RFR: 8329433: Reduce nmethod header size [v3]

[Dean Long]

On Mon, 15 Apr 2024 03:24:07 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 one additional commit since the last revision:
> 
>   Union fields which usages do not overlap

src/hotspot/share/code/nmethod.cpp line 1235:

> 1233:     int skipped_insts_size   = code_buffer->total_skipped_instructions_size();
> 1234: #ifdef ASSERT
> 1235:     assert(((skipped_insts_size >> 16) == 0), "size is bigger than 64Kb: %d", skipped_insts_size);

Suggestion:


I think it's simpler just to use checked_cast below.

src/hotspot/share/code/nmethod.cpp line 1240:

> 1238:     int consts_offset = code_buffer->total_offset_of(code_buffer->consts());
> 1239:     assert(consts_offset == 0, "const_offset: %d", consts_offset);
> 1240: #endif

Suggestion:

src/hotspot/share/code/nmethod.cpp line 1241:

> 1239:     assert(consts_offset == 0, "const_offset: %d", consts_offset);
> 1240: #endif
> 1241:     _skipped_instructions_size = (uint16_t)skipped_insts_size;

Suggestion:

    _skipped_instructions_size = checked_cast<uint16_t>(code_buffer->total_skipped_instructions_size());

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

PR Review Comment: https://git.openjdk.org/jdk/pull/18768#discussion_r1566764300
PR Review Comment: https://git.openjdk.org/jdk/pull/18768#discussion_r1566765068
PR Review Comment: https://git.openjdk.org/jdk/pull/18768#discussion_r1566759786