RFR: 8256999: Add C2 intrinsic for Reference.refersTo and PhantomReference::refersTo

[Vladimir Kozlov]

On Wed, 25 Nov 2020 15:13:11 GMT, Erik Österlund <eosterlund at openjdk.org> wrote:

>> src/hotspot/share/gc/g1/c2/g1BarrierSetC2.cpp line 623:
>> 
>>> 621:   // Also we need to add memory barrier to prevent commoning reads
>>> 622:   // from this field across safepoint since GC can change its value.
>>> 623:   bool need_read_barrier = (((on_weak || on_phantom) && !no_keepalive) ||
>> 
>> There's a slight change: `in_heap && (on_weak || ...)` turns into `(on_weak ...) || (in_heap ...)`. It will introduce  a read barrier for `!in_heap && on_weak` case. Does it occur in practice?
>> 
>> Another one: `on_weak` turns into ((on_weak ...) && !no_keepalive). 
>> My interpretation is no read barrier needed when `NO_KEEPALIVE` flag is used and currently a redundant barrier is issued. 
>> 
>> Maybe replace `!no_keepalive` with just `keep_alive`? The former is harder to parse.
>> 
>> The check grows bigger and bigger. Maybe it's time to split it?
>> 
>> Turn `on_weak || on_phantom` into `!is_strong`?
>
> I don't think we have any !in_heap && on_weak loads today. But if we did, they would indeed need read barriers.
> We need read barrier if the the reference isn't provably strong... unless it's an AS_NO_KEEPALIVE access. That also reflects why the variable is called no_keepalive instead of keepalive; it is to reflect the shared decorator name used all over the place. I don't mind inverting it though, but personally found it easier to read when the names match our decorators.

>From this conversation the only change I can do is 'Turn (on_weak || on_phantom) into !on_strong'.
@fisk Is this correct? I am concern that it will include `unknown` decorator too.
I agree with Erik to keep !no_keepalive because he prefer it and this is code supported by GC group.

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PR: https://git.openjdk.java.net/jdk/pull/1425