Use a union for type-punning; enforce 64bit integer

- gcc 4.6.3 was throwing warnings; it seems the recommended way to pun
types is with unions as otherwise compiler optimizations can do
unexpected things.
- use uint64_t rather than unsigned long to enforce 64bit-ness (to
match that of double)
- We could just use R_IsNaN or R_IsNA, but both are much slower (around
2x faster if I use these unions)

Author: kevinushey

ChangeLog

@@ -1,5 +1,8 @@
 2013-12-20  Kevin Ushey  <kevinushey@gmail.com>
 
+  * inst/include/Rcpp/sugar/tools/mapcompare.h: Use a union for
+    type punning, to avoid compiler warning on aliasing. Also
+    switch to uint64_t to enforce 64bit comparison.
   * inst/include/Rcpp/RObject.h: Add missing *this return for
     RObject_impl::operator=
 

inst/include/Rcpp/sugar/tools/mapcompare.h

@@ -22,18 +22,28 @@
 #ifndef Rcpp__sugar__tools__mapcompare_h
 #define Rcpp__sugar__tools__mapcompare_h
 
+#include <cstdint>
+using std::uint64_t;
+
 namespace Rcpp {
 namespace sugar {
 
-static unsigned long const R_UNSIGNED_LONG_NA_REAL = *(unsigned long*)(&NA_REAL);
-static unsigned long const R_UNSIGNED_LONG_NAN_REAL = *(unsigned long*)(&R_NaN);
+union DoublePunner {
+  double d;
+  uint64_t p;
+};
+
+static const DoublePunner NA_REAL_PUN = { NA_REAL };
+static const DoublePunner NAN_REAL_PUN = { R_NaN };
 
 inline bool Rcpp_IsNA(double x) {
-  return *reinterpret_cast<unsigned long*>(&x) == R_UNSIGNED_LONG_NA_REAL;
+  DoublePunner xp = { x };
+  return xp.p == NA_REAL_PUN.p;
 }
 
 inline bool Rcpp_IsNaN(double x) {
-  return *reinterpret_cast<unsigned long*>(&x) == R_UNSIGNED_LONG_NAN_REAL;
+  DoublePunner xp = { x };
+  return xp.p == NAN_REAL_PUN.p;
 }
 
 inline int StrCmp(SEXP x, SEXP y) {