Optimize bytecode for combination enhanced-for and enums

[Roel Spilker]

Rémi, are you sure the clone call can be removed? I agree that that would
even be better for hotspot JVMs that use escape analysis. That said, apart
from engineering effort, if a synthetic inner class is used you get the
benefit immediately, even on VMs that cannot afford escape analysis.

Roel


On Wed, Mar 28, 2012 at 4:13 PM, Rémi Forax <forax at univ-mlv.fr> wrote:

> On 03/28/2012 03:39 PM, Roel Spilker wrote:
>
>> Hi all,
>>
>> TL;DR: for (Edge edge : Edge.values()) copies an array every time it is
>> executed. This can be prevented. Is that a good idea and how to proceed
>> from here?
>>
>> I'm new to this list, so please let me know if this is not the place to
>> post this.
>>
>> When I was profiling our application I notices that we allocate quite
>> some memory when iterating over enum values. The pattern used was:
>>
>> class  Foo {
>>  void bar() {
>>    for (Edge e : Edge.values()) {
>>      // do some work
>>    }
>>  }
>> }
>>
>> The call to Edge.values() obviously creates a new clone of the Edge[]
>> containing all enum constants. So we've modified our code:
>>
>> class  Foo {
>>  private static final Edge[] EDGES = Edge.values();
>>  void bar() {
>>    for (Edge e : EDGES) {
>>      // do some work
>>    }
>>  }
>> }
>>
>> This solves our allocation problem, but it is not a nice solution. Since
>> code in the enhanced-for has no way to modify the array this pattern can be
>> done at compile-time. A synthetic inner class can be generated to keep the
>> copy of the array. The desugared (pseudo) code would then look something
>> like this:
>>
>> /* syncthetic */ class Foo$0 {
>>  static final Edge[] EDGES = Edge.values();
>> }
>>
>> class  Foo {
>>  void bar() {
>>    for (Edge e : Foo$0.EDGES) {
>>      // do some work
>>    }
>>  }
>> }
>>
>> There is precedence for this kind of desugaring/optimization: When you
>> use an enum-in-switch, a synthetic class is generated containing an int
>> array for the ordinals.
>>
>> I have a few questions:
>> - Do you think this is a good optimization? The trade-off here is
>> creating a copy every time the enhanced-for is used (could be in an inner
>> loop) versus the overhead of loading an extra class.
>> - Is there a better optimization possible/required? EnumSet uses
>> SharedSecrets.**getJavaLangAccess().**getEnumConstantsShared(**elementType),
>> but that won't work in user code.
>> - If it is a good idea, how do I proceed from here? Possibly create a
>> JEP? How can I contribute? Who wants to support?
>>
>> Roel Spilker
>>
>>
> Hi Roel,
> There are several issues with your template, EDGES is initialized too
> soon, i.e.
> even if you don't call bar() and stay too long, i.e even if you never call
> bar() more than once.
>
> I think it's better to let the the escape analysis pass done by the JIT to
> inline
> values() and remove the call to clone().
>
> Rémi
>
>
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mail.openjdk.java.net/pipermail/compiler-dev/attachments/20120328/e6c6c08a/attachment.html