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 1441:

> 1439:     int deps_size     = align_up((int)dependencies->size_in_bytes(), oopSize);
> 1440:     int sum_size      = oops_size + metadata_size + deps_size;
> 1441:     assert((sum_size >> 16) == 0, "data size is bigger than 64Kb: %d", sum_size);

I suggest using checked_cast for the assignment below, rather than special-purpose checks here.

src/hotspot/share/code/nmethod.cpp line 1445:

> 1443:     _metadata_offset      = (uint16_t)oops_size;
> 1444:     _dependencies_offset  = _metadata_offset      + (uint16_t)metadata_size;
> 1445:     _scopes_pcs_offset    = _dependencies_offset  + (uint16_t)deps_size;

Use checked_cast instead of raw casts.

src/hotspot/share/code/nmethod.cpp line 1459:

> 1457:     assert((data_offset() + data_end_offset) <= nmethod_size, "wrong nmethod's size: %d < %d", nmethod_size, (data_offset() + data_end_offset));
> 1458: 
> 1459:     _entry_offset          = (uint16_t)offsets->value(CodeOffsets::Entry);

Use checked_cast.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18768#discussion_r1566771026
PR Review Comment: https://git.openjdk.org/jdk/pull/18768#discussion_r1566772567
PR Review Comment: https://git.openjdk.org/jdk/pull/18768#discussion_r1566773477