Withdrawn: 8273392: Improve usability of stack-less exceptions due to -XX:+OmitStackTraceInFastThrow

duke duke at openjdk.java.net
Thu Nov 25 10:50:10 UTC 2021


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.

This pull request has been closed without being integrated.

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


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