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

[Jatin Bhateja]

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`.**

Verified patch with the following configurations, we now enable APX only on APX-capable targets with -XX:+UnlockExperimentalVMOptions


CPROMPT>sde64 -dmr -ptr_raise -- java -XX:+PrintFlagsFinal -XX:+UnlockExperimentalVMOptions -XX:-UseAPX --version | grep UseAPX
     bool UseAPX                                   = false                           {ARCH experimental} {command line}
CPROMPT>sde64 -dmr -ptr_raise -- java -XX:+PrintFlagsFinal -XX:+UnlockExperimentalVMOptions -XX:+UseAPX --version | grep UseAPX
     bool UseAPX                                   = true                            {ARCH experimental} {command line}
CPROMPT>sde64 -dmr -ptr_raise -- java -XX:+PrintFlagsFinal -XX:+UnlockExperimentalVMOptions  --version | grep UseAPX
     bool UseAPX                                   = false                           {ARCH experimental} {default}
CPROMPT>sde64 -gnr -ptr_raise -- java -XX:+PrintFlagsFinal -XX:+UnlockExperimentalVMOptions -XX:+UseAPX --version | grep UseAPX
OpenJDK 64-Bit Server VM warning: UseAPX is not supported on this CPU, setting it to false
     bool UseAPX                                   = false                           {ARCH experimental} {command line}
CPROMPT>sde64 -gnr -ptr_raise -- java -XX:+PrintFlagsFinal -XX:+UnlockExperimentalVMOptions -XX:-UseAPX --version | grep UseAPX
     bool UseAPX                                   = false                           {ARCH experimental} {command line}

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PR Comment: https://git.openjdk.org/jdk/pull/26029#issuecomment-3015108073