Nim without GC

What do ParaSail, "modern" C++ and Rust have in common? They focus on "pointer free programming" (ok, maybe Rust doesn't, but it uses similar mechanisms). In this blog post I am exploring how we can move Nim into this direction. My goals are:

• Memory safety without GC.
• Make passing data between threads more efficient.
• Make it more natural to write code with excellent performance.
• A simpler programming model: Pointers introduce aliasing, this means programs are hard to reason about, this affects optimizers as well as programmers.

The title gave it away: We are going to get into this state of programming Valhalla by eliminating pointers. Of course for low level programming Nim's ptr type is here to stay but I hope to avoid ref as far as reasonable in the standard library. (ref might become an atomic RC'ed pointer.) As a nice side-effect, nil ceases to be a problem too. Instead of ref object we will use object more, this implies the var vs "no var" distinction will be used more often, another benefit in my opinion.

What's wrong with Nim's GC?

Not much per se (hey, it's likely faster than the alternatives that I'm exploring here) but it makes interoperability with most of what's outside of Nim's ecosystem harder:

• Python has its own GC and while building a Nim DLL that Python can load works, it would be even easier if the DLL wouldn't need special code that ensures the GC's conservative stack scanning works.
• C++ game engines are based on RAII and wrapping a C++ object in a Nim ref object that calls a C++ destructor in a GC finalizer adds overhead. This applies to almost every big C or C++ project.
• The conservative stack scanning can fail for more unusual targets like Emscripten. (Workarounds exist though.)
• I have spent far more time now in fixing GC related bugs or optimizing the GC than I ever spent in hunting down memory leaks or corruptions. Memory safety is not negotiable but we should attempt to get it without a runtime that grows ever more complex.

Containers

Nim's containers should be value types. Explicit move semantics as well as a special optimizer will eliminate most copies.

Almost all containers keep the number of elements they hold and so instead of nil we get a much nicer state len == 0 that is not as prone to crashes as nil. When a container is moved, its length becomes 0.

Slicing

Strings and seqs will support O(1) slicing, other containers might also produce a "view" into their interiors. Slices break up the clear ownership semantics that we're after and so will probably be restricted to parameters much like openArray.

Opt

Trees do not require pointers to be constructed, a seq can do the same:

type
  Node = object  ## note the absence of ``ref`` here
    children: seq[Node]
    payload: string

However often only 1 or 0 entries are possible and so a seq would be overkill. opt is a container that can be full or empty, just like the well known Option type from other languages.

type
  Node = object  ## note the absence of ``ref`` here
    left, right: opt[Node]
    payload: string

Under the hood opt[Note] uses a pointer, it has to, otherwise a construct like the above would take up an infinite amount of memory ("a node contains nodes which contain nodes which ..."). But since this pointer is not exposed, it doesn't destroy the value semantics. It can be argued that opt[T] is very much a unique pointer that adheres to the copy vs move distinction.

Destructors, assignment and moves

The existing Nim supports moving via shallowCopy, this is a bit ugly so from now on a move shall be written as <-. Note that <- is not a real new operator here, I used it only to emphasize in the examples where a move occurs.

Value semantics make it easy to determine the lifetime of an object, when it goes out of scope, its attached resources can be freed, that means its destructor is called. If it was moved (if it escapes) instead, some internal state in the object or container reflects this and the destruction can be prevented. An optimization pass is allowed to remove destructor calls, likewise a copy propagation pass is allowed to remove assignments.

There are in fact two places where destruction can occur: At scope exit and at assignment, x = y means "destroy x; copy y into x". This is often inefficient:

proc put(t: var Table; key, val: string) =
  # outline of a hash table implementation:
  let h = hash(key)
  # these are destructive assignments:
  t.a[h].key = key
  t.a[h].val = val

proc main =
  let key <- stdin.readLine()
  let val <- stdin.readLine()
  var t = createTable()
  t.put key, val

This constructs 2 strings via the readLine calls that are then copied into the table t. At the scope exit of main the original strings key and val are freed.

This naive code does 2 copies and 4 destructions. We can do much better with swap:

proc put(t: var Table; key, val: var string) =
  # outline of a hash table implementation:
  let h = hash(key)
  swap t.a[h].key, key
  swap t.a[h].val, val

proc main =
  var key <- stdin.readLine()
  var val <- stdin.readLine()
  var t = createTable()
  t.put key, val

This code now only does the required minimum of 2 destructions. It also quite ugly, key and val are forced to be var's and after the move into the table t they can be accessed and contain the old table entries. This can occasionally be useful but more often we would like to keep the let and instead accessing the value after it was moved should produce a compile-time error.

