C1 and @Stable array elements

[Vladimir Ivanov]

Paul,

It is definitely doable, but my estimate is that it could require 
extensive changes.

The first question I'd like to ask you is what use cases do you have in 
mind?

What I'm trying to understand is what level of optimization you are 
looking for.

(1) With field loads it is simple
   getfield T.f // class T { @Stable int f; }

all information is local, so it can be constant folded during bytecode 
parsing.

(2) For arrays it becomes slightly harder:
   getfield T.arr:[I // class T { @Stable int[] arr; }
   iconst_0
   iaload

The value produced by getfield should be annotated as "stable", so when 
aaload is parsed there is a way to find out the array on stack is "stable".

(3) And for arrays of arrays it becomes complex:
  getfield T.arr:[[[I // class T { @Stable int[][][] arr; }
  iconst_0
  aaload    // => int[][]
  iconst_0
  aaload    // => int[]
  iconst_0
  iaload    // => int

The compiler has to track not only "stable" bit, but also array 
dimension to be able to constant fold the very last load.

And then we have to add control flow into consideration. Ends up pretty 
complex, isn't it? :-)

Best regards,
Vladimir Ivanov

PS: I deliberately tried to be pessimistic in my estimates since I don't 
have extensive experience in C1 code base.

On 11/19/15 7:22 PM, Paul Sandoz wrote:
> Hi,
>
> Currently C1 does not support constant folding of array elements of an array held by a @Stable field.
>
> How easy or hard would it be for C1 to support such constant folding?
>
> Naively and intuitively, since C1 appears to constant fold an array length (i.e. it knows the array itself is stable or static final), it would in theory seem possible to extend that logic for array elements, such that if the referenced element value is not the default value, produce a constant holding the element value.
>
> Paul.
>