RFR: 8373794: Move nmethod header from CodeCache

[Andrew Haley]

On Wed, 17 Dec 2025 11:34:46 GMT, Mikhail Ablakatov <mablakatov at openjdk.org> wrote:

> nmethod objects in the CodeCache have the following layout:
> 
> 
> | CodeBlob header | NMethod header | Constants | MainCode | StubCode | Data/oops |
> 
> 
> Although mutable and immutable metadata have already been moved out of the code cache by JDK-8343789 and JDK-8331087 respectively, the embedded `nmethod` header fields still occupy ~160 B (with the `CodeBlob` header adding another 64 B). In JDK25 the total header footprint is 224 B. This space is reserved inside executable memory, which decreases overall executable code density.
> 
> This patch relocates the `nmethod` header to a C-heap-allocated structure and keeps only 8-byte pointer to that header in the CodeCache. The resulting layout is:
> 
> 
> | CodeBlob header | Ptr to NMethodHeader | Constants | MainCode | StubCode | Data/oops |
> 
> 
> This change reduces the size of the CodeCache-resident header from 224 B to 72 B (64 B `CodeBlob` header + 8 B pointer), achieving roughly a **3x reduction** in header footprint.
> 
> This change follows the direction established by JDK-7072317, JDK-8331087 and JDK-8343789.
> 
> ## Testing
> 
> The patch has passed `tier1-3` and `hotspot_all` tests on AArch64 and x86_64.

src/hotspot/share/code/nmethod.cpp line 487:

> 485: void nmethod::set_deoptimized_done() {
> 486:   ConditionalMutexLocker ml(NMethodState_lock, !NMethodState_lock->owned_by_self(), Mutex::_no_safepoint_check_flag);
> 487:   if (_hdr->_deoptimization_status != NMethodHeader::deoptimize_done) { // can't go backwards

Suggestion:

  if (deoptimization_status() != deoptimize_done) { // can't go backwards

Create an accessor for `deoptimization_status`. Why is it now necessary to use `NMethodHeader::`?

src/hotspot/share/code/nmethod.cpp line 1301:

> 1299:     _hdr->_compiler_type           = type;
> 1300:     _hdr->_orig_pc_offset          = 0;
> 1301:     _hdr->_num_stack_arg_slots     = 0;

It'd be better to move this into a separate initializer for `NMethodHeader`.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/28866#discussion_r2731224628
PR Review Comment: https://git.openjdk.org/jdk/pull/28866#discussion_r2731235594