RFR: 8373794: Move nmethod header from CodeCache [v2]

[Andrew Haley]

On Wed, 18 Feb 2026 17:02:39 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.
>
> Mikhail Ablakatov has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:
> 
>  - cleanup: remove redundant comments
>  - Merge branch 'master' at a86a847f57a
>  - 8373794: Move nmethod header from CodeCache:
>    
>    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.

Can you check that `nm(addr)` still works with both an nmethod address and a code pointer? Thanks.

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PR Review: https://git.openjdk.org/jdk/pull/28866#pullrequestreview-3848151346