[OpenBLAS] conanfile.py assumes gfortran compiler

[leezu]

conan-center-index/recipes/openblas/all/conanfile.py

Lines 87 to 88 in 5e30b91

	if self.options.build_lapack:
	self.cpp_info.system_libs.append("gfortran")

gfortran is not the only Fortran compiler supported to compile OpenBLAS. Sometimes gfortran is a no-go as it introduces a runtime dependency on the GPL libgfortran.so. When redistributing artifacts with runtime dependency on libgfortran.so, libgfortran.so would need to be redistributed as well. However, many organizations (eg. Apache Software Foundation) have policies against redistributing GPL artifacts.

Relying on a system libgfortran.so is not an option either, as gfortran breaks ABI regularly:

• https://github.com/gcc-mirror/gcc/blob/releases/gcc-6/libgfortran/gfortran.map
• https://github.com/gcc-mirror/gcc/blob/releases/gcc-7/libgfortran/gfortran.map
• https://github.com/gcc-mirror/gcc/blob/releases/gcc-8/libgfortran/gfortran.map

Flang Fortran compiler can be used instead of gfortran and does not introduce runtime dependencies on GPL libraries. Flang toolchain can be obtained via https://github.com/flang-compiler/flang/releases and for example OpenBLAS compiled via CC=gcc make DYNAMIC_ARCH=1 USE_OPENMP=1 FC=flang

What's the best way to structure a conanfile.py to support different fortran toolchains? Should I open an issue about fortran compile support at https://github.com/conan-io/conan/ as well?

[Croydon]

This is a hidden system requirement anyway which we should avoid in the mid-term

I believe once we have LLVM packages we should package Flang and add it as a requirement

[leezu]

@Croydon your suggestion is a good mid-term solution. Do you have any recommendation how to extend the existing approach until then? Would the following be appropriate:

• Add flang option to conanfile (based on https://github.com/conan-community/conan-openblas cf [OpenBLAS] Conan uses experimental cmake build instead of recommended build option #1769)
• If flang is specified, export FC=flang to the cmake file if FC is not defined in users environment
• Change the linking line from -lgfortran to -lflang -lflangrti -lpgmath

[Croydon]

I don't have a strong opinion either way, but I think such an option could be fine

[Croydon]

We now have a gfortan package which re-packages official binaries for Windows, Linux and macOS. Better than nothing

#4249