For nullable/double typed fields compare with null for deoptimization, else assume double.

runtime/vm/intermediate_language_mips.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_MIPS.
 #if defined(TARGET_ARCH_MIPS)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/dart_entry.h"
@@ skipping 3024 matching lines @@
       new LocationSummary(kNumInputs, kNumTemps, LocationSummary::kNoCall);
   summary->set_in(0, needs_writable_input
                      ? Location::WritableRegister()
                      : Location::RequiresRegister());
   summary->set_out(0, Location::RequiresFpuRegister());
   return summary;
 }
 
 
 void UnboxDoubleInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
-  const intptr_t value_cid = value()->Type()->ToCid();
+  CompileType* value_type = value()->Type();
+  const intptr_t value_cid = value_type->ToCid();
   const Register value = locs()->in(0).reg();
   const DRegister result = locs()->out(0).fpu_reg();
 
   if (value_cid == kDoubleCid) {
     __ LoadDFromOffset(result, value, Double::value_offset() - kHeapObjectTag);
   } else if (value_cid == kSmiCid) {
     __ SmiUntag(value);  // Untag input before conversion.
     __ mtc1(value, STMP1);
     __ cvtdw(result, STMP1);
   } else {
     Label* deopt = compiler->AddDeoptStub(deopt_id_,
                                           ICData::kDeoptBinaryDoubleOp);
-    Label is_smi, done;
+    if (value_type->is_nullable() && (value_type->cid() == kDoubleCid)) {
+      __ BranchEqual(value, reinterpret_cast<int32_t>(Object::null()), deopt);
+      // It must be double now.
+      __ LoadDFromOffset(result, value,
+          Double::value_offset() - kHeapObjectTag);
+    } else {
+      Label is_smi, done;
 
-    __ andi(CMPRES1, value, Immediate(kSmiTagMask));
-    __ beq(CMPRES1, ZR, &is_smi);
-    __ LoadClassId(CMPRES1, value);
-    __ BranchNotEqual(CMPRES1, kDoubleCid, deopt);
-    __ LoadDFromOffset(result, value, Double::value_offset() - kHeapObjectTag);
-    __ b(&done);
-    __ Bind(&is_smi);
-    // TODO(regis): Why do we preserve value here but not above?
-    __ sra(TMP, value, 1);
-    __ mtc1(TMP, STMP1);
-    __ cvtdw(result, STMP1);
-    __ Bind(&done);
+      __ andi(CMPRES1, value, Immediate(kSmiTagMask));
+      __ beq(CMPRES1, ZR, &is_smi);
+      __ LoadClassId(CMPRES1, value);
+      __ BranchNotEqual(CMPRES1, kDoubleCid, deopt);
+      __ LoadDFromOffset(result, value,
+          Double::value_offset() - kHeapObjectTag);
+      __ b(&done);
+      __ Bind(&is_smi);
+      // TODO(regis): Why do we preserve value here but not above?
+      __ sra(TMP, value, 1);
+      __ mtc1(TMP, STMP1);
+      __ cvtdw(result, STMP1);
+      __ Bind(&done);
+    }
   }
 }
 
 
 LocationSummary* BoxFloat32x4Instr::MakeLocationSummary(bool opt) const {
   UNIMPLEMENTED();
   return NULL;
 }
 
 
@@ skipping 1405 matching lines @@
   compiler->GenerateCall(token_pos(),
                          &label,
                          PcDescriptors::kOther,
                          locs());
   __ Drop(ArgumentCount());  // Discard arguments.
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_MIPS