8029015: PPC64 (part 216): opto: trap based null and range checks

Summary: On PPC64 use tdi instruction that does a compare and raises SIGTRAP for NULL and range checks.
Reviewed-by: kvn

src/cpu/ppc/vm/globals_ppc.hpp

@@ -38,6 +38,7 @@ define_pd_global(bool, NeedsDeoptSuspend,     false); // Only register window ma
 
 
 define_pd_global(bool, ImplicitNullChecks,    true);  // Generate code for implicit null checks.
+define_pd_global(bool, TrapBasedNullChecks,   true);
 define_pd_global(bool, UncommonNullCast,      true);  // Uncommon-trap NULLs passed to check cast.
 
 // Use large code-entry alignment.
@@ -100,12 +101,6 @@ define_pd_global(uintx, TypeProfileLevel, 0);
   product(bool, TrapBasedNotEntrantChecks, true,                            \
           "Raise and handle SIGTRAP if calling not entrant or zombie"       \
           " method.")                                                       \
-  product(bool, TrapBasedNullChecks, true,                                  \
-          "Generate code for null checks that uses a cmp and trap "         \
-          "instruction raising SIGTRAP.  This is only used if an access to" \
-          "null (+offset) will not raise a SIGSEGV.")                       \
-  product(bool, TrapBasedRangeChecks, true,                                 \
-          "Raise and handle SIGTRAP if array out of bounds check fails.")   \
   product(bool, TraceTraps, false, "Trace all traps the signal handler"     \
           "handles.")                                                       \
                                                                             \

src/cpu/ppc/vm/nativeInst_ppc.hpp

@@ -76,11 +76,13 @@ class NativeInstruction VALUE_OBJ_CLASS_SPEC {
   }
   static bool is_sigill_zombie_not_entrant_at(address addr);
 
+#ifdef COMPILER2
   // SIGTRAP-based implicit range checks
   bool is_sigtrap_range_check() {
     assert(UseSIGTRAP && TrapBasedRangeChecks, "precondition");
     return MacroAssembler::is_trap_range_check(long_at(0));
   }
+#endif
 
   // 'should not reach here'.
   bool is_sigtrap_should_not_reach_here() {

src/cpu/ppc/vm/vm_version_ppc.cpp

@@ -77,14 +77,17 @@ void VM_Version::initialize() {
     MSG(TrapBasedICMissChecks);
     MSG(TrapBasedNotEntrantChecks);
     MSG(TrapBasedNullChecks);
-    MSG(TrapBasedRangeChecks);
     FLAG_SET_ERGO(bool, TrapBasedNotEntrantChecks, false);
     FLAG_SET_ERGO(bool, TrapBasedNullChecks,       false);
     FLAG_SET_ERGO(bool, TrapBasedICMissChecks,     false);
-    FLAG_SET_ERGO(bool, TrapBasedRangeChecks,      false);
   }
 
 #ifdef COMPILER2
+  if (!UseSIGTRAP) {
+    MSG(TrapBasedRangeChecks);
+    FLAG_SET_ERGO(bool, TrapBasedRangeChecks, false);
+  }
+
   // On Power6 test for section size.
   if (PowerArchitecturePPC64 == 6)
     determine_section_size();

src/cpu/sparc/vm/c2_globals_sparc.hpp

@@ -90,6 +90,8 @@ define_pd_global(uint64_t,MaxRAM,                    4ULL*G);
 define_pd_global(uintx, CodeCacheMinBlockLength,     4);
 define_pd_global(uintx, CodeCacheMinimumUseSpace,    400*K);
 
+define_pd_global(bool,  TrapBasedRangeChecks,        false); // Not needed on sparc.
+
 // Heap related flags
 define_pd_global(uintx,MetaspaceSize,    ScaleForWordSize(16*M));
 

src/cpu/sparc/vm/globals_sparc.hpp

@@ -43,7 +43,8 @@ define_pd_global(bool, CountInterpCalls,            false); // not implemented i
 define_pd_global(bool, NeedsDeoptSuspend,           true); // register window machines need this
 
 define_pd_global(bool, ImplicitNullChecks,          true);  // Generate code for implicit null checks
-define_pd_global(bool, UncommonNullCast,            true);  // Uncommon-trap NULLs past to check cast
+define_pd_global(bool, TrapBasedNullChecks,         false); // Not needed on sparc.
+define_pd_global(bool, UncommonNullCast,            true);  // Uncommon-trap NULLs passed to check cast
 
 define_pd_global(intx, CodeEntryAlignment,    32);
 // The default setting 16/16 seems to work best.

