RFR: 8373794: Move nmethod header from CodeCache

[Dean Long]

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.

Wouldn't it be better to move the whole CodeBlob header out of the code cache?  Instead of nmethod having a _hdr pointer, CodeBlob would be in malloc space and have a _code pointer into the CodeCache.

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PR Comment: https://git.openjdk.org/jdk/pull/28866#issuecomment-3668010082