More efficient use of space in AllocationSite.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 69969bdf28113dde6f0a47a23dd1db63536d4a0b..a52b7b992a0696e58a3eb2d5ccd49140b67249f7 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -2640,17 +2640,21 @@ void HGraphBuilder::BuildCreateAllocationMemento(
       HObjectAccess::ForAllocationMementoSite(),
       allocation_site);
   if (FLAG_allocation_site_pretenuring) {
-    HValue* memento_create_count = Add<HLoadNamedField>(
+    // We want to add 1 to the memento create count. The field is actually a smi
+    // bitfield, but the memento create count is in the lower 14 bits. Therefore
+    // we don't need to mask and shift.
+    STATIC_ASSERT(AllocationSite::MementoCreateCountBits::kShift == 0);
+    HValue* pretenure_data = Add<HLoadNamedField>(
         allocation_site, HObjectAccess::ForAllocationSiteOffset(
-            AllocationSite::kMementoCreateCountOffset));
-    memento_create_count = AddUncasted<HAdd>(
-        memento_create_count, graph()->GetConstant1());
+            AllocationSite::kPretenureDataOffset));
+    pretenure_data = AddUncasted<HAdd>(
+        pretenure_data, graph()->GetConstant1());
     // This smi value is reset to zero after every gc, overflow isn't a problem
     // since the counter is bounded by the new space size.
-    memento_create_count->ClearFlag(HValue::kCanOverflow);
+    pretenure_data->ClearFlag(HValue::kCanOverflow);
     HStoreNamedField* store = Add<HStoreNamedField>(
         allocation_site, HObjectAccess::ForAllocationSiteOffset(
-            AllocationSite::kMementoCreateCountOffset), memento_create_count);
+            AllocationSite::kPretenureDataOffset), pretenure_data);
     // No write barrier needed to store a smi.
     store->SkipWriteBarrier();
   }