RFR 8218142 [lworld] C1 support for array covariance for aaload

[Karen Kinnear]

Many thanks Srikanth for the covariance/non-covariance support.

And the current flags:
-XDallowWithFieldOperator: __WithField construct is allowed only when on
-XDallowGenericsOverValues: For values to parameterize generics, not for values them
selves to be generic
-XDnonCovariantValueArrays: For experiments where VT[] is not a subtype of j.l.O[]
-XDallowEmptyValues: Allow values that have no state

(I agree with getting rid of -XDdisallowValueConstructors if you didn’t do that already)

thanks,
Karen

> On Feb 5, 2019, at 8:34 AM, Srikanth <srikanth.adayapalam at oracle.com> wrote:
> 
> Javac by default allows covariance now.
> 
> Srikanth
> 
> On 05/02/19 5:24 PM, Ioi Lam wrote:
>> If I remember correctly, some of them failed to compile or run after changes in javac or hotspot-runtime that disallowed covariance. I'll re-enable them and see what happens.
>> 
>> Thanks
>> 
>> - Ioi
>> 
>> On 2/5/19 3:06 AM, Tobias Hartmann wrote:
>>> Hi Ioi,
>>> 
>>> this looks good to me. Should we re-enable the tests marked with "TODO Re-enable if value type
>>> arrays become covariant with object arrays"? C2 already supports array covariance.
>>> 
>>> Thanks,
>>> Tobias
>>> 
>>> On 31.01.19 13:52, Ioi Lam wrote:
>>>> http://cr.openjdk.java.net/~iklam/valhalla/8218142-aaload-covariance.v01/ 
>>>> https://bugs.openjdk.java.net/browse/JDK-8218142
>>>> 
>>>> When loading from an array declared as Object[], now the C1 code checks if the array is flattened.
>>>> If so, the loading is done via a runtime call.
>>>> 
>>>> Here's an example of the generated code
>>>> 
>>>>        mov    0x8(%rsi),%eax      ; rsi = array
>>>>        mov    $0x800000000,%r12
>>>>        add    %r12,%rax           ; rax = array klass
>>>>        xor    %r12,%r12           ; why is this here??
>>>>        mov    0xc(%rax),%eax      ; layout_helper
>>>>        sar    $0x1d,%eax
>>>>        cmp    $0xfffffffd,%eax    ; check for Klass::_lh_array_tag_vt_value
>>>>        je      L_3                ; slow case
>>>> 
>>>> L_1   movslq %edx,%rdx           ; non-flattened load
>>>>        mov    0x10(%rsi,%rdx,4),%eax
>>>>        shl    $0x3,%rax
>>>> L_2   ....
>>>> 
>>>> L_3   mov    %rsi,0x8(%rsp)      ; LoadFlattenedArrayStub slow case
>>>>        mov    %rdx,(%rsp)
>>>>        callq  0x00007fe6489c8840  ; runtime_call load_flattened_array
>>>>        jmpq    L_2
>>>> 
>>>> 
>>>> The code is pretty big and sub-optimal. My plan is to also get aastore working first, and then
>>>> optimize.
>>>> 
>>>> One possibility is to check if the array is actually an Object[] array. That should help with the
>>>> erased generic types.
>>>> 
>>>>        mov    0x8(%rsi),%eax      ; rsi = array
>>>> +     cmp    $0x00001234, %eax   ; compressed klass for [Ljava/lang/Object;
>>>> +     je     L_1                 ; go to non-flattened load
>>>>        mov    $0x800000000,%r12
>>>> 
>>>> 
>>>> Thanks
>>>> - Ioi
>