Sharing the markword (aka Valhalla's markword use)

[John Rose]

On 5 Mar 2024, at 8:28, Thomas Stüfe wrote:

> Hi Dan,
>
>  In addition to Roman's answer, we plan to reduce the Klasspointer to 22 bits [1]. For 64-bit headers, this would give us 31-bit i-hash back and still leave us with 4 unused bits.

Thank you for making that clear.

It looks like there are plenty of bits in the 64-bit header for Valhalla, so some of the tricky bit-twiddling suggested in my previous mail is not needed, unless (at some future date) we go to 32-bit headers.  I’m going to make a wild guess that Valhalla is likely to come before 32-bit headers.

IMO, the proposal about loading the prototype header bits out of the Klass metadata is still reasonable to consider, regardless of header size.