Reland: track global accesses to constant types

src/hydrogen.cc

 // Copyright 2013 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/hydrogen.h"
 
 #include <sstream>
 
 #include "src/v8.h"
 
@@ skipping 5446 matching lines @@
           return ast_context()->ReturnInstruction(result, expr->id());
         }
       }
 
       LookupIterator it(global, variable->name(), LookupIterator::OWN);
       GlobalPropertyAccess type = LookupGlobalProperty(variable, &it, LOAD);
 
       if (type == kUseCell) {
         Handle<PropertyCell> cell = it.GetPropertyCell();
         top_info()->dependencies()->AssumePropertyCell(cell);
-        if (it.property_details().cell_type() == PropertyCellType::kConstant) {
+        auto cell_type = it.property_details().cell_type();
+        if (cell_type == PropertyCellType::kConstant ||
+            cell_type == PropertyCellType::kUndefined) {
           Handle<Object> constant_object(cell->value(), isolate());
           if (constant_object->IsConsString()) {
             constant_object =
                 String::Flatten(Handle<String>::cast(constant_object));
           }
           HConstant* constant = New<HConstant>(constant_object);
           return ast_context()->ReturnInstruction(constant, expr->id());
         } else {
+          auto access = HObjectAccess::ForPropertyCellValue();
+          UniqueSet<Map>* field_maps = nullptr;
+          if (cell_type == PropertyCellType::kConstantType) {
+            switch (cell->GetConstantType()) {
+              case PropertyCellConstantType::kSmi:
+                access = access.WithRepresentation(Representation::Smi());
+                break;
+              case PropertyCellConstantType::kStableMap: {
+                // Check that the map really is stable. The heap object could
+                // have mutated without the cell updating state. In that case,
+                // make no promises about the loaded value except that it's a
+                // heap object.
+                access =
+                    access.WithRepresentation(Representation::HeapObject());
+                Handle<Map> map(HeapObject::cast(cell->value())->map());
+                if (map->is_stable()) {
+                  field_maps = new (zone())
+                      UniqueSet<Map>(Unique<Map>::CreateImmovable(map), zone());
+                }
+                break;
+              }
+            }
+          }
           HConstant* cell_constant = Add<HConstant>(cell);
-          HLoadNamedField* instr = New<HLoadNamedField>(
-              cell_constant, nullptr, HObjectAccess::ForPropertyCellValue());
+          HLoadNamedField* instr;
+          if (field_maps == nullptr) {
+            instr = New<HLoadNamedField>(cell_constant, nullptr, access);
+          } else {
+            instr = New<HLoadNamedField>(cell_constant, nullptr, access,
+                                         field_maps, HType::HeapObject());
+          }
           instr->ClearDependsOnFlag(kInobjectFields);
           instr->SetDependsOnFlag(kGlobalVars);
           return ast_context()->ReturnInstruction(instr, expr->id());
         }
       } else {
         HValue* global_object = Add<HLoadNamedField>(
             context(), nullptr,
             HObjectAccess::ForContextSlot(Context::GLOBAL_OBJECT_INDEX));
         HLoadGlobalGeneric* instr =
             New<HLoadGlobalGeneric>(global_object,
@@ skipping 1153 matching lines @@
       Add<HSimulate>(ast_id, REMOVABLE_SIMULATE);
       return;
     }
   }
 
   LookupIterator it(global, var->name(), LookupIterator::OWN);
   GlobalPropertyAccess type = LookupGlobalProperty(var, &it, STORE);
   if (type == kUseCell) {
     Handle<PropertyCell> cell = it.GetPropertyCell();
     top_info()->dependencies()->AssumePropertyCell(cell);
-    if (it.property_details().cell_type() == PropertyCellType::kConstant) {
+    auto cell_type = it.property_details().cell_type();
+    if (cell_type == PropertyCellType::kConstant ||
+        cell_type == PropertyCellType::kUndefined) {
       Handle<Object> constant(cell->value(), isolate());
       if (value->IsConstant()) {
         HConstant* c_value = HConstant::cast(value);
         if (!constant.is_identical_to(c_value->handle(isolate()))) {
           Add<HDeoptimize>(Deoptimizer::kConstantGlobalVariableAssignment,
                            Deoptimizer::EAGER);
         }
       } else {
         HValue* c_constant = Add<HConstant>(constant);
         IfBuilder builder(this);
         if (constant->IsNumber()) {
           builder.If<HCompareNumericAndBranch>(value, c_constant, Token::EQ);
         } else {
           builder.If<HCompareObjectEqAndBranch>(value, c_constant);
         }
         builder.Then();
         builder.Else();
         Add<HDeoptimize>(Deoptimizer::kConstantGlobalVariableAssignment,
                          Deoptimizer::EAGER);
         builder.End();
       }
     }
     HConstant* cell_constant = Add<HConstant>(cell);
-    HInstruction* instr = Add<HStoreNamedField>(
-        cell_constant, HObjectAccess::ForPropertyCellValue(), value);
+    auto access = HObjectAccess::ForPropertyCellValue();
+    if (cell_type == PropertyCellType::kConstantType) {
+      switch (cell->GetConstantType()) {
+        case PropertyCellConstantType::kSmi:
+          access = access.WithRepresentation(Representation::Smi());
+          break;
+        case PropertyCellConstantType::kStableMap: {
+          // The map may no longer be stable, deopt if it's ever different from
+          // what is currently there, which will allow for restablization.
+          Handle<Map> map(HeapObject::cast(cell->value())->map());
+          Add<HCheckHeapObject>(value);
+          value = Add<HCheckMaps>(value, map);
+          access = access.WithRepresentation(Representation::HeapObject());
+          break;
+        }
+      }
+    }
+    HInstruction* instr = Add<HStoreNamedField>(cell_constant, access, value);
     instr->ClearChangesFlag(kInobjectFields);
     instr->SetChangesFlag(kGlobalVars);
     if (instr->HasObservableSideEffects()) {
       Add<HSimulate>(ast_id, REMOVABLE_SIMULATE);
     }
   } else {
     HValue* global_object = Add<HLoadNamedField>(
         context(), nullptr,
         HObjectAccess::ForContextSlot(Context::GLOBAL_OBJECT_INDEX));
     HStoreNamedGeneric* instr =
@@ skipping 6393 matching lines @@
   if (ShouldProduceTraceOutput()) {
     isolate()->GetHTracer()->TraceHydrogen(name(), graph_);
   }
 
 #ifdef DEBUG
   graph_->Verify(false);  // No full verify.
 #endif
 }
 
 } }  // namespace v8::internal