[stdlib] add two-operand version of max and use max2 in stdlib when appropriate.

This helps array append's performance by ~ 2x. The generic max with a variadic
argument creates a temporary array then iterates over the array.

rdar://17140639 rdar://17073827


Swift SVN r18764

stdlib/core/Algorithm.swift

@@ -312,6 +312,14 @@ func min<T : Comparable>(x: T, y: T, rest: T...) -> T {
   return r
 }
 
+func max2<T : Comparable>(x: T, y: T) -> T {
+  var r = y
+  if y < x {
+    r = x
+  }
+  return r
+}
+
 func max<T : Comparable>(x: T, y: T, rest: T...) -> T {
   var r = y
   if y < x {

stdlib/core/Arrays.swift.gyb

@@ -507,7 +507,7 @@ where C.GeneratorType.Element == A._Buffer.Element
   // Ensure uniqueness, mutability, and sufficient storage.  Note that
   // for consistency, we need unique lhs even if rhs is empty.
   lhs.reserveCapacity(
-    newCount > capacity ? max(newCount, capacity * 2) : newCount)
+    newCount > capacity ? max2(newCount, capacity * 2) : newCount)
   
   var p = lhs._buffer.elementStorage + oldCount
   for x in rhs {
@@ -541,14 +541,14 @@ func _demandUniqueMutableBuffer<_Buffer: ArrayBufferType>(
 
   _sanityCheck(newCount >= 0)
   
-  let requiredCapacity = max(newCount, minimumCapacity)
+  let requiredCapacity = max2(newCount, minimumCapacity)
   
   if let b = source.requestUniqueMutableBuffer(requiredCapacity) {
     source.count = newCount
     return nil
   }
 
-  let minimumCapacity = max(
+  let minimumCapacity = max2(
     requiredCapacity,
     newCount > source.capacity ? source.capacity * 2 : source.capacity)
 

stdlib/core/ContiguousArrayBuffer.swift

@@ -57,7 +57,7 @@ struct ContiguousArrayBuffer<T> : ArrayBufferType, LogicValue {
     _base = HeapBuffer(
       ContiguousArrayStorage<T>.self,
       _ArrayBody(),
-      max(count, minimumCapacity))
+      max2(count, minimumCapacity))
 
     var bridged = false
     if _canBeClass(T.self) {

stdlib/core/Dictionary.swift

@@ -405,7 +405,7 @@ struct _NativeDictionaryStorage<KeyType : Hashable, ValueType> :
   static func getMinCapacity(
       requestedCount: Int, _ maxLoadFactorInverse: Double) -> Int {
     // `requestedCount + 1` below ensures that we don't fill in the last hole
-    return max(Int(Double(requestedCount) * maxLoadFactorInverse),
+    return max2(Int(Double(requestedCount) * maxLoadFactorInverse),
                 requestedCount + 1)
   }
 

stdlib/core/StringBuffer.swift

@@ -90,7 +90,7 @@ struct _StringBuffer {
 
     // Allocate storage
     self = _StringBuffer(
-      capacity: max(utf16Count, minimumCapacity),
+      capacity: max2(utf16Count, minimumCapacity),
       initialSize: utf16Count,
       elementWidth: isAscii ? 1 : 2)
 

stdlib/core/StringCore.swift

@@ -339,7 +339,7 @@ struct _StringCore {
       }
       else if newSize > buffer.capacity {
         // Growth failed because of insufficient storage; double the size
-        return (max(buffer.capacity * 2, newSize), .null())
+        return (max2(buffer.capacity * 2, newSize), .null())
       }
     }
     return (newSize, .null())