8370049: [s390x] G1 barrier compareAndExchange does not return old value when compareExchange fails

Backport-of: 5609ee11a2daf888d02c0c1b2b70eb4df817582c

Author: Vladimir Petko

src/hotspot/cpu/s390/gc/g1/g1_s390.ad

@@ -356,7 +356,7 @@ instruct g1CompareAndExchangeP(iRegP mem_ptr, rarg5RegP oldval, iRegP_N2P newval
 
     __ z_lgr($res$$Register, $oldval$$Register); // previous content
 
-    __ z_csg($oldval$$Register, $newval$$Register, 0, $mem_ptr$$reg);
+    __ z_csg($res$$Register, $newval$$Register, 0, $mem_ptr$$reg);
 
     write_barrier_post(masm, this,
                        $mem_ptr$$Register /* store_addr */,

test/hotspot/jtreg/compiler/gcbarriers/TestG1BarrierGeneration.java

@@ -880,6 +880,16 @@ public void runAtomicTests() {
             Asserts.assertEquals(oldVal, oldVal2);
             Asserts.assertEquals(o.f, newVal);
         }
+        {
+            Outer o = new Outer();
+            Object oldVal = new Object();
+            o.f = oldVal;
+            Object cmpVal = new Object();
+            Object newVal = new Object();
+            Object oldVal2 = testCompareAndExchange(o, cmpVal, newVal);
+            Asserts.assertEquals(oldVal2, oldVal);
+            Asserts.assertEquals(o.f, oldVal);
+        }
         {
             Outer o = new Outer();
             Object oldVal = new Object();
@@ -889,6 +899,16 @@ public void runAtomicTests() {
             Asserts.assertTrue(b);
             Asserts.assertEquals(o.f, newVal);
         }
+        {
+            Outer o = new Outer();
+            Object oldVal = new Object();
+            o.f = oldVal;
+            Object cmpVal = new Object();
+            Object newVal = new Object();
+            boolean b = testCompareAndSwap(o, cmpVal, newVal);
+            Asserts.assertFalse(b);
+            Asserts.assertEquals(o.f, oldVal);
+        }
         {
             Outer o = new Outer();
             Object oldVal = new Object();