[clang] [z/OS] Set the default arch for z/OS to be arch10 (PR #89854)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> @FHe
> 
> > > > @MaskRay Got it.
> > > > The problem with that solution is that if you use --target you won't get the correct arch. This would be a problem for any cross compilation. For example, say you cross compile from zLinux (which wouldn't have the config file), the arch would be arch8. And if you cross compiled from z/OS (with the config file) to zLinux then the default arch would be arch10. Both of these would be incorrect.
> > > > We also use the config files for installation specific information. It is common for users to have their own config files. If we require that the option be in the config file then we would be creating a very error prone situation.
> > > > We need to be able to change the arch default based on the target triple.
> > > 
> > > 
> > > Sorry for the late reply. Such driver defaults via cmake variable would make testing brittle. Users expect that `check-clang` pass regardless of the cmake variable setting. If a test changes behavior due to different `-march=`, there are a few choices:
> > > 
> > > * add a `REQUIRES: zos-new-default-march` directive
> > > * hard code a `-march=`
> > > 
> > > Neither is immediately noticeable. In the past clang/test/Driver has had many such tests that require fixup. We have tried removing some unnecessary `CLANG_DEFAULT_*`.
> > 
> > 
> > I just realized that [reviews.llvm.org/D75914](https://reviews.llvm.org/D75914) added `CLANG_SYSTEMZ_DEFAULT_ARCH` for Ubuntu. @xnox @uweigand
> > I think we want to avoid such CMake options/clang `config.h`
> > As an alternative, if the clang executable is at `/tmp/Rel/bin/clang`, you can add `/tmp/Rel/bin/s390x-unknown-linux-gnu.cfg` with content `-march=z13`.
> > Once we drop `CLANG_SYSTEMZ_DEFAULT_ARCH`, the `config.h` include will not be needed.
> 
> I don't understand your proposal. In Ubuntu we need to set built-in default arch value, to a higher one than upstream has it at, for all builds without any toolchain configs. Such that any builds with any toolchain configs by default target a higher arch setting.
> 
> arch10 is way too old and slow.
> 
> and z13 might not be enough either for latest devel distributions, i.e. next development ubuntu release may be bumping further above z13.
> 
> Does your proposal address above needs?

Yes. See this part:

> As an alternative, if the clang executable is at /tmp/Rel/bin/clang, you can add /tmp/Rel/bin/s390x-unknown-linux-gnu.cfg with content -march=z13.

We should then remove  CLANG_SYSTEMZ_DEFAULT_ARCH (https://reviews.llvm.org/D75914)

https://github.com/llvm/llvm-project/pull/89854