RFR: 8266845: Shenandoah: Simplify SBS::load_reference_barrier implementation

[Aleksey Shipilev]

On Mon, 10 May 2021 20:14:14 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> Early return on null obj simplifies implementation and makes code more readable.
> 
> Test:
> - [x] hotspot_gc_shenandoah

Changes requested by shade (Reviewer).

src/hotspot/share/gc/shenandoah/shenandoahBarrierSet.inline.hpp line 105:

> 103:   if (obj == nullptr) {
> 104:     return nullptr;
> 105:   }

Let's use `NULL` consistently? Yes, it is an old style, but it is consistent with the code in the same file.

src/hotspot/share/gc/shenandoah/shenandoahBarrierSet.inline.hpp line 131:

> 129:   oop fwd = load_reference_barrier(obj);
> 130:   assert(fwd != nullptr, "Sanity");
> 131:   if (ShenandoahSelfFixing && load_addr != NULL) {

I think dropping `fwd != obj` is not about the null-check. It is about not doing the CAS when evacuation fails. It is a minor performance optimization: do not try to perform obviously no-op update. Let's leave it as is.

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PR: https://git.openjdk.java.net/jdk/pull/3956