Fix 23563: double unary- operator unstable for NANs

runtime/lib/double.cc

Index: runtime/lib/double.cc
diff --git a/runtime/lib/double.cc b/runtime/lib/double.cc
index 4173c1403d034ad917f12dea13d7f23cd2fc1f4b..e6f8596c022781352b1490979253e83c6097d0b6 100644
--- a/runtime/lib/double.cc
+++ b/runtime/lib/double.cc
@@ -300,6 +300,16 @@ DEFINE_NATIVE_ENTRY(Double_getIsNegative, 1) {
   return Bool::Get(signbit(dval) && !isnan(dval)).raw();
 }
 
+
+DEFINE_NATIVE_ENTRY(Double_flipSignBit, 1) {
+  const Double& arg = Double::CheckedHandle(arguments->NativeArgAt(0));
+  const double in_val = arg.value();
+  const int64_t bits =
+      *(reinterpret_cast<const int64_t*>(&in_val)) ^ kSignBitDouble;
+  const double out_val = *(reinterpret_cast<const double*>(&bits));
+  return Double::New(out_val);

[Anders Johnsen, 2015/06/03 06:37:36]

I may have missed out here, but my experience tells me that 

  -in_val

should do it. Is that not the case?

[koda, 2015/06/03 17:28:11]

Even if the C++ standard would mandate the behavior we want (does it?), and all
reasonable compilers correctly implement it (do they?), I still think being
explicit about the exact semantics is preferable, and any performance loss is
noise compared to the runtime call overhead.

+}
+
 // Add here only functions using/referring to old-style casts.
 
 }  // namespace dart