llvm dev - Jun 2017 - re: How to deal with UNDEF SDNode?

Chenwj,
             In ISD the UNDEF node (i.e. SDValue) is an "undefined value”,
not “undefined behavior”, i.e. not scary!. You can think of it as an
uninitialized / don’t-care register, or in a vector an uninitialized /
don’t-care element.

Peter Lawrence.
> On Jun 22, 2017, at 8:35 AM, via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Date: Thu, 22 Jun 2017 21:21:09 +0800
> Subject: [llvm-dev] How to deal with UNDEF SDNode?
> 
> 
> Hi All,
> 
>  The comment of UNDEF in ISDOpcodes.h [1] is pretty simple,
> 
>      /// UNDEF - An undefined node.
> 
>      UNDEF,
> The name make me think of C undefined behavior, which is ... scary. But I
> also see there are
> APIs like isBuildVectorAllOnes, isBuildVectorAllZeros, ... etc, seems UNDEF
> just can be any
> value and we don't care about what it is. The most case I see UNDEF is
in
> BUULD_VECTOR,
> VECTOR_SHUFFLE, ... etc. I would like to know usually how we deal with
> those UNDEF nodes?
> 
> Thanks.
> 
> [1] http://llvm.org/doxygen/ISDOpcodes_8h_source.html
<http://llvm.org/doxygen/ISDOpcodes_8h_source.html>
> 
> Regards,
> chenwj
> 
> -- 
> Wei-Ren Chen (陳韋任)
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Hi Peter,

2017-06-23 1:13 GMT+08:00 Peter Lawrence via llvm-dev <
llvm-dev at lists.llvm.org>:
> Chenwj,
>              In ISD the UNDEF node (i.e. SDValue) is an "undefined
value”,
> not “undefined behavior”, i.e. not scary!. You can think of it as an
> uninitialized / don’t-care register, or in a vector an uninitialized /
> don’t-care element.
>
So the UNDEF node corresponds to undefined value in LLVM LangRef [1], right?
And I can do whatever I want with those UNDEF nodes, right?

Thanks.

[1] http://llvm.org/docs/LangRef.html#undefined-values

Regards,
chenwj
​
-- 
Wei-Ren Chen (陳韋任)
Homepage: https://people.cs.nctu.edu.tw/~chenwj
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