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8382377: [lworld] The JvmtiHeapwalkObject(obj) set incorrect layout #2473

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8382377: [lworld] The JvmtiHeapwalkObject(obj) set incorrect layout #2473

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19 changes: 12 additions & 7 deletions src/hotspot/share/prims/jvmtiTagMapTable.hpp
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
/*
* Copyright (c) 2020, 2025, Oracle and/or its affiliates. All rights reserved.
* Copyright (c) 2020, 2026, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
Expand Down Expand Up @@ -35,22 +35,27 @@ class JvmtiEnv;

// Describes an object which can be tagged during heap walk operation.
// - generic heap object: _obj: oop, offset == 0, _inline_klass == nullptr;
// - value heap object: _obj: oop, offset == 0, _inline_klass == _obj.klass();
// - flat value object: _obj: holder object, offset == offset in the holder, _inline_klass == klass of the flattened object;
// - value heap object: _obj: oop, offset == 0, _inline_klass == _obj.klass();
class JvmtiHeapwalkObject {
oop _obj; // for flattened value object this is holder object
int _offset; // == 0 for heap objects
InlineKlass* _inline_klass; // for value object, nullptr otherwise
LayoutKind _layout_kind; // layout kind in holder object, used only for flat->heap conversion

static InlineKlass* inline_klass_or_null(oop obj) {
Klass* k = obj->klass();
return k->is_inline_klass() ? InlineKlass::cast(k) : nullptr;
}
public:
JvmtiHeapwalkObject(): _obj(nullptr), _offset(0), _inline_klass(nullptr), _layout_kind(LayoutKind::UNKNOWN) {}
JvmtiHeapwalkObject(oop obj): _obj(obj), _offset(0), _inline_klass(inline_klass_or_null(obj)), _layout_kind(LayoutKind::REFERENCE) {}
JvmtiHeapwalkObject(oop obj, int offset, InlineKlass* ik, LayoutKind lk): _obj(obj), _offset(offset), _inline_klass(ik), _layout_kind(lk) {}
JvmtiHeapwalkObject(oop obj): _obj(obj), _offset(0) {
Klass* k = obj->klass();
if (k->is_inline_klass()) {
_inline_klass = InlineKlass::cast(k);
_layout_kind = LayoutKind::BUFFERED;
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@fparain fparain May 26, 2026

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LayoutKind::BUFFERED is just a synthetic concept, not a real layout.
It is used to easily get the characteristics of the most constraining layout supported by a class, in order to know how heap allocated instances should be sized.
In this method, I think the _layout_kind should be set to REFERENCE for both cases.

} else {
_inline_klass = nullptr;
_layout_kind = LayoutKind::REFERENCE;
}
}

inline bool is_empty() const { return _obj == nullptr; }
inline bool is_value() const { return _inline_klass != nullptr; }
Expand Down