Integrated: 8334229: Optimize InterpreterOopMap layout

[Xiaolong Peng]

On Mon, 1 Jul 2024 21:23:35 GMT, Xiaolong Peng <xpeng at openjdk.org> wrote:

> Hi all,
>    This PR is re-arrange the fields in InterpreterOopMap to optimize the layout of the class; by moving ```unsigned short _bci ``` to the place after ```int _expression_stack_size```, and ```int _num_oops``` to the place before ```intptr_t _bit_mask[4];```, most of the holes are removed. 
>    
>     Layout before the fix:
> 
> (gdb) ptype /xo InterpreterOopMap
> /* offset      |    size */  type = class InterpreterOopMap : private ResourceObj {
>                              private:
> /* 0x0000      |  0x0008 */    class Method *_method;
> /* 0x0008      |  0x0002 */    unsigned short _bci;
> /* XXX  2-byte hole      */
> /* 0x000c      |  0x0004 */    int _mask_size;
> /* 0x0010      |  0x0004 */    int _expression_stack_size;
>                              protected:
> /* XXX  4-byte hole      */
> /* 0x0018      |  0x0020 */    intptr_t _bit_mask[4];
> /* 0x0038      |  0x0004 */    int _num_oops;
> /* XXX  4-byte padding   */
> 
>                                /* total size (bytes):   64 */
>                              }
> 
> 
> After fix:
> 
> (gdb) ptype /xo  InterpreterOopMap
> /* offset      |    size */  type = class InterpreterOopMap : private ResourceObj {
>                              private:
> /* 0x0000      |  0x0008 */    class Method *_method;
> /* 0x0008      |  0x0004 */    int _mask_size;
> /* 0x000c      |  0x0004 */    int _expression_stack_size;
> /* 0x0010      |  0x0002 */    unsigned short _bci;
>                              protected:
> /* XXX  2-byte hole      */
> /* 0x0014      |  0x0004 */    int _num_oops;
> /* 0x0018      |  0x0020 */    intptr_t _bit_mask[4];
> 
>                                /* total size (bytes):   56 */
>                              }
> 
> 
> (Also moved ```bool _resource_allocate_bit_mask``` to before ```intptr_t _bit_mask[4];```, so it will use 1 byte of the 2-byte hole, resulting in 1 byte hole in fastdebug build.)
> 
> Layout in fastdebug build:
> 
> (gdb) ptype /xo InterpreterOopMap
> /* offset      |    size */  type = class InterpreterOopMap : private ResourceObj {
>                              private:
> /* 0x0000      |  0x0008 */    class Method *_method;
> /* 0x0008      |  0x0004 */    int _mask_size;
> /* 0x000c      |  0x0004 */    int _expression_stack_size;
> /* 0x0010      |  0x0002 */    unsigned short _bci;
>                              protected:
> /* 0x0012      |  0x0001 */    bool _resource_allocate_bit_mask;
> /* XXX  1-byte hole      */
> /* 0x0014      |  0x0004 */    int _num_oops;
> /* 0x0018      |  0x0020 */    intptr_t _bit_mask[4];
> 
>                            ...

This pull request has now been integrated.

Changeset: fac74b11
Author:    Xiaolong Peng <xpeng at openjdk.org>
Committer: David Holmes <dholmes at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/fac74b118f5fda4ec297e46238d34ce5b9be1e21
Stats:     10 lines in 1 file changed: 5 ins; 5 del; 0 mod

8334229: Optimize InterpreterOopMap layout

Reviewed-by: coleenp, dholmes

-------------

PR: https://git.openjdk.org/jdk/pull/19979