ReduceAllocationMerges patch causes miscompiles

[IThundxr]

Describe the bug
A clear and concise description of what the bug is.

Desktop (please complete the following information):

OS & version [e.g. Windows 10, 1822]:
Windows - Unknown versions

MSFT Build of OpenJDK Version:
17.0.8+7

Screenshots, Logs etc
The following gist contains multiple jvm crash logs, and multiple people who've encountered this issue have verified that adding -XX:-ReduceAllocationMerges to their jvm args does indeed fix this.
https://gist.github.com/IThundxr/508a9c4e57ef4956777ac8d69a67f525

Additional context
Providing reproduction instructions for this isn't really possible, seems to only happen on specific machines with the common denominator being that everyone is using microsoft's build of openjdk.

Although all the logs are for minecraft this issue likely occurs outside it aswell in specific situations, i'm just unable to get a small test case setup for it as i've never managed to reproduce this issue on my own machine.

ReduceAllocationMerges has been disabled in openjdk due to causing a number of issues, and it ends up resulting in miscompiles which causes the issues in the attached logs, this issue likely affects the java 21 build aswell.

I propose that ReduceAllocationMerges is disabled by default and requires explicitly opting into it.

[karianna]

@brianjstafford & @brunoborges - IIRC we did disable this by default in a later 17.0.x build, but checking with you to confirm.

[brianjstafford]

Hey @IThundxr - thanks for the report and the suggestion. Although this optimization was disabled by default in JDK 22, it was re-enabled by default in JDK 23 onwards. It was disabled briefly due to some bugs that were found late in the JDK 22 release period which couldn't be fixed in time. It was re-enabled because it provides some good performance benefits and it will bit-rot if it doesn't get used. There have been some bug fixes in the last few months that we are backporting to the Microsoft Build of OpenJDK for the January release. Hopefully this takes care of the issues you've reported and we'll continue to monitor for additional issues. Please continue to disable the optimization as a work around if you come across any issues after the January release, and provide us with any details you have that will allow us to investigate and fix.

Thank you!