[cfe-dev] How to report bug when compiler hangs

mats petersson via cfe-dev cfe-dev at lists.llvm.org
Wed Oct 4 01:40:20 PDT 2017 

The size of the source is not in itself indicative of the overall
complexity [or simplicity, as I believe large basic-blocks is the main
culprit due to something along the lines of "for each instruction I in BB {
for each instruction J (except I) in BB { do something } }"] - it's easy to
produce long sequences with for example a simple macro that repeats some
code.

However, 40 minutes is rather a long time, so I'd say you probably have a
hang.

--
Mats

On 3 October 2017 at 20:22, Steven Peters <scpeters at openrobotics.org> wrote:

> Mats,
>
> That's a good point. I can't prove it's an infinite loop. Compiling with
> `-O1` or `-march=native` allows it to complete in less than 0.2 seconds
> with an object file size of 12 kB. I just tested the problematic case again
> and let it go for 40 minutes before I stopped it. I'll try running it a
> little longer on a computer I don't need to use, but it's not guaranteed to
> fix it I guess.
>
> It's tough to instrospect without debug symbols in the compiler toolchain.
>
> Steve
>
> On Tue, Oct 3, 2017 at 3:17 AM, mats petersson <mats at planetcatfish.com>
> wrote:
>
>> Are you absolutely sure it hangs, and not just "taking a very long time"?
>> I've hit the O(n^2) type problem with instruction selection, so if you have
>> some code that produces very long basic-blocks, it could take ages to
>> compile them - and the call-stack in that case will look very much like the
>> one you posted.
>>
>> I'm not saying you that your case isn't a genuine bug, I'm just asking so
>> that the "right" problem is reported.
>>
>> --
>> Mats
>>
>> On 2 October 2017 at 21:03, Steven Peters via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>
>>> Thanks for the idea. It looks like another infinite loop in DAG Combine
>>> (like r206873
>>> <http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20140421/213744.html>
>>> and D26605 <https://reviews.llvm.org/D26605>) when I compile that file
>>> with -O2 and for the core2 target cpu. Is there any advice on introspecting
>>> SelectionDAG? I'm not sure how to identify which part of the source code is
>>> triggering the infinite loop.
>>>
>>> example stacktrace:
>>> ~~~
>>> (lldb) thread backtrace all
>>> * thread #1, queue = 'com.apple.main-thread', stop reason = signal
>>> SIGSTOP
>>>   * frame #0: 0x00000001079c39d1 clang`llvm::SelectionDAG::Combine(llvm::CombineLevel,
>>> llvm::AAResults&, llvm::CodeGenOpt::Level) + 1441
>>>     frame #1: 0x00000001079c242e clang`llvm::SelectionDAGISel::CodeGenAndEmitDAG()
>>> + 974
>>>     frame #2: 0x000000010790f726 clang`llvm::SelectionDAGISel::
>>> SelectAllBasicBlocks(llvm::Function const&) + 6118
>>>     frame #3: 0x0000000107907846 clang`llvm::SelectionDAGISel::
>>> runOnMachineFunction(llvm::MachineFunction&) + 998
>>>     frame #4: 0x000000010829cd54 clang`(anonymous
>>> namespace)::X86DAGToDAGISel::runOnMachineFunction(llvm::MachineFunction&)
>>> + 20
>>>     frame #5: 0x000000010790590d clang`llvm::MachineFunctionPas
>>> s::runOnFunction(llvm::Function&) + 125
>>>     frame #6: 0x00000001078c2562 clang`llvm::FPPassManager::runOnFunction(llvm::Function&)
>>> + 498
>>>     frame #7: 0x00000001078eb943 clang`llvm::FPPassManager::runOnModule(llvm::Module&)
>>> + 67
>>>     frame #8: 0x00000001078c67f5 clang`llvm::legacy::PassManagerImpl::run(llvm::Module&)
>>> + 693
