RFR (XS) 8232778: Shenandoah: SBSA::arraycopy_prologue checks wrong register

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8232778

Fix:

diff -r 24d411cb3a90 src/hotspot/cpu/aarch64/gc/shenandoah/shenandoahBarrierSetAssembler_aarch64.cpp
--- a/src/hotspot/cpu/aarch64/gc/shenandoah/shenandoahBarrierSetAssembler_aarch64.cpp   Tue Oct 22
08:57:41 2019 +0200
+++ b/src/hotspot/cpu/aarch64/gc/shenandoah/shenandoahBarrierSetAssembler_aarch64.cpp   Tue Oct 22
13:39:05 2019 +0200
@@ -58,7 +58,7 @@
       Address gc_state(rthread, in_bytes(ShenandoahThreadLocalData::gc_state_offset()));
       __ ldrb(rscratch1, gc_state);
       if (dest_uninitialized) {
-        __ tbz(rscratch2, ShenandoahHeap::HAS_FORWARDED_BITPOS, done);
+        __ tbz(rscratch1, ShenandoahHeap::HAS_FORWARDED_BITPOS, done);
       } else {
         __ mov(rscratch2, ShenandoahHeap::HAS_FORWARDED | ShenandoahHeap::MARKING);
         __ tst(rscratch1, rscratch2);

The load happens into rscratch1, yet we are testing rscratch2. I think this silently breaks
arraycopy to-space guarantees, as rscratch2 may contain garbage.

Testing: aarch64 hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey