[llvm] r319894 - [InstSimplify] Fold insertelement into undef if index is out of bounds

[Michel Dänzer via llvm-commits]

Looks like this has become a case for Nicolai or Marek.

To give some context to Marek, the issue below can be reproduced with
the piglit test fcc-read-to-pbo-after-clear.


On 2017-12-12 04:25 PM, Sanjay Patel wrote:
> For reference, the extractelement with bad index should already have
> been simplified.
> https://reviews.llvm.org/rL319910 recently made that stronger, but
> raised some concern for AMDGPU backend tests.
> 
> On Tue, Dec 12, 2017 at 8:01 AM, Igor Laevsky via llvm-commits
> <llvm-commits at lists.llvm.org <mailto:llvm-commits at lists.llvm.org>> wrote:
> 
>     Hi Michael,
> 
>     In the source code I see the following operations:
>       %122 = extractelement <4 x i32> %121, i32 6
>     and:
>       %124 = insertelement <4 x i32> %121, i32 %123, i32 6
> 
>     This operations has undefined result
>     (see https://llvm.org/docs/LangRef.html#extractelement-instruction
>     <https://llvm.org/docs/LangRef.html#extractelement-instruction>).
>     Looks like a bug in the frontend. Do you know what’s the best
>     approach here?
> 
> 
>>     On 12 Dec 2017, at 13:45, Michel Dänzer <michel at daenzer.net
>>     <mailto:michel at daenzer.net>> wrote:
>>
>>     On 2017-12-12 11:08 AM, Igor Laevsky wrote:
>>>     I reverted problematic changes in the
>>>     https://reviews.llvm.org/rL320466 <https://reviews.llvm.org/rL320466>
>>
>>     Thanks.
>>
>>
>>>     Is it possible to obtain an llvm-ir of the failing test case
>>>     before the
>>>     optimizations? I can see the miscompile, but I can’t reproduce it
>>>     using
>>>     optimised good case.
>>
>>     The attached should be the LLVM IR before optimizations.
>>
>>
>>     -- 
>>     Earthling Michel Dänzer               |
>>                   http://www.amd.com <http://www.amd.com>
>>     Libre software enthusiast             |             Mesa and X
>>     developer
>>     <fcc-read-to-pbo-after-clear.ll>
> 
> 
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> 
> 


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer