Fix 23563: double unary- operator unstable for NANs

runtime/lib/double.cc

 // Copyright (c) 2011, 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/bootstrap_natives.h"
 
 #include "platform/math.h"
 
 #include "vm/code_generator.h"  // DartModulo.
 #include "vm/dart_entry.h"
@@ skipping 282 matching lines @@
 }
 
 
 DEFINE_NATIVE_ENTRY(Double_getIsNegative, 1) {
   const Double& arg = Double::CheckedHandle(arguments->NativeArgAt(0));
   // Include negative zero, infinity.
   double dval = arg.value();
   return Bool::Get(signbit(dval) && !isnan(dval)).raw();
 }
 
+
+DEFINE_NATIVE_ENTRY(Double_flipNANsign, 1) {
+  const Double& arg = Double::CheckedHandle(arguments->NativeArgAt(0));
+  ASSERT(isnan(arg.value()));
+  double res;
+  if (signbit(arg.value()) == 1) {
+    res = NAN;

[koda, 2015/06/02 21:20:57]

Here, you are also normalizing to a particular NAN constant. I was under the
impression that it should flip the sign bit of the input itself and return that.
Please clarify.

Also, you are assuming a particular behavior from the C++ compiler. Why not
simply reinterpret_cast and then xor with the proper bit pattern?

[srdjan, 2015/06/02 22:32:10]

Renamed method to Double_flipSignBit. Flipping sign bit of any double,
simplified the double's unary- code.

+  } else {
+    res = -NAN;
+  }
+  return Double::New(res);
+}
+
 // Add here only functions using/referring to old-style casts.
 
 }  // namespace dart