Optimize bytecode for combination enhanced-for and enums

[Rémi Forax]

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