[llvm] r347917 - [DAGCombiner] narrow truncated binops

[David Jones via llvm-commits]

Sadly, I don't think r348508 fixes the issue. I still see the large memory
growth. I collected some more traces from the allocator, and it looks like
there is still quite a bit of allocation happening
in hoistLogicOpWithSameOpcodeHands, but most of the growth is actually
coming from further up the stack (SelectionDAG::Combine).

On Thu, Dec 6, 2018 at 10:27 AM Sanjay Patel <spatel at rotateright.com> wrote:

> 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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