[Rd] configure output with flexiblas [was Re: About FlexiBLAS in the R-admin docs]

[Tomas Kalibera]

On 10/31/23 10:45, Iñaki Ucar wrote:
> On Tue, 24 Oct 2023 at 12:53, Tomas Kalibera <tomas.kalibera using gmail.com> wrote:
>> The output of session info is based on that flexiblas is used and on
>> what flexiblas tells R is the backend it uses. R does not attempt to
>> check that optimized LAPACK functions from the backend really end up
>> called via flexiblas, and I don't think it could be realistically checked.
>>
>> But I've checked one case manually in Fedora 38 using Linux perf tool.
>> The following code:
>>
>> S <- toeplitz((10:1)/10)
>> repeat { R <- rWishart(10, 20, S) }
>>
>> uses dpotrf from LAPACK, which is optimized in OpenBLAS and ATLAS and
>> the corresponding optimized implementations really appeared on the
>> sampling profile for me from the backend libraries.
>>
>> The comment from R Admin has been removed now and if anyone runs into
>> the problem (that an optimized LAPACK function is not called from a
>> backend that provides it), it would be best to report it with sufficient
>> detail to flexiblas.
> Thanks, Tomas. Yes, if an expected redirection to an optimized
> function does not happen, that would be a bug in FlexiBLAS and should
> be reported upstream.
>
> Related to this, a small detail... I noticed that the R configure
> script reports "BLAS(FlexiBlas)" correctly as an external library, but
> then "LAPACK(generic)". This should be FlexiBLAS too instead of
> "generic". It doesn't make any difference, since the LAPACK symbols in
> FlexiBLAS are called anyway, but it's misleading.

This depends on how you configure R. On Fedora 38,

../trunk/configure --with-blas=flexiblas --with-lapack

reports "BLAS(FlexiBlas), LAPACK(in blas)" and from my reading of 
R-admin, this is the recommended way of configuring with flexiblas.

Configuring via (is this what you had in mind?):

../trunk/configure --with-blas=flexiblas --with-lapack=flexiblas

reports "BLAS(FlexiBlas), LAPACK(generic)". The build would actually do 
"-lflexiblas -lflexiblas", the former for LAPACK (from --with-lapack 
value) and the latter for BLAS (from --with-blas value). The "generic" 
here means not from BLAS, but from an extra standalone library. Compare 
with the first (recommended) use when the build would only link 
"-lflexiblas" once.

Best,
Tomas

>
> Best,