[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:
> > 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.


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

https://reviews.llvm.org/D158218