RFR(XS): 8229450: C2 compilation fails with assert(found_sfpt) failed

[Liu Xin]

hi, Tobias,

Thank you for providing this information. I will work on it with this
guideline.

thanks,
--lx



On Mon, Aug 19, 2019 at 11:46 PM Tobias Hartmann <tobias.hartmann at oracle.com>
wrote:

>
> On 19.08.19 10:26, Liu Xin wrote:
> > I still have 2 questions.  Could reviewers help me out?
> >
> > 1. I'd like to  add a testcase,  but JVM won't hit it even though I
> repeat
> > invoke that function thousands of times. I can see that function is
> > compiled by C2, but C2 succeeds to compile it.
> > The only way to reproduce that problem is using replay file. Is that
> > possible to build a testcase from a replay file?  or What kinda of
> > information should I pull out of the replay file?
>
> Generating a simple regression test is a science in itself but such tests
> are extremely valuable.
>
> If a replay compilation file reproduces the failure, I usually attempt the
> following to create a
> simple regression test:
> - Try to simplify the replay compilation file as far as possible by
> removing inline statements
> - Disable as many C1/C2 optimizations [1] as possible so that the issue
> still reproduces (this will
> simplify the graph)
> - Debug the issue using replay compilation until you get a good
> understanding of the root cause and
> the required conditions (inlining, optimistic optimizations based on
> profiling, IR shape, ...)
> - Look at the Java code that is compiled and the way it is
> invoked/profiled. Not being able to
> reproduce with a normal run usually suggests that the profiling is
> different.
> - Write a test that invokes the same method in the same way (same
> arguments, same inlining). Use
> CompileCommands to control inling if necessary. Compare the IR generated
> for your test to the IR
> generated with the replay file (flags like -XX:+TraceLoopOpts also help)
> and modify your test to get
> closer. Sometimes this takes days but it's worth it.
> - Once the failure triggers, simplify the test as good as possible.
> - If you don't have a fix yet, use the simple regression test to debug
> further.
>
> > 2. I found you guys sometimes paste a sub-graph of Ideal nodes in JBS
> > issues. Do you have a script to render a IdealLoopTree? So far, I only
> have
> > idealgraphvisualizer. It renders the whole function.Too big to
> understand.
>
> Do you mean sub-graphs like [2]? That graph is created by the
> IdealGraphVisualizer. You can search
> for nodes or apply filters to find the interesting parts of the usually
> large graph and then click
> on nodes to expand/collapse other parts of the graph.
>
> I usually print the ids of offending nodes by modifying the sources and
> then look at the surrounding
> nodes with the IdealGraphVisualizer or directly at the -XX:+PrintIdeal
> output. Stepping through the
> optimization phases might help as well.
>
> Best regards,
> Tobias
>
> [1] Here are some C2 optimizations you might want to try to disable
> -XX:-OptimizePtrCompare -XX:-OptoPeephole -XX:LoopUnrollLimit=0
> -XX:LoopMaxUnroll=0
> -XX:-SuperWordLoopUnrollAnalysis -XX:-UseCountedLoopSafepoints
> -XX:-UseLoopPredicate
> -XX:-PartialPeelAtUnsignedTests -XX:-LoopUnswitching -XX:-UseSuperWord
> -XX:-SubsumeLoads
> -XX:-OptimizeStringConcat -XX:-SplitIfBlocks -XX:-RangeCheckElimination
> [2] http://cr.openjdk.java.net/~thartmann/8228888/8228888_graph.png
>