Which optimizations does Hotspot apply?

Vitaly Davidovich vitalyd at gmail.com
Sat Jan 25 17:35:34 UTC 2014

One thing that would be very useful (and I've asked Vladimir Kozlov for it)
is to expose a product flag (I believe there's one now but it's develop) to
trace EA decisions/reasons.  Maybe that's already planned, but if not,
hopefully it makes it at some point.

Sent from my phone
On Jan 25, 2014 5:15 AM, "Remi Forax" <forax at univ-mlv.fr> wrote:

> On 01/24/2014 11:01 PM, Vitaly Davidovich wrote:
>> This would be predicated on escape analysis, whose effectiveness is in
>> turn
>> driven by sufficient inlining whereby compiler can see that instance
>> doesn't escape.  Inlining, in turn, can be screwed up by (amongst other
>> things) polymorphic call sites.  So in the end, it's all quite brittle and
>> sensitive :).  Personally, in places where it matters, I try to allocate
>> as
>> little as possible explicitly - a lot of times much more effective than
>> hoping compiler helps out and/or turning GC knobs.
>> Sent from my phone
> Hi Andrej,
> if you see a lot of instances of ArrayList iterator it means that you have
> a code path that create an iterator and use it later and the JIT is not
> able to see the creation and the use in the same inlining horizon. As
> Vitaly said, it's perhaps because you have the same code called with
> several different kinds of List (teach your programmer to stop to use
> LinkedList :) or because the code create an Iterator too far from where you
> use it (by example if the iterator is stored in a List or a Map).
> If you use -XX:+PrintInlining, the VM will print the reason why something
> is not inlined.
> cheers,
> Rémi
>  On Jan 24, 2014 4:36 PM, "Andrej Golovnin" <golovnin at gmx.net> wrote:
>>  Hi Rémi,
>>>  With latest jdk8, it's not true anymore.
>>>> (and most of the time the iterator object is not created anymore at
>>> least with jdk7+).
>>> Could you please explain it a little bit more? When is that optimization
>>> applied,
>>> e.g. what conditions are required for this optimization, since which
>>> version
>>> of JDK/Hotspot it is supported, where it is implemented in JDK?
>>> When I take look at a product I'm working on, I see a lot instances of
>>> ArrayList$Itr objects,
>>> which are created by for-each loops (we use JDK 7u51).
>>> Thanks in advance!
>>> Best regards,
>>> Andrej Golovnin

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