Hydrogen array constructor cleanup and improvements

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index b699b43fe38b78dc0098e50657ce8e401befa0f3..503b56c5f44f737f4914e0e8822ea9ba02a823d4 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -1755,11 +1755,12 @@ HInstruction* HGraphBuilder::BuildGetArrayFunction(HValue* context) {
 HGraphBuilder::JSArrayBuilder::JSArrayBuilder(HGraphBuilder* builder,
                                               ElementsKind kind,
                                               HValue* allocation_site_payload,
+                                              HValue* constructor_function,
                                               bool disable_allocation_sites) :
         builder_(builder),
         kind_(kind),
         allocation_site_payload_(allocation_site_payload),
-        constructor_function_(NULL) {
+        constructor_function_(constructor_function) {
   mode_ = disable_allocation_sites
       ? DONT_TRACK_ALLOCATION_SITE
       : AllocationSiteInfo::GetMode(kind);
@@ -1778,17 +1779,23 @@ HGraphBuilder::JSArrayBuilder::JSArrayBuilder(HGraphBuilder* builder,
 
 
 HValue* HGraphBuilder::JSArrayBuilder::EmitMapCode(HValue* context) {
-  HInstruction* native_context = builder()->BuildGetNativeContext(context);
+  if (kind_ == FAST_SMI_ELEMENTS) {
+    // Optimization: the initial Array map is for FAST_SMI_ELEMENTS.
+    // No need for a context lookup in this case.

[danno, 2013/06/27 08:53:04]

Why not? What is special about smi elements?

[mvstanton, 2013/06/27 15:33:29]

Because the initial map on the constructor is setup for smi elements. I added an
assert in bootstrapper.cc to protect this (the assert can't live here because
this code is called before the global object is fully set up). I also changed
the code to check against GetInitialFastElementsKind() instead of
FAST_SMI_ELEMENTS.

+    HObjectAccess access = HObjectAccess::ForPrototypeOrInitialMap();
+    return AddInstruction(
+        builder()->BuildLoadNamedField(constructor_function_,
+                                       access,
+                                       Representation::Tagged()));
+  }
 
+  HInstruction* native_context = builder()->BuildGetNativeContext(context);
   HInstruction* index = builder()->AddInstruction(new(zone())
       HConstant(Context::JS_ARRAY_MAPS_INDEX));
-
   HInstruction* map_array = builder()->AddInstruction(new(zone())
       HLoadKeyed(native_context, index, NULL, FAST_ELEMENTS));
-
   HInstruction* kind_index = builder()->AddInstruction(new(zone())
-      HConstant(kind_));
-
+                                                       HConstant(kind_));
   return builder()->AddInstruction(new(zone())
       HLoadKeyed(map_array, kind_index, NULL, FAST_ELEMENTS));
 }
@@ -1885,7 +1892,7 @@ HValue* HGraphBuilder::JSArrayBuilder::AllocateArray(HValue* size_in_bytes,
 
   // Fill in the fields: map, properties, length
   HValue* map;
-  if (constructor_function_ != NULL) {
+  if (allocation_site_payload_ == NULL) {
     map = EmitInternalMapCode();
   } else {
     map = EmitMapCode(context);