8345273: Fix -Wzero-as-null-pointer-constant warnings in s390 code

src/hotspot/cpu/s390/assembler_s390.hpp

@@ -122,14 +122,14 @@ class RelAddr {
     return is_in_range_of_RelAddr(target, pc, true);
   }
   static bool is_in_range_of_RelAddr16(ptrdiff_t distance) {
-    return is_in_range_of_RelAddr((address)distance, 0, true);
+    return is_in_range_of_RelAddr((address)distance, nullptr, true);
   }
 
   static bool is_in_range_of_RelAddr32(address target, address pc) {
     return is_in_range_of_RelAddr(target, pc, false);
   }
   static bool is_in_range_of_RelAddr32(ptrdiff_t distance) {
-    return is_in_range_of_RelAddr((address)distance, 0, false);
+    return is_in_range_of_RelAddr((address)distance, nullptr, false);
   }
 
   static int pcrel_off(address target, address pc, bool shortForm) {
@@ -149,14 +149,14 @@ class RelAddr {
     return pcrel_off(target, pc, true);
   }
   static int pcrel_off16(ptrdiff_t distance) {
-    return pcrel_off((address)distance, 0, true);
+    return pcrel_off((address)distance, nullptr, true);
   }
 
   static int pcrel_off32(address target, address pc) {
     return pcrel_off(target, pc, false);
   }
   static int pcrel_off32(ptrdiff_t distance) {
-    return pcrel_off((address)distance, 0, false);
+    return pcrel_off((address)distance, nullptr, false);
   }
 
   static ptrdiff_t inv_pcrel_off16(int offset) {

src/hotspot/cpu/s390/frame_s390.cpp

@@ -265,7 +265,7 @@ void frame::patch_pc(Thread* thread, address pc) {
                   p2i(&((address*) _sp)[-1]), p2i(((address*) _sp)[-1]), p2i(pc));
   }
   assert(!Continuation::is_return_barrier_entry(*pc_addr), "return barrier");
-  assert(_pc == *pc_addr || pc == *pc_addr || 0 == *pc_addr,
+  assert(_pc == *pc_addr || pc == *pc_addr || nullptr == *pc_addr,
          "must be (pc: " INTPTR_FORMAT " _pc: " INTPTR_FORMAT " pc_addr: " INTPTR_FORMAT
          " *pc_addr: " INTPTR_FORMAT  " sp: " INTPTR_FORMAT ")",
          p2i(pc), p2i(_pc), p2i(pc_addr), p2i(*pc_addr), p2i(sp()));
@@ -296,10 +296,10 @@ void frame::patch_pc(Thread* thread, address pc) {
 bool frame::is_interpreted_frame_valid(JavaThread* thread) const {
   assert(is_interpreted_frame(), "Not an interpreted frame");
   // These are reasonable sanity checks
-  if (fp() == 0 || (intptr_t(fp()) & (wordSize-1)) != 0) {
+  if (fp() == nullptr || (intptr_t(fp()) & (wordSize-1)) != 0) {
     return false;
   }
-  if (sp() == 0 || (intptr_t(sp()) & (wordSize-1)) != 0) {
+  if (sp() == nullptr || (intptr_t(sp()) & (wordSize-1)) != 0) {
     return false;
   }
   int min_frame_slots = (z_common_abi_size + z_ijava_state_size) / sizeof(intptr_t);
@@ -420,7 +420,7 @@ void frame::back_trace(outputStream* st, intptr_t* start_sp, intptr_t* top_pc, u
                            ? (address) top_pc
                            : (address) *((intptr_t*)(((address) current_sp) + _z_abi(return_pc)));
 
-    if ((intptr_t*) current_fp != 0 && (intptr_t*) current_fp <= current_sp) {
+    if ((intptr_t*) current_fp != nullptr && (intptr_t*) current_fp <= current_sp) {
       st->print_cr("ERROR: corrupt stack");
       return;
     }
@@ -503,7 +503,7 @@ void frame::back_trace(outputStream* st, intptr_t* start_sp, intptr_t* top_pc, u
       case 0: // C frame:
         {
           st->print("    ");
-          if (current_pc == 0) {
+          if (current_pc == nullptr) {
             st->print("? ");
           } else {
              // name

src/hotspot/cpu/s390/macroAssembler_s390.cpp

@@ -4108,7 +4108,7 @@ void MacroAssembler::encode_klass_not_null(Register dst, Register src) {
   Register current = (src != noreg) ? src : dst; // Klass is in dst if no src provided. (dst == src) also possible.
   address  base    = CompressedKlassPointers::base();
   int      shift   = CompressedKlassPointers::shift();
-  bool     need_zero_extend = base != 0;
+  bool     need_zero_extend = base != nullptr;
   assert(UseCompressedClassPointers, "only for compressed klass ptrs");
 
   BLOCK_COMMENT("cKlass encoder {");

src/hotspot/cpu/s390/nativeInst_s390.cpp

@@ -59,7 +59,7 @@ void NativeInstruction::verify() {
   //  - any address in first page (0x0000 .. 0x0fff)
   //  - odd address (will cause a "specification exception")
   address addr = addr_at(0);
-  if ((addr == 0) || (((unsigned long)addr & ~0x0fff) == 0) || ((intptr_t)addr & 1) != 0) {
+  if ((addr == nullptr) || (((unsigned long)addr & ~0x0fff) == 0) || ((intptr_t)addr & 1) != 0) {
     tty->print_cr(INTPTR_FORMAT ": bad instruction address", p2i(addr));
     fatal("not an instruction address");
   }

src/hotspot/cpu/s390/relocInfo_s390.cpp

@@ -164,7 +164,6 @@ void Relocation::pd_set_call_destination(address x) {
 
 address* Relocation::pd_address_in_code() {
  ShouldNotReachHere();
- return 0;
 }
 
 address Relocation::pd_get_address_from_code() {

src/hotspot/cpu/s390/stubGenerator_s390.cpp

@@ -688,7 +688,7 @@ class StubGenerator: public StubCodeGenerator {
     // code (code_size == 0) confuses opjitconv
     // StubCodeMark mark(this, "StubRoutines", "verify_oop_stub");
 
-    address start = 0;
+    address start = nullptr;
 
 #if !defined(PRODUCT)
     start = CAST_FROM_FN_PTR(address, verify_oop_helper);