RFR: 8241825: Make compressed oops and compressed class pointers independent on x86_64

[Frederic Parain]

Hi Erik,

Why did you removed the code handling classes with hard coded layouts?
This code was there to provide the freedom to play with layout algorithms
without having to deal with these special cases. Without this code, any
modification is now constrained by these special classes.

And I’d prefer to keep the dispatch in FieldLayoutBuilder::build_layout()
because we have more cases to handle there for the Valhalla project (the
layout algorithm for inline types is different than the one in
compute_regular_layout()).

Regards,

Fred


> On Apr 24, 2020, at 04:17, Erik Österlund <erik.osterlund at oracle.com> wrote:
> 
> Hi,
> 
> Today, the use of compressed class pointers assumes the use of compressed oops.
> This patch loosens up the relationship between compressed oops and compressed
> class pointers, so that compressed class pointers can be used without compressed
> oops. This benefits for example ZGC that wants compressed class pointers, but
> not compressed oops, effectively giving ZGC 4 bytes smaller objects.
> 
> Much of the complexity of the change is that r12 used to be used by compressed
> class pointers as some kind of semi-fast temp register that one would restore
> to the compressed oops heap base (or zero). That made it effectively a slightly
> optimized spilling mechanism used to find a temp register. I replaced that
> mechanism with a plain old normal temp register that you pass in as a parameter.
> The bad news is that I had to find available temp registers in a lot of places.
> The good news is that almost always were there temp registers available for free,
> and hence the new temp register is even faster than the old optimized spilling
> mechanism. Because we almost never need any spilling at all in the contexts where
> this is used.
> 
> Since I want the 4 new bytes to actually make objects smaller, I poked the new
> layout code a bit so that the klass gap is made available for fields. That used
> to be made available only with compressed oops enabled, due to restrictions in
> the layout code. Our new layout code does not have such restrictions, and so
> I will make the 4 bytes available for fields when the new layout code is used
> and compressed class pointers are used.
> 
> Now I'm only fixing this for HotSpot x86_64. Ideally the use of compressed oops
> and compressed class pointers should not be entangled in other platforms and
> Graal. But that is beyond the scope of this change, and I will let them behave
> the way that they used to, to be potentially fixed later.
> 
> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8241825
> 
> Webrev:
> http://cr.openjdk.java.net/~eosterlund/8241825/webrev.00/
> 
> Testing:
> hs-tier1-7
> 
> Thanks,
> /Erik