nim-wiki/Destructors.rest

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This page is a follow-up of https://nim-lang.org/araq/destructors.html and further outlines of where Nim is heading in the future. (Did I hear anyone say "Nim v2"?)
Nim's strings and sequences should become "GC-free" implementations and are exemplary for how Nim's core should work. Strings and sequences are value-based that means ``=`` performs a copy (conceptually). In practice many copies can be optimized away (see my blog post). The "optimized" copy is called a "move" and is supported via the type bound operator ``=sink``.
Rewrite rules (simplified)
==========================
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======== ==================== ===========================================================
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Rule Pattern Transformed into
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======== ==================== ===========================================================
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1 var x; stmts var x; try stmts finally: `=destroy`(x)
2 x = f() `=sink`(x, f())
3 x = lastReadOf z `=sink`(x, z)
4 x = y `=`(x, y) # a copy
5 f(g()) var tmp; `=sink`(tmp, g()); f(tmp); `=destroy`(tmp)
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======== ==================== ===========================================================
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Rule (5) can be optimized further to ``var tmp = bitwiseCopy(g()); f(tmp); =destroy(tmp)``.
Sink parameters
===============
A ``sink`` parameter conveys a transfer of ownership. The parameter will be *consumed*.
A ``sink`` parameter is internally **not** mapped to ``var``, instead the
usual "pass-by-copy" / "optimize to by-ref if more efficient" implementation
is used. However, similar rules apply -- you cannot pass a ``const`` to
a ``sink`` parameter.
A ``sink`` parameter **must** be **consumed** exactly once within the
proc's body. The compiler will use a dataflow analysis to prove this fact.
For a ``sink`` parameter called ``sp`` a **consume** looks like:
.. code-block:: nim
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proc consume(c: var Container; sp: sink T) =
locationDerivedFrom(c) = sp
This assignment is mapped to the ``=sink`` operator.
A consume can also be forwarded, "pass sp to a different proc as a sink parameter":
.. code-block:: nim
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proc consume(c: var Container; sp: sink T) =
c.takeAsSink(sp)
Use after consume
-----------------
Locations passed to a ``sink`` parameter are invalidated after the call
and the compiler tries to prove that it is not used again afterwards. For
local variables this is quite easy to prove:
.. code-block:: nim
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proc consume(c: var Container; element: sink T) =
c[i] = element
proc main() =
var x = initT()
for i in 0..3:
container.consume(x) # Error: attempt to re-use already moved value 'x'
For arbitrary locations involving array accesses etc it is too hard to prove
it is not used afterwards. The compiler transforms ``takeAsSink(sp)`` into
``takeAsSink(sp); reset(sp)``. ``reset`` sets the value back into its default
value. For locals the ``reset`` can be optimized away (stores to a dead object),
for function calls there is no location to reset at all.
For a location that has had its value moved into a sink parameter no
destructor call needs to be injected. This is an important optimization
to keep the produced code small.
Sink for locals
---------------
``sink T`` is also a valid type for locals. For a variable ``v`` of
type ``sink T`` no destructor call is injected and it is statically
ensured that every code path leads to its consumption.
Lent type
---------
``proc p(x: sink T)`` means that the proc ``p`` takes ownership of ``x``.
To eliminate even more creation/copy <-> destruction pairs, a proc's return
type can be annotated as ``lent T``. This is useful for "getter" accessors
that seek to allow an immutable view into a container.
Like ``sink T`` ``lent T`` is a valid annotation for local variables too.
For a variable ``v`` of type ``lent T`` it is statically ensured that no code
path leads to its consumption, in other words that it must not escape its
local stack frame (either directly or indirectly via passing to a ``sink``
parameter). For ``v`` no destructor call is injected since it doesn't own
the object.
The ``sink`` and ``lent`` annotations allow us to remove most (if not all)
superfluous copies and destructions.
``lent T`` is like ``var T`` a hidden pointer that the compiler needs to prove that
it doesn't outlive its origin.
.. code-block:: nim
type
Tree = object
kids: seq[Tree]
proc construct(kids: sink seq[Tree]): Tree =
result = Tree(kids: kids)
# converted into:
`=sink`(result.kids, kids)
proc `[]`*(x: Tree; i: int): lent Tree =
result = x.kids[i]
# borrows from 'x', this is transformed into:
result = addr x.kids[i]
# This means 'lent' is like 'var T' a hidden pointer.
# Unlike 'var' this cannot be used to mutate the object.
iterator children*(t: Tree): lent Tree =
for x in t.kids: yield x
proc main =
# everything turned into moves:
let t = construct(@[construct(@[]), construct(@[])])
echo t[0] # accessor does not copy the element!
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``sink T`` and ``lent T`` introduce further rewrite rules but lead to more efficient code. Even better, these rules optimize away create/copy <-> destroy pairs and so can also make atomic reference counting more efficient by eliminating incref <-> decref pairs.
Rewrite rules (extended)
========================
======== ==================== ===========================================================
Rule Pattern Transformed into
======== ==================== ===========================================================
1.1 var x: T; stmts var x: T; try stmts finally: `=destroy`(x)
1.2 var x: sink T; stmts var x: sink T; stmts; ensureEmpty(x)
2 x = f() `=sink`(x, f())
3 x = lastReadOf z `=sink`(x, z)
4.1 sinkParam = y `=sink`(sinkParam, y)
4.2 x = y `=`(x, y) # a copy
5.1 f_sink(g()) f_sink(g())
5.2 f_sink(y) f_sink(y); reset(y)
# 'reset(y)' for locals usually optimized away
5.3 f_noSink(g()) var tmp = bitwiseCopy(g()); f(tmp); `=destroy`(tmp)
======== ==================== ===========================================================
``sink T`` also affects overloading resolution rules; by the time type checking is performed we have no control flow graph yet so the property ``lastReadOf z`` is not available. However, passing a call expression ``f()`` to a ``g`` taking a sink parameter is a syntactic property and so is available for overloading resolution. Thus I propose the following rule:
.. code-block:: nim
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proc add(c: var Container; x: T) # version A
proc add(c: var Container; x: sink T) # version B
var c: Container
var x: T
c.add x # calls version A
c.add f() # calls version B
# object construction counts as proc call:
c.add T() # calls version B