[swift-dev] statically initialized arrays

[David Smith]

> On Jun 14, 2017, at 4:04 PM, Michael Gottesman via swift-dev <swift-dev at swift.org> wrote:
> 
>> 
>> On Jun 14, 2017, at 11:24 AM, Erik Eckstein via swift-dev <swift-dev at swift.org <mailto: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.
>> 
>> 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.
> 
> I was thinking about this a little bit. IMO, we probably actually want a bit in the header. The reason why is that even though setting the ref count to be unbalanced makes an object immortal, retain/release will still modify the reference count meaning that the statically initialized constant can not be in read only memory. On the other hand, if we had a bit in the header, we could use read only memory.
> 
> Michael

This is also surprisingly perf-relevant in concurrent situations. We ran into a scenario in Foundation this year where contention on the refcount of a global object was enough to cause an order of magnitude slowdown when the test was run on 10 threads.

	David

> 
>> 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
>> 
>> Thanks,
>> Erik
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> 
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