Simplify type argument instantiation cache lookup by introducing an array

runtime/vm/intermediate_language_arm.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_ARM.
 #if defined(TARGET_ARCH_ARM)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/dart_entry.h"
@@ skipping 2179 matching lines @@
   Label type_arguments_instantiated;
   const intptr_t len = type_arguments().Length();
   if (type_arguments().IsRawInstantiatedRaw(len)) {
     __ LoadImmediate(IP, reinterpret_cast<intptr_t>(Object::null()));
     __ cmp(instantiator_reg, ShifterOperand(IP));
     __ b(&type_arguments_instantiated, EQ);
   }
 
   __ LoadObject(R2, type_arguments());
   __ ldr(R2, FieldAddress(R2, TypeArguments::instantiations_offset()));
-  __ ldr(R3, FieldAddress(R2, Array::length_offset()));
   __ AddImmediate(R2, Array::data_offset() - kHeapObjectTag);
-  __ add(R3, R2, ShifterOperand(R3, LSL, 1));  // R3 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);
-  __ cmp(R2, ShifterOperand(R3));
-  __ b(&slow_case, CS);  // Unsigned higher or equal.
   __ ldr(R1, Address(R2, 0 * kWordSize));  // Cached instantiator.
   __ cmp(R1, ShifterOperand(R0));
   __ b(&found, EQ);
   __ CompareImmediate(R1, Smi::RawValue(StubCode::kNoInstantiator));
   __ b(&slow_case, EQ);
   __ AddImmediate(R2, 2 * kWordSize);
   __ b(&loop);
   __ Bind(&found);
   __ ldr(R0, Address(R2, 1 * kWordSize));  // Cached instantiated args.
   __ b(&type_arguments_instantiated);
@@ skipping 2681 matching lines @@
   compiler->GenerateCall(token_pos(),
                          &label,
                          PcDescriptors::kOther,
                          locs());
   __ Drop(2);  // Discard type arguments and receiver.
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_ARM