track global accesses to constant types

src/code-stubs-hydrogen.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/bailout-reason.h"
 #include "src/code-stubs.h"
 #include "src/field-index.h"
 #include "src/hydrogen.h"
@@ skipping 1306 matching lines @@
   StoreGlobalStub* stub = casted_stub();
   HParameter* value = GetParameter(StoreDescriptor::kValueIndex);
   if (stub->check_global()) {
     // Check that the map of the global has not changed: use a placeholder map
     // that will be replaced later with the global object's map.
     HParameter* proxy = GetParameter(StoreDescriptor::kReceiverIndex);
     HValue* proxy_map =
         Add<HLoadNamedField>(proxy, nullptr, HObjectAccess::ForMap());
     HValue* global =
         Add<HLoadNamedField>(proxy_map, nullptr, HObjectAccess::ForPrototype());
-    Handle<Map> placeholder_map = isolate()->factory()->meta_map();
-    HValue* cell = Add<HConstant>(Map::WeakCellForMap(placeholder_map));
-    HValue* expected_map =
-        Add<HLoadNamedField>(cell, nullptr, HObjectAccess::ForWeakCellValue());
+    HValue* map_cell = Add<HConstant>(isolate()->factory()->NewWeakCell(
+        StoreGlobalStub::global_map_placeholder(isolate())));
+    HValue* expected_map = Add<HLoadNamedField>(
+        map_cell, nullptr, HObjectAccess::ForWeakCellValue());
     HValue* map =
         Add<HLoadNamedField>(global, nullptr, HObjectAccess::ForMap());
     IfBuilder map_check(this);
     map_check.IfNot<HCompareObjectEqAndBranch>(expected_map, map);
     map_check.ThenDeopt(Deoptimizer::kUnknownMap);
     map_check.End();
   }
 
   HValue* weak_cell = Add<HConstant>(isolate()->factory()->NewWeakCell(
       StoreGlobalStub::property_cell_placeholder(isolate())));
   HValue* cell = Add<HLoadNamedField>(weak_cell, nullptr,
                                       HObjectAccess::ForWeakCellValue());
   Add<HCheckHeapObject>(cell);
   HObjectAccess access = HObjectAccess::ForPropertyCellValue();
+  // Load the payload of the global parameter cell. A hole indicates that the
+  // cell has been invalidated and that the store must be handled by the
+  // runtime.
   HValue* cell_contents = Add<HLoadNamedField>(cell, nullptr, access);
 
-  if (stub->is_constant()) {
+  auto cell_type = stub->cell_type();
+  if (cell_type == PropertyCellType::kConstant ||
+      cell_type == PropertyCellType::kUndefined) {
+    // This is always valid for all states a cell can be in.
     IfBuilder builder(this);
     builder.If<HCompareObjectEqAndBranch>(cell_contents, value);
     builder.Then();
     builder.ElseDeopt(
         Deoptimizer::kUnexpectedCellContentsInConstantGlobalStore);
     builder.End();
   } else {
-    // Load the payload of the global parameter cell. A hole indicates that the
-    // property has been deleted and that the store must be handled by the
-    // runtime.
     IfBuilder builder(this);
     HValue* hole_value = graph()->GetConstantHole();
     builder.If<HCompareObjectEqAndBranch>(cell_contents, hole_value);
     builder.Then();
     builder.Deopt(Deoptimizer::kUnexpectedCellContentsInGlobalStore);
     builder.Else();
+    // When dealing with constant types, the type may be allowed to change, as
+    // long as optimized code remains valid.
+    if (cell_type == PropertyCellType::kConstantType) {
+      switch (stub->constant_type()) {
+        case PropertyCellConstantType::kSmi:
+          access = access.WithRepresentation(Representation::Smi());
+          break;
+        case PropertyCellConstantType::kStableMap: {
+          // It is sufficient here to check that the value and cell contents
+          // have identical maps, no matter if they are stable or not or if they
+          // are the maps that were originally in the cell or not. If optimized
+          // code will deopt when a cell has a unstable map and if it has a
+          // dependency on a stable map, it will deopt if the map destabilizes.
+          Add<HCheckHeapObject>(value);
+          Add<HCheckHeapObject>(cell_contents);
+          HValue* expected_map = Add<HLoadNamedField>(cell_contents, nullptr,
+                                                      HObjectAccess::ForMap());
+          HValue* map =
+              Add<HLoadNamedField>(value, nullptr, HObjectAccess::ForMap());
+          IfBuilder map_check(this);
+          map_check.IfNot<HCompareObjectEqAndBranch>(expected_map, map);
+          map_check.ThenDeopt(Deoptimizer::kUnknownMap);
+          map_check.End();
+          access = access.WithRepresentation(Representation::HeapObject());
+          break;
+        }
+      }
+    }
     Add<HStoreNamedField>(cell, access, value);
     builder.End();
   }
 
   return value;
 }
 
 
 Handle<Code> StoreGlobalStub::GenerateCode() {
   return DoGenerateCode(this);
@@ skipping 659 matching lines @@
   // need.
   info()->MarkMustNotHaveEagerFrame();
 
   // Probe the stub cache.
   Add<HTailCallThroughMegamorphicCache>(receiver, name);
 
   // We never continue.
   return graph()->GetConstant0();
 }
 } }  // namespace v8::internal