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

[Tobias Hartmann]

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