This concern mainly the problem where the number of rows of A is smaller than the number of columns, and x is sparse.
sudo apt update sudo apt upgrade sudo apt install cmake libopenblas-dev liblapack-dev
wget http://sourceforge.net/projects/arma/files/armadillo-9.800.4.tar.xz
cd arma* cmake . make sudo make install
rm -rf Debug ; mkdir Debug ; cd Debug ; cmake -GNinja -DCMAKE_BUILD_TYPE=Debug .. ; ninja
rm -rf Release ; mkdir Release ; cd Release ; cmake -GNinja -DCMAKE_BUILD_TYPE=Release .. ; ninja
rm -rf Debug ; mkdir Debug ; cd Debug ; cmake -DCMAKE_BUILD_TYPE=Debug .. ; make
rm -rf Release ; mkdir Release ; cd Release ; cmake -DCMAKE_BUILD_TYPE=Release .. ; make