[llvm] r347917 - [DAGCombiner] narrow truncated binops

Fri Dec 7 08:08:08 PST 2018

I added a debug flag to disable this transform by default, so I can more
easily investigate the bug:
https://reviews.llvm.org/rL348604

On Fri, Dec 7, 2018 at 8:08 AM Sanjay Patel <spatel at rotateright.com> wrote:

> Thanks. Yes, I agree - now that we have a repro test and know it's not
> limited to PPC, we should revert. I'll do that shortly.
>
> On Fri, Dec 7, 2018 at 5:30 AM Hans Wennborg via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
>> I just bisected an x86 Android Chrome build failure to this revision,
>> so it's not PPC specific.
>>
>> I've uploaded a repro here:
>> https://bugs.chromium.org/p/chromium/issues/detail?id=912878#c3
>>
>> Before this change, the test compiles in about 20 s on my machine.
>> After the change, it runs for several minutes and then seems to hit an
>> assert in SmallVector (due to out-of-memory I suppose).
>>
>> Since we now have a non-PPC test case that clearly shows this is a
>> regression, I think the change should be reverted until it's been
>> investigated.
>>
>> On Fri, Dec 7, 2018 at 2:01 AM Nemanja Ivanovic via llvm-commits
>> <llvm-commits at lists.llvm.org> wrote:
>> >
>> > I am sorry that I am jumping into this so late and I don't know how it
>> is that I am missing some of the original context. At some point there were
>> mentions of a PPC specific problem. However this doesn't seem to be the
>> case at this point. If the group on this thread still feels that there is a
>> PPC specific component to this, please let me know how to reproduce and I
>> will gladly do the PPC investigation.
>> >
>> > On Dec 6, 2018 7:26 PM, "Craig Topper" <craig.topper at gmail.com> wrote:
>> >
>> > DAGCombiner::Run makes sure all nodes are visited at least once. It
>> does this by checking the operands for everything that's visited. So if the
>> node that's being added to the worklist was created just before the add to
>> worklist then I don't think it needs to be explicitly added.
>> >
>> > ~Craig
>> >
>> >
>> > On Thu, Dec 6, 2018 at 4:13 PM Sanjay Patel <spatel at rotateright.com>
>> wrote:
>> >>
>> >> Ok...there are still some seemingly obvious bugs in
>> hoistLogicOpWithSameOpcodeHands(), so I'm going to keep trying to kill
>> those.
>> >>
>> >> But I've hit a couple of things I don't understand:
>> >> 1. Does that code need to explicitly call AddToWorklist()?
>> >> 2. How did this happen?
>> >> https://reviews.llvm.org/rL348552
>> >>
>> >> On Thu, Dec 6, 2018 at 3:52 PM David Jones <dlj at google.com> wrote:
>> >>>
>> >>> 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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