Design question: why not reuse the CMS slots in mark word?

[Aleksey Shipilev]

On 09/19/2016 11:00 PM, Christine Flood wrote:
> The theory was to make the read barrier as efficient as possible.
> It's always just a single read of a fixed offset.  If we tried to
> reuse the mark word we'd have to deal with locked objects and hash
> codes and all that nonsense potentially making read barriers much
> more expensive.
> 
> If you have a protocol that uses the mark word and doesn't introduce
> extra overhead in the common case (a conditional in a read barrier
> would be deadly) I'd love to hear it.

Okay, that makes sense, thanks.

Is this a hunch about the read barriers cost, or we actually tried more
complicated read barriers? Trying to understand if potentially worth
investigating, or this road was taken already.

Thanks,
-Aleksey