src/cpu/x86/vm/c2_globals_x86.hpp

@@ -88,6 +88,8 @@ define_pd_global(intx, ReservedCodeCacheSize,        48*M);
 define_pd_global(uintx, CodeCacheMinBlockLength,     4);
 define_pd_global(uintx, CodeCacheMinimumUseSpace,    400*K);
 
+define_pd_global(bool,  TrapBasedRangeChecks,        false); // Not needed on x86.
+
 // Heap related flags
 define_pd_global(uintx,MetaspaceSize,    ScaleForWordSize(16*M));
 

src/cpu/x86/vm/globals_x86.hpp

@@ -37,7 +37,8 @@ define_pd_global(bool, CountInterpCalls,         true);
 define_pd_global(bool, NeedsDeoptSuspend,        false); // only register window machines need this
 
 define_pd_global(bool, ImplicitNullChecks,       true);  // Generate code for implicit null checks
-define_pd_global(bool, UncommonNullCast,         true);  // Uncommon-trap NULLs past to check cast
+define_pd_global(bool, TrapBasedNullChecks,      false); // Not needed on x86.
+define_pd_global(bool, UncommonNullCast,         true);  // Uncommon-trap NULLs passed to check cast
 
 // See 4827828 for this change. There is no globals_core_i486.hpp. I can't
 // assign a different value for C2 without touching a number of files. Use

src/cpu/zero/vm/globals_zero.hpp

@@ -38,6 +38,7 @@ define_pd_global(bool,  CountInterpCalls,     true);
 define_pd_global(bool,  NeedsDeoptSuspend,    false);
 
 define_pd_global(bool,  ImplicitNullChecks,   true);
+define_pd_global(bool,  TrapBasedNullChecks,  false);
 define_pd_global(bool,  UncommonNullCast,     true);
 
 define_pd_global(intx,  CodeEntryAlignment,   32);

src/share/vm/adlc/main.cpp

@@ -272,6 +272,7 @@ int main(int argc, char *argv[])
   AD.addInclude(AD._CPP_PIPELINE_file, "adfiles", get_basename(AD._HPP_file._name));
   AD.addInclude(AD._DFA_file, "precompiled.hpp");
   AD.addInclude(AD._DFA_file, "adfiles", get_basename(AD._HPP_file._name));
+  AD.addInclude(AD._DFA_file, "opto/cfgnode.hpp");  // Use PROB_MAX in predicate.
   AD.addInclude(AD._DFA_file, "opto/matcher.hpp");
   AD.addInclude(AD._DFA_file, "opto/opcodes.hpp");
   // Make sure each .cpp file starts with include lines:

src/share/vm/adlc/output_h.cpp

@@ -1615,15 +1615,19 @@ void ArchDesc::declareClasses(FILE *fp) {
     Attribute *attr = instr->_attribs;
     bool avoid_back_to_back = false;
     while (attr != NULL) {
-      if (strcmp (attr->_ident,"ins_cost") &&
-          strncmp(attr->_ident,"ins_field_", 10) != 0 &&
-          strcmp (attr->_ident,"ins_short_branch")) {
-        fprintf(fp,"  virtual int            %s() const { return %s; }\n",
-                attr->_ident, attr->_val);
+      if (strcmp (attr->_ident, "ins_cost") != 0 &&
+          strncmp(attr->_ident, "ins_field_", 10) != 0 &&
+          // Must match function in node.hpp: return type bool, no prefix "ins_".
+          strcmp (attr->_ident, "ins_is_TrapBasedCheckNode") != 0 &&
+          strcmp (attr->_ident, "ins_short_branch") != 0) {
+        fprintf(fp, "  virtual int            %s() const { return %s; }\n", attr->_ident, attr->_val);
       }
       // Check value for ins_avoid_back_to_back, and if it is true (1), set the flag
-      if (!strcmp(attr->_ident,"ins_avoid_back_to_back") && attr->int_val(*this) != 0)
+      if (!strcmp(attr->_ident, "ins_avoid_back_to_back") != 0 && attr->int_val(*this) != 0)
         avoid_back_to_back = true;
+      if (strcmp (attr->_ident, "ins_is_TrapBasedCheckNode") == 0)
+        fprintf(fp, "  virtual bool           is_TrapBasedCheckNode() const { return %s; }\n", attr->_val);
+
       attr = (Attribute *)attr->_next;
     }
 

src/share/vm/opto/block.cpp

@@ -530,18 +530,27 @@ void PhaseCFG::insert_goto_at(uint block_no, uint succ_no) {
 
