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

[Erik Österlund]

On Wed, 25 Nov 2020 08:43:04 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>> JDK-8188055 added the function Reference.refersTo. For performance, the supporting native methods Reference.refersTo0 and PhantomReference.refersTo0 should be intrinsified by C2.
>> 
>> Initial patch was prepared by @fisk.
>> 
>> Tested hs-tier1-4. Added new compiler tests to test intrinsics.
>> 
>> Ran new test with Shenandoah. Found only one issue. As result I disable  PhantomReference::refersTo intrinsic for COOP+ Shenandoah combination. Someone from Shenandoah team have to test changes if that is enough.
>
> 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.

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