RFR: 8343789: Move mutable nmethod data out of CodeCache

[Dean Long]

On Tue, 1 Oct 2024 02:10:37 GMT, Boris Ulasevich <bulasevich at openjdk.org> wrote:

> This change relocates mutable data (such as relocations, oops, and metadata) from the nmethod. The change follows the recent PR #18984, which relocated immutable nmethod data from the CodeCache.
> 
> The core idea remains the same: use the CodeCache for executable code while moving additional data to the C heap. The primary motivations are improving security and enhancing code density.
> 
> Although performance is not the main focus, testing on AArch64 CPUs, where code density plays a significant role, has shown a 1–2% performance improvement in specific scenarios, such as the CodeCacheStress test and the Renaissance Dotty benchmark.
> 
> The numbers. Immutable data constitutes **~30%** on the nmehtod. Mutable data constitutes **~8%** of nmethod. Example (statistics collected on the CodeCacheStress benchmark):
> - nmethod_count:134000, total_compilation_time: 510460ms
> - total allocation time malloc_mutable/malloc_immutable/CodeCache_alloc: 62ms/114ms/6333ms,
> - total allocation size (mutable/immutable/nmentod): 64MB/192MB/488MB
> 
> Functional testing: jtreg on arm/aarch/x86.
> Performance testing: renaissance/dacapo/SPECjvm2008 benchmarks.
> 
> Alternative solution (see comments): In the future, relocations can be moved to _immutable_data.

src/hotspot/share/code/codeBlob.hpp line 135:

> 133:   CodeBlob(const char* name, CodeBlobKind kind, CodeBuffer* cb, int size, uint16_t header_size,
> 134:            int16_t frame_complete_offset, int frame_size, OopMapSet* oop_maps, bool caller_must_gc_arguments,
> 135:            int mutable_data_size = 0);

If we want to allow the default for mutable data size to be the relocations size, then instead of using = 0 here, you could do this instead:

 CodeBlob(const char* name, CodeBlobKind kind, CodeBuffer* cb, int size, uint16_t header_size,
           int16_t frame_complete_offset, int frame_size, OopMapSet* oop_maps, bool caller_must_gc_arguments,
           int mutable_data_size);

 CodeBlob(const char* name, CodeBlobKind kind, CodeBuffer* cb, int size, uint16_t header_size,
           int16_t frame_complete_offset, int frame_size, OopMapSet* oop_maps, bool caller_must_gc_arguments) : 
    CodeBlob(name, kind, cb, size, header_size,
           frame_complete_offset, frame_size, oop_maps, caller_must_gc_arguments,
           cb->total_relocation_size)
{
}

but I would prefer not to treat relocations as special, and have the caller always pass the correct value.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21276#discussion_r1853214675