RFR: 8360776: Disable Intel APX by default and enable it only if requested by the user using -XX:+UnlockExperimentalVMOptions -XX:+UseAPX

[David Holmes]

On Fri, 27 Jun 2025 22:13:47 GMT, Srinivas Vamsi Parasa <sparasa at openjdk.org> wrote:

> Currently, APX is not enabled consistently between product and debug builds.
> 
> If the hardware supports Intel APX:
> 
> 1) In product builds, APX is disabled by default, even if the user explicitly enables it using `-XX:+UnlockExperimentalVMOptions -XX:+UseAPX`.
> 
> 2) In debug builds, APX is enabled by default regardless of whether the user explicitly enables it or not.
> 
> **The goal of this PR is to enable APX for both product and debug builds if and only if the user explicitly enables it using `-XX:+UnlockExperimentalVMOptions -XX:+UseAPX`.**

src/hotspot/cpu/x86/vm_version_x86.cpp line 3155:

> 3153:   }
> 3154:   // Enable APX support for product builds after
> 3155:   // completion of planned features listed in JDK-8329030.

So you have decided not to follow the original plan ( as JDK-8329030 is not complete) and instead go ahead and enable APX in product mode now. Why? Was this discussed anywhere?

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PR Review Comment: https://git.openjdk.org/jdk/pull/26029#discussion_r2174355658