Bug 1248555: Introduce variants of SpecificNaN / BitwiseCast that pre…

…serve the signaling NaN bit; r=froydnj

MozReview-Commit-ID: 5A8p06nBqyI

mfbt/Casting.h

@@ -17,16 +17,29 @@
 namespace mozilla {
 
 /**
- * Return a value of type |To|, containing the underlying bit pattern of
+ * Sets the outparam value of type |To| with the same underlying bit pattern of
  * |aFrom|.
  *
  * |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.
+ *
+ * There is also a variant that returns the value directly.  In most cases, the
+ * two variants should be identical.  However, in the specific case of x86
+ * chips, the behavior differs: returning floating-point values directly is done
+ * through the x87 stack, and x87 loads and stores turn signaling NaNs into
+ * quiet NaNs... silently.  Returning floating-point values via outparam,
+ * however, is done entirely within the SSE registers when SSE2 floating-point
+ * is enabled in the compiler, which has semantics-preserving behavior you would
+ * expect.
+ *
+ * If preserving the distinction between signaling NaNs and quiet NaNs is
+ * important to you, you should use the outparam version.  In all other cases,
+ * you should use the direct return version.
  */
 template<typename To, typename From>
-inline To
-BitwiseCast(const From aFrom)
+inline void
+BitwiseCast(const From aFrom, To* aResult)
 {
   static_assert(sizeof(From) == sizeof(To),
                 "To and From must have the same size");
@@ -36,7 +49,16 @@ BitwiseCast(const From aFrom)
     To mTo;
   } u;
   u.mFrom = aFrom;
-  return u.mTo;
+  *aResult = u.mTo;
+}
+
+template<typename To, typename From>
+inline To
+BitwiseCast(const From aFrom)
+{
+  To temp;
+  BitwiseCast<To, From>(aFrom, &temp);
+  return temp;
 }
 
 namespace detail {

mfbt/FloatingPoint.h

@@ -245,20 +245,44 @@ NegativeInfinity()
 }
 
 
-/** Constructs a NaN value with the specified sign bit and significand bits. */
+/**
+ * Constructs a NaN value with the specified sign bit and significand bits.
+ *
+ * There is also a variant that returns the value directly.  In most cases, the
+ * two variants should be identical.  However, in the specific case of x86
+ * chips, the behavior differs: returning floating-point values directly is done
+ * through the x87 stack, and x87 loads and stores turn signaling NaNs into
+ * quiet NaNs... silently.  Returning floating-point values via outparam,
+ * however, is done entirely within the SSE registers when SSE2 floating-point
+ * is enabled in the compiler, which has semantics-preserving behavior you would
+ * expect.
+ *
+ * If preserving the distinction between signaling NaNs and quiet NaNs is
+ * important to you, you should use the outparam version.  In all other cases,
+ * you should use the direct return version.
+ */
 template<typename T>
-static MOZ_ALWAYS_INLINE T
-SpecificNaN(int signbit, typename FloatingPoint<T>::Bits significand)
+static MOZ_ALWAYS_INLINE void
+SpecificNaN(int signbit, typename FloatingPoint<T>::Bits significand, T* result)
 {
   typedef FloatingPoint<T> Traits;
   MOZ_ASSERT(signbit == 0 || signbit == 1);
   MOZ_ASSERT((significand & ~Traits::kSignificandBits) == 0);
   MOZ_ASSERT(significand & Traits::kSignificandBits);
 
-  T t = BitwiseCast<T>((signbit ? Traits::kSignBit : 0) |
-                       Traits::kExponentBits |
-                       significand);
-  MOZ_ASSERT(IsNaN(t));
+  BitwiseCast<T>((signbit ? Traits::kSignBit : 0) |
+                  Traits::kExponentBits |
+                  significand,
+                  result);
+  MOZ_ASSERT(IsNaN(*result));
+}
+
+template<typename T>
+static MOZ_ALWAYS_INLINE T
+SpecificNaN(int signbit, typename FloatingPoint<T>::Bits significand)
+{
+  T t;
+  SpecificNaN(signbit, significand, &t);
   return t;
 }