llvm dev - May 2017 - The right usage of llvm::ArrayRef

Hi All,

  I debug a Release build LLVM crash, and find the culprit. The code
snippet below,

    llvm::ArrayRef<SDValue> Ops = { Lo, Hi }; // should be `SDValue Ops[2]
{ Lo, Hi };`
    return DAG.getMergeValues(Ops, dl);

Turns out a nullptr is dereferenced in DAG.getMergeValues, thus I have
segfault.
>From the comment on [1], it says:
    This class does not own the underlying data, it is expected to be used
in situations
    where the data resides in some other buffer, whose lifetime extends
past that of the
    ArrayRef. For this reason, it is not in general safe to store an
ArrayRef.

I feel somehow the comment explain why I get the segfault, but not sure
about that.
Would someone kindly explain why?

  Thanks.

[1] http://llvm.org/doxygen/classllvm_1_1ArrayRef.html

Regards,
chenwj

-- 
Wei-Ren Chen (陳韋任)
Homepage: https://people.cs.nctu.edu.tw/~chenwj
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> llvm::ArrayRef<SDValue> Ops = { Lo, Hi };
{Lo, Hi} is a temporary object in stack and it is supposed to vanish after
the statement.
You may write like ; (See X86ISelLowering.cpp:21709)

  SDValue Ops[] = { Lo, Hi };
  return DAG.getMergeValues(Ops, dl);

ArrayRef's constructor recognizes and infers array.


On Mon, May 22, 2017 at 8:27 PM 陳韋任 via llvm-dev <llvm-dev at
lists.llvm.org>
wrote:
> Hi All,
>
>   I debug a Release build LLVM crash, and find the culprit. The code
> snippet below,
>
>     llvm::ArrayRef<SDValue> Ops = { Lo, Hi }; // should be `SDValue
> Ops[2] = { Lo, Hi };`
>     return DAG.getMergeValues(Ops, dl);
>
> Turns out a nullptr is dereferenced in DAG.getMergeValues, thus I have
> segfault.
>
> From the comment on [1], it says:
>
>     This class does not own the underlying data, it is expected to be used
> in situations
>     where the data resides in some other buffer, whose lifetime extends
> past that of the
>     ArrayRef. For this reason, it is not in general safe to store an
> ArrayRef.
>
> I feel somehow the comment explain why I get the segfault, but not sure
> about that.
> Would someone kindly explain why?
>
>   Thanks.
>
> [1] http://llvm.org/doxygen/classllvm_1_1ArrayRef.html
>
> Regards,
> chenwj
>
> --
> Wei-Ren Chen (陳韋任)
> Homepage: https://people.cs.nctu.edu.tw/~chenwj
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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