[llvm-dev] Discrepancy between Debug and Release+Asserts versions of Clang/LLVM

[Craig Topper via llvm-dev]

I agree that the ArrayRef is likely the issue. I've debugged a crash caused
by a temporary ArrayRef like that a couple times.

Either do what David suggested or use a normal array:

Metadata *mdArray[] =  {ConstantInt::get(Int64Ty, 0), newMD};

~Craig


On Thu, Apr 30, 2020 at 9:56 AM David Blaikie via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
>
> On Thu, Apr 30, 2020 at 9:51 AM Shishir V Jessu via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hello,
>>
>> I am editing the LowerTypeTests pass in LLVM, and part of my additions
>> include the following 3 lines of code:
>>
>> // newTypeName is a std::string
>> MDString* newMD = MDString::get(M.getContext(), newTypeName);
>> ArrayRef<Metadata*> mdArray {ConstantInt::get(Int64Ty, 0), newMD};
>> auto* node = MDTuple::get(M.getContext(), mdArray);
>>
>
> If I had to hazard a guess, it's that mdArray is left with dangling
> pointers to an array (initializer list) that ceases to exist once the ';'
> on line 2 is reached. So when the elements the ArrayRef references are
> accessed, it gets crashy.
>
> I'd say try writing it like:
>
> auto *node = MDTuple::get(M.getContext(), {ConstantInt::get(Int64Ty, 0),
> newMD});
>
> Also try testing with ASan or MSan, perhaps?
>
>
>>
>> Thus far, I have been developing on a version of Clang with debugging
>> symbols enabled, and this code has worked. However, on the Release+Asserts
>> version, the third line results in a segmentation fault. The backtrace for
>> the error, up to the method I'm working on, is as follows:
>>
>> // signal handling...
>>  #4 0x00005556e48d6120
>> llvm::ReplaceableMetadataImpl::getOrCreate(llvm::Metadata&)
>> (/home/sjessu/build-no-debug/bin/clang-10+0x2787120)
>>  #5 0x00005556e48db8c2 llvm::MetadataTracking::track(void*,
>> llvm::Metadata&, llvm::PointerUnion<llvm::MetadataAsValue*,
>> llvm::Metadata*>) (/home/sjessu/build-no-debug/bin/clang-10+0x278c8c2)
>>  #6 0x00005556e48dbc12 llvm::MDNode::MDNode(llvm::LLVMContext&, unsigned
>> int, llvm::Metadata::StorageType, llvm::ArrayRef<llvm::Metadata*>,
>> llvm::ArrayRef<llvm::Metadata*>)
>> (/home/sjessu/build-no-debug/bin/clang-10+0x278cc12)
>>  #7 0x00005556e48dd70d llvm::MDTuple::getImpl(llvm::LLVMContext&,
>> llvm::ArrayRef<llvm::Metadata*>, llvm::Metadata::StorageType, bool)
>> (/home/sjessu/build-no-debug/bin/clang-10+0x278e70d)
>>  #8 0x00005556e4a4b293 (anonymous
>> namespace)::LowerTypeTestsModule::lower() (.part.1843)
>> (/home/sjessu/build-no-debug/bin/clang-10+0x28fc293)
>>
>>
>> Crucially, *this error does not occur in the Debug version*, so I am
>> lost as to how to diagnose it. I have checked that newMD is non-null and
>> contains the value I expect. Are there any discrepancies in the Debug
>> version as compared to the Release + Asserts version that would cause such
>> an error? Thanks for your help!
>>
>> Best,
>> Shishir Jessu
>>
>>
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>> llvm-dev at lists.llvm.org
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>>
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