My Patch for Defect 6478991

[rgougol at email.sjsu.edu]

Hello everybody and thanks for the feedbacks so far,

Here comes my suggested patch for the defect of NullCheck Elimination. Basically
it sets a flag if a type check operation is iterated. This flag prevents the 
optimizations of folding the NullChecks and let the elimination of the latter
NullCheck exception instead of the former. The flag is unset when the last
explicit exception is set. I will be very thankful if I get any feedback? Shall
I extend the patch to all the trap instruction besides type checks? Can also
somebody explain me what is the purpose of folding null check exception and why
should the former null check be eliminated instead of the latter? 

--- openjdk/hotspot/src/share/vm/c1/c1_Optimizer.cpp	2007-10-12
00:46:03.000000000 -0700
+++ nullcheck-openjdk/hotspot/src/share/vm/c1/c1_Optimizer.cpp	2007-12-19
14:22:58.000000000 -0800
@@ -490,6 +490,9 @@
   // Returns true if caused a change in the block's state.
   bool      merge_state_for(BlockBegin* block,
                             ValueSet*   incoming_state);
+  Instruction *  _prior_type_check;
+  Instruction *  prior_type_check() const { return _prior_type_check; }
+  void set_prior_type_check(Instruction * instr) { _prior_type_check = instr; } 
 
  public:
   // constructor
@@ -498,7 +501,8 @@
     , _set(new ValueSet())
     , _last_explicit_null_check(NULL)
     , _block_states(BlockBegin::number_of_blocks(), NULL)
-    , _work_list(new BlockList()) {
+    , _work_list(new BlockList())
+    , _prior_type_check (NULL)  {
     _visitable_instructions = new ValueSet();
     _visitor.set_eliminator(this);
   }
@@ -715,6 +719,9 @@
     // visiting instructions which are references in other blocks or
     // visiting instructions more than once.
     mark_visitable(instr);
+    if (instr->as_TypeCheck() != NULL ) {
+	set_prior_type_check(instr);
+    }
     if (instr->is_root() || instr->can_trap() || (instr->as_NullCheck() != NULL)) {
       mark_visited(instr);
       instr->input_values_do(&NullCheckEliminator::do_value);
@@ -769,13 +776,14 @@
   Value obj = x->obj();
   if (set_contains(obj)) {
     // Value is non-null => update AccessField
-    if (last_explicit_null_check_obj() == obj && !x->needs_patching()) {
+    if (last_explicit_null_check_obj() == obj && !x->needs_patching() && !
prior_type_check()) {
     } else {
       x->set_explicit_null_check(NULL);
       x->set_needs_null_check(false);
@@ -800,7 +808,7 @@
   Value array = x->array();
   if (set_contains(array)) {
     // Value is non-null => update AccessArray
-    if (last_explicit_null_check_obj() == array) {
+    if (last_explicit_null_check_obj() == array && ! prior_type_check()) {
       x->set_explicit_null_check(consume_last_explicit_null_check());
       x->set_needs_null_check(true);
       if (PrintNullCheckElimination) {
@@ -831,7 +839,7 @@
   Value array = x->array();
   if (set_contains(array)) {
     // Value is non-null => update AccessArray
-    if (last_explicit_null_check_obj() == array) {
+    if (last_explicit_null_check_obj() == array && ! prior_type_check()) {
       x->set_explicit_null_check(consume_last_explicit_null_check());
       x->set_needs_null_check(true);
       if (PrintNullCheckElimination) {
@@ -898,6 +906,7 @@
     if (PrintNullCheckElimination) {
       tty->print_cr("NullCheck %d of value %d proves value to be non-null",
x->id(), obj->id());
     }
+    set_prior_type_check(NULL);
   }
 }
 

Sincerely,

Rouhollah Gougol