Flang should handle the -mtune option

[tukss]

flang-new already knows about the -march option but not about -mtune. clang (and also gcc/gfortran) handles both options. For context, -mtune produces code that's optimized for a certain subarchitecture (say haswell or zen3) while retaining compatibility with other subarchitectures, whereas -march might produce illegal instructions when run on the wrong hardware.

My reason for asking is that is makes it hard to use flang-new within Spack. Spack handles flang-new as the Fortran component of the clang compiler but unfortunately doesn't provide a way to specify different compiler flags for the C, C++, and Fortran compilers. -mtune is used as an option for most architectures (e.g., here), which breaks using flang-new to build anything with Spack.

Even if -mtune wasn't implemented immediately it would at least be helpful if flang-new wouldn't just stop compilation but just display a warning and continue.

[AlexisPerry]

PR for basic mtune support: #95043

[llvmbot]

@llvm/issue-subscribers-flang-driver

Author: Philipp Edelmann (tukss)

`flang-new` already knows about the `-march` option but not about `-mtune`. `clang` (and also `gcc`/`gfortran`) handles both options. For context, `-mtune` produces code that's optimized for a certain subarchitecture (say `haswell` or `zen3`) while retaining compatibility with other subarchitectures, whereas `-march` might produce illegal instructions when run on the wrong hardware.

My reason for asking is that is makes it hard to use flang-new within Spack. Spack handles flang-new as the Fortran component of the clang compiler but unfortunately doesn't provide a way to specify different compiler flags for the C, C++, and Fortran compilers. -mtune is used as an option for most architectures (e.g., here), which breaks using flang-new to build anything with Spack.

Even if -mtune wasn't implemented immediately it would at least be helpful if flang-new wouldn't just stop compilation but just display a warning and continue.

[llvmbot]

@llvm/issue-subscribers-flang-ir

Author: Philipp Edelmann (tukss)

`flang-new` already knows about the `-march` option but not about `-mtune`. `clang` (and also `gcc`/`gfortran`) handles both options. For context, `-mtune` produces code that's optimized for a certain subarchitecture (say `haswell` or `zen3`) while retaining compatibility with other subarchitectures, whereas `-march` might produce illegal instructions when run on the wrong hardware.

My reason for asking is that is makes it hard to use flang-new within Spack. Spack handles flang-new as the Fortran component of the clang compiler but unfortunately doesn't provide a way to specify different compiler flags for the C, C++, and Fortran compilers. -mtune is used as an option for most architectures (e.g., here), which breaks using flang-new to build anything with Spack.

Even if -mtune wasn't implemented immediately it would at least be helpful if flang-new wouldn't just stop compilation but just display a warning and continue.

[banach-space]

@AlexisPerry , thanks for working on this! Given that #95043 has landed, can we close this ticket?

[tukss]

I was just about to test it but then I've seen it has been reverted in #96678.
So I guess we leave this open for now.

[AlexisPerry]

#95043 was reverted. A fixed version of the patch has been posted: #96688

[AlexisPerry]

PR #98517 has been merged. @tukss Could you check to see if the issue still persists? Thank you.

[llvmbot]

@llvm/issue-subscribers-flang-frontend

Author: Philipp Edelmann (tukss)

`flang-new` already knows about the `-march` option but not about `-mtune`. `clang` (and also `gcc`/`gfortran`) handles both options. For context, `-mtune` produces code that's optimized for a certain subarchitecture (say `haswell` or `zen3`) while retaining compatibility with other subarchitectures, whereas `-march` might produce illegal instructions when run on the wrong hardware.

My reason for asking is that is makes it hard to use flang-new within Spack. Spack handles flang-new as the Fortran component of the clang compiler but unfortunately doesn't provide a way to specify different compiler flags for the C, C++, and Fortran compilers. -mtune is used as an option for most architectures (e.g., here), which breaks using flang-new to build anything with Spack.

Even if -mtune wasn't implemented immediately it would at least be helpful if flang-new wouldn't just stop compilation but just display a warning and continue.

[tukss]

Yes, the option works fine for me on the current main branch. I can't speak to how much of an optimization benefit this brings but I mainly care about flang-new accepting the option for things like spack. That works great now. Thank you!