R devel - Mar 2021 - boneheaded BLAS questions

I've been going around in circles trying to get BLAS-switching 
working on a current r-devel, I'm sure I'm doing something dumb.  Any 
ideas about what I might be doing wrong, or suggestions for further 
diagnosis, would be welcome!

   tl;dr  I am compiling R-devel with (to the best of my knowledge) 
options set to allow BLAS-switching, but getting "undefined symbol"
errors.

  ----

   Latest R-devel (via SVN), PopOS!/Ubuntu 20.10

   I have read Dirk E's post: https://github.com/eddelbuettel/mkl4deb
   I have attempted to read the relevant section of R Installation & 
Administration several times: 
https://cran.r-project.org/doc/manuals/r-release/R-admin.html#BLAS
   https://wiki.debian.org/DebianScience/LinearAlgebraLibraries


   I have installed MKL and OpenBLAS on my system via 'apt install' 
(libopenblas-dev, libopenblas-base, and TWO versions of intel-mkl-64bit)

   When I build R without BLAS everything is OK;

	rm -Rf r-build; mkdir r-build; cd r-build; ../r-devel/configure 
--without-blas --enable-R-shlib --enable-BLAS-shlib; make -j 6


Matrix products: default
BLAS:   /usr/local/lib/R/lib/libRblas.so
LAPACK: /usr/local/lib/R/lib/libRlapack.so


    When I look at my BLAS alternatives I don't see anything obviously 
wrong:


sudo update-alternatives --config libblas.so.3-x86_64-linux-gnu
There are 3 choices for the alternative libblas.so.3-x86_64-linux-gnu 
(providing /usr/lib/x86_64-linux-gnu/libblas.so.3).

   Selection    Path 
Priority   Status
------------------------------------------------------------
* 0            /opt/intel/mkl/lib/intel64/libmkl_rt.so 
150       auto mode
   1            /opt/intel/mkl/lib/intel64/libmkl_rt.so 
  150       manual mode
   2            /usr/lib/x86_64-linux-gnu/blas/libblas.so.3 
  10        manual mode
   3            /usr/lib/x86_64-linux-gnu/openblas-pthread/libblas.so.3 
  100       manual mode


   When I rebuild R with --with-blas:

	rm -Rf r-build; mkdir r-build; cd r-build; ../r-devel/configure 
--with-blas --enable-R-shlib --enable-BLAS-shlib; make -j 6

  I end up with this:

gcc -I../../../r-devel/src/extra -I/usr/include/tirpc -I. 
-I../../src/include -I../../../r-devel/src/include  -I/usr/local/include 
-I../../../r-devel/src/nmath -DHAVE_CONFIG_H   -fopenmp -fpic  -g -O2 
-c ../../../r-devel/src/main/Rmain.c -o Rmain.o
gcc -Wl,--export-dynamic -fopenmp  -L"../../lib" -L/usr/local/lib -o 
R.bin Rmain.o  -lR -lRblas


/usr/bin/ld: ../../lib/libR.so: undefined reference to `zgemm_'
/usr/bin/ld: ../../lib/libR.so: undefined reference to `daxpy_'
/usr/bin/ld: ../../lib/libR.so: undefined reference to `dgemv_'
/usr/bin/ld: ../../lib/libR.so: undefined reference to `dscal_'


    If

======================intel-mkl-64bit-2018.2-046/all,now 2018.2-046 amd64
[installed]
intel-mkl-64bit-2020.4-912/all,now 2020.4-912 amd64 [installed]

<... lots more intel-mkl stuff>

libblas-dev/groovy,now 3.9.0-3ubuntu1 amd64 [installed,automatic]
libblas3/groovy,now 3.9.0-3ubuntu1 amd64 [installed,automatic]
libgraphblas3/groovy,now 1:5.8.1+dfsg-2 amd64 [installed,automatic]
libgslcblas0/groovy,now 2.6+dfsg-2 amd64 [installed,automatic]
libopenblas-base/groovy,now 0.3.10+ds-3ubuntu1 amd64 [installed]
libopenblas-dev/groovy,now 0.3.10+ds-3ubuntu1 amd64 [installed]
libopenblas-pthread-dev/groovy,now 0.3.10+ds-3ubuntu1 amd64 
[installed,automatic]
libopenblas0-pthread/groovy,now 0.3.10+ds-3ubuntu1 amd64 
[installed,automatic]
libopenblas0/groovy,now 0.3.10+ds-3ubuntu1 amd64 [installed]

Ben,

This stuff has worked unchanged since the 1990s when we had a _really_ far
sighted fellow in Debian come up with the 'switch the links' scheme
which was
(and is) subsequently deployed by many numerical applications within Debian,
R and e.g. Octave included.

And I used this ability to switch over a decade ago in a never-quite-finished
paper which resulted in a package as well as a vignette as paper draft on
CRAN: gcbd [1] It used the ability to switch between implementation to time
and compare and benchmark the various BLAS and LAPACK libraries -- which was
then motivated by a comparison with GPUs. (The actual code / package is
stale-ish as some of the underlying packages have gone as eg the GPU one --
but the mechanics you are after still work the exact same way on Debian and
derivarives including Ubuntu and PopOS.)

(As a complete aside, the state of the art here is now one level up in
libraries based on flame/blis (a riff on blas) which can do a similar logical
switch _at runtime_ (rather than by flipping softlinks and restarting the
app). Julia and some other languages uses that, I think Fedora may have it in
its R build as well. Inaki may know more...)

That said, from the top of my head, I think you error may just be with the
second R compilation -- I always (i.e. for the Debian package) use both
  --with-blas --with-lapack
and not just --with-blas. And I do there is public: if you know where to look
you can see the exact invocation of the Debian build of the R package (which
Ubuntu and Pop and ... then shadow) [2]

Hth, Dirk

[1] https://cran.r-project.org/package=gcbd
[2] https://sources.debian.org/src/r-base/4.0.4-1/debian/rules/
    (and I apologise for how messy this still is)
-- 
https://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org