[stdlib] Add default payload to Signaling NaN (#2494)

* [stdlib] Add default payload to Signaling NaN

Or it will have inifinity bit pattern.

* [stdlib] Skip isSignaling test on i386 arch

see: #2494

* [stdlib] Allow 0 payload for signaling NaN

Instead, use `_quietNaNMask >> 1` as the payload.
That is, we use only significandBitCount - 2 bits for NaN payload.

Author: rintaro

stdlib/public/core/FloatingPointTypes.swift.gyb

@@ -459,10 +459,11 @@ extension ${Self}: BinaryFloatingPoint {
   /// Compares not equal to every value, including itself.  Most operations
   /// with a NaN operand will produce a NaN result.
   public init(nan payload: RawSignificand, signaling: Bool) {
-    _precondition(payload < ${Self}._quietNaNMask,
+    // We use significandBitCount - 2 bits for NaN payload.
+    _precondition(payload < (${Self}._quietNaNMask >> 1),
       "NaN payload is not encodable.")
     var significand = payload
-    if !signaling { significand |= ${Self}._quietNaNMask }
+    significand |= ${Self}._quietNaNMask >> (signaling ? 1 : 0)
     self.init(sign: .plus,
               exponentBitPattern: ${Self}._infinityExponent,
               significandBitPattern: significand)

test/1_stdlib/Float.swift

@@ -222,13 +222,24 @@ func checkQNaN(_ qnan: TestFloat) {
   _precondition(qnan.floatingPointClass == .quietNaN)
 }
 
+func checkSNaN(_ snan: TestFloat) {
+  checkNaN(snan)
+// sNaN cannot be fully supported on i386.
+#if !arch(i386)
+  _precondition(snan.isSignaling)
+  _precondition(snan.floatingPointClass == .signalingNaN)
+#endif
+}
+
 func testNaN() {
   var stdlibDefaultNaN = TestFloat.nan
   checkQNaN(stdlibDefaultNaN)
 
   var stdlibQNaN = TestFloat.quietNaN
   checkQNaN(stdlibQNaN)
 
+  var stdlibSNaN = TestFloat.signalingNaN
+  checkSNaN(stdlibSNaN)
   print("testNaN done")
 }
 testNaN()