One of the things that JVMs can do is create a small cache for the most
recently seen classes in instanceof checks. I believe both OpenJDK and the
Oracle JVM do this.
So if you check for both ISeq and Seqable, you may find that you get twice
as many classes cached, and therefore see a performance benefit. Of course
this is implementation dependant so YMMV.
On Friday, 19 May 2017 11:28:27 UTC+8, Tianxiang Xiong wrote:
>
> But if something is `ISeq`, it's `Seqable`, so checking for `Seqable`
> alone should be sufficient.
>
> Am I missing something here? Is there a performance performance benefit of
> checking for `ISeq` *and* `Seqable` that I'm not aware of?
>
> On Wednesday, May 3, 2017 at 2:19:42 AM UTC-7, Mikera wrote:
>>
>> Clearly not necessary from a functional perspective.
>>
>> However I believe the ordering of these tests will affect JVM
>> optimisations. You want to test the common/fast cases first. And the JVM
>> does some clever things with caching most recently used lookups, which will
>> again behave differently if you test things in different orders.
>>
>> Benchmarking on realistic workloads would typically be required to
>> determine the optimal order.
>>
>> FWIW I find it odd that the null check is third. This is extremely fast
>> (certainly faster than instance checks) and is a very common case given the
>> amount of nil usage in idiomatic Clojure code (as an empty seq), so I would
>> probably put it first.
>>
>> On Wednesday, 3 May 2017 11:59:29 UTC+8, Tianxiang Xiong wrote:
>>>
>>> Why does `clojure.lang.RT/canSeq` need to check both `ISeq` _and_
>>> `Seqable` when `ISeq <- IPersistentCollection <- Seqable`?
>>>
>>> static public boolean canSeq(Object coll){
>>> return coll instanceof ISeq
>>> || coll instanceof Seqable
>>> || coll == null
>>> || coll instanceof Iterable
>>> || coll.getClass().isArray()
>>> || coll instanceof CharSequence
>>> || coll instanceof Map;
>>> }
>>>
>>>
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