--- a/src/hotspot/cpu/aarch64/globalDefinitions_aarch64.hpp	Mon Sep 02 12:10:21 2019 +0200
+++ b/src/hotspot/cpu/aarch64/globalDefinitions_aarch64.hpp	Mon Aug 12 19:20:12 2019 +0200
@@ -34,6 +34,12 @@
 
 #define SUPPORTS_NATIVE_CX8
 
+// Aarch64 was not originally defined as multi-copy-atomic, but now is.
+// See: "Simplifying ARM Concurrency: Multicopy-atomic Axiomatic and
+// Operational Models for ARMv8"
+// So we could #define CPU_MULTI_COPY_ATOMIC but historically we have
+// not done so.
+
 // According to the ARMv8 ARM, "Concurrent modification and execution
 // of instructions can lead to the resulting instruction performing
 // any behavior that can be achieved by executing any sequence of

--- a/src/hotspot/cpu/arm/globalDefinitions_arm.hpp	Mon Sep 02 12:10:21 2019 +0200
+++ b/src/hotspot/cpu/arm/globalDefinitions_arm.hpp	Mon Aug 12 19:20:12 2019 +0200
@@ -45,6 +45,9 @@
 #define SUPPORTS_NATIVE_CX8
 #endif
 
+// arm32 is not specified as multi-copy-atomic
+// So we must not #define CPU_MULTI_COPY_ATOMIC
+
 #define STUBROUTINES_MD_HPP    "stubRoutines_arm.hpp"
 #define INTERP_MASM_MD_HPP     "interp_masm_arm.hpp"
 #define TEMPLATETABLE_MD_HPP   "templateTable_arm.hpp"

--- a/src/hotspot/cpu/ppc/globalDefinitions_ppc.hpp	Mon Sep 02 12:10:21 2019 +0200
+++ b/src/hotspot/cpu/ppc/globalDefinitions_ppc.hpp	Mon Aug 12 19:20:12 2019 +0200
@@ -41,8 +41,8 @@
 
 #define SUPPORTS_NATIVE_CX8
 
-// The PPC CPUs are NOT multiple-copy-atomic.
-#define CPU_NOT_MULTIPLE_COPY_ATOMIC
+// PPC64 is not specified as multi-copy-atomic
+// So we must not #define CPU_MULTI_COPY_ATOMIC
 
 // The expected size in bytes of a cache line, used to pad data structures.
 #define DEFAULT_CACHE_LINE_SIZE 128

--- a/src/hotspot/cpu/s390/globalDefinitions_s390.hpp	Mon Sep 02 12:10:21 2019 +0200
+++ b/src/hotspot/cpu/s390/globalDefinitions_s390.hpp	Mon Aug 12 19:20:12 2019 +0200
@@ -42,6 +42,8 @@
 
 #define SUPPORTS_NATIVE_CX8
 
+#define CPU_MULTI_COPY_ATOMIC
+
 // Indicates whether the C calling conventions require that
 // 32-bit integer argument values are extended to 64 bits.
 // This is the case on z/Architecture.

--- a/src/hotspot/cpu/sparc/globalDefinitions_sparc.hpp	Mon Sep 02 12:10:21 2019 +0200
+++ b/src/hotspot/cpu/sparc/globalDefinitions_sparc.hpp	Mon Aug 12 19:20:12 2019 +0200
@@ -36,6 +36,8 @@
 
 #define SUPPORTS_NATIVE_CX8
 
+#define CPU_MULTI_COPY_ATOMIC
+
 // The expected size in bytes of a cache line, used to pad data structures.
 #if defined(TIERED)
   // tiered, 64-bit, large machine

--- a/src/hotspot/cpu/x86/globalDefinitions_x86.hpp	Mon Sep 02 12:10:21 2019 +0200
+++ b/src/hotspot/cpu/x86/globalDefinitions_x86.hpp	Mon Aug 12 19:20:12 2019 +0200
@@ -33,6 +33,8 @@
 
 #define SUPPORTS_NATIVE_CX8
 
+#define CPU_MULTI_COPY_ATOMIC
+
 // The expected size in bytes of a cache line, used to pad data structures.
 #if defined(TIERED)
   #ifdef _LP64

--- a/src/hotspot/share/gc/shared/taskqueue.inline.hpp	Mon Sep 02 12:10:21 2019 +0200
+++ b/src/hotspot/share/gc/shared/taskqueue.inline.hpp	Mon Aug 12 19:20:12 2019 +0200
@@ -207,7 +207,7 @@
   // Architectures with weak memory model require a barrier here
   // to guarantee that bottom is not older than age,
   // which is crucial for the correctness of the algorithm.
-#if !(defined SPARC || defined IA32 || defined AMD64)
+#ifndef CPU_MULTI_COPY_ATOMIC
   OrderAccess::fence();
 #endif
   uint localBot = OrderAccess::load_acquire(&_bottom);

--- a/src/hotspot/share/utilities/globalDefinitions.hpp	Mon Sep 02 12:10:21 2019 +0200
+++ b/src/hotspot/share/utilities/globalDefinitions.hpp	Mon Aug 12 19:20:12 2019 +0200
@@ -481,10 +481,13 @@
 // assure their ordering, instead of after volatile stores.
 // (See "A Tutorial Introduction to the ARM and POWER Relaxed Memory Models"
 // by Luc Maranget, Susmit Sarkar and Peter Sewell, INRIA/Cambridge)
-#ifdef CPU_NOT_MULTIPLE_COPY_ATOMIC
-const bool support_IRIW_for_not_multiple_copy_atomic_cpu = true;
+#ifdef CPU_MULTI_COPY_ATOMIC
+// Not needed.
+const bool support_IRIW_for_not_multiple_copy_atomic_cpu = false;
 #else
-const bool support_IRIW_for_not_multiple_copy_atomic_cpu = false;
+// From all non-multi-copy-atomic architectures, only PPC64 supports IRIW at the moment.
+// Final decision is subject to JEP 188: Java Memory Model Update.
+const bool support_IRIW_for_not_multiple_copy_atomic_cpu = PPC64_ONLY(true) NOT_PPC64(false);
 #endif
 
 // The expected size in bytes of a cache line, used to pad data structures.