Access double fields in C++ as uint64_t fields to preserve signaling bit of a NaN.

src/crankshaft/hydrogen-instructions.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/crankshaft/hydrogen-instructions.h"
 
 #include "src/base/bits.h"
 #include "src/base/ieee754.h"
 #include "src/base/safe_math.h"
 #include "src/codegen.h"
@@ skipping 2159 matching lines @@
 
 
 HConstant::HConstant(Special special)
     : HTemplateInstruction<0>(HType::TaggedNumber()),
       object_(Handle<Object>::null()),
       object_map_(Handle<Map>::null()),
       bit_field_(HasDoubleValueField::encode(true) |
                  InstanceTypeField::encode(kUnknownInstanceType)),
       int32_value_(0) {
   DCHECK_EQ(kHoleNaN, special);
+  // Manipulating the signaling NaN used for the hole in C++, e.g. with bit_cast
+  // will change its value on ia32 (the x87 stack is used to return values
+  // and stores to the stack silently clear the signalling bit).
+  // Therefore we have to use memcpy for initializing |double_value_| with
+  // kHoleNanInt64 here.
   std::memcpy(&double_value_, &kHoleNanInt64, sizeof(double_value_));
   Initialize(Representation::Double());
 }
 
 
 HConstant::HConstant(Handle<Object> object, Representation r)
     : HTemplateInstruction<0>(HType::FromValue(object)),
       object_(Unique<Object>::CreateUninitialized(object)),
       object_map_(Handle<Map>::null()),
       bit_field_(
@@ skipping 1859 matching lines @@
     case HObjectAccess::kExternalMemory:
       os << "[external-memory]";
       break;
   }
 
   return os << "@" << access.offset();
 }
 
 }  // namespace internal
 }  // namespace v8