RFR: 8273392: Improve usability of stack-less exceptions due to -XX:+OmitStackTraceInFastThrow

[Richard Reingruber]

On Tue, 7 Sep 2021 15:25:46 GMT, Volker Simonis <simonis at openjdk.org> wrote:

> If running with `-XX:+OmitStackTraceInFastThrow` (which is the default) C2 will optimize certain "hot" implicit exceptions (i.e. AIOOBE, NullPointerExceptions,..) and replace them by a static, pre-allocated exception without any stacktrace.
> 
> However, we can actually do better. Instead of using a single, pre-allocated exception object for all methods we can let the compiler allocate specific exceptions for each compilation unit (i.e. nmethod) and fill them with at least one stack frame with the method /line-number information of the currently compiled method. If the method in question is being inlined (which often happens), we can add stackframes for all callers up to the inlining depth of the method in question.
> 
> For the attached JTreg test, we get the following exception in interpreter mode:
> 
> java.lang.NullPointerException: Cannot read the array length because "<parameter2>" is null
>         at compiler.exceptions.StackFrameInFastThrow.throwImplicitException(StackFrameInFastThrow.java:76)
>         at compiler.exceptions.StackFrameInFastThrow.level2(StackFrameInFastThrow.java:95)
>         at compiler.exceptions.StackFrameInFastThrow.level1(StackFrameInFastThrow.java:99)
>         at compiler.exceptions.StackFrameInFastThrow.main(StackFrameInFastThrow.java:233)
> 
> Once the method gets compiled with `-XX:+OmitStackTraceInFastThrow` the same exception will look as follows:
> 
> java.lang.NullPointerException
> 
> After this change, if `StackFrameInFastThrow.throwImplicitException()` will be compiled stand alone, we will get:
> 
> java.lang.NullPointerException
>         at compiler.exceptions.StackFrameInFastThrow.throwImplicitException(StackFrameInFastThrow.java:76)
> 
> and if `StackFrameInFastThrow.throwImplicitException()` will be inlined into `level2()` and `level2()` into `level1()` we will get the following exception (altough we're still running with `-XX:+OmitStackTraceInFastThrow`):
> 
> java.lang.NullPointerException
>         at compiler.exceptions.StackFrameInFastThrow.throwImplicitException(StackFrameInFastThrow.java:76)
>         at compiler.exceptions.StackFrameInFastThrow.level2(StackFrameInFastThrow.java:95)
>         at compiler.exceptions.StackFrameInFastThrow.level1(StackFrameInFastThrow.java:99)
> 
> The new functionality is guarded by `-XX:+/-StackFrameInFastThrow`, but switched on by default (I'll create a CSR for the new option once reviewers are comfortable with the change). Notice that the optimization comes at no run-time costs because all the extra work will be done at compile time.
> 
> ## Implementation details
> 
> - Already the current implementation of `-XX:+OmitStackTraceInFastThrow` potentially lazy-allocates the empty singleton exceptions like AIOOBE in `ciEnv::ArrayStoreException_instance()`. With this change, if running with `-XX:+StackFrameInFastThrow` we will always allocate new exception objects and populate them with the stack frames which are statically available at compile time (see `java_lang_Throwable::fill_in_stack_trace_of_implicit_exception()`).
> - Because nmethods don't act as strong GC roots, we have to create a global JNI handle for every newly generated exception to prevent GC from collecting them.
> - In order to avoid a memory leak we have to release these global JNI handles once a nmethod gets unloaded. In order to achieve this, I've added a new section "implicit exceptions" to the nmethod which holds these JNI handles.
> - While adding the new  "implicit exceptions" section to the corresponding stats (`print_nmethod_stats()` and printing routines (`nmethod::print()`) I realized that a previous change ([JDK-8254231: Implementation of Foreign Linker API (Incubator)](https://bugs.openjdk.java.net/browse/JDK-8254231)) had already introduced a new nmethod section ("native invokers") but missed to add it to the corresponding stats and printing routines so I've added that section as well.
> - The `#ifdef COMPILER2` guards are only required to not break the `zero`/`minimal` builds.
> - The JTreg test is using `-XX:PerMethodTrapLimit=0` to handle all implicit exceptions as "hot". This makes the test simpler and at the same time provokes the allocation of more implicit exceptions.
> - Manually verified that the created Exception objects are freed by GC once the corresponding nmethods have been flushed.
> - Manual "stress" test with a very small heap and continuous recompilation of methods with explicit exceptions to provoke GCs during compilation didn't reveal any issues.

Hi Volker,

> 
> _Mailing list message from [Volker Simonis](mailto:volker.simonis at gmail.com) on [hotspot-dev](mailto:hotspot-dev at mail.openjdk.java.net):_
> 
> Hi Richard,
> 
> thanks a lot for looking into this change.
> 
> Nmethod unloading does still work with this change, just take a look
> at the associated JTreg test which compiles and then unloads a method
> with a generated implicit exception.

Yes it works in your test because you explicitly make the compiled method not
entrant. Think of another test where a nmethod would be unloaded because the
corresponding classloader isn't reachable anymore. The change prevents this
because the loader will be kept alive by the preallocated exception if one exists.

A test with a class leak would repeatedly create a loader, c2 compile a method
with preallocated exception that was loaded by the loader and then drop the
reference to the classloader. All the loaders would be kept alive by the
preallocated exceptions.

> Once the nmethod has been
> unloaded, the global JNI handle will be released and the class can be
> unloaded as well. But I agree that it might be too late and class
> unloading shouldn't depend on unloading of all nmethods which
> reference that class. I'll have a look if I can fix that somehow.
> 
> Best regards,
> Volker
> 
> On Mon, Sep 20, 2021 at 10:53 AM Richard Reingruber
> <rrich at openjdk.java.net> wrote:

Cheers, Richard.

-------------

PR: https://git.openjdk.java.net/jdk/pull/5392