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

[Erik Österlund]

On Wed, 25 Nov 2020 08:30:46 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/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.

The information that there was a barrier attached, is implicit in the ins_encode block due to it being run at all. In other words, since we matched the mach node to our ZGC access instead of a normal access, we already know that there was barrier data attached, and that we no longer have such barrier data.

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