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

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 3147 matching lines @@
   const bool needs_writable_input = (value()->Type()->ToCid() != kDoubleCid);
   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 XmmRegister result = locs()->out(0).fpu_reg();
 
   if (value_cid == kDoubleCid) {
     __ movsd(result, FieldAddress(value, Double::value_offset()));
   } else if (value_cid == kSmiCid) {
     __ SmiUntag(value);  // Untag input before conversion.
     __ cvtsi2sd(result, value);
   } else {
     Label* deopt = compiler->AddDeoptStub(deopt_id_,
                                           ICData::kDeoptBinaryDoubleOp);
-    Label is_smi, done;
-    __ testq(value, Immediate(kSmiTagMask));
-    __ j(ZERO, &is_smi);
-    __ CompareClassId(value, kDoubleCid);
-    __ j(NOT_EQUAL, deopt);
-    __ movsd(result, FieldAddress(value, Double::value_offset()));
-    __ jmp(&done);
-    __ Bind(&is_smi);
-    __ SmiUntag(value);
-    __ cvtsi2sd(result, value);
-    __ Bind(&done);
+    if (value_type->is_nullable() &&
+        (value_type->ToNullableCid() == kDoubleCid)) {
+      const Immediate& raw_null =
+          Immediate(reinterpret_cast<intptr_t>(Object::null()));
+      __ cmpq(value, raw_null);
+      __ j(EQUAL, deopt);
+      // It must be double now.
+      __ movsd(result, FieldAddress(value, Double::value_offset()));
+    } else {
+      Label is_smi, done;
+      __ testq(value, Immediate(kSmiTagMask));
+      __ j(ZERO, &is_smi);
+      __ CompareClassId(value, kDoubleCid);
+      __ j(NOT_EQUAL, deopt);
+      __ movsd(result, FieldAddress(value, Double::value_offset()));
+      __ jmp(&done);
+      __ Bind(&is_smi);
+      __ SmiUntag(value);
+      __ cvtsi2sd(result, value);
+      __ Bind(&done);
+    }
   }
 }
 
 
 LocationSummary* BoxFloat32x4Instr::MakeLocationSummary(bool opt) const {
   const intptr_t kNumInputs = 1;
   const intptr_t kNumTemps = 0;
   LocationSummary* summary =
       new LocationSummary(kNumInputs,
                           kNumTemps,
@@ skipping 2388 matching lines @@
                          PcDescriptors::kOther,
                          locs());
   __ Drop(ArgumentCount());  // Discard arguments.
 }
 
 }  // namespace dart
 
 #undef __
 
 #endif  // defined TARGET_ARCH_X64