Simplify type argument instantiation cache lookup by introducing an array

runtime/vm/intermediate_language_x64.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"  // Needed here to get TARGET_ARCH_X64.
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/dart_entry.h"
@@ skipping 2055 matching lines @@
   if (type_arguments().IsRawInstantiatedRaw(len)) {
     __ CompareObject(instantiator_reg, Object::null_object(), PP);
     __ j(EQUAL, &type_arguments_instantiated, Assembler::kNearJump);
   }
 
   // Lookup cache before calling runtime.
   // TODO(fschneider): Consider moving this into a shared stub to reduce
   // generated code size.
   __ LoadObject(RDI, type_arguments(), PP);
   __ movq(RDI, FieldAddress(RDI, TypeArguments::instantiations_offset()));
-  __ movq(RBX, FieldAddress(RDI, Array::length_offset()));
   __ leaq(RDI, FieldAddress(RDI, Array::data_offset()));
-  __ leaq(RBX, Address(RDI, RBX, TIMES_2, 0));  // RBX is smi.
+  // The instantiations cache is initialized with Object::zero_array() and is
+  // therefore guaranteed to contain kNoInstantiator. No length check needed.
   Label loop, found, slow_case;
   __ Bind(&loop);
-  __ cmpq(RDI, RBX);
-  __ j(ABOVE_EQUAL, &slow_case);
   __ movq(RDX, Address(RDI, 0 * kWordSize));  // Cached instantiator.
   __ cmpq(RDX, RAX);
   __ j(EQUAL, &found, Assembler::kNearJump);
   __ cmpq(RDX, Immediate(Smi::RawValue(StubCode::kNoInstantiator)));
   __ j(EQUAL, &slow_case);
   __ addq(RDI, Immediate(2 * kWordSize));
   __ jmp(&loop, Assembler::kNearJump);
   __ Bind(&found);
   __ movq(RAX, Address(RDI, 1 * kWordSize));  // Cached instantiated args.
   __ jmp(&type_arguments_instantiated);
@@ skipping 2899 matching lines @@
                          PcDescriptors::kOther,
                          locs());
   __ Drop(2);  // Discard type arguments and receiver.
 }
 
 }  // namespace dart
 
 #undef __
 
 #endif  // defined TARGET_ARCH_X64