openblas: try clang build

[junghans]

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[carlocab]

This won't work as-is, because gfortran will still try to link with libgomp, which we don't want.

We would either need to patch some Makefiles, or handle this in brew/superenv.

Thoughts here, @Homebrew/core? I'm personally in favour of this change, along with using libomp everywhere on macOS. This allows us to use Clang instead of GCC when OpenMP is needed, and avoids the difficulties caused by software that have C++ code but also use OpenMP. [*]

Additional context at #129100, #129648.

[carlocab]

I guess, alternatively, we could try building with clang but also using libgomp with that for OpenMP, but I'm a little less clear on how to make that work. I imagine it's possible though.

[fxcoudert]

What is the benefit? It would be adding a dependency, and requiring us to hack the build, but I do not clearly see the gain. Also, how does that impact dependents? Specially those that rely on the Fortran OpenMP, which isn't available in libomp (but is part of libgomp).

[junghans]

Can we use flang?

[ywwry66]

gfortran will still try to link with libgomp, which we don't want.

It doesn't seem like that with my local clang+libomp build:

libopenblasp-r0.3.23.dylib:
	/opt/homebrew/opt/openblas/lib/libopenblas.0.dylib (compatibility version 0.0.0, current version 0.0.0)
	/opt/homebrew/opt/gcc/lib/gcc/current/libgfortran.5.dylib (compatibility version 6.0.0, current version 6.0.0)
	/opt/homebrew/opt/libomp/lib/libomp.dylib (compatibility version 5.0.0, current version 5.0.0)
	/opt/homebrew/opt/gcc/lib/gcc/current/libquadmath.0.dylib (compatibility version 1.0.0, current version 1.0.0)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1319.100.3)

What is the benefit?

Adding one more reason: libgomp is not fork safe on macOS, and will probably never be. See:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58378
OpenMathLib/OpenBLAS#2197 (comment)

Can we use flang?

LLVM flang is still WIP and not production ready.

[carlocab]

I've outlined some benefits above, but to make it more explicit: currently, building OpenBLAS with GCC means that any formula, particularly ones with dependents, with C++ code effectively has to choose between using OpenBLAS or OpenMP. If you need to use both (which seems to be the case for suite-sparse), then you are stuck linking against libstdc++, which makes things very difficult for dependents.

Unless we can build suite-sparse with GCC but link with libc++ instead?

[cho-m]

Unless we can build suite-sparse with GCC but link with libc++ instead?

I see recommendations for -nostdinc++ -nodefaultlibs and manually re-adding libraries, e.g. -lc++ -lc++abi -lm -lc -lgcc_s -lgcc

Or maybe rebuilding gcc with -stdlib=libc++ support enabled like option in MacPorts (https://github.com/macports/macports-ports/blob/master/lang/gcc-devel/Portfile#L269-L281).

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I guess, alternatively, we could try building with clang but also using libgomp with that for OpenMP

Not sure if this is possible. You can force linkage with LLVM Clang's -fopenmp=libgomp, but it doesn't seem to enable OpenMP. Maybe someone else knows of a way.

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along with using libomp everywhere on macOS.

I think Conda Forge has done something similar. Not familiar with reading their recipes, but they may have drop-in replaced libgomp with libomp inside their gfortran.

Ref: https://conda-forge.org/docs/maintainer/knowledge_base.html#switching-openmp-implementation
Ref: https://github.com/conda-forge/gfortran_impl_osx-64-feedstock/pull/11/files