 // Does this block end in a multiway branch that cannot have the default case
 // flipped for another case?
-static bool no_flip_branch( Block *b ) {
+static bool no_flip_branch(Block *b) {
   int branch_idx = b->number_of_nodes() - b->_num_succs-1;
-  if( branch_idx < 1 ) return false;
-  Node *bra = b->get_node(branch_idx);
-  if( bra->is_Catch() )
+  if (branch_idx < 1) {
+    return false;
+  }
+  Node *branch = b->get_node(branch_idx);
+  if (branch->is_Catch()) {
     return true;
-  if( bra->is_Mach() ) {
-    if( bra->is_MachNullCheck() )
+  }
+  if (branch->is_Mach()) {
+    if (branch->is_MachNullCheck()) {
+      return true;
+    }
+    int iop = branch->as_Mach()->ideal_Opcode();
+    if (iop == Op_FastLock || iop == Op_FastUnlock) {
       return true;
-    int iop = bra->as_Mach()->ideal_Opcode();
-    if( iop == Op_FastLock || iop == Op_FastUnlock )
+    }
+    // Don't flip if branch has an implicit check.
+    if (branch->as_Mach()->is_TrapBasedCheckNode()) {
       return true;
+    }
   }
   return false;
 }
@@ -700,6 +709,57 @@ void PhaseCFG::remove_empty_blocks() {
   } // End of for all blocks
 }
 
+Block *PhaseCFG::fixup_trap_based_check(Node *branch, Block *block, int block_pos, Block *bnext) {
+  // Trap based checks must fall through to the successor with
+  // PROB_ALWAYS.
+  // They should be an If with 2 successors.
+  assert(branch->is_MachIf(),   "must be If");
+  assert(block->_num_succs == 2, "must have 2 successors");
+
+  // Get the If node and the projection for the first successor.
+  MachIfNode *iff   = block->get_node(block->number_of_nodes()-3)->as_MachIf();
+  ProjNode   *proj0 = block->get_node(block->number_of_nodes()-2)->as_Proj();
+  ProjNode   *proj1 = block->get_node(block->number_of_nodes()-1)->as_Proj();
+  ProjNode   *projt = (proj0->Opcode() == Op_IfTrue)  ? proj0 : proj1;
+  ProjNode   *projf = (proj0->Opcode() == Op_IfFalse) ? proj0 : proj1;
+
+  // Assert that proj0 and succs[0] match up. Similarly for proj1 and succs[1].
+  assert(proj0->raw_out(0) == block->_succs[0]->head(), "Mismatch successor 0");
+  assert(proj1->raw_out(0) == block->_succs[1]->head(), "Mismatch successor 1");
+
+  ProjNode *proj_always;
+  ProjNode *proj_never;
+  // We must negate the branch if the implicit check doesn't follow
+  // the branch's TRUE path. Then, the new TRUE branch target will
+  // be the old FALSE branch target.
+  if (iff->_prob <= 2*PROB_NEVER) {   // There are small rounding errors.
+    proj_never  = projt;
+    proj_always = projf;
+  } else {
+    // We must negate the branch if the trap doesn't follow the
+    // branch's TRUE path. Then, the new TRUE branch target will
+    // be the old FALSE branch target.
+    proj_never  = projf;
+    proj_always = projt;
+    iff->negate();
+  }
+  assert(iff->_prob <= 2*PROB_NEVER, "Trap based checks are expected to trap never!");
+  // Map the successors properly
+  block->_succs.map(0, get_block_for_node(proj_never ->raw_out(0)));   // The target of the trap.
+  block->_succs.map(1, get_block_for_node(proj_always->raw_out(0)));   // The fall through target.
+
+  // Place the fall through block after this block.
+  Block *bs1 = block->non_connector_successor(1);
+  if (bs1 != bnext && move_to_next(bs1, block_pos)) {
+    bnext = bs1;
+  }
+  // If the fall through block still is not the next block, insert a goto.
+  if (bs1 != bnext) {
+    insert_goto_at(block_pos, 1);
+  }
+  return bnext;
+}
+
 // Fix up the final control flow for basic blocks.
 void PhaseCFG::fixup_flow() {
   // Fixup final control flow for the blocks.  Remove jump-to-next
@@ -723,25 +783,39 @@ void PhaseCFG::fixup_flow() {
     // Check for multi-way branches where I cannot negate the test to
     // exchange the true and false targets.
     if (no_flip_branch(block)) {
-      // Find fall through case - if must fall into its target
+      // Find fall through case - if must fall into its target.
+      // Get the index of the branch's first successor.
       int branch_idx = block->number_of_nodes() - block->_num_succs;
-      for (uint j2 = 0; j2 < block->_num_succs; j2++) {
-        const ProjNode* p = block->get_node(branch_idx + j2)->as_Proj();
-        if (p->_con == 0) {
-          // successor j2 is fall through case
-          if (block->non_connector_successor(j2) != bnext) {
-            // but it is not the next block => insert a goto
-            insert_goto_at(i, j2);
-          }
-          // Put taken branch in slot 0
-          if (j2 == 0 && block->_num_succs == 2) {
-            // Flip targets in succs map
-            Block *tbs0 = block->_succs[0];
-            Block *tbs1 = block->_succs[1];
-            block->_succs.map(0, tbs1);
-            block->_succs.map(1, tbs0);
+
+      // The branch is 1 before the branch's first successor.
+      Node *branch = block->get_node(branch_idx-1);
+
+      // Handle no-flip branches which have implicit checks and which require
+      // special block ordering and individual semantics of the 'fall through
+      // case'.
+      if ((TrapBasedNullChecks || TrapBasedRangeChecks) &&
+          branch->is_Mach() && branch->as_Mach()->is_TrapBasedCheckNode()) {
+        bnext = fixup_trap_based_check(branch, block, i, bnext);
+      } else {
+        // Else, default handling for no-flip branches
+        for (uint j2 = 0; j2 < block->_num_succs; j2++) {
+          const ProjNode* p = block->get_node(branch_idx + j2)->as_Proj();
+          if (p->_con == 0) {
+            // successor j2 is fall through case
+            if (block->non_connector_successor(j2) != bnext) {
+              // but it is not the next block => insert a goto
+              insert_goto_at(i, j2);
+            }
+            // Put taken branch in slot 0
+            if (j2 == 0 && block->_num_succs == 2) {
+              // Flip targets in succs map
+              Block *tbs0 = block->_succs[0];
+              Block *tbs1 = block->_succs[1];
+              block->_succs.map(0, tbs1);
+              block->_succs.map(1, tbs0);
+            }
+            break;
           }
-          break;
         }
       }
 

