Round Float.pi down so that it's in [0,π].

* Change value of Float.pi so that it's rounded down; this helps insure that angles computed via trigonometry end up in the correct quadrant.

* Missed commit of corresponding test change.

Author: stephentyrone

stdlib/public/core/FloatingPoint.swift.gyb

@@ -122,6 +122,9 @@ public protocol FloatingPoint: Comparable, IntegerLiteralConvertible,
 
   /// The mathematical constant pi = 3.14159...
   ///
+  /// This value should be rounded towards zero to keep user computations
+  /// with angles from inadvertently ending up in the wrong quadrant.
+  ///
   /// Extensible floating-point types might provide additional APIs to obtain
   /// this value to caller-specified precision.
   static var pi: Self { get }

stdlib/public/core/FloatingPointTypes.swift.gyb

@@ -341,7 +341,12 @@ extension ${Self}: BinaryFloatingPoint {
 
   public static var pi: ${Self} {
 %if bits == 32:
-    return Float(0x1.921fb6p1)
+    // Note: this is not the correctly-rounded (to nearest) value of pi,
+    // because pi would round *up* in Float precision, which can result
+    // in angles in the wrong quadrant if users aren't careful.  This is
+    // not a problem for Double or Float80, as pi rounds down in both of
+    // those formats.
+    return Float(0x1.921fb4p1)
 %elif bits == 64:
     return Double(0x1.921fb54442d18p1)
 %elif bits == 80:

test/1_stdlib/FloatingPoint.swift.gyb

@@ -104,7 +104,7 @@ FloatingPoint.test("Float/staticProperties") {
   expectBitwiseEqual(bitPattern: 0x7fa0_0000, Ty.signalingNaN)
   expectBitwiseEqual(bitPattern: 0x7f80_0000, Ty.infinity)
   expectBitwiseEqual(0x1.ffff_fe__p127, Ty.greatestFiniteMagnitude)
-  expectBitwiseEqual(0x1.921f_b6__p1, Ty.pi)
+  expectBitwiseEqual(0x1.921f_b4__p1, Ty.pi)
   expectBitwiseEqual(0x1.0p-23, Ty.ulpOfOne)
   expectBitwiseEqual(0x1.0p-126, Ty.leastNormalMagnitude)
 #if arch(arm)