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/memory/heap.hpp line 58:

> 56:   void set_length(size_t length)                 {
> 57:     LP64_ONLY( assert(((length >> 32) == 0), "sanity"); )
> 58:     _header._length = (uint32_t)length;

Suggestion:

    _header._length = checked_cast<uint32_t>length;

src/hotspot/share/memory/heap.hpp line 63:

> 61:   // Accessors
> 62:   void* allocated_space() const                  { return (void*)(this + 1); }
> 63:   size_t length() const                          { return (size_t)_header._length; }

This cast looks unnecessary.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18768#discussion_r1566784458
PR Review Comment: https://git.openjdk.org/jdk/pull/18768#discussion_r1566784587