RFR: Fix and rewrite update-refs barrier selector

[Aleksey Shipilev]

On 01/24/2018 05:31 PM, Aleksey Shipilev wrote:
> After Traversal GC commit, normal cycle on Compiler.compiler fails within:
> 
> V  [libjvm.so+0x153c713]  oop ShenandoahHeap::evac_update_oop_ref<unsigned int>(unsigned int*,
> bool&)+0x333
> V  [libjvm.so+0x1539082]  ShenandoahBarrierSet::write_ref_array(HeapWord*, unsigned long)+0x852
> V  [libjvm.so+0x133be02]  void ObjArrayKlass::do_copy<unsigned int>(arrayOop, unsigned int*,
> arrayOop, unsigned int*, int, Thread*)+0x142
> V  [libjvm.so+0x133932b]  ObjArrayKlass::copy_array(arrayOop, int, arrayOop, int, int, Thread*)+0x72b
> V  [libjvm.so+0xf23325]  JVM_ArrayCopy+0x1e5
> 
> The troubling bit is why do we even get here:
> 
>   inline void do_oop_work(T* p) {
>     oop o;
>     if (STOREVAL_WRITE_BARRIER) {
>       bool evac;
>       o = _heap->evac_update_oop_ref(p, evac); <--- ????
>       if ((ALWAYS_ENQUEUE || evac) && !oopDesc::is_null(o)) {
>         ShenandoahBarrierSet::enqueue(o);
>       }
>     } else {
>       o = _heap->maybe_update_oop_ref(p);
>     }
>     if (UPDATE_MATRIX && !oopDesc::is_null(o)) {
>       _heap->connection_matrix()->set_connected(p, o);
>     }
>   }
> 
> It happens because the condition in selector is wrong:
>   http://cr.openjdk.java.net/~shade/shenandoah/traversal-regr-1/webrev.01/

Actually, let's rewrite the damn fragile thing:
  http://cr.openjdk.java.net/~shade/shenandoah/traversal-regr-1/webrev.02/

-Aleksey