Lazy removal of dead HValues in GVN from use lists.

BUG=v8:1969
TEST=regress/regress-1969

Review URL: https://chromiumcodereview.appspot.com/9455011

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10812 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/hydrogen-instructions.cc

@@ -276,6 +276,15 @@ bool HValue::IsDefinedAfter(HBasicBlock* other) const {
 }
 
 
+HUseListNode* HUseListNode::tail() {
+  // Skip and remove dead items in the use list.
+  while (tail_ != NULL && tail_->value()->CheckFlag(HValue::kIsDead)) {
+    tail_ = tail_->tail_;
+  }
+  return tail_;
+}
+
+
 HUseIterator::HUseIterator(HUseListNode* head) : next_(head) {
   Advance();
 }
@@ -374,7 +383,10 @@ void HValue::DeleteAndReplaceWith(HValue* other) {
   // We replace all uses first, so Delete can assert that there are none.
   if (other != NULL) ReplaceAllUsesWith(other);
   ASSERT(HasNoUses());
-  ClearOperands();
+  // Clearing the operands includes going through the use list of each operand
+  // to remove this HValue, which can be expensive.  Instead, we simply mark it
+  // as dead and remove it lazily from the operands' use lists.
+  SetFlag(kIsDead);
   DeleteFromGraph();
 }
 

src/hydrogen-instructions.h

@@ -448,7 +448,7 @@ class HUseListNode: public ZoneObject {
       : tail_(tail), value_(value), index_(index) {
   }
 
-  HUseListNode* tail() const { return tail_; }
+  HUseListNode* tail();
   HValue* value() const { return value_; }
   int index() const { return index_; }
 
@@ -530,7 +530,8 @@ class HValue: public ZoneObject {
     kDeoptimizeOnUndefined,
     kIsArguments,
     kTruncatingToInt32,
-    kLastFlag = kTruncatingToInt32
+    kIsDead,
+    kLastFlag = kIsDead
   };
 
   STATIC_ASSERT(kLastFlag < kBitsPerInt);

test/mjsunit/mjsunit.status

-# Copyright 2011 the V8 project authors. All rights reserved.
+# Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
@@ -65,6 +65,9 @@ regress/regress-524: (PASS || TIMEOUT), SKIP if $mode == debug
 debug-liveedit-check-stack: SKIP
 debug-liveedit-patch-positions-replace: SKIP
 
+# Test Crankshaft compilation time.  Expected to take too long in debug mode.
+regress/regress-1969: PASS, SKIP if $mode == debug
+
 ##############################################################################
 [ $isolates ]