> On Feb 3, 2017, at 11:55 AM, Andrew Trick via swift-dev <swift-dev@swift.org>
> wrote:
>
>>> #3b. (lazy resolution) Offset tables can be completely localized.
>>>
>>> method_index = immediate
>>> { // common per-class method lookup
>>> isa = load[obj]
>>> offset = load[@local_class_method_table + method_index]
>>> if (!isInitializedOffset(offset)) {
>>> offset = @resolveMethodOffset(@class_id, method_index)
>>> store [@local_class_method_table + method_index]
>>> }
>>> if (isVtableOffset(offset))
>>> method_entry = load[isa + offset]
>>> else
>>> method_entry = @resolveMethodAddress(isa, @class_id, method_index)
>>> }
>>> call method_entry
>>
>> The size of @local_class_method_table is not statically knowable.
>> Fortunately, it doesn't matter, because this mechanism does not actually
>> care about the table being contiguous; the lookup function could be
>> passed a per-method cache variable. This would also allow the lookup
>> function to be shared between classes.
Hmm... I thought the local method offset table size could be statically
knowable because it will only be accesd for methods that were publicly
available at build time, based on the sorted method index.
We could simplify the method resolution API with a single exported symbol
per-class (maybe that's what you're getting at):
method_entry = resolveMethodAddress_ForAClass(isa, method_index, &vtable_offset)
The problem with that is the client-side code can’t hoist and combine the
method offset lookup anymore.
-Andy
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