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

[Iñaki Ucar]

On Wed, 1 Nov 2023 at 12:57, Tomas Kalibera <tomas.kalibera using gmail.com> wrote:
>
>
> 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.

I see. I didn't understand that part then. I thought that a library
should be specified.

-- 
Iñaki Úcar