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

[Vladimir Ivanov]

On Wed, 25 Nov 2020 03:31:36 GMT, Vladimir Kozlov <kvn 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/cpu/x86/gc/z/z_x86_64.ad line 123:

> 121: 
> 122:   ins_encode %{
> 123:     if (barrier_data() != 0) { // barrier could be elided by ZBarrierSetC2::analyze_dominating_barriers()

Maybe keep a bit reserved for `ZLoadBarrierElided` to just map it to `0`? The former is preferred because it keeps the info that there was a barrier data attached in the first place.

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`?

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