[stdlib] DoubleWidth formatting and bit shift audit

Author: natecook1000

stdlib/public/core/DoubleWidth.swift.gyb

@@ -318,8 +318,8 @@ public struct DoubleWidth<Base : FixedWidthInteger> :
     let initialOffset = q.leadingZeroBitCount +
       (DoubleWidth.bitWidth - rhs.leadingZeroBitCount) - 1
 
-    // TODO(performance): Use &>> instead here?
     // Start with remainder capturing the high bits of q.
+    // (These need to be smart shifts, as initialOffset can be > q.bitWidth)
     var r = q >> Magnitude(DoubleWidth.bitWidth - initialOffset)
     q <<= Magnitude(initialOffset)
 
@@ -401,8 +401,9 @@ public struct DoubleWidth<Base : FixedWidthInteger> :
     let mid2 = sum(b.carry, c.carry, d.partial)
 
     let low = DoubleWidth<Low>((mid1.partial, a.partial))
-    let high = DoubleWidth(
-      (High(mid2.carry + d.carry), mid1.carry + mid2.partial))
+    let high = DoubleWidth((
+      High(mid2.carry + d.carry), mid1.carry + mid2.partial
+    ))
 
     if isNegative {
       let (lowComplement, overflow) = (~low).addingReportingOverflow(1)
@@ -438,6 +439,7 @@ public struct DoubleWidth<Base : FixedWidthInteger> :
       return
     }
 
+    // Shift is larger than this type's bit width.
     if rhs._storage.high != (0 as High) ||
       rhs._storage.low >= DoubleWidth.bitWidth
     {
@@ -481,34 +483,30 @@ public struct DoubleWidth<Base : FixedWidthInteger> :
   }
 
   public static func &<<=(lhs: inout DoubleWidth, rhs: DoubleWidth) {
+    // Need to use smart shifts here, since rhs can be > Base.bitWidth
     let rhs = rhs & DoubleWidth(DoubleWidth.bitWidth - 1)
 
     lhs._storage.high <<= High(rhs._storage.low)
-    if Base.bitWidth > rhs._storage.low {
-      let h = lhs._storage.low >>
-        (numericCast(Base.bitWidth) - rhs._storage.low)
-      lhs._storage.high |= High(extendingOrTruncating: h)
-    } else {
-      let h = lhs._storage.low <<
-        (rhs._storage.low - numericCast(Base.bitWidth))
-      lhs._storage.high |= High(extendingOrTruncating: h)
-    }
+
+    let t = Base.bitWidth > rhs._storage.low
+      ? lhs._storage.low >> (numericCast(Base.bitWidth) - rhs._storage.low)
+      : lhs._storage.low << (rhs._storage.low - numericCast(Base.bitWidth))
+    lhs._storage.high |= High(extendingOrTruncating: t)
+
     lhs._storage.low <<= rhs._storage.low
   }
 
   public static func &>>=(lhs: inout DoubleWidth, rhs: DoubleWidth) {
+    // Need to use smart shifts here, since rhs can be > Base.bitWidth
     let rhs = rhs & DoubleWidth(DoubleWidth.bitWidth - 1)
 
     lhs._storage.low >>= rhs._storage.low
-    if Base.bitWidth > rhs._storage.low {
-      let l = lhs._storage.high <<
-        numericCast(numericCast(Base.bitWidth) - rhs._storage.low)
-      lhs._storage.low |= Low(extendingOrTruncating: l)
-    } else {
-      let l = lhs._storage.high >>
-        numericCast(rhs._storage.low - numericCast(Base.bitWidth))
-      lhs._storage.low |= Low(extendingOrTruncating: l)
-    }
+
+    let t = Base.bitWidth > rhs._storage.low
+      ? lhs._storage.high << (numericCast(Base.bitWidth) - rhs._storage.low)
+      : lhs._storage.high >> (rhs._storage.low - numericCast(Base.bitWidth))
+    lhs._storage.low |= Low(extendingOrTruncating: t)
+
     lhs._storage.high >>= High(extendingOrTruncating: rhs._storage.low)
   }