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/codeBlob.cpp line 88:

> 86:   S390_ONLY(_ctable_offset(0) COMMA)
> 87:   _header_size((uint16_t)header_size),
> 88:   _frame_complete_offset((int16_t)frame_complete_offset),

Rather than a raw cast, it would be better to use checked_cast here, or better yet, change the incoming parameter types to match the field type.  That way, if the caller is passing a constant, the compiler can check it at compile time.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18768#discussion_r1566601934