[llvm] r347917 - [DAGCombiner] narrow truncated binops

[Sanjay Patel via llvm-commits]

First attempt at damage control in that function:
https://reviews.llvm.org/rL348508

If I've guessed correctly, that's enough to prevent the OOM. Still looking
at fixing that better though.


On Thu, Dec 6, 2018 at 8:12 AM Sanjay Patel <spatel at rotateright.com> wrote:

> #10 0x000055680f8d7466 (anonymous
> namespace)::DAGCombiner::SimplifyBinOpWithSameOpcodeHands(llvm::SDNode*)
>
> Thanks! That's a good lead. I'm looking at that code now, and it doesn't
> appear to check the number of uses before doing transforms. That means it
> can increase the instruction count at each invocation. Let me try some
> experiments in there.
>
> On Wed, Dec 5, 2018 at 7:51 PM David Jones <dlj at google.com> wrote:
>
>>
>>
>> On Wed, Dec 5, 2018 at 6:34 PM Friedman, Eli <efriedma at codeaurora.org>
>> wrote:
>>
>>> On 12/5/2018 5:47 PM, David Jones wrote:
>>>
>>>
>>> On Wed, Dec 5, 2018 at 5:40 PM Sanjay Patel <spatel at rotateright.com>
>>> wrote:
>>>
>>>> Hi David,
>>>>
>>>> Thanks for reporting the problem. I don’t have any guesses as to how
>>>> this could cause OOM. Cc’ing some people that might.
>>>>
>>>
>>> +a few more, thanks! :-)
>>>
>>>
>>>> If the problem is only showing up on PPC, a quick hack would be to add
>>>> a TLI hook and disable the transform for PPC. I’d prefer that to a full
>>>> revert (especially since I’ve already enhanced this code to handle some
>>>> vectors, and another enhancement is in progress)...unless we have a test
>>>> case that shows it’s not a PPC-specific bug?
>>>>
>>>>
>>> Agreed on avoiding a revert... this looks like a pretty hard-fought
>>> change, based on https://bugs.llvm.org/show_bug.cgi?id=32023.
>>>
>>> I do believe this is PPC-specific (although, as is often the case, your
>>> change may simply be tickling a bug somewhere else).
>>>
>>> If it's PPC-specific, my first guess here would be there's some
>>> PowerPC-specific target combine that's trying to combine the opposite way,
>>> or something like that.  Assuming that's the case, it probably runs out of
>>> memory pretty quickly, so it should be fast to bisect if you restrict the
>>> memory usage with ulimit.
>>>
>>
>> So I did try something like this... instead of just ulimit, I patched my
>> allocator to crash when it sees a sufficiently large allocation (so I can
>> see the large allocation itself, not just a random allocation when close to
>> the limit).
>>
>> Here's a stacktrace I got (slightly edited for clarity):
>>
>> 1. <eof> parser at end of file
>> 2. Code generation
>> 3. Running pass 'Function Pass Manager' on module
>> 'FrontendAction-9a7810.cpp'.
>> 4. Running pass 'PowerPC DAG->DAG Pattern Instruction Selection' on
>> function
>> '@_ZN5clang14FrontendAction15BeginSourceFileERNS_16CompilerInstanceERKNS_17FrontendInputFileE'
>> #0 0x000055681055d388 llvm::sys::RunSignalHandlers()
>> #1 0x000055681055f486 SignalHandler(int)
>> #2 0x00007f88bd9f89a0 __restore_rt
>> #3 0x00007f88bd77a602 __GI_raise
>> #4 0x00007f88bd77c320 __GI_abort
>> #5 0x00005568105e2a70 tcmalloc::Logger::Add(tcmalloc::LogItem const&)
>> #6 0x00005568105d5838 (anonymous namespace)::do_malloc_pages(unsigned
>> long, unsigned long)
>> #7 0x000055681068a56d realloc
>> #8 0x000055681055c789 llvm::SmallVectorBase::grow_pod(void*, unsigned
>> long, unsigned long)
>> #9 0x000055680f895119 (anonymous
>> namespace)::DAGCombiner::AddToWorklist(llvm::SDNode*)
>> #10 0x000055680f8d7466 (anonymous
>> namespace)::DAGCombiner::SimplifyBinOpWithSameOpcodeHands(llvm::SDNode*)
>> #11 0x000055680f8a1791 (anonymous
>> namespace)::DAGCombiner::visitAND(llvm::SDNode*)
>> #12 0x000055680f89729b (anonymous
>> namespace)::DAGCombiner::combine(llvm::SDNode*)
>> #13 0x000055680f8960cf llvm::SelectionDAG::Combine(llvm::CombineLevel,
>> llvm::AAResults*, llvm::CodeGenOpt::Level)
>> #14 0x000055680f78cdbc llvm::SelectionDAGISel::CodeGenAndEmitDAG()
>> #15 0x000055680f78bbde
>> llvm::SelectionDAGISel::SelectAllBasicBlocks(llvm::Function const&)
>> #16 0x000055680f788c56
>> llvm::SelectionDAGISel::runOnMachineFunction(llvm::MachineFunction&)
>> #17 0x000055680f418574 (anonymous
>> namespace)::PPCDAGToDAGISel::runOnMachineFunction(llvm::MachineFunction&)
>> #18 0x000055680fa0955b
>> llvm::MachineFunctionPass::runOnFunction(llvm::Function&)
>> #19 0x0000556810478c6c llvm::FPPassManager::runOnFunction(llvm::Function&)
>> #20 0x0000556810478f23 llvm::FPPassManager::runOnModule(llvm::Module&)
>> #21 0x00005568104793f5 llvm::legacy::PassManagerImpl::run(llvm::Module&)
>> #22 0x000055680da6c9ff
>> clang::EmitBackendOutput(clang::DiagnosticsEngine&,
>> clang::HeaderSearchOptions const&, clang::CodeGenOptions const&,
>> clang::TargetOptions const&, clang::LangOptions const&, llvm::DataLayout
>> const&, llvm::Module*, clang::BackendAction,
>> std::unique_ptr<llvm::raw_pwrite_stream,
>> std::default_delete<llvm::raw_pwrite_stream> >)
>> #23 0x000055680da43542
>> clang::BackendConsumer::HandleTranslationUnit(clang::ASTContext&)
>> #24 0x000055680e47c7b3 clang::ParseAST(clang::Sema&, bool, bool)
>> #25 0x000055680e2bc0fa clang::FrontendAction::Execute()
>> #26 0x000055680e142428
>> clang::CompilerInstance::ExecuteAction(clang::FrontendAction&)
>> #27 0x000055680d80b562
>> clang::ExecuteCompilerInvocation(clang::CompilerInstance*)
>> #28 0x000055680d7fab05 cc1_main(llvm::ArrayRef<char const*>, char const*,
>> void*)
>> #29 0x000055680d8088ea main
>> #30 0x00007f88bd766bbd __libc_start_main
>> #31 0x000055680d7fa469 _start
>>
>>
>> (I've been poking at this in a few other ways... the stacks vary
>> somewhat, but they all seem to point to an ever-growing work list
>> in DAGCombiner, inside of PPCDAGToDAGISel -- so #13-#31 seem to be the
>> common parts of the stacks.)
>>
>>>
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