JDK 8 code review request for 7140820 Add covariant overrides to Collections clone methods

[Rémi Forax]

On 01/30/2012 10:17 PM, Vitaly Davidovich wrote:
> I would also expect clone to run a bit faster than copy constructor, if for
> nothing else than clone not executing any constructor; this perf diff would
> probably be more noticeable in interpreter as compiler may inline
> constructor.  In addition, I'd also think that clone can basically be
> equivalent to memcpy which should be faster.

It depends if the class if final or not.
If the class is not final the VM will have to add a guard before calling 
Object.clone().
Object.clone() is intrinsified (lookup for 'intrinsics' in the source code)
so it will do a memcopy. A far as I remember, memcopy is slower that copying
fields one by one if there is a few fields (otherwise it's faster).
Then you need a checkcast at the end and as far as I remember, the VM
doesn't remove it.

So as Tom said, if the class is final, using a copy constructor is 
usually faster.

Anyway, this is too Hotspot specific and may change in the future,
moreover I've never seen a call clone() or to a copy constructor
being the performance bottleneck.
Stupid algorithms and bad choices of the data structures are far more 
frequent.

Rémi

>
> Sent from my phone
> On Jan 30, 2012 4:08 PM, "Ulf Zibis"<Ulf.Zibis at gmx.de>  wrote:
>
>> Am 30.01.2012 14:28, schrieb Tom Hawtin:
>>
>>> On 30/01/2012 13:16, Ulf Zibis wrote:
>>>
>>>> Isn't cloning faster than normal instantiation?
>>>> I can imagine, that behind the scenes cloning mainly only needs to
>>>> duplicate the binary footprint of an object.
>>>>
>>> I don't see a good reason why it should be (equally, I've not tried
>>> benchmarking).
>>>
>>> For the immediate fields of an object, (partial) bitwise copying "by
>>> hand" should be of comparable performance to a bitwise clone. For copying
>>> the referenced objects, there is no benefit for the clone.
>>>
>> Is there anybody, who knows this exactly, e.g. in reference to Hotspot
>> runtime?
>>
>> -Ulf
>>
>>