Detect compilers from environment variables

[awvwgk]

Build systems like CMake and meson can read the compiler from the FC and CC environment variables or detect the archiver from the AR environment variable. Similarly the FFLAGS and CFLAGS variables can be used to pass flags for the compilation.

For example, below are environment variables usually set in a conda-build process:

ADDR2LINE=$BUILD_PREFIX/bin/x86_64-conda-linux-gnu-addr2line
AR=$BUILD_PREFIX/bin/x86_64-conda-linux-gnu-ar
AS=$BUILD_PREFIX/bin/x86_64-conda-linux-gnu-as
CXXFILT=$BUILD_PREFIX/bin/x86_64-conda-linux-gnu-c++filt
CC=$BUILD_PREFIX/bin/x86_64-conda-linux-gnu-cc
CFLAGS=-march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O2 -ffunction-sections -pipe -isystem $PREFIX/include -fdebug-prefix-map=$SRC_DIR=/usr/local/src/conda/fpm-0.3.0 -fdebug-prefix-map=$PREFIX=/usr/local/src/conda-prefix
CPPFLAGS=-DNDEBUG -D_FORTIFY_SOURCE=2 -O2 -isystem $PREFIX/include
CPP=$BUILD_PREFIX/bin/x86_64-conda-linux-gnu-cpp
# ...
LDFLAGS=-Wl,-O2 -Wl,--sort-common -Wl,--as-needed -Wl,-z,relro -Wl,-z,now -Wl,--disable-new-dtags -Wl,--gc-sections -Wl,-rpath,$PREFIX/lib -Wl,-rpath-link,$PREFIX/lib -L$PREFIX/lib
INFO: activate-gfortran_linux-64.sh made the following environmental changes:
F77=$BUILD_PREFIX/bin/x86_64-conda-linux-gnu-gfortran
F90=$BUILD_PREFIX/bin/x86_64-conda-linux-gnu-gfortran
F95=$BUILD_PREFIX/bin/x86_64-conda-linux-gnu-f95
FC=$BUILD_PREFIX/bin/x86_64-conda-linux-gnu-gfortran
FFLAGS=-fopenmp -march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O2 -ffunction-sections -pipe -isystem $PREFIX/include -fdebug-prefix-map=$SRC_DIR=/usr/local/src/conda/fpm-0.3.0 -fdebug-prefix-map=$PREFIX=/usr/local/src/conda-prefix
FORTRANFLAGS=-fopenmp -march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O2 -ffunction-sections -pipe -isystem $PREFIX/include -fdebug-prefix-map=$SRC_DIR=/usr/local/src/conda/fpm-0.3.0 -fdebug-prefix-map=$PREFIX=/usr/local/src/conda-prefix
GFORTRAN=$BUILD_PREFIX/bin/x86_64-conda-linux-gnu-gfortran

Automatically picking up some of this information would reduce the packaging of any fpm project for conda to just:

fpm install --prefix $PREFIX

[urbanjost]

In an early PR that got pulled this was done and the consensus at the time was that using environment variables extensively went against making a reproducible package and that this would be better done in the fpm.toml file and subsequently via the --profile flag. That will hopefully occur soon; and there were a number of issues including whether the FC variable could include full pathnames and how that would be handled. Hopefully, custom --profile is not far off.

But instead of applying those environment options unconditionally when present what if there were a "compiler" name that set all it's values from the variables? fpm --compiler env perhaps?

PS; could not find the earlier discussions ( I was the one that orginally had it reading the variables) but there were discussions about issues like inadvertently picking up options, how different compilers needed different values anyway, and how someone could not as easily reproduce someone else's build, and how since a unique directory is built for each combination of options fpm is aware of you would either have to take the variables into account when determining the build directory name or someone could make a build that was a mixture of build options with no easy way to reproduce or record that.

Those are the ones I remember (the original introduction of alternate compilers to the Fortran version allowed for the variables to be used and introduced --flag and --cflag and a few others, but those options were rejected at the time).

Also note there are differences between what Make and Cmake and other utilities do with the same variable names and what the contents are allowed to be, those variables are a de-facto standard mostly derived from common make and gmake usage (and those are not always compatible either), so the question arises whether there should be FPM_* versions of the variables. Note several compilers have their own variables as well, so even now you can set compiler options via config files and environment variables when using ifort (for example) that fpm is unaware of and so does not make a new build subdirectory for. Whether that is a feature or a flaw depends on the results.