RFR: 8346433: Cannot use DllMain in hotspot for static builds

[Magnus Ihse Bursie]

On Tue, 17 Dec 2024 13:48:59 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> To be able to properly support static builds on Windows in [JDK-8346377](https://bugs.openjdk.org/browse/JDK-8346377), we cannot use DllMain, for two reasons:
> 
> 1) This is not called for statically linked libraries, and
> 2) There are multiple DllMain definitions throughout the JDK native libraries, causing name collisions.
> 
> While it could have been possible to keep the DllMain function for non-static builds and just use an alternative solution for static builds, I think it is preferable to have a single solution that works as well for both static and dynamic builds.
> 
> The DllMain in hotspot is doing work both at DLL load time, and at DLL unload time. Let's go through them to see why this patch is safe.
> 
> During DLL load time, the library handle is set, and the hi-res timer is initialized. The `pre_initialize` method from `WindowsDbgHelp` and `SymbolEngine` is called. These two are basically identical; both setup a critical section (a Windows mutex). However, this mutex is only used from a stack local guard object, which is allocated only when calls are made into these classes, which are not happening at bootstrapping time, so this shift in initialization time is harmless.
> 
> That shift in time is also very small. The `DllMain` method is called by Windows when the DLL is loaded (by libjli), and the very first thing that JLI does after that is to call `JNI_CreateJavaVM`, which ends up calling  `Threads::create_vm`, which calls `os::init` early on.

So you'd prefer to have a duplicate this code, conditional on that we're running a static build?

What is this problem with ForceTimeHighResolution you are referring to? There is no incompatibility problem with the static launcher (since it has not existed), but it would be nice to avoid building into some bad behavior, if we can avoid that.

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PR Comment: https://git.openjdk.org/jdk/pull/22793#issuecomment-2550008058