Add fake data dependencies (instead of disabling GVN) to fix code motion

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index dff96b4f7b488b89f030eca4540b8ea7c5d41846..d73eb07c98ca39c286ffb916fd81357b850ad539 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -3264,7 +3264,7 @@ void HGraphBuilder::VisitArrayLiteral(ArrayLiteral* expr) {
 
     // Load the elements array before the first store.
     if (elements == NULL)  {
-     elements = new(zone()) HLoadElements(literal);
+     elements = new(zone()) HLoadElements(literal, literal);

[Kevin Millikin (Chromium), 2011/07/11 15:03:54]

The second occurrence of literal is used as a token to indicate that you've
discharged your obligation to check this map ("trust me, I know what I'm
doing").  I wonder if the undefined value or something is better to use here?

[Jakob Kummerow, 2011/07/19 14:55:44]

Obsolete (I undid the changes to HLoadElements).

      AddInstruction(elements);
     }
 
@@ -3895,8 +3895,8 @@ HInstruction* HGraphBuilder::BuildMonomorphicElementAccess(HValue* object,
                     : BuildLoadKeyedGeneric(object, key);
   }
   AddInstruction(new(zone()) HCheckNonSmi(object));
-  AddInstruction(new(zone()) HCheckMap(object, map));
-  HInstruction* elements = new(zone()) HLoadElements(object);
+  HInstruction* mapcheck = AddInstruction(new(zone()) HCheckMap(object, map));
+  HInstruction* elements = new(zone()) HLoadElements(object, mapcheck);
   HInstruction* length = NULL;
   HInstruction* checked_key = NULL;
   if (map->has_external_array_elements()) {
@@ -3911,7 +3911,7 @@ HInstruction* HGraphBuilder::BuildMonomorphicElementAccess(HValue* object,
   }
   ASSERT(map->has_fast_elements());
   if (map->instance_type() == JS_ARRAY_TYPE) {
-    length = AddInstruction(new(zone()) HJSArrayLength(object));
+    length = AddInstruction(new(zone()) HJSArrayLength(object, mapcheck));
     checked_key = AddInstruction(new(zone()) HBoundsCheck(key, length));
     AddInstruction(elements);
   } else {
@@ -3962,6 +3962,7 @@ HValue* HGraphBuilder::HandlePolymorphicElementAccess(HValue* object,
 
   HInstruction* elements_kind_instr =
       AddInstruction(new(zone()) HElementsKind(object));
+  HCompareConstantEqAndBranch* elements_kind_branch = NULL;
   HInstruction* elements = NULL;
   HLoadExternalArrayPointer* external_elements = NULL;
   HInstruction* checked_key = NULL;
@@ -3978,14 +3979,8 @@ HValue* HGraphBuilder::HandlePolymorphicElementAccess(HValue* object,
                   JSObject::LAST_ELEMENTS_KIND);
     if (elements_kind == JSObject::FIRST_EXTERNAL_ARRAY_ELEMENTS_KIND
         && todo_external_array) {
-      elements = AddInstruction(new(zone()) HLoadElements(object));
-      // We need to forcibly prevent some ElementsKind-dependent instructions
-      // from being hoisted out of any loops they might occur in, because
-      // the current loop-invariant-code-motion algorithm isn't clever enough
-      // to deal with them properly.
-      // There's some performance to be gained by developing a smarter
-      // solution for this.
-      elements->ClearFlag(HValue::kUseGVN);
+      elements = AddInstruction(
+          new(zone()) HLoadElements(object, elements_kind_branch));

[Kevin Millikin (Chromium), 2011/07/11 15:03:54]

Should we assert the check instruction is not NULL here?

[Jakob Kummerow, 2011/07/19 14:55:44]

Good catch! Depending on the JS code being executed, the check instruction
actually could be NULL at this point. While investigating, I found that hoisting
prevention for HLoadElements no longer seems to be necessary, so I removed it.

       HInstruction* length =
           AddInstruction(new(zone()) HExternalArrayLength(elements));
       checked_key = AddInstruction(new(zone()) HBoundsCheck(key, length));
@@ -3995,13 +3990,11 @@ HValue* HGraphBuilder::HandlePolymorphicElementAccess(HValue* object,
     if (type_todo[elements_kind]) {
       HBasicBlock* if_true = graph()->CreateBasicBlock();
       HBasicBlock* if_false = graph()->CreateBasicBlock();
-      HCompareConstantEqAndBranch* compare =
-          new(zone()) HCompareConstantEqAndBranch(elements_kind_instr,
-                                                  elements_kind,
-                                                  Token::EQ_STRICT);
-      compare->SetSuccessorAt(0, if_true);
-      compare->SetSuccessorAt(1, if_false);
-      current_block()->Finish(compare);
+      elements_kind_branch = new(zone()) HCompareConstantEqAndBranch(
+          elements_kind_instr, elements_kind, Token::EQ_STRICT);
+      elements_kind_branch->SetSuccessorAt(0, if_true);
+      elements_kind_branch->SetSuccessorAt(1, if_false);
+      current_block()->Finish(elements_kind_branch);
 
       set_current_block(if_true);
       HInstruction* access;
@@ -4015,12 +4008,11 @@ HValue* HGraphBuilder::HandlePolymorphicElementAccess(HValue* object,
         current_block()->Finish(typecheck);
 
         set_current_block(if_jsarray);
-        HInstruction* length = new(zone()) HJSArrayLength(object);
+        HInstruction* length = new(zone()) HJSArrayLength(object, typecheck);
         AddInstruction(length);
-        length->ClearFlag(HValue::kUseGVN);
         checked_key = AddInstruction(new(zone()) HBoundsCheck(key, length));
-        elements = AddInstruction(new(zone()) HLoadElements(object));
-        elements->ClearFlag(HValue::kUseGVN);
+        elements =
+            AddInstruction(new(zone()) HLoadElements(object, checked_key));
         if (is_store) {
           access = AddInstruction(
               new(zone()) HStoreKeyedFastElement(elements, checked_key, val));
@@ -4036,8 +4028,7 @@ HValue* HGraphBuilder::HandlePolymorphicElementAccess(HValue* object,
         if_jsarray->Goto(join);
 
         set_current_block(if_fastobject);
-        elements = AddInstruction(new(zone()) HLoadElements(object));
-        elements->ClearFlag(HValue::kUseGVN);
+        elements = AddInstruction(new(zone()) HLoadElements(object, typecheck));
         length = AddInstruction(new(zone()) HFixedArrayLength(elements));
         checked_key = AddInstruction(new(zone()) HBoundsCheck(key, length));
         if (is_store) {
@@ -4170,8 +4161,9 @@ void HGraphBuilder::VisitProperty(Property* expr) {
   if (expr->IsArrayLength()) {
     HValue* array = Pop();
     AddInstruction(new(zone()) HCheckNonSmi(array));
-    AddInstruction(HCheckInstanceType::NewIsJSArray(array));
-    instr = new(zone()) HJSArrayLength(array);
+    HInstruction* mapcheck =
+        AddInstruction(HCheckInstanceType::NewIsJSArray(array));
+    instr = new(zone()) HJSArrayLength(array, mapcheck);
 
   } else if (expr->IsStringLength()) {
     HValue* string = Pop();