This is made possible by sink parameters. A sink parameter is like a var parameter but let variables can be passed to it and afterwards a simple control flow analysis prohibits accesses to the location. With sink the example looks as follows:

proc put(t: var Table; key, val: sink string) =
  # outline of a hash table implementation:
  let h = hash(key)
  swap t.a[h].key, key
  swap t.a[h].val, val

proc main =
  let key <- stdin.readLine()
  let val <- stdin.readLine()
  var t = createTable()
  t.put key, val

Alternatively we can simply allow to pass a let to a var parameter and then it means it's moved.

Btw let key = stdin.readLine() will always be transformed into let key <- stdin.readLine().

Optimizing copies into moves

Consider this example:

let key = stdin.readLine()
var a: array[10, string]
a[0] = key
echo key

Since key is accessed after the assignment a[0] = key it has to be copied into the array slot. But without the echo key statement the value can be moved. And so that's what the compiler does for us. Blurring the distinction between moves and copies means that code can evolve without "friction".

Destructors

Every construction needs to be paired with a destruction in order to prevent memory leaks. It also must be destroyed exactly once in order to prevent corruptions. The secret to get memory safety from this model lies in the fact that calls to destructors are always inserted by the compiler.

But what is a construction? Nim has no traditional constructors. The answer is that the result of every proc counts as construction. This is no big loss as return values tend to be bad for high performance code. More on this later.

Code generation for destructors

Naive destructors for trees are recursive. This means they can lead to stack overflows and can lead to missed deadlines in a realtime setting. The default code generation for them thus uses an explicit stack that interacts with the memory allocator to implement lazy freeing. Or maybe we can introduce a lazyDestroy proc that should be used in strategic places. The implementation could look like this:

type Destructor = proc (data: pointer) {.nimcall.}

var toDestroy {.threadvar.}: seq[(Destructor, pointer)]

proc lazyDestroy(arg: pointer; destructor: Destructor) =
  if toDestroy.len >= 100:
    # too many pending destructor calls, run immediately:
    destructor(arg)
  else:
    toDestroy.add((destructor, arg))

proc `=destroy`(x: var T) =
  lazyDestroy cast[pointer](x), proc (p: pointer) =
    let x = cast[var T](p)
    `=destroy`(x.le)
    `=destroy`(x.ri)
    dealloc(p)

proc constructT(): T =
  if toDestroy.len > 0:
    let (d, p) = toDestroy.pop()
    d(p)

This is really just a variant of "object pooling".

Move rules

Now that we have gained these insights, we can finally write down the precise rules when copies, moves and destroys happen:

Rule	Pattern	Meaning
1	var x; stmts	var x; try stmts finally: destroy(x)
2	x = f()	move(x, f())
3	x = lastReadOf z	move(x, z)
4	x = y	copy(x, y)
5	f(g())	f((move(tmp, g()); tmp)); destroy(tmp)

var x = y is handled as var x; x = y. x, y here are arbitrary locations, f and g are routines that take an arbitrary number of arguments, z a local variable.

In the current implementation lastReadOf z is approximated by "z is read and written only once and that is done in the same basic block". Later versions of the Nim compiler will detect this case more precisely.

The key insight here is that assignments are resolved into several distinct semantics that do "the right thing". Containers should thus be written to leverage the builtin assignment!

To see what this means, let's look at C++: In C++ there is a distinction between moves and copies and this distinction bubbles up in the APIs, for example std::vector has

void push_back(const value_type& x); // copies the element
void push_back(value_type&& x); // moves the element

In Nim we can do better thanks to its template feature (which has nothing to do with C++'s templates):

proc reserveSlot(x: var seq[T]): ptr T =
  if x.len >= x.cap: resize(x)
  result = addr(x.data[x.len])
  inc x.len

template add*[T](x: var seq[T]; y: T) =
  reserveSlot(x)[] = y

Thanks to add being a template the final assignment is not hidden from the compiler and so it is allowed to use the most effective form. The implementation uses the unsafe ptr and addr constructs, but it is generally accepted now that a language's core containers are allowed to do that.

