Eliminate overflow check after integer add and sub if result is truncated to int32.

src/hydrogen-instructions.cc

Index: src/hydrogen-instructions.cc
===================================================================
--- src/hydrogen-instructions.cc	(revision 10863)
+++ src/hydrogen-instructions.cc	(working copy)
@@ -285,6 +285,14 @@
 }
 
 
+bool HValue::CheckUsesForFlag(Flag f) {
+  for (HUseIterator it(uses()); !it.Done(); it.Advance()) {
+    if (!it.value()->CheckFlag(f)) return false;
+  }
+  return true;
+}
+
+
 HUseIterator::HUseIterator(HUseListNode* head) : next_(head) {
   Advance();
 }
@@ -831,12 +839,12 @@
 
 
 HValue* HConstant::Canonicalize() {
-  return HasNoUses() && !IsBlockEntry() ? NULL : this;
+  return HasNoUses() ? NULL : this;
 }
 
 
 HValue* HTypeof::Canonicalize() {
-  return HasNoUses() && !IsBlockEntry() ? NULL : this;
+  return HasNoUses() ? NULL : this;
 }
 
 
@@ -858,6 +866,20 @@
 }
 
 
+HValue* HAdd::Canonicalize() {
+  if (!representation().IsInteger32()) return this;
+  if (CheckUsesForFlag(kTruncatingToInt32)) ClearFlag(kCanOverflow);
+  return this;
+}
+
+
+HValue* HSub::Canonicalize() {
+  if (!representation().IsInteger32()) return this;
+  if (CheckUsesForFlag(kTruncatingToInt32)) ClearFlag(kCanOverflow);
+  return this;
+}
+
+
 HValue* HChange::Canonicalize() {
   return (from().Equals(to())) ? value() : this;
 }