {
+                } else if (check->is_Con()) {
                   if (is_derived) {
                     // Derived is NULL+offset
                     assert(!is_derived || check->bottom_type()->is_ptr()->ptr() == TypePtr::Null,"Bad derived pointer");
@@ -346,8 +351,8 @@
                              check->bottom_type()->is_ptr()->ptr() == TypePtr::Null,"Bad base pointer");
                     }
                   }
-                } else if( check->bottom_type()->is_ptr()->_offset == 0 ) {
-                  if(check->is_Proj() || (check->is_Mach() &&
+                } else if (check->bottom_type()->is_ptr()->_offset == 0) {
+                  if (check->is_Proj() || (check->is_Mach() &&
                      (check->as_Mach()->ideal_Opcode() == Op_CreateEx ||
                       check->as_Mach()->ideal_Opcode() == Op_ThreadLocal ||
                       check->as_Mach()->ideal_Opcode() == Op_CMoveP ||
@@ -381,7 +386,7 @@
 }
 
 // Verify that graphs and base pointers are still sane.
-void PhaseChaitin::verify( ResourceArea *a, bool verify_ifg ) const {
+void PhaseChaitin::verify(ResourceArea *a, bool verify_ifg) const {
 #ifdef ASSERT
   if (VerifyRegisterAllocator) {
     _cfg.verify();
diff -r 355f4f42dda5 -r 95a99e617f28 src/hotspot/share/opto/reg_split.cpp
--- a/src/hotspot/share/opto/reg_split.cpp	Thu Nov 14 10:55:46 2019 +0100
+++ b/src/hotspot/share/opto/reg_split.cpp	Thu Nov 14 15:24:35 2019 +0100
@@ -1271,10 +1271,12 @@
         // it contains no members which compress to defidx.  Finding such an
         // instance may be a case to add liveout adjustment in compress_uf_map().
         // See 5063219.
-        uint member;
-        IndexSetIterator isi(liveout);
-        while ((member = isi.next()) != 0) {
-          assert(defidx != _lrg_map.find_const(member), "Live out member has not been compressed");
+        if (!liveout->is_empty()) {
+          uint member;
+          IndexSetIterator isi(liveout);
+          while ((member = isi.next()) != 0) {
+            assert(defidx != _lrg_map.find_const(member), "Live out member has not been compressed");
+          }
         }
 #endif
         Reachblock[slidx] = NULL;