diff --git a/stdlib/public/core/ArrayBufferProtocol.swift b/stdlib/public/core/ArrayBufferProtocol.swift
index cf6512badb16b..db464227a0160 100644
--- a/stdlib/public/core/ArrayBufferProtocol.swift
+++ b/stdlib/public/core/ArrayBufferProtocol.swift
@@ -24,6 +24,8 @@ internal protocol _ArrayBufferProtocol
   /// Adopt the entire buffer, presenting it at the provided `startIndex`.
   init(_buffer: _ContiguousArrayBuffer<Element>, shiftedToStartIndex: Int)
 
+  init(copying buffer: Self)
+
   /// Copy the elements in `bounds` from this buffer into uninitialized
   /// memory starting at `target`.  Return a pointer "past the end" of the
   /// just-initialized memory.
@@ -124,7 +126,7 @@ internal protocol _ArrayBufferProtocol
   var endIndex: Int { get }
 }
 
-extension _ArrayBufferProtocol {
+extension _ArrayBufferProtocol where Indices == CountableRange<Int>{
 
   @_inlineable
   @_versioned
@@ -132,6 +134,19 @@ extension _ArrayBufferProtocol {
     return firstElementAddress
   }
 
+  // Make sure the compiler does not inline _copyBuffer to reduce code size.
+  @_inlineable
+  @inline(never)
+  @_versioned
+  internal init(copying buffer: Self) {
+    let newBuffer = _ContiguousArrayBuffer<Element>(
+      _uninitializedCount: buffer.count, minimumCapacity: buffer.count)
+    buffer._copyContents(
+      subRange: Range(buffer.indices),
+      initializing: newBuffer.firstElementAddress)
+    self = Self( _buffer: newBuffer, shiftedToStartIndex: buffer.startIndex)
+  }
+
   @_inlineable
   @_versioned
   internal mutating func replaceSubrange<C>(
diff --git a/stdlib/public/core/Arrays.swift.gyb b/stdlib/public/core/Arrays.swift.gyb
index 1c06f5513a244..8502f7a0bd0ac 100644
--- a/stdlib/public/core/Arrays.swift.gyb
+++ b/stdlib/public/core/Arrays.swift.gyb
@@ -835,27 +835,12 @@ extension ${Self} {
     return Builtin.unsafeCastToNativeObject(_buffer.owner)
   }
 
-  // FIXME(ABI)#12 : move to an initializer on _Buffer.
-  // Make sure the compiler does not inline _copyBuffer to reduce code size.
-  @inline(never)
-  @_inlineable
-  @_versioned
-  static internal func _copyBuffer(_ buffer: inout _Buffer) {
-    let newBuffer = _ContiguousArrayBuffer<Element>(
-      _uninitializedCount: buffer.count, minimumCapacity: buffer.count)
-    buffer._copyContents(
-      subRange: buffer.indices,
-      initializing: newBuffer.firstElementAddress)
-    buffer = _Buffer(
-      _buffer: newBuffer, shiftedToStartIndex: buffer.startIndex)
-  }
-
   @_inlineable
   @_versioned
   @_semantics("array.make_mutable")
   internal mutating func _makeMutableAndUnique() {
     if _slowPath(!_buffer.isMutableAndUniquelyReferenced()) {
-      ${Self}._copyBuffer(&_buffer)
+      _buffer = _Buffer(copying: _buffer)
     }
   }
 
@@ -864,7 +849,7 @@ extension ${Self} {
   @_semantics("array.make_mutable")
   internal mutating func _makeMutableAndUniqueOrPinned() {
     if _slowPath(!_buffer.isMutableAndUniquelyReferencedOrPinned()) {
-      ${Self}._copyBuffer(&_buffer)
+      _buffer = _Buffer(copying: _buffer)
     }
   }
 
@@ -1543,8 +1528,8 @@ extension ${Self} : RangeReplaceableCollection, ArrayProtocol {
   @_inlineable
   public mutating func popLast() -> Element? {
     let newCount = _getCount() - 1
-    guard newCount >= 0 else { return nil }
-    _makeUniqueAndReserveCapacityIfNotUnique()
+    guard !isEmpty else { return nil }
+    return _customRemoveLast().unsafelyUnwrapped
     let pointer = (_buffer.firstElementAddress + newCount)
     let element = pointer.move()
     _buffer.count = newCount
diff --git a/test/SILOptimizer/swap_refcnt.swift b/test/SILOptimizer/swap_refcnt.swift
deleted file mode 100644
index 8153ca0c9e9ab..0000000000000
--- a/test/SILOptimizer/swap_refcnt.swift
+++ /dev/null
@@ -1,12 +0,0 @@
-// RUN: %target-swift-frontend -O -emit-sil %s | %target-sil-opt -assume-parsing-unqualified-ownership-sil -simplify-unreachable-containing-blocks | %FileCheck %s
-
-// Make sure we can swap two values in an array without retaining anything along non-fatalerror paths.
-
-// CHECK-LABEL: sil @$S11swap_refcnt0A7ByIndex1A1x1yySays4Int8VGz_S2itF : $@convention(thin) (@inout Array<Int8>, Int, Int) -> () {
-// CHECK-NOT: strong_retain
-// CHECK-NOT: strong_release
-// CHECK: } // end sil function '$S11swap_refcnt0A7ByIndex1A1x1yySays4Int8VGz_S2itF'
-public func swapByIndex(A: inout [Int8], x : Int, y : Int) {
-  swap(&A[x],&A[y])
-}
-