Optimize array literal boilerplate copy for fast cases.

src/ia32/full-codegen-ia32.cc

Index: src/ia32/full-codegen-ia32.cc
diff --git a/src/ia32/full-codegen-ia32.cc b/src/ia32/full-codegen-ia32.cc
index aa70dcb16b19aa77d47382f2c9e640bfb167d238..c1ef22edcf867bcb3e418bee23ec66fd792c5524 100644
--- a/src/ia32/full-codegen-ia32.cc
+++ b/src/ia32/full-codegen-ia32.cc
@@ -1477,10 +1477,9 @@ void FullCodeGenerator::VisitArrayLiteral(ArrayLiteral* expr) {
   int length = subexprs->length();
   Handle<FixedArray> constant_elements = expr->constant_elements();
   ASSERT_EQ(2, constant_elements->length());
-#if DEBUG
   ElementsKind constant_elements_kind =
       static_cast<ElementsKind>(Smi::cast(constant_elements->get(0))->value());
-#endif
+  bool has_constant_fast_elements = constant_elements_kind == FAST_ELEMENTS;
   Handle<FixedArrayBase> constant_elements_values(
       FixedArrayBase::cast(constant_elements->get(1)));
 
@@ -1488,7 +1487,17 @@ void FullCodeGenerator::VisitArrayLiteral(ArrayLiteral* expr) {
   __ push(FieldOperand(ebx, JSFunction::kLiteralsOffset));
   __ push(Immediate(Smi::FromInt(expr->literal_index())));
   __ push(Immediate(constant_elements));
-  if (expr->depth() > 1) {
+  Heap* heap = isolate()->heap();
+  if (has_constant_fast_elements &&
+      constant_elements_values->map() == heap->fixed_cow_array_map()) {
+    // If the elements are already FAST_ELEMENTS, the boilerplate cannot
+    // change, so it's possible to specialize the stub in advance.
+    __ IncrementCounter(isolate()->counters()->cow_arrays_created_stub(), 1);
+    FastCloneShallowArrayStub stub(
+        FastCloneShallowArrayStub::COPY_ON_WRITE_ELEMENTS,
+        length);
+    __ CallStub(&stub);
+  } else if (expr->depth() > 1) {
     __ CallRuntime(Runtime::kCreateArrayLiteral, 3);
   } else if (length > FastCloneShallowArrayStub::kMaximumClonedLength) {
     __ CallRuntime(Runtime::kCreateArrayLiteralShallow, 3);
@@ -1496,13 +1505,12 @@ void FullCodeGenerator::VisitArrayLiteral(ArrayLiteral* expr) {
     ASSERT(constant_elements_kind == FAST_ELEMENTS ||
            constant_elements_kind == FAST_SMI_ONLY_ELEMENTS ||
            FLAG_smi_only_arrays);
-    if (constant_elements_values->map() ==
-        isolate()->heap()->fixed_cow_array_map()) {
-      __ IncrementCounter(isolate()->counters()->cow_arrays_created_stub(),
-                          1);
-    }
-    FastCloneShallowArrayStub stub(
-        FastCloneShallowArrayStub::CLONE_ANY_ELEMENTS, length);
+    // If the elements are already FAST_ELEMENTS, the boilerplate cannot
+    // change, so it's possible to specialize the stub in advance.
+    FastCloneShallowArrayStub::Mode mode = has_constant_fast_elements
+        ? FastCloneShallowArrayStub::CLONE_ELEMENTS
+        : FastCloneShallowArrayStub::CLONE_ANY_ELEMENTS;
+    FastCloneShallowArrayStub stub(mode, length);
     __ CallStub(&stub);
   }