RFR 8208163 [lworld] performance regression on String(char[]) constructor

[Tobias Hartmann]

Hi Ioi,

On 10.08.2018 23:37, Ioi Lam wrote:
> https://bugs.openjdk.java.net/browse/JDK-8208163
> http://cr.openjdk.java.net/~iklam/valhalla/8208163_string_charary_ctor_slowdown.v01/

It seems that with your change, we now mark all non-array allocations as "unknown value", right?
This will cause PhaseMacroExpand::expand_allocate_common() to add additional runtime checks. It
might be better to check for primitive arrays (TypeArrayKlass) explicitly.

Also, your comment is a bit misleading because it suggests that the set_unknown_value() code is only
used for reflective array creation. However, that code should be invoked whenever we create an array
and don't know if the element type is a value type.

To improve readability, I think it would also be good to wrap the entire code starting from line
3972 into 'if (EnableValhalla)' and not only the else branch.

Thanks,
Tobias