[llvm] r347917 - [DAGCombiner] narrow truncated binops

[David Jones via llvm-commits]

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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