llvm dev - Dec 2016 - Use case of undef

Hello all,

I am wondering the use case of undef. Is there any C code that I could get
undef if it is compiled by clang.

Thanks
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Here is an example:
https://godbolt.org/g/wKoXWW

-- Sean Silva

On Sat, Dec 3, 2016 at 4:28 PM, LYU YH via llvm-dev <llvm-dev at
lists.llvm.org
> wrote:
> Hello all,
>
> I am wondering the use case of undef. Is there any C code that I could get
> undef if it is compiled by clang.
>
> Thanks
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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Hello Sean,

I read several LLVM undef materials[1][2][3] but still have some questions
of it. I can understand "add %X, undef -> undef". But why the
equation "and
%X, undef -> 0" is valid? In LLVM Language Reference Manual, the
explanation of such equation is that it is safe to assume that all bits of
the ‘undef‘ could be 0. If so, why can't we assume all bits of the
'undef'
is 0 in the former equation. Therefore we can get "add %X, 0 -> %X"
instead?

By the way, is there any document that describe how 'undef' should be
handled with other LLVM value? Is the 'undef' in LLVM the same as the
UNDEF[4] (the top element in lattice) in constant propagation?

[1] http://www.nondot.org/sabre/LLVMNotes/UndefinedValue.txt
[2] https://www.cs.utah.edu/~regehr/llvm-ub.pdf
[3] http://sunfishcode.github.io/blog/2014/07/14/undef-introduction.html
[4] http://infolab.stanford.edu/~ullman/dragon/w06/lectures/cp.pdf

Thanks for your reply
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