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

[Ioi Lam]

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