This way of writing containers works for more complex cases too:

template put(t: var Table; key, val: string) =
  # ensure 'key' is evaluated only once:
  let k = key
  
  let h = hash(k)
  t.a[h].key = k    # move (rule 3)
  t.a[h].val = val  # move (rule 3)

proc main =
  var key = stdin.readLine() # move (rule 2)
  var val = stdin.readLine() # move (rule 2)
  var t = createTable()
  t.put key, val

Note how rule 3 ensures that t.a[h].key = k is transformed into a move since k is never used again afterwards. (Optimizing away the temporary k completely is a story for another time.)

Given these new insights, I assume that sink parameters are not required at all. Keeps the language simpler.

Getters

Templates also help in avoiding copies introduced by getters:

template get(x: Container): T = x.field

echo get() # no copy, no move

If we replace template get with proc get here rule 5 would apply and produce:

proc get(x: Container): T =
  copy result, x.field

echo((var tmp; move(tmp, get()); tmp))
destroy(tmp)

Strings

Here is an outline of how Nim's standard strings can be implemented with this new scheme. The code is reasonable straight-forward, but you always need to keep two things in mind:

• Assignments and copies need to destroy the old destination.
• Self assignments need to work.

type
  string = object
    len, cap: int
    data: ptr UncheckedArray[char]

proc add*(s: var string; c: char) =
  if s.len >= s.cap: resize(s)
  s.data[s.len] = c

proc `=destroy`*(s: var string) =
  if s.data != nil:
    dealloc(s.data)
    s.data = nil
    s.len = 0
    s.cap = 0

proc `=move`*(a, b: var string) =
  # we hope this is optimized away for not yet alive objects:
  if a.data != nil and a.data != b.data: dealloc(a.data)
  a.len = b.len
  a.cap = b.cap
  a.data = b.data
  # we hope these are optimized away for dead objects:
  b.len = 0
  b.cap = 0
  b.data = nil

proc `=`*(a: var string; b: string) =
  if a.data != nil and a.data != b.data:
    dealloc(a.data)
    a.data = nil
  a.len = b.len
  a.cap = b.cap
  if b.data != nil:
    a.data = alloc(a.cap)
    copyMem(a.data, b.data, a.cap)

Unfortunately the signatures do not match, =move takes 2 var parameters but according to the transformation rules move(a, f()) or move(a, lastRead b) are produced and these are not addressable locations! So we need different type-bound operator called =sink that is used instead.

proc `=sink`*(a: var string, b: string) =
  if a.data != nil and a.data != b.data: dealloc(a.data)
  a.len = b.len
  a.cap = b.cap
  a.data = b.data

The compiler only invokes sink. move is an explicit programmer optimization. Which can usually also be written as swap operation.

Return values are harmful

Nim's stdlib contains the following coding pattern for the toString $ operator:

proc helper(x: Node; result: var string) =
  case x.kind
  of strLit: result.add x.strVal
  of intLit: result.add $x.intVal
  of arrayLit:
    result.add "["
    for i in 0 ..< x.len:
      if i > 0: result.add ", "
      helper(x[i], result)
    result.add "]"

proc `$`(x: Node): string =
  result = ""
  helper(x, result)

(The declaration of the Node type is left as an excercise for the reader.) The reason for this workaround with the helper proc is that it lets us use result: var string, a single string buffer we keep appending to. The naive implementation would instead produce much more allocations and concatenations. We gain a lot by constructing (or in this case: appending) the result directly where it will end up.

Now imagine we want to embed this string in a larger context like an HTML page, helper is actually the much more useful interface for speed. This answers the old question "should procs operate inplace or return a new value?".

Excessive inplace operations do lead to a code style that is completely statement-based, the dataflow is much harder to see than in the more FP'ish expression-based style. What Nim needs is a transformation from expression based style to statement style. This transformation is really simple, given a proc like:

proc p(args; result: var T): void

A call to it missing the final parameter p(args) is rewritten to (var tmp: T; p(args, tmp); tmp). Ideally the compiler would introduce the minimum of required temporaries in nested calls but such an optimization is far away and one can always choose to write the more efficient version directly.

Reification

Second class types or parameter passing modes like var or the imagined sink have the problem that they cannot be put into an object. This is more severe than it first seems as any kind of threading or tasking system requires a "reification" of the argument list into a task object that is then sent to a queue or thread. In fact in the current Nim neither await nor spawn supports invoking a proc with var parameters and even capturing such a parameter in a closure does not work! The current workaround is to use ptr for these. Maybe somebody will come up with a better solution.