RFR: 8334220: Optimize Klass layout after JDK-8180450 [v2]

[duke]

On Tue, 2 Jul 2024 17:25:53 GMT, Xiaolong Peng <xpeng at openjdk.org> wrote:

>> Hi all, 
>>     This PR is created to optimize the layout of Klass in hotspot, after JDK-8180450 the layout of Klsss seems broken, there are 3 holes, they are caused by alignment issue introduced by the 1 byte ```_hash_slot```. 
>> 
>> 
>> (gdb) ptype /ox Klass
>> /* offset      |    size */  type = class Klass : public Metadata {
>>                              public:
>>                                static const uint KLASS_KIND_COUNT;
>>                              protected:
>> /* 0x000c      |  0x0004 */    jint _layout_helper;
>> /* 0x0010      |  0x0004 */    const enum Klass::KlassKind _kind;
>> /* 0x0014      |  0x0004 */    jint _modifier_flags;
>> /* 0x0018      |  0x0004 */    juint _super_check_offset;
>> /* XXX  4-byte hole      */
>> /* 0x0020      |  0x0008 */    class Symbol *_name;
>> /* 0x0028      |  0x0008 */    class Klass *_secondary_super_cache;
>> /* 0x0030      |  0x0008 */    class Array<Klass*> *_secondary_supers;
>> /* 0x0038      |  0x0040 */    class Klass *_primary_supers[8];
>> /* 0x0078      |  0x0008 */    class OopHandle {
>>                                  private:
>> /* 0x0078      |  0x0008 */        class oop *_obj;
>> 
>>                                    /* total size (bytes):    8 */
>>                                } _java_mirror;
>> /* 0x0080      |  0x0008 */    class Klass *_super;
>> /* 0x0088      |  0x0008 */    class Klass * volatile _subklass;
>> /* 0x0090      |  0x0008 */    class Klass * volatile _next_sibling;
>> /* 0x0098      |  0x0008 */    class Klass *_next_link;
>> /* 0x00a0      |  0x0008 */    class ClassLoaderData *_class_loader_data;
>> /* 0x00a8      |  0x0008 */    uintx _bitmap;
>> /* 0x00b0      |  0x0001 */    uint8_t _hash_slot;
>> /* XXX  3-byte hole      */
>> /* 0x00b4      |  0x0004 */    int _vtable_len;
>> /* 0x00b8      |  0x0004 */    class AccessFlags {
>>                                  private:
>> /* 0x00b8      |  0x0004 */        jint _flags;
>> 
>>                                    /* total size (bytes):    4 */
>>                                } _access_flags;
>> /* XXX  4-byte hole      */
>> /* 0x00c0      |  0x0008 */    traceid _trace_id;
>>                              private:
>> /* 0x00c8      |  0x0002 */    s2 _shared_class_path_index;
>> /* 0x00ca      |  0x0002 */    u2 _shared_class_flags;
>> /* 0x00cc      |  0x0004 */    int _archived_mirror_index;
>>                              public:
>>                                static const int SECONDARY_SUPERS_TABLE_SIZE;
>>                         ...
>
> Xiaolong Peng has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Move both _bitmap and _hash_slot together

@pengxiaolong 
Your change (at version ce1560c1fcdca237d594cdbd9e0ea59572964509) is now ready to be sponsored by a Committer.

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PR Comment: https://git.openjdk.org/jdk/pull/19958#issuecomment-2204347799