8355364: [REDO] Missing REX2 prefix accounting in ZGC barriers leads to incorrect encoding

Co-authored-by: Axel Boldt-Christmas <aboldtch@openjdk.org>
Reviewed-by: aboldtch, sviswanathan

Author: Jatin Bhateja

src/hotspot/cpu/x86/gc/z/zBarrierSetAssembler_x86.cpp

@@ -1328,7 +1328,13 @@ void ZBarrierSetAssembler::patch_barrier_relocation(address addr, int format) {
   const uint16_t value = patch_barrier_relocation_value(format);
   uint8_t* const patch_addr = (uint8_t*)addr + offset;
   if (format == ZBarrierRelocationFormatLoadGoodBeforeShl) {
-    *patch_addr = (uint8_t)value;
+    if (VM_Version::supports_apx_f()) {
+      NativeInstruction* instruction = nativeInstruction_at(addr);
+      uint8_t* const rex2_patch_addr = patch_addr + (instruction->has_rex2_prefix() ? 1 : 0);
+      *rex2_patch_addr = (uint8_t)value;
+    } else {
+      *patch_addr = (uint8_t)value;
+    }
   } else {
     *(uint16_t*)patch_addr = value;
   }