Bug 798179 - Implement To BitwiseCast<To>(From), abstracting the treatment of a value's bits as being of another type. r=froydnj

--HG--
extra : rebase_source : e6c57c10f7035b2654eafdd5fa2b3c5d8882f282

mfbt/Casting.h

@@ -15,6 +15,27 @@
 
 namespace mozilla {
 
+/**
+ * Return a value of type |To|, containing the underlying bit pattern of |from|.
+ *
+ * |To| and |From| must be types of the same size; be careful of cross-platform
+ * size differences, or this might fail to compile on some but not all
+ * platforms.
+ */
+template<typename To, typename From>
+inline To
+BitwiseCast(const From from)
+{
+  MOZ_STATIC_ASSERT(sizeof(From) == sizeof(To),
+                    "To and From must have the same size");
+  union {
+    From from;
+    To to;
+  } u;
+  u.from = from;
+  return u.to;
+}
+
 namespace detail {
 
 enum ToSignedness { ToIsSigned, ToIsUnsigned };

mfbt/tests/TestCasting.cpp

@@ -6,8 +6,33 @@
 #include "mozilla/Casting.h"
 #include "mozilla/StandardInteger.h"
 
+using mozilla::BitwiseCast;
 using mozilla::detail::IsInBounds;
 
+template<typename Uint, typename Ulong, bool = (sizeof(Uint) == sizeof(Ulong))>
+struct UintUlongBitwiseCast;
+
+template<typename Uint, typename Ulong>
+struct UintUlongBitwiseCast<Uint, Ulong, true>
+{
+    static void test() {
+      MOZ_ASSERT(BitwiseCast<Ulong>(Uint(8675309)) == Ulong(8675309));
+    }
+};
+
+template<typename Uint, typename Ulong>
+struct UintUlongBitwiseCast<Uint, Ulong, false>
+{
+    static void test() { }
+};
+
+static void
+TestBitwiseCast()
+{
+  MOZ_ASSERT(BitwiseCast<int>(int(8675309)) == int(8675309));
+  UintUlongBitwiseCast<unsigned int, unsigned long>::test();
+}
+
 static void
 TestSameSize()
 {
@@ -74,6 +99,8 @@ TestToSmallerSize()
 int
 main()
 {
+  TestBitwiseCast();
+
   TestSameSize();
   TestToBiggerSize();
   TestToSmallerSize();