RFR: 8329433: Reduce nmethod header size [v6]

[Cesar Soares Lucas]

On Wed, 17 Apr 2024 00:56:33 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> This is part of changes which try to reduce size of `nmethod` and `codeblob` data vs code in CodeCache.
>> These changes reduced size of `nmethod` header from 288 to 232 bytes. From 304 to 248 in optimized VM:
>> 
>> Statistics for 1282 bytecoded nmethods for C2:
>>  total in heap = 5560352 (100%)
>>  header = 389728 (7.009053%)
>> 
>> vs
>> 
>> Statistics for 1322 bytecoded nmethods for C2:
>>  total in heap  = 8307120 (100%)
>>  header         = 327856 (3.946687%)
>> 
>> 
>> Several unneeded fields in `nmethod` and `CodeBlob` were removed. Some fields were changed from `int` to `int16_t` with added corresponding asserts to make sure their values are fit into 16 bits.
>> 
>> I did additional cleanup after recent `CompiledMethod` removal.
>> 
>> Tested tier1-7,stress,xcomp and performance testing.
>
> Vladimir Kozlov has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - remove trailing space
>  - Shuffle fields initialization

src/hotspot/share/code/nmethod.hpp line 259:

> 257:   int _orig_pc_offset;
> 258: 
> 259:   int          _compile_id;            // which compilation made this nmethod

NIT: are these fields always needed?

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

PR Review Comment: https://git.openjdk.org/jdk/pull/18768#discussion_r1569185473