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

Tue Oct 3 12:22:59 PDT 2017

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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