src/share/vm/opto/block.hpp

@@ -590,6 +590,7 @@ class PhaseCFG : public Phase {
 
   // Remove empty basic blocks
   void remove_empty_blocks();
+  Block *fixup_trap_based_check(Node *branch, Block *block, int block_pos, Block *bnext);
   void fixup_flow();
 
   // Insert a node into a block at index and map the node to the block

src/share/vm/opto/c2_globals.hpp

@@ -654,7 +654,11 @@
           "Propagate type improvements in callers of inlinee if possible")  \
                                                                             \
   experimental(bool, UseTypeSpeculation, false,                             \
-          "Speculatively propagate types from profiles")
+          "Speculatively propagate types from profiles")                    \
+                                                                            \
+  product_pd(bool, TrapBasedRangeChecks,                                    \
+          "Generate code for range checks that uses a cmp and trap "        \
+          "instruction raising SIGTRAP. Used on PPC64.")                    \
 
 C2_FLAGS(DECLARE_DEVELOPER_FLAG, DECLARE_PD_DEVELOPER_FLAG, DECLARE_PRODUCT_FLAG, DECLARE_PD_PRODUCT_FLAG, DECLARE_DIAGNOSTIC_FLAG, DECLARE_EXPERIMENTAL_FLAG, DECLARE_NOTPRODUCT_FLAG)
 

src/share/vm/opto/compile.cpp

@@ -860,6 +860,10 @@ Compile::Compile( ciEnv* ci_env, C2Compiler* compiler, ciMethod* target, int osr
   int next_slot = _orig_pc_slot + (sizeof(address) / VMRegImpl::stack_slot_size);
   set_fixed_slots(next_slot);
 
+  // Compute when to use implicit null checks. Used by matching trap based
+  // nodes and NullCheck optimization.
+  set_allowed_deopt_reasons();
+
   // Now generate code
   Code_Gen();
   if (failing())  return;
@@ -948,7 +952,8 @@ Compile::Compile( ciEnv* ci_env,
     _inlining_incrementally(false),
     _print_inlining_list(NULL),
     _print_inlining_idx(0),
-    _preserve_jvm_state(0) {
+    _preserve_jvm_state(0),
+    _allowed_reasons(0) {
   C = this;
 
