Question about SATB barrier

[Roman Kennke]

Hi Kelvin,

The form where 'pre-val' is passed as an input instead of loaded from a
field is used by the cases where the previous-value needs to be
explicitely kept-alive:

- Most prominently, in the intrinsics for Reference.get(): we need to
keep the referent alive because Referent.get() makes it strongly
reachable, and it may otherwise get lost by SATB
- Stuff like xchg and cmpxchg, where the previous value is loaded
anyway, and we only need to enqueue it.
- a few other cases, especially in the runtime impl

It is not meant for the compiler-optimization. I believe the compiler
would figure it out if it can deduce the previously-stored-value in a
safe way.

There is also the scenario of the I-U mode. This is basically the
inverse of SATB, and we always enqueue the update-value of a store.
There we only ever need the simpler form of the enqueueing barrier. As
a bonus we don't need to keep-alive Referent.get() there, which means
we can even reclaim weakrefs that are accessed during conc-mark. Some
users love that feature ;-)

Cheers,
Roman

On Sat, 2020-07-18 at 23:09 +0000, Nilsen, Kelvin wrote:
> I've observed that the various SATB barrier implementations have two
> different forms.
> 
> In one form, the address to be overwritten is supplied as an argument
> and the previous content held at that address is fetched and stored
> into a SATB buffer.  In the other form, the address to be overwritten
> is not supplied and the "keep-alive" value is supplied as the pre-
> initialized content of the pre-val register.  In this second form,
> the content of the pre-val register is stored into the SATB buffer.
> 
> My question is whether the second form is used with write operations
> for which the "compiler" is able to determine through static analysis
> the previous content of the value to be overwritten, or is it used
> only for other situations that require the logging of a keep-alive
> pointer value?
> 
> I have been assuming the latter, but I need to make sure I understand
> how these two forms are used.
> 
> Thanks.
> 
> 
> 
> 
>