>>>     frame #9: 0x000000010789e09c clang`clang::EmitBackendOutput(clang::DiagnosticsEngine&,
>>> clang::HeaderSearchOptions const&, clang::CodeGenOptions const&,
>>> clang::TargetOptions const&, clang::LangOptions const&, llvm::DataLayout
>>> const&, llvm::Module*, clang::BackendAction, std::__1::unique_ptr<llvm::raw_pwrite_stream,
>>> std::__1::default_delete<llvm::raw_pwrite_stream> >) + 2700
>>>     frame #10: 0x000000010786f00a clang`clang::BackendConsumer::
>>> HandleTranslationUnit(clang::ASTContext&) + 410
>>>     frame #11: 0x000000010754b249 clang`clang::ParseAST(clang::Sema&,
>>> bool, bool) + 249
>>>     frame #12: 0x00000001075482bc clang`clang::FrontendAction::Execute()
>>> + 44
>>>     frame #13: 0x00000001074fe666 clang`clang::CompilerInstance:
>>> :ExecuteAction(clang::FrontendAction&) + 294
>>>     frame #14: 0x00000001074fc7b9 clang`clang::ExecuteCompilerIn
>>> vocation(clang::CompilerInstance*) + 2329
>>>     frame #15: 0x00000001074c6974 clang`cc1_main(llvm::ArrayRef<char
>>> const*>, char const*, void*) + 1492
>>>     frame #16: 0x00000001074c24ac clang`main + 13820
>>>     frame #17: 0x00007fff6206b145 libdyld.dylib`start + 1
>>>     frame #18: 0x00007fff6206b145 libdyld.dylib`start + 1
>>> ~~~
>>>
>>> Regards,
>>> Steve
>>>
>>> On Fri, Sep 29, 2017 at 12:57 AM, Csaba Raduly <rcsaba at gmail.com> wrote:
>>>
>>>> Hi Steven,
>>>>
>>>> If the compiler hangs, try to attach the debugger (gdb --pid). You
>>>> should be able to get a stack trace, which would carry more
>>>> information.
>>>>
>>>> On Thu, Sep 28, 2017 at 10:03 PM, Steven Peters via cfe-dev
>>>> <cfe-dev at lists.llvm.org> wrote:
>>>> > Hello,
>>>> >
>>>> > I've been digging into a compilation problem using the version of
>>>> clang
>>>> > distrubuted with XCode 9 on macOS High Sierra. I narrowed the problem
>>>> down
>>>> > to the transition from `-O1` to `-O2` for a target cpu of core2, and
>>>> then I
>>>> > further narrowed it down to the `-vectorize-loops` option for clang.
>>>> The
>>>> > challenge is that this problem manifests with clang failing to
>>>> terminate. I
>>>> > don't know if it's a deadlock or getting stuck in an infinite loop,
>>>> but I
>>>> > didn't see any guidance in the support documents about how to submit
>>>> a bug
>>>> > report in a case like this.
>>>> >
>>>> > Current discussion of the compilation failure:
>>>> > https://github.com/bulletphysics/bullet3/issues/1347
>>>> >
>>>> > ~~~
>>>> > $ cc --version
>>>> > Apple LLVM version 9.0.0 (clang-900.0.37)
>>>> > Target: x86_64-apple-darwin17.0.0
>>>> > Thread model: posix
>>>> > InstalledDir: /Library/Developer/CommandLineTools/usr/bin
>>>> > ~~~
>>>> >
>>>> > Thanks for your time.
>>>> >
>>>> > Regards,
>>>> > Steve Peters
>>>> >
>>>> >
>>>> > _______________________________________________
>>>> > cfe-dev mailing list
>>>> > cfe-dev at lists.llvm.org
>>>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>> >
>>>>
>>>>
>>>>
>>>> --
>>>> GCS a+ e++ d- C++ ULS$ L+$ !E- W++ P+++$ w++$ tv+ b++ DI D++ 5++
>>>> The Tao of math: The numbers you can count are not the real numbers.
>>>> Life is complex, with real and imaginary parts.
>>>> "Ok, it boots. Which means it must be bug-free and perfect. " -- Linus
>>>> Torvalds
>>>> "People disagree with me. I just ignore them." -- Linus Torvalds
>>>>
>>>
>>>
>>> _______________________________________________
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>>>
>>>
>>
>
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