 #ifndef PRODUCT
@@ -3350,6 +3355,19 @@ bool Compile::too_many_recompiles(ciMethod* method,
   }
 }
 
+// Compute when not to trap. Used by matching trap based nodes and
+// NullCheck optimization.
+void Compile::set_allowed_deopt_reasons() {
+  _allowed_reasons = 0;
+  if (is_method_compilation()) {
+    for (int rs = (int)Deoptimization::Reason_none+1; rs < Compile::trapHistLength; rs++) {
+      assert(rs < BitsPerInt, "recode bit map");
+      if (!too_many_traps((Deoptimization::DeoptReason) rs)) {
+        _allowed_reasons |= nth_bit(rs);
+      }
+    }
+  }
+}
 
 #ifndef PRODUCT
 //------------------------------verify_graph_edges---------------------------

src/share/vm/opto/compile.hpp

@@ -855,6 +855,11 @@ class Compile : public Phase {
                       ciMethodData* logmd = NULL);
   // Report if there were too many recompiles at a method and bci.
   bool too_many_recompiles(ciMethod* method, int bci, Deoptimization::DeoptReason reason);
+  // Return a bitset with the reasons where deoptimization is allowed,
+  // i.e., where there were not too many uncommon traps.
+  int _allowed_reasons;
+  int      allowed_deopt_reasons() { return _allowed_reasons; }
+  void set_allowed_deopt_reasons();
 
   // Parsing, optimization
   PhaseGVN*         initial_gvn()               { return _initial_gvn; }

src/share/vm/opto/gcm.cpp

@@ -1330,15 +1330,6 @@ void PhaseCFG::global_code_motion() {
   // 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.
   if (C->is_method_compilation()) {
-    // Don't do it for natives, adapters, or runtime stubs
-    int allowed_reasons = 0;
-    // ...and don't do it when there have been too many traps, globally.
-    for (int reason = (int)Deoptimization::Reason_none+1;
-         reason < Compile::trapHistLength; reason++) {
-      assert(reason < BitsPerInt, "recode bit map");
-      if (!C->too_many_traps((Deoptimization::DeoptReason) reason))
-        allowed_reasons |= nth_bit(reason);
-    }
     // By reversing the loop direction we get a very minor gain on mpegaudio.
     // Feel free to revert to a forward loop for clarity.
     // for( int i=0; i < (int)matcher._null_check_tests.size(); i+=2 ) {
@@ -1346,7 +1337,7 @@ void PhaseCFG::global_code_motion() {
       Node* proj = _matcher._null_check_tests[i];
       Node* val  = _matcher._null_check_tests[i + 1];
       Block* block = get_block_for_node(proj);
-      implicit_null_check(block, proj, val, allowed_reasons);
+      implicit_null_check(block, proj, val, C->allowed_deopt_reasons());
       // The implicit_null_check will only perform the transformation
       // if the null branch is truly uncommon, *and* it leads to an
       // uncommon trap.  Combined with the too_many_traps guards

src/share/vm/opto/machnode.hpp

@@ -315,6 +315,9 @@ public:
   static const Pipeline *pipeline_class();
   virtual const Pipeline *pipeline() const;
 
+  // Returns true if this node is a check that can be implemented with a trap.
+  virtual bool is_TrapBasedCheckNode() const { return false; }
+
 #ifndef PRODUCT
   virtual const char *Name() const = 0; // Machine-specific name
   virtual void dump_spec(outputStream *st) const; // Print per-node info

src/share/vm/opto/matcher.cpp

@@ -2395,6 +2395,69 @@ bool Matcher::post_store_load_barrier(const Node* vmb) {
   return false;
 }
 
