[swift-dev] statically initialized arrays

[John McCall]

> On Jun 15, 2017, at 10:52 AM, Arnold via swift-dev <swift-dev at swift.org> wrote:
> 
> 
> 
>> On Jun 14, 2017, at 2:56 PM, Dave Abrahams via swift-dev <swift-dev at swift.org> wrote:
>> 
>> 
>>> on Wed Jun 14 2017, Erik Eckstein <swift-dev-AT-swift.org> wrote:
>>> 
>>> Hi,
>>> 
>>> I’m about implementing statically initialized arrays. It’s about
>>> allocating storage for arrays in the data section rather than on the
>>> heap.
>> 
>> W00t!  I'd like to do the same for String, i.e. encode the entire buffer
>> in the data section.  I was looking for Array example code to follow but
>> couldn't find it.
> 
> 
> We have support for constant string buffers as of  PR 8701 and PR 8692. The former PR shows the protocol that has to be implemented.
> 
> (The implementation currently exposes the ref count ABI. This can/needs to be fixed when we move to a stable abi by running an once initializer)

It would be reasonable to arrange for a specific bit pattern to be a guaranteed "do not reference-count this" pattern even under a stable ABI.  We just have to be careful about picking it.

If we didn't want to do that, the best solution would be an absolute symbol — although we'd have to use an entire word for the refcount, even on 64-bit.

John.

>>> Info: the array storage is a heap object. So in the following I’m
>>> using the general term “object” but the optimization will (probably)
>>> only handle array buffers.
>>> 
>>> This optimization can be done for array literals containing only other
>>> literals as elements.  Example:
>>> 
>>> func createArray() -> [Int] {
>>> return [1, 2, 3]
>>> }
>>> 
>>> The compiler can allocate the whole array buffer as a statically
>>> initialized global llvm-variable with a reference count of 2 to make
>>> it immortal.
>> 
>> Why not 1
> 
> Mutation must force copying.
> 
>>> It avoids heap allocations for array literals in cases stack-promotion
>>> can’t kick in. It also saves code size.
>>> 
>>> What’s needed for this optimization?
>>> 
>>> 1) An optimization pass (GlobalOpt) which detects such array literal
>>> initialization patterns and “outlines” those into a statically
>>> initialized global variable
>>> 
>>> 2) A representation of statically initialized global variables in SIL
>>> 
>>> 3) IRGen to create statically initialized objects as global
>>> llvm-variables
>>> 
>>> ad 2) Changes in SIL:
>>> 
>>> Currently a static initialized sil_global is represented by having a reference to a globalinit
>>> function which has to match a very specific pattern (e.g. must contain a single store to the
>>> global).
>>> This is somehow quirky and would get even more complicated for statically initialized objects.
>>> 
>>> I’d like to change that so that the sil_global itself contains the initialization value.
>>> This part is not yet related to statically initialized objects. It just improves the representation
>>> of statically initialized global in general.
>>> 
>>> @@ -1210,7 +1210,9 @@ Global Variables
>>> ::
>>> 
>>>  decl ::= sil-global-variable
>>> +  static-initializer ::= '{' sil-instruction-def* '}'
>>>  sil-global-variable ::= 'sil_global' sil-linkage identifier ':' sil-type
>>> +                             (static-initializer)?
>>> 
>>> SIL representation of a global variable.
>>> 
>>> @@ -1221,6 +1223,19 @@ SIL instructions. Prior to performing any access on the global, the
>>> Once a global's storage has been initialized, ``global_addr`` is used to
>>> project the value.
>>> 
>>> +A global can also have a static initializer if it's initial value can be
>>> +composed of literals. The static initializer is represented as a list of
>>> +literal and aggregate instructions where the last instruction is the top-level
>>> +value of the static initializer::
>>> +
>>> +  sil_global hidden @_T04test3varSiv : $Int {
>>> +    %0 = integer_literal $Builtin.Int64, 27
>>> +    %1 = struct $Int (%0 : $Builtin.Int64)
>>> +  }
>>> +
>>> +In case a global has a static initializer, no ``alloc_global`` is needed before
>>> +it can be accessed.
>>> +
>>> 
>>> Now to represent a statically initialized object, we need a new instruction. Note that this
>>> “instruction" can only appear in the initializer of a sil_global.
>>> 
>>> +object
>>> +``````
>>> +::
>>> +
>>> +  sil-instruction ::= 'object' sil-type '(' (sil-operand (',' sil-operand)*)? ')'
>>> +
>>> +  object $T (%a : $A, %b : $B, ...)
>>> +  // $T must be a non-generic or bound generic reference type
>>> +  // The first operands must match the stored properties of T
>>> +  // Optionally there may be more elements, which are tail-allocated to T
>>> +
>>> +Constructs a statically initialized object. This instruction can only appear
>>> +as final instruction in a global variable static initializer list.
>>> 
>>> Finally we need an instruction to use such a statically initialized global object.
>>> 
>>> +global_object
>>> +`````````````
>>> +::
>>> +
>>> +  sil-instruction ::= 'global_object' sil-global-name ':' sil-type
>>> +
>>> +  %1 = global_object @v : $T
>>> +  // @v must be a global variable with a static initialized object
>>> +  // $T must be a reference type
>>> +
>>> +Creates a reference to the address of a global variable which has a static
>>> +initializer which is an object, i.e. the last instruction of the global's
>>> +static initializer list is an ``object`` instruction.
>>> 
>>> ad 3) IRGen support
>>> 
>>> Generating statically initialized globals is already done today for structs and tuples.
>>> What’s needed is the handling of objects.
>>> In addition to creating the global itself, we also need a runtime call to initialize the object
>>> header. In other words: the object is statically initialized, except the header.
>>> 
>>> HeapObject *swift::swift_initImmortalObject(HeapMetadata const *metadata, HeapObject *object)
>>> 
>>> There are 2 reasons for that: first, the object header format is not part of the ABI. And second, in
>>> case of a bound generic type (e.g. array buffers) the metadata is not statically available.
>>> 
>>> One way to call this runtime function is dynamically at the global_object instruction whenever the
>>> metadata pointer is still null (via swift_once).
>>> Another possibility would be to call it in a global constructor.
>>> 
>>> If you have any feedback, please let me know
>> 
>> I just ask that you keep in mind that we'll eventually want the same
>> capability for other types, and try to write the code to make that
>> feasible.
>> 
>> Thanks,
>> 
>> -- 
>> -Dave
>> 
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