DBC: Add Smi fastpath to AssertAssignable

runtime/vm/simulator_dbc.cc

 // Copyright (c) 2016, 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 <setjmp.h>  // NOLINT
 #include <stdlib.h>
 
 #include "vm/globals.h"
 #if defined(TARGET_ARCH_DBC)
 
@@ skipping 2672 matching lines @@
       INVOKE_RUNTIME(DRT_BadTypeError, native_args);
       UNREACHABLE();
     }
     SP -= 3;
     DISPATCH();
   }
 
   {
     BYTECODE(AssertAssignable, A_D);  // Stack: instance, type args, type, name
     RawObject** args = SP - 3;
-    if (args[0] != null_value) {
+    const bool may_be_smi = (rA == 1);
+    const bool is_smi =
+        ((reinterpret_cast<intptr_t>(args[0]) & kSmiTagMask) == kSmiTag);
+    const bool smi_ok = is_smi && may_be_smi;
+    if (!smi_ok && (args[0] != null_value)) {
       RawSubtypeTestCache* cache =
           static_cast<RawSubtypeTestCache*>(LOAD_CONSTANT(rD));
       if (cache != null_value) {
         RawInstance* instance = static_cast<RawInstance*>(args[0]);
         RawTypeArguments* instantiator_type_arguments =
             static_cast<RawTypeArguments*>(args[1]);
 
         const intptr_t cid = SimulatorHelpers::GetClassId(instance);
 
         RawTypeArguments* instance_type_arguments =
@@ skipping 785 matching lines @@
   pc_ = pc;
   special_[kExceptionSpecialIndex] = raw_exception;
   special_[kStacktraceSpecialIndex] = raw_stacktrace;
   buf->Longjmp();
   UNREACHABLE();
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_DBC