RFR: 8334229: Optimize InterpreterOopMap layout [v2]

[Xiaolong Peng]

> 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];
> 
>                            ...

Xiaolong Peng has updated the pull request incrementally with one additional commit since the last revision:

  Move _resource_allocate_bit_mask to the place before _num_oops for better layout in debug build

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/19979/files
  - new: https://git.openjdk.org/jdk/pull/19979/files/a6deded4..db1170ac

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19979&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19979&range=00-01

  Stats: 4 lines in 1 file changed: 1 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/19979.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19979/head:pull/19979

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