8340620: Fix -Wzero-as-null-pointer-constant warnings for CompressedOops

Reviewed-by: shade, stefank, mli, amitkumar

Author: Kim Barrett

src/hotspot/cpu/ppc/ppc.ad

@@ -6598,7 +6598,7 @@ instruct encodeP_not_null_Ex(iRegNdst dst, iRegPsrc src) %{
 instruct encodeP_not_null_base_null(iRegNdst dst, iRegPsrc src) %{
   match(Set dst (EncodeP src));
   predicate(CompressedOops::shift() != 0 &&
-            CompressedOops::base() ==0);
+            CompressedOops::base() == nullptr);
 
   format %{ "SRDI    $dst, $src, #3 \t// encodeP, $src != nullptr" %}
   size(4);
@@ -6695,7 +6695,7 @@ instruct decodeN_Ex(iRegPdst dst, iRegNsrc src, flagsReg crx) %{
   predicate((n->bottom_type()->is_oopptr()->ptr() != TypePtr::NotNull &&
              n->bottom_type()->is_oopptr()->ptr() != TypePtr::Constant) &&
             CompressedOops::shift() != 0 &&
-            CompressedOops::base() != 0);
+            CompressedOops::base() != nullptr);
   ins_cost(4 * DEFAULT_COST); // Should be more expensive than decodeN_Disjoint_isel_Ex.
   effect(TEMP crx);
 
@@ -6707,7 +6707,7 @@ instruct decodeN_Ex(iRegPdst dst, iRegNsrc src, flagsReg crx) %{
 instruct decodeN_nullBase(iRegPdst dst, iRegNsrc src) %{
   match(Set dst (DecodeN src));
   predicate(CompressedOops::shift() != 0 &&
-            CompressedOops::base() == 0);
+            CompressedOops::base() == nullptr);
 
   format %{ "SLDI    $dst, $src, #3 \t// DecodeN (zerobased)" %}
   size(4);
@@ -6825,7 +6825,7 @@ instruct decodeN_notNull_addBase_Ex(iRegPdst dst, iRegNsrc src) %{
   predicate((n->bottom_type()->is_oopptr()->ptr() == TypePtr::NotNull ||
              n->bottom_type()->is_oopptr()->ptr() == TypePtr::Constant) &&
             CompressedOops::shift() != 0 &&
-            CompressedOops::base() != 0);
+            CompressedOops::base() != nullptr);
   ins_cost(2 * DEFAULT_COST);
 
   format %{ "DecodeN $dst, $src \t// $src != nullptr, postalloc expanded" %}

src/hotspot/cpu/s390/s390.ad

@@ -4628,7 +4628,7 @@ instruct encodeP(iRegN dst, iRegP src, flagsReg cr) %{
   match(Set dst (EncodeP src));
   effect(KILL cr);
   predicate((n->bottom_type()->make_ptr()->ptr() != TypePtr::NotNull) &&
-            (CompressedOops::base() == 0 ||
+            (CompressedOops::base() == nullptr ||
              CompressedOops::base_disjoint() ||
              !ExpandLoadingBaseEncode));
   ins_cost(MEMORY_REF_COST+3 * DEFAULT_COST);
@@ -4651,7 +4651,7 @@ instruct encodeP_NN(iRegN dst, iRegP src, flagsReg cr) %{
   match(Set dst (EncodeP src));
   effect(KILL cr);
   predicate((n->bottom_type()->make_ptr()->ptr() == TypePtr::NotNull) &&
-            (CompressedOops::base() == 0 ||
+            (CompressedOops::base() == nullptr ||
              CompressedOops::base_disjoint() ||
              !ExpandLoadingBaseEncode_NN));
   ins_cost(MEMORY_REF_COST+3 * DEFAULT_COST);

src/hotspot/share/oops/compressedOops.cpp

@@ -61,7 +61,7 @@ void CompressedOops::initialize(const ReservedHeapSpace& heap_space) {
   }
   if ((uint64_t)heap_space.end() <= OopEncodingHeapMax) {
     // Did reserve heap below 32Gb. Can use base == 0;
-    set_base(0);
+    set_base(nullptr);
   } else {
     set_base((address)heap_space.compressed_oop_base());
   }
@@ -115,7 +115,7 @@ CompressedOops::Mode CompressedOops::mode() {
     return DisjointBaseNarrowOop;
   }
 
-  if (base() != 0) {
+  if (base() != nullptr) {
     return HeapBasedNarrowOop;
   }
 
@@ -166,7 +166,7 @@ void CompressedOops::print_mode(outputStream* st) {
 
   st->print(", Compressed Oops mode: %s", mode_to_string(mode()));
 
-  if (base() != 0) {
+  if (base() != nullptr) {
     st->print(": " PTR_FORMAT, p2i(base()));
   }