[vm] Treat static final fields with trivial initializers as const

When loading a value of a static final field with trivial initializer
generate a Constant instruction instead of LoadStaticField.

Initializer of a static field is considered trivial if it is null, int,
double, String or bool literal.

TEST=vm/cc/StreamingFlowGraphBuilder_StaticGetFinalFieldWithTrivialInitializer
Fixes dart-lang/sdk#47120

Change-Id: Id2bfc3da8c14376f7f1ef829265cca29a018bad1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/212873
Reviewed-by: Slava Egorov <[email protected]>
Commit-Queue: Alexander Markov <[email protected]>

Author: alexmarkov

runtime/vm/compiler/backend/type_propagator_test.cc

@@ -552,7 +552,7 @@ ISOLATE_UNIT_TEST_CASE(TypePropagator_NonNullableLoadStaticField) {
 
   const char* kScript = R"(
     const y = 0xDEADBEEF;
-    final int x = 0xFEEDFEED;
+    final int x = int.parse('0xFEEDFEED');
 
     void main(List<String> args) {
       print(x);

runtime/vm/compiler/frontend/kernel_binary_flowgraph.cc

@@ -2776,6 +2776,10 @@ Fragment StreamingFlowGraphBuilder::BuildStaticGet(TokenPosition* p) {
              Class::Handle(field.Owner()).Name() == Symbols::ClassID().ptr());
       return Constant(Instance::ZoneHandle(
           Z, Instance::RawCast(field.StaticConstFieldValue())));
+    } else if (field.is_final() && field.has_trivial_initializer()) {
+      // Final fields with trivial initializers are effectively constant.
+      return Constant(Instance::ZoneHandle(
+          Z, Instance::RawCast(field.StaticConstFieldValue())));
     } else {
       const Class& owner = Class::Handle(Z, field.Owner());
       const String& getter_name = H.DartGetterName(target);

runtime/vm/compiler/frontend/kernel_binary_flowgraph_test.cc

@@ -340,4 +340,44 @@ ISOLATE_UNIT_TEST_CASE(StreamingFlowGraphBuilder_TypedClosureCall) {
   // clang-format on
 }
 
+ISOLATE_UNIT_TEST_CASE(
+    StreamingFlowGraphBuilder_StaticGetFinalFieldWithTrivialInitializer) {
+  const char* kScript = R"(
+    final int x = 0xFEEDFEED;
+    test() {
+      return x;
+    }
+  )";
+
+  const auto& root_library = Library::Handle(LoadTestScript(kScript));
+  const auto& function = Function::Handle(GetFunction(root_library, "test"));
+
+  Invoke(root_library, "test");
+
+  TestPipeline pipeline(function, CompilerPass::kJIT);
+  FlowGraph* flow_graph = pipeline.RunPasses({
+      CompilerPass::kComputeSSA,
+  });
+
+  auto entry = flow_graph->graph_entry()->normal_entry();
+  EXPECT(entry != nullptr);
+
+  ReturnInstr* return_instr = nullptr;
+
+  ILMatcher cursor(flow_graph, entry);
+  RELEASE_ASSERT(cursor.TryMatch({
+      kMatchAndMoveFunctionEntry,
+      kMatchAndMoveCheckStackOverflow,
+      kMoveDebugStepChecks,
+      {kMatchReturn, &return_instr},
+  }));
+
+  EXPECT(return_instr != nullptr);
+  ConstantInstr* const_value =
+      return_instr->value()->definition()->AsConstant();
+  EXPECT(const_value != nullptr);
+  EXPECT(const_value->value().IsInteger());
+  EXPECT_EQ(0xFEEDFEED, Integer::Cast(const_value->value()).AsInt64Value());
+}
+
 }  // namespace dart

runtime/vm/kernel_loader.cc

@@ -881,7 +881,8 @@ void KernelLoader::CheckForInitializer(const Field& field) {
     SimpleExpressionConverter converter(&H, &helper_);
     const bool has_simple_initializer =
         converter.IsSimple(helper_.ReaderOffset() + 1);
-    if (!has_simple_initializer || !converter.SimpleValue().IsNull()) {
+    if (!has_simple_initializer ||
+        (!field.is_static() && !converter.SimpleValue().IsNull())) {
       field.set_has_nontrivial_initializer(true);
     }
     return;

runtime/vm/object.cc

@@ -11172,7 +11172,8 @@ ErrorPtr Field::InitializeStatic() const {
 }
 
 ObjectPtr Field::StaticConstFieldValue() const {
-  ASSERT(is_static() && is_const());
+  ASSERT(is_static() &&
+         (is_const() || (is_final() && has_trivial_initializer())));
 
   auto thread = Thread::Current();
   auto zone = thread->zone();
@@ -11183,7 +11184,11 @@ ObjectPtr Field::StaticConstFieldValue() const {
   auto& value = Object::Handle(
       zone, initial_field_table->At(field_id(), /*concurrent_use=*/true));
   if (value.ptr() == Object::sentinel().ptr()) {
+    // Fields with trivial initializers get their initial value
+    // eagerly when they are registered.
+    ASSERT(is_const());
     ASSERT(has_initializer());
+    ASSERT(has_nontrivial_initializer());
     value = EvaluateInitializer();
     if (!value.IsError()) {
       ASSERT(value.IsNull() || value.IsInstance());