[petsc-dev] Meaning of PETSc matrices with zero rows but nonzero columns?

[Stefano Zampini]

MKL fails  on gemms calls too. At least in the past, not sure about the latest version.  

> On Jun 1, 2020, at 10:48 PM, Jeff Hammond <jeff.science at gmail.com> wrote:
> 
> I assume it's MKL sparse that fails for empty matrices, because the BLAS and LAPACK should follow the Netlib convention.
> 
> I'm sorry that MKL is unwilling to do nothing for you :-)
> 
> Jeff
> 
> On Sat, May 30, 2020 at 4:26 PM Mills, Richard Tran via petsc-dev <petsc-dev at mcs.anl.gov <mailto:petsc-dev at mcs.anl.gov>> wrote:
> Thanks for the replies, everyone. It suppose is not actually that hard for me to handle these dimensions properly -- I just hadn't personally encountered or thought much about when such operations with empty matrices might arise, and was initially puzzled about what multiplication by an "empty" matrix even means. I think I see now why I need to put in the work to handle these cases properly. (Sure wish that MKL could just do it, though!)
> 
> --Richard
> 
> On 5/30/20 4:09 PM, Stefano Zampini wrote:
>>> On May 31, 2020, at 1:03 AM, Jed Brown <jed at jedbrown.org> <mailto:jed at jedbrown.org> wrote:
>>> 
>>> Stefano Zampini <stefano.zampini at gmail.com> <mailto:stefano.zampini at gmail.com> writes:
>>> 
>>>> If A is 0x8 and B is 8x5 then C is correct to be of size 0x5. The rows and columns of the resulting matrix have to follow the rules.
>>> Right, I think if you said C is 0x0 (which seems like Richard's proposal), you'd need to relax shape compatibility logic in many places, including in ways that might produce confusing errors.
>>> 
>> Richard
>> 
>> In the triple matrix product case, your code will break, because the operation will no longer be associative
>> 
>> A 3x0, B 0x8, C 8x7 -> (ABC) is a valid 3x7 matrix (empty) 
>> 
>> If I understand you right, (AB)  would be  a 0x0 matrix, and it can no longer be multiplied against C
>> 
>>> Richard, what is the hardship in preserving the shape relations?
> 
> 
> 
> -- 
> Jeff Hammond
> jeff.science at gmail.com <mailto:jeff.science at gmail.com>
> http://jeffhammond.github.io/ <http://jeffhammond.github.io/>
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