Readme update

[gareth-nx]

This pull request explains some CMake options that affect stdlib builds.

• It is designed to address these issues in a simple way.

The first issue is that user-specified compiler optimizations in FFLAGS are overridden by the default the stdlib CMake setup. This means that, in the original README, the use of FFLAGS was not having any effect.

• To fix this, the updated README discusses -DCMAKE_BUILD_TYPE. This can be setup to avoid default optimizations (while retaining flags that are essential to compile stdlib). This allows optimization flags in FFLAGS to be used.

The second issue is that by default, the user does not see what commands CMake is using to compile the code. This makes it difficult to notice problems like those above.

• The solution is to set -DCMAKE_VERBOSE_MAKEFILE=On, which prints the compiler options.

I am a CMake novice, so it would be good to have this checked by an expert like @awvwgk .

[milancurcic]

These look good to me, thanks!

[awvwgk]

What happens if you set -DCMAKE_BUILD_TYPE=Release instead of using NoConfig and setting FFLAGS manually?

[gareth-nx]

What happens if you set -DCMAKE_BUILD_TYPE=Release instead of using NoConfig and setting FFLAGS manually?

It seems to build without any optimizations.

To be specific, I delete the build and installation directories, then run the following from inside the directory /home/gareth/Code_Experiments/fortran/stdlib/my_branch/stdlib/

export FC="gfortran" cmake -B build -DCMAKE_MAXIMUM_RANK=7 -DCMAKE_INSTALL_PREFIX=/home/gareth/Code_Experiments/fortran/stdlib/my_branch/stdlib/local_install_gfortran/ -DCMAKE_VERBOSE_MAKEFILE=On -DCMAKE_BUILD_TYPE=Release 

Then the printed compiler commands in the build process look like this example -- (no optimization flags)

/usr/bin/gfortran -I/home/gareth/Code_Experiments/fortran/stdlib/my_branch/stdlib/build/src/mod_files -fimplicit-none -ffree-line-length-132 -Jmod_files/ -fPIC -fno-range-check -c /home/gareth/Code_Experiments/fortran/stdlib/my_branch/stdlib/build/src/stdlib_hash_64bit_fnv.f90 -o CMakeFiles/fortran_stdlib.dir/stdlib_hash_64bit_fnv.f90.o 

Incidentally I believe that this behaviour has changed since last year sometime (when Release led to -O3 type optimization). Back then, it also had the shortcoming of overriding the FFLAGS optimizations.

[jvdp1]

It sounds good to me. IMO it could be merged, pending @awvwgk 's comments.

[awvwgk]

The Release flags are currently empty

stdlib/config/DefaultFlags.cmake

Lines 7 to 9 in 3028a36

	set(
	CMAKE_Fortran_FLAGS_RELEASE_INIT
	)

You could add the preferred options there and we can rely on -DCMAKE_BUILD_TYPE=Release rather than reverting to the NoConfig which is the usual CMake default.

[jvdp1]

The Release flags are currently empty

stdlib/config/DefaultFlags.cmake

Lines 7 to 9 in 3028a36

	set(
	CMAKE_Fortran_FLAGS_RELEASE_INIT
	)

You could add the preferred options there and we can rely on -DCMAKE_BUILD_TYPE=Release rather than reverting to the NoConfig which is the usual CMake default.

I would propose to use the same compiler options as for the release option of fpm:

 -O3 -funroll-loops -Wimplicit-interface -fPIC -fmax-errors=1 -fcoarray=single 

[jvdp1]

@awvwgk Are you good with these changes? Can this PR be merged?

[jvdp1]

Thank you all. I will merge it.