Optimize testing for an index's existence in packed Arrays

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
diff --git a/src/x64/full-codegen-x64.cc b/src/x64/full-codegen-x64.cc
index ac31cfda545d0bd8e80ac8627056cf2e14d92f81..b619d34405e894110d045c8b4c3c2e385b971a93 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -4340,6 +4340,35 @@ void FullCodeGenerator::EmitDebugIsActive(CallRuntime* expr) {
 }
 
 
+void FullCodeGenerator::EmitHasFastPackedElements(CallRuntime* expr) {
+  ZoneList<Expression*>* args = expr->arguments();
+  DCHECK(args->length() == 1);
+
+  VisitForAccumulatorValue(args->at(0));
+
+  Label materialize_true, materialize_false;
+  Label* if_true = NULL;
+  Label* if_false = NULL;
+  Label* fall_through = NULL;
+  context()->PrepareTest(&materialize_true, &materialize_false, &if_true,
+                         &if_false, &fall_through);
+
+  __ JumpIfSmi(rax, if_false);
+  __ movp(rax, FieldOperand(rax, HeapObject::kMapOffset));
+  __ movzxbl(rax, FieldOperand(rax, Map::kBitField2Offset));
+  __ shrp(rax, Immediate(Map::ElementsKindBits::kShift));
+  __ cmpb(rax, Immediate(FAST_SMI_ELEMENTS));
+  __ j(equal, if_true);
+  __ cmpb(rax, Immediate(FAST_ELEMENTS));
+  __ j(equal, if_true);
+  __ cmpb(rax, Immediate(FAST_DOUBLE_ELEMENTS));
+  PrepareForBailoutBeforeSplit(expr, true, if_true, if_false);
+  Split(equal, if_true, if_false, fall_through);
+
+  context()->Plug(if_true, if_false);
+}
+
+
 void FullCodeGenerator::VisitCallRuntime(CallRuntime* expr) {
   if (expr->function() != NULL &&
       expr->function()->intrinsic_type == Runtime::INLINE) {