[async] Reduce generated size of `await` builtins.

There's a AsyncBuiltinsAssembler::InitializeNativeClosure() helper which
is used by the `await` builtins to initialize the native closures, which
are registered as fulfill/reject handlers. This helper initializes a
JSFunction based on the builtin SharedFunctionInfo. Since we are dealing
with builtins here, there's no point in using the fully generic helper
CodeStubAssembler::GetSharedFunctionInfoCode(), but we can immediately
assume that the SharedFunctionInfo::function_data() field contains a Smi
builtin index (guarded by TNode CASTs).

This almost cuts the generated code size for the `await` builtins in a
half, and might also yield some performance improvements due to reduced
register and instruction cache pressure.

Bug: v8:7253, v8:8238
Change-Id: I3415c2f9e0f04a7154c4bf4c3fe8156854dbfe13
Reviewed-on: https://chromium-review.googlesource.com/c/1281604Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56662}

src/builtins/builtins-async-gen.cc

@@ -250,8 +250,8 @@ void AsyncBuiltinsAssembler::InitializeNativeClosure(Node* context,
                                                      Node* native_context,
                                                      Node* function,
                                                      Node* context_index) {
-  Node* const function_map = LoadContextElement(
-      native_context, Context::STRICT_FUNCTION_WITHOUT_PROTOTYPE_MAP_INDEX);
+  TNode<Map> function_map = CAST(LoadContextElement(
+      native_context, Context::STRICT_FUNCTION_WITHOUT_PROTOTYPE_MAP_INDEX));
   // Ensure that we don't have to initialize prototype_or_initial_map field of
   // JSFunction.
   CSA_ASSERT(this, WordEqual(LoadMapInstanceSizeInWords(function_map),
@@ -266,13 +266,20 @@ void AsyncBuiltinsAssembler::InitializeNativeClosure(Node* context,
   StoreObjectFieldRoot(function, JSFunction::kFeedbackCellOffset,
                        RootIndex::kManyClosuresCell);
 
-  Node* shared_info = LoadContextElement(native_context, context_index);
-  CSA_ASSERT(this, IsSharedFunctionInfo(shared_info));
+  TNode<SharedFunctionInfo> shared_info =
+      CAST(LoadContextElement(native_context, context_index));
   StoreObjectFieldNoWriteBarrier(
       function, JSFunction::kSharedFunctionInfoOffset, shared_info);
   StoreObjectFieldNoWriteBarrier(function, JSFunction::kContextOffset, context);
 
-  Node* const code = GetSharedFunctionInfoCode(shared_info);
+  // For the native closures that are initialized here (for `await`)
+  // we know that their SharedFunctionInfo::function_data() slot
+  // contains a builtin index (as Smi), so there's no need to use
+  // CodeStubAssembler::GetSharedFunctionInfoCode() helper here,
+  // which almost doubles the size of `await` builtins (unnecessarily).
+  TNode<Smi> builtin_id = LoadObjectField<Smi>(
+      shared_info, SharedFunctionInfo::kFunctionDataOffset);
+  TNode<Code> code = LoadBuiltin(builtin_id);
   StoreObjectFieldNoWriteBarrier(function, JSFunction::kCodeOffset, code);
 }