Which optimizations does Hotspot apply?

[David Holmes]

Hotspot questions belong on hotspot lists. cc'ing hotspot-dev and 
(trying to) bcc core-libs-dev.

David

On 23/01/2014 9:37 AM, Remi Forax wrote:
> On 01/22/2014 11:37 PM, Robert Stupp wrote:
>> Is there any documentation available which optimizations Hotspot can
>> perform and what "collecting a garbage" object costs?
>> I know that these are two completely different areas ;)
>>
>> I was inspecting whether the following code
>>      for (Object o : someArrayList) { ... }
>> would be faster than
>>      for (int i=0, l=someArrayList.size(); i<l; i++) { Object
>> o=someArrayList.get(i); }
>> for RandomAccessList implementations. The challenge here is not just
>> to track the CPU time spent creating & using the iterator vs. size() &
>> get() calls but also track GC effort (which is at least complicated if
>> not impossible due to the variety of GC configuration options).
>
> For a long time, using a for with an index (if you are *sure* that it's
> an ArrayList) was faster.
> With latest jdk8, it's not true anymore.
> (and most of the time the iterator object is not created anymore at
> least with jdk7+).
>
>>
>> For example:
>> - Does it help Hotspot to declare parameters/variables as "final" or
>> can Hotspot identify that?
>
> no, it's an urban myth.
> You can test it by yourself, if you declare a local variable final or
> not the exact same bytecode is produced by javac. The keyword final for
> a local variable (or a parameter) is not stored in the bytecode.
>
> BTW, final was introduced in 1.1 mostly to allow to capture the value of
> a variable to be used by an anonymous class, Java 8 doesn't require this
> kind of variable to be declared final anymore.
>
>> - When does Hotspot inline method calls in general and getters/setters
>> especially?
>
> In general, up to a depth of 10 by default and 1 for a recursive method.
> Roughly, a method call is not inlined either if the call is virtual and
> can call too many implementations or if the generated assembly code will
> be too big.
>
>>
>> I think such a piece of documentation (just what Hotspot can do in
>> which release) would be really helpful when someone tries to optimize
>> code - want to say: the question is: "Is something worth to spend time
>> on or is this special situation handled by Hotspot?"
>
> It never worth it.
> Choose the right algorithms and shape your data to be easily consumed by
> your algorithms is enough.
>
>>
>> -Robert
>
> Rémi
>