[PATCH] D158218: [CMake] Deprecate DEFAULT_SYSROOT and GCC_INSTALL_PREFIX

[Fangrui Song via Phabricator via cfe-commits]

MaskRay added inline comments.


================
Comment at: clang/CMakeLists.txt:179-183
+if(DEFAULT_SYSROOT)
+  message(WARNING "DEFAULT_SYSROOT is deprecated and will be removed. Use "
+    "configuration files (https://clang.llvm.org/docs/UsersManual.html#configuration-files)"
+    "to specify the default --sysroot=")
+endif()
----------------
bcain wrote:
> MaskRay wrote:
> > MaskRay wrote:
> > > bcain wrote:
> > > > MaskRay wrote:
> > > > > bcain wrote:
> > > > > > At one time I believe that the clang configuration files could not specify paths relative to the clang executable.  AFAICT `DEFAULT_SYSROOT` does support this.
> > > > > > 
> > > > > > But if I'm mistaken about that can we add an example to the docs at https://clang.llvm.org/docs/UsersManual.html#configuration-files illustrating how to use a relative sysroot?
> > > > > Clang configuration files just complement user-specified command line options. As one can do `--sysroot=./sysroot`, one can add `--sysroot=./sysroot` to a configuration file, too.
> > > > > 
> > > > > If you think having a sysroot example is useful, I can add
> > > > > 
> > > > > ```
> > > > > # Relative --sysroot
> > > > > --sysroot=./sysroot
> > > > > ```
> > > > > before
> > > > > clang/docs/UsersManual.rst:1018 `-c --target=x86_64-unknown-linux-gnu`
> > > > IIUC: when clang takes a `--sysroot=./sysroot` argument, it will interpret that path as a prefix to the files it wants to access.  So the system will treat it as relative to the environment's `cwd`, correct?
> > > > 
> > > > But when `DEFAULT_SYSROOT` is set to a relative path, that relative path is considered to be relative to `clang`.  Therefore a convenient feature that we take advantage of is setting it to something like `../target/hexagon-unknown-linux-musl` in order to have anyone who invokes `hexagon-unknown-linux-clang` from any path be able to find the includes and libraries distributed with the toolchain without having to specify the sysroot.
> > > > 
> > > > Maybe there's a better way to achieve this without the need for a relative `DEFAULT_SYSROOT` but it's been very useful and the config files do not seem suited to replace it.
> > > `--sysroot=` is used as a prefix to certain files, primarily libc and GCC installations.
> > > 
> > > `-DDEFAULT_SYSROOT=...` just changes `clang/lib/Driver/Driver.cpp:203` `SysRoot(DEFAULT_SYSROOT)`.
> > > There is no magic related to the `clang` executable path. CMake doesn't do any magic, either.
> > Ah, sorry. There is magic: D76653 (@sbc100).
> > Ah, sorry. There is magic: D76653 (@sbc100).
> 
> Okay, so should we abandon the plan to deprecate `DEFAULT_SYSROOT`?  Or could we add a corresponding feature to config files if we want to get rid of it?
The way relative `DEFAULT_SYSROOT` looks strange to me as it is not exactly equivalent to user-specified `--sysroot=`. I hope that there is a way to achieve @sbc100's goal.

I think a proper mechanism will take time, and we probably can just deprecated `GCC_INSTALL_PREFIX` for now.
And I wish that RHEL/CentOS can migrate to a modern practice.


Repository:
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https://reviews.llvm.org/D158218