8335062: NMT: Make StackIndex non-opaque

Reviewed-by: stuefe, gziemski

src/hotspot/share/nmt/nmtNativeCallStackStorage.hpp

@@ -28,6 +28,7 @@
 #include "nmt/arrayWithFreeList.hpp"
 #include "utilities/growableArray.hpp"
 #include "utilities/nativeCallStack.hpp"
+#include <limits>
 
 // Virtual memory regions that are tracked by NMT also have their NativeCallStack (NCS) tracked.
 // NCS:s are:
@@ -41,19 +42,19 @@
 // We achieve this by using a closed hashtable for finding previously existing NCS:s and referring to them by an index that's smaller than a pointer.
 class NativeCallStackStorage : public CHeapObjBase {
 public:
-  struct StackIndex {
-    friend NativeCallStackStorage;
-    int32_t _stack_index;
-  public:
-    static constexpr const int32_t invalid = -1;
-    static bool equals(const StackIndex& a, const StackIndex& b) {
-      return a._stack_index == b._stack_index;
-    }
+  using StackIndex = int;
 
-    bool is_invalid() {
-      return _stack_index == invalid;
-    }
-  };
+private:
+  constexpr static const StackIndex invalid = std::numeric_limits<StackIndex>::max() - 1;
+
+public:
+  static bool equals(const StackIndex a, const StackIndex b) {
+    return a == b;
+  }
+
+  static bool is_invalid(StackIndex a) {
+    return a == invalid;
+  }
 
 private:
   struct TableEntry;
@@ -83,16 +84,16 @@ public:
   StackIndex push(const NativeCallStack& stack) {
     // Not in detailed mode, so not tracking stacks.
     if (!_is_detailed_mode) {
-      return StackIndex{StackIndex::invalid};
+      return invalid;
     }
     return put(stack);
   }
 
   const inline NativeCallStack& get(StackIndex si) {
-    if (si._stack_index == -1) {
+    if (is_invalid(si)) {
       return _fake_stack;
     }
-    return _stacks.at(si._stack_index);
+    return _stacks.at(si);
   }
 
   NativeCallStackStorage(bool is_detailed_mode, int table_size = default_table_size);

src/hotspot/share/nmt/vmatree.hpp

@@ -78,7 +78,7 @@ public:
 
     static bool equals(const RegionData& a, const RegionData& b) {
       return a.flag == b.flag &&
-             NativeCallStackStorage::StackIndex::equals(a.stack_idx, b.stack_idx);
+             NativeCallStackStorage::equals(a.stack_idx, b.stack_idx);
     }
   };
 
@@ -112,7 +112,7 @@ private:
       return RegionData{sidx, flag()};
     }
 
-    const NativeCallStackStorage::StackIndex stack() const {
+    NativeCallStackStorage::StackIndex stack() const {
      return sidx;
     }
   };

test/hotspot/gtest/nmt/test_nmt_nativecallstackstorage.cpp

@@ -35,7 +35,7 @@ TEST_VM_F(NMTNativeCallStackStorageTest, DoNotStoreStackIfNotDetailed) {
   NativeCallStack ncs{};
   NCSS ncss(false);
   NCSS::StackIndex si = ncss.push(ncs);
-  EXPECT_TRUE(si.is_invalid());
+  EXPECT_TRUE(NCSS::is_invalid(si));
   NativeCallStack ncs_received = ncss.get(si);
   EXPECT_TRUE(ncs_received.is_empty());
 }
@@ -57,7 +57,7 @@ TEST_VM_F(NMTNativeCallStackStorageTest, CollisionsReceiveDifferentIndexes) {
   for (int i = 0; i < nr_of_stacks; i++) {
     for (int j = 0; j < nr_of_stacks; j++) {
       if (i == j) continue;
-      EXPECT_FALSE(NCSS::StackIndex::equals(si_arr[i],si_arr[j]));
+      EXPECT_FALSE(NCSS::equals(si_arr[i],si_arr[j]));
     }
   }
 }