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

Jonas Devlieghere via cfe-dev cfe-dev at lists.llvm.org
Wed Oct 4 11:08:25 PDT 2017 

Hi Steve,

I had a quick look and with the help of creduce I was able to reproduce this with https://reviews.llvm.org/P8041

It looks like this is already fixed upstream and in the swift-4.1-branch. 

Cheers,
Jonas

> On Oct 4, 2017, at 9:40 AM, mats petersson via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> 
> 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 <mailto: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 <mailto: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 <mailto: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::MachineFunctionPass::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::ExecuteCompilerInvocation(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 <mailto: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 <mailto: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 <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
> >
> >
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> >
> 
> 
> 
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