+// Check whether node n is a branch to an uncommon trap that we could
+// optimize as test with very high branch costs in case of going to
+// the uncommon trap. The code must be able to be recompiled to use
+// a cheaper test.
+bool Matcher::branches_to_uncommon_trap(const Node *n) {
+  // Don't do it for natives, adapters, or runtime stubs
+  Compile *C = Compile::current();
+  if (!C->is_method_compilation()) return false;
+
+  assert(n->is_If(), "You should only call this on if nodes.");
+  IfNode *ifn = n->as_If();
+
+  Node *ifFalse = NULL;
+  for (DUIterator_Fast imax, i = ifn->fast_outs(imax); i < imax; i++) {
+    if (ifn->fast_out(i)->is_IfFalse()) {
+      ifFalse = ifn->fast_out(i);
+      break;
+    }
+  }
+  assert(ifFalse, "An If should have an ifFalse. Graph is broken.");
+
+  Node *reg = ifFalse;
+  int cnt = 4; // We must protect against cycles.  Limit to 4 iterations.
+               // Alternatively use visited set?  Seems too expensive.
+  while (reg != NULL && cnt > 0) {
+    CallNode *call = NULL;
+    RegionNode *nxt_reg = NULL;
+    for (DUIterator_Fast imax, i = reg->fast_outs(imax); i < imax; i++) {
+      Node *o = reg->fast_out(i);
+      if (o->is_Call()) {
+        call = o->as_Call();
+      }
+      if (o->is_Region()) {
+        nxt_reg = o->as_Region();
+      }
+    }
+
+    if (call &&
+        call->entry_point() == SharedRuntime::uncommon_trap_blob()->entry_point()) {
+      const Type* trtype = call->in(TypeFunc::Parms)->bottom_type();
+      if (trtype->isa_int() && trtype->is_int()->is_con()) {
+        jint tr_con = trtype->is_int()->get_con();
+        Deoptimization::DeoptReason reason = Deoptimization::trap_request_reason(tr_con);
+        Deoptimization::DeoptAction action = Deoptimization::trap_request_action(tr_con);
+        assert((int)reason < (int)BitsPerInt, "recode bit map");
+
+        if (is_set_nth_bit(C->allowed_deopt_reasons(), (int)reason)
+            && action != Deoptimization::Action_none) {
+          // This uncommon trap is sure to recompile, eventually.
+          // When that happens, C->too_many_traps will prevent
+          // this transformation from happening again.
+          return true;
+        }
+      }
+    }
+
+    reg = nxt_reg;
+    cnt--;
+  }
+
+  return false;
+}
+
 //=============================================================================
 //---------------------------State---------------------------------------------
 State::State(void) {

src/share/vm/opto/matcher.hpp

@@ -485,6 +485,8 @@ public:
   // retain the Node to act as a compiler ordering barrier.
   static bool post_store_load_barrier(const Node* mb);
 
+  // Does n lead to an uncommon trap that can cause deoptimization?
+  static bool branches_to_uncommon_trap(const Node *n);
 
 #ifdef ASSERT
   void dump_old2new_map();      // machine-independent to machine-dependent

src/share/vm/opto/output.cpp

@@ -1459,6 +1459,12 @@ void Compile::fill_buffer(CodeBuffer* cb, uint* blk_starts) {
           // Intel all the time, with add-to-memory kind of opcodes.
           previous_offset = current_offset;
         }
+
+        // Not an else-if!
+        // If this is a trap based cmp then add its offset to the list.
+        if (mach->is_TrapBasedCheckNode()) {
+          inct_starts[inct_cnt++] = current_offset;
+        }
       }
 
       // Verify that there is sufficient space remaining
@@ -1725,6 +1731,12 @@ void Compile::FillExceptionTables(uint cnt, uint *call_returns, uint *inct_start
       _inc_table.append(inct_starts[inct_cnt++], blk_labels[block_num].loc_pos());
       continue;
     }
+    // Handle implicit exception table updates: trap instructions.
+    if (n->is_Mach() && n->as_Mach()->is_TrapBasedCheckNode()) {
+      uint block_num = block->non_connector_successor(0)->_pre_order;
+      _inc_table.append(inct_starts[inct_cnt++], blk_labels[block_num].loc_pos());
+      continue;
+    }
   } // End of for all blocks fill in exception table entries
 }
 

src/share/vm/runtime/globals.hpp

@@ -2500,6 +2500,12 @@ class CommandLineFlags {
   develop_pd(bool, ImplicitNullChecks,                                      \
           "Generate code for implicit null checks")                         \
                                                                             \
+  product_pd(bool, TrapBasedNullChecks,                                     \
+          "Generate code for null checks that uses a cmp and trap "         \
+          "instruction raising SIGTRAP.  This is only used if an access to" \
+          "null (+offset) will not raise a SIGSEGV, i.e.,"                  \
+          "ImplicitNullChecks don't work (PPC64).")                         \
+                                                                            \
   product(bool, PrintSafepointStatistics, false,                            \
           "Print statistics about safepoint synchronization")               \
                                                                             \