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

[David Blaikie via llvm-dev]

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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