:orphan:
   
================================================
 Copy-On-Write Optimization of ``inout`` Values
================================================

:Authors: Dave Abrahams, Joe Groff
          
:Summary: Our writeback model interacts with Copy-On-Write (COW) to
          cause some surprising ineffiencies, such as O(N) performance
          for ``x[0][0] = 1``. We propose a modified COW optimization
          that recovers O(1) performance for these cases and supports
          the efficient use of slices in algorithm implementation.

Whence the Problem?
===================

The problem is caused as follows:

* COW depends on the programmer being able to mediate all writes (so
  she can copy if necessary)
  
* Writes to container elements and slices are mediated through
  subscript setters, so in ::

    x[0].mutate()

  we “``subscript get``” ``x[0]`` into a temporary, mutate the
  temporary, and “``subscript set``” it back into ``x[0]``.

* When the element itself is a COW type, that temporary implies a
  retain count of at least 2 on the element's buffer.

* Therefore, mutating such an element causes an expensive copy, *even
  when the element's buffer isn't otherwise shared*.

Naturally, this problem generalizes to any COW value backed by a
getter/setter pair, such as a computed or resilient ``String``
property::

  anObject.title.append('.') // O(N)

Interaction With Slices
=======================

Consider the classic divide-and-conquer algorithm QuickSort, which
could be written as follows:

.. parsed-literal::

  protocol Sliceable {
    ...
    @mutating
    func quickSort(compare: (StreamType.Element, StreamType.Element)->Bool) {
      let (start,end) = (startIndex, endIndex)
      if start != end && start.succ() != end {
        let pivot = self[start]
        let mid = partition({compare($0, pivot)})
        **self[start...mid].quickSort(compare)**
        **self[mid...end].quickSort(compare)**
      }
    }
  }

The implicit ``inout`` on the target of the recursive ``quickSort``
calls currently forces two allocations and O(N) copies in each layer
of the QuickSort implementation.  Note that this problem applies to
simple containers such as ``Int[]``, not just containers of COW
elements.

Without solving this problem, mutating algorithms must operate on
``MutableCollection``\ s and pairs of their ``Index`` types, and we
must hope the ARC optimizer is able to eliminate the additional
reference at the top-level call.  However, that does nothing for the
cases mentioned in the previous section.

Our Solution
============

We need to prevent lvalues created in an ``inout`` context from
forcing a copy-on-write.  To accomplish that:

* In the class instance header, we reserve a bit ``INOUT``.
  
* When a unique reference to a COW buffer ``b`` is copied into
  an ``inout`` lvalue, we save the value of the ``b.INOUT`` bit and set it.

* When a reference to ``b`` is taken that is not part of an ``inout``
  value, ``b.INOUT`` is cleared.

* When ``b`` is written-back into ``r``, ``b.INOUT`` is restored to the saved
  value.

* A COW buffer can be modified in-place when it is uniquely referenced
  *or* when ``INOUT`` is set.

We believe this can be done with little user-facing change; the author
of a COW type would add an attribute to the property that stores the
buffer, and we would use a slightly different check for in-place
writability.
  
Other Considered Solutions
--------------------------

Move optimization seemed like a potential solution when we first considered
this problem--given that it is already unspecified to reference a property
while an active ``inout`` reference can modify it, it seems natural to move
ownership of the value to the ``inout`` when entering writeback and move it
back to the original value when exiting writeback. We do not think it is viable
for the following reasons:

- In general, relying on optimizations to provide performance semantics is
  brittle.
- Move optimization would not be memory safe if either the original value or
  ``inout`` slice were modified to give up ownership of the original backing
  store.  Although observing a value while it has inout aliases is unspecified,
  it should remain memory-safe to do so. This should remain memory safe, albeit
  unspecified::

    var arr = [1,2,3]
    func mutate(x: inout Int[]) -> Int[] {
      x = [3...4]
      return arr[0...2]
    }
    mutate(&arr[0...2])

  Inout slices thus require strong ownership of the backing store independent
  of the original object, which must also keep strong ownership of the backing
  store.
- Move optimization requires unique referencing and would fail when there are
  multiple concurrent, non-overlapping ``inout`` slices. ``swap(&x.a, &x.b)``
  should be well-defined if ``x.a`` and ``x.b`` do not access overlapping
  state, and so should be ``swap(&x[0...50], &x[50...100])``.  More
  generally, we would like to use inout slicing to implement
  divide-and-conquer parallel algorithms, as in::

    async { mutate(&arr[0...50]) }
    async { mutate(&arr[50...100]) }

Language Changes
================

Builtin.isUniquelyReferenced
----------------------------

A mechanism needs to be exposed to library writers to allow them to check
whether a buffer is uniquely referenced. This check requires primitive access
to the layout of the heap object, and can also potentially be reasoned about
by optimizations, so it makes sense to expose it as a ``Builtin`` which lowers
to a SIL ``is_uniquely_referenced`` instruction.

The ``@cow`` attribute
----------------------

A type may declare a stored property as being ``@cow``::

  class ArrayBuffer { /* ... */ }

  struct Array {
    @cow var buffer : ArrayBuffer
  }

The property must meet the following criteria:

- It must be a stored property.
- It must be of a pure Swift class type. (More specifically, at the
  implementation level, it must have a Swift refcount.)
- It must be mutable. A ``@cow val`` property would not be useful.

Values with ``@cow`` properties have special implicit behavior when they are
used in ``inout`` contexts, described below.

Implementation of @cow properties
=================================

inout SIL operations
--------------------

To maintain the ``INOUT`` bit of a class instance, we need new SIL operations
that update the ``INOUT`` bit. Because the state of the bit needs to be
saved and restored through every writeback scope, we can have::

  %former = inout_retain %b : $ClassType

increase the retain count, save the current value of ``INOUT``, set ``INOUT``,
and produce the ``%former`` value as its ``Int1`` result. To release,
we have::

  inout_release %b : $ClassType, %former : $Builtin.Int1

both reduce the retain count and change the value of ``INOUT`` back to the
value saved in ``%former``. Furthermore::

  strong_retain %b : $ClassType

must always clear the ``INOUT`` bit.

To work with opaque types, ``copy_addr`` must also be able to perform an
``inout`` initialization of a writeback buffer as well as reassignment to
an original value. This can be an additional attribute on the source, mutually
exclusive with ``[take]``::

  copy_addr [inout] %a to [initialization] %b

This implies that value witness tables will need witnesses for
inout-initialization and inout-reassignment.

Copying of @cow properties for writeback
----------------------------------------

When a value is copied into a writeback buffer, its ``@cow`` properties must
be retained for the new value using ``inout_retain`` instead of
``strong_retain`` (or ``copy_addr [inout] [initialization]`` instead of plain
``copy_addr [initialization]``). When the value is written back, the
property values should be ``inout_release``\ d, or the value should be
written back using ``copy_addr [inout]`` reassignment.
