C2 doesn't hoist out flattened value array guard out of loops

Tobias Hartmann tobias.hartmann at oracle.com
Fri Mar 29 07:28:27 UTC 2019

Hi Sergey,

thanks for reporting. Is this a regression from LW1?

I'm currently working on LW2 array support for C2 which will change how we detect the array layout:

I'll investigate once the LW2 array changes are ready.

Best regards,

On 28.03.19 22:37, Sergey Kuksenko wrote:
> Please, look into that issue https://bugs.openjdk.java.net/browse/JDK-8221647
> That issue is real showstopper for any other valhalla performance evaluations.
> Large performance regression is observed on numerous legacy (non-value) code working with Object[]
> arrays comparing -XX:+EnableValhalla vs -XX:-EnableValhalla
> For example:
> - Arrays.hasCode(Object[]) - 60% slowdown
> - for loop Object[] to Object[] copying - 2x times slowdown
> - pass elements of Object[] - 90% slowdown.
> - etc....
> Small set of simple benchmarks could be found
> herehttp://cr.openjdk.java.net/~skuksenko/valhalla/arrayloop/benchmarks/
> The reason of that that check if arrays is flattened value array was not hoisted out of the loop and
> checked on each iteration.
>  asm for check (e.g.):
>  mov 0x8(%r10),%r8d
>  sar $0x1d,%r8d
>  cmp $0xfffffffd,%r8d
>  jne 0x00007f4d008cc2ad

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