llvm dev - Aug 2015 - how LLVM deals with 'undef'

Hi,

I'm using LLVM-3.8 (current SVN version) to optimize my code. After loop
unrolling, I find there is 'undef' in the IR. For example,
"*%.lcssa21.unr
= phi double [ undef, %.lr.ph9 ], [ %13, %7 ]*". I have two questions:

(1) Why this 'undef' is generated?  Because before 'loop
unrolling', there
is no such 'undef'.
(2) How LLVM backend deals with this 'undef'?

Any suggestion is appreciable.

Regards,

Xiangyang
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On 8/25/2015 2:09 PM, Xiangyang Guo via llvm-dev wrote:
>
> I'm using LLVM-3.8 (current SVN version) to optimize my code. After
loop
> unrolling, I find there is 'undef' in the IR. For example,
> "/%.lcssa21.unr = phi double [ undef, %.lr.ph9 ], [ %13, %7 ]/".
I have
> two questions:
>
> (1) Why this 'undef' is generated?  Because before 'loop
unrolling',
> there is no such 'undef'.
Most likely to satisfy the syntax of a phi node.  If everything is 
correct, that value will never be used.  If things weren't exactly 
correct, this may be a result of a load from an uninitialized memory, or 
something like that.
> (2) How LLVM backend deals with this 'undef'?
In general, undef will propagate itself through instructions.  How it 
will deal with this specific one, it's impossible to say without the 
context.

-Krzysztof

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, 
hosted by The Linux Foundation

Thanks for reply.

First, I use Clang to compile my source code without any optimization so
that I get one .bc file, in which there is no 'undef' showing up. Then,
I
use opt to optimize the IR. So I assume things are correct and the
'undef'
is introduced to satisfy the syntax of phi node, as you mentioned. But I
really don't want this 'undef' in IR. Do you have any idea to get
rid of
'undef'? Thanks a lot.

Regards,

Xiangyang

On Tue, Aug 25, 2015 at 3:16 PM, Krzysztof Parzyszek via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> On 8/25/2015 2:09 PM, Xiangyang Guo via llvm-dev wrote:
>
>>
>> I'm using LLVM-3.8 (current SVN version) to optimize my code. After
loop
>> unrolling, I find there is 'undef' in the IR. For example,
>> "/%.lcssa21.unr = phi double [ undef, %.lr.ph9 ], [ %13, %7
]/". I have
>> two questions:
>>
>> (1) Why this 'undef' is generated?  Because before 'loop
unrolling',
>> there is no such 'undef'.
>>
>
> Most likely to satisfy the syntax of a phi node.  If everything is
> correct, that value will never be used.  If things weren't exactly
correct,
> this may be a result of a load from an uninitialized memory, or something
> like that.
>
> (2) How LLVM backend deals with this 'undef'?
>>
>
> In general, undef will propagate itself through instructions.  How it will
> deal with this specific one, it's impossible to say without the
context.
>
> -Krzysztof
>
> --
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted
> by The Linux Foundation
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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