escape analysis issue with nested objects

[Vladimir Kozlov]

Hi Attila,

What Java version you are using?

I tried jdk 7, 8, 9.

What I see is that in both cases only arrays are eliminated 
(-XX:+PrintEliminateAllocations with debug version of VM):

BEGIN single
Scalar  73	AllocateArray	===  51  46  47  8  1 ( 71  59  21  58  41  1 
1 ) [[ 74  75  76  83  84  85 ]]  rawptr:NotNull ( int:>=0, 
java/lang/Object:NotNull *, bool, int ) NewMain::single @ bci:9 !jvms: 
NewMain::single @ bci:9
++++ Eliminated: 73 AllocateArray
END single
BEGIN multi
Scalar  116	AllocateArray	===  91  86  87  8  1 ( 114  103  21  102  41 
76  1  1  1 ) [[ 117  118  119  126  127  128 ]]  rawptr:NotNull ( 
int:>=0, java/lang/Object:NotNull *, bool, int ) NewMain::multi @ bci:17 
!jvms: NewMain::multi @ bci:17
++++ Eliminated: 116 AllocateArray
DONE

But based on code non of objects should be allocated - both methods 
should have only 'return' generated.

Looks like EA missing such case when objects are stored into small array.

Thanks,
Vladimir

On 5/25/17 9:16 AM, Hontvári Attila wrote:
> When creating a non-escaping array and putting a newly created, 
> non-escaping object in it, the EA works, there are no heap allocations.
> 
>      private static void single() {
>          Object x = new Object();
>          Object[] array = new Object[]{x};
>          Object a = array[0];
>      }
> 
> But if we do the same with two or more objects, the array will be 
> allocated on the heap, and not eliminated.
> 
>      private static void multi() {
>          Object x = new Object();
>          Object y = new Object();
>          Object[] array = new Object[]{x, y};
>          Object a = array[0];
>          Object b = array[1];
>      }
> 
> Is there a reason why it is only working in the first case?
> 
> This would be useful for example, MethodHandle::invokeWithArguments, 
> when the primitive types are boxed, and put into a varargs array, see my 
> older email [1].
> 
> A complete test source code is in [2], if we run it with -verbose:gc, we 
> can see there are many GCs in the second case, but there are no GCs in 
> the first case.
> 
> [1] 
> http://mail.openjdk.java.net/pipermail/jigsaw-dev/2017-January/010933.html
> 
> [2] https://gist.github.com/anonymous/bd46075ef1ebd858dae49fe6cfe39da8
>