MaskRay wrote:

> @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_*`.

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