[libcxx-commits] [PATCH] D99484: Use `GNUInstallDirs` to support custom installation dirs.

[Petr Hosek via Phabricator via libcxx-commits]

phosek added inline comments.


================
Comment at: libcxx/cmake/Modules/HandleLibCXXABI.cmake:66
           install(FILES "${LIBCXX_BINARY_INCLUDE_DIR}/${fpath}"
-            DESTINATION ${LIBCXX_INSTALL_HEADER_PREFIX}include/c++/v1/${dstdir}
+            DESTINATION ${LIBCXX_INSTALL_HEADER_PREFIX}${CMAKE_INSTALL_INCLUDEDIR}/c++/v1/${dstdir}
             COMPONENT cxx-headers
----------------
phosek wrote:
> ldionne wrote:
> > Ericson2314 wrote:
> > > ldionne wrote:
> > > > compnerd wrote:
> > > > > Ericson2314 wrote:
> > > > > > compnerd wrote:
> > > > > > > @ldionne - how is the `LIBCXX_INSTALL_HEADER_PREFIX` used?  Can altering the value of `CMAKE_INSTALL_INCLUDEDIR` serve the purpose?
> > > > > > It is sometimes modified to be per target when multiple targets are being used at once. All things `CMAKE_INSTALL_*` are globally scoped so in general the combination builds are quite awkward.
> > > > > > 
> > > > > > (Having worked on Meson, I am really missing https://mesonbuild.com/Subprojects.html which is exactly what's needed to do this without these bespoke idioms that never work well enough . Alas...)
> > > > > I don't think that bringing up other build systems is particularly helpful.
> > > > > 
> > > > > I do expect it to be modified, and I suspect that this is used specifically for builds that @ldionne supports.
> > > > Actually, I've never used it myself, but @phosek seems to be using it for the Runtimes build to output one set of headers for each target, as mentioned above.
> > > > 
> > > > It seems to me that tweaking `CMAKE_INSTALL_PREFIX` globally when driving the libc++ build from the runtimes build would be more in-line with the CMake way of doing things (one configuration == one build), but I'm curious to hear what @phosek has to say about that.
> > > > It seems to me that tweaking CMAKE_INSTALL_PREFIX globally when driving the libc++ build from the runtimes build would be more in-line with the CMake way of doing things (one configuration == one buid)
> > > 
> > > You mean trying to mutate it during the libc++ CMake eval and then set it back after? That would keep the intended meaning of  `CMAKE_INSTALL_PREFIX` being the actual prefix, but that feels awfully fragile to me. Or do you mean something else?
> > I keep forgetting that the runtimes build uses `add_subdirectory` to include each sub-project instead of driving a proper CMake build for each of those.
> > 
> > Basically, what I'm saying is that whatever place we decide to build for N multiple targets, we should perform N different CMake builds setting the appropriate `CMAKE_INSTALL_PREFIX`, one for each target. But that discussion should happen elsewhere, not on this review.
> > 
> > As far as this review is concerned, I do believe you want instead:
> > 
> > ```
> > ${CMAKE_INSTALL_INCLUDEDIR}${LIBCXX_INSTALL_HEADER_PREFIX}
> > ```
> > 
> > (reversed the order of variables)
> > 
> > We should have @phosek confirm.
> @ldionne `CMAKE_INSTALL_PREFIX` cannot be used for this purpose. When using the multiarch layout with the runtimes build, we want to place build artifacts in `${BUILD_DIR}/{include,lib}/<target>/...` and install them to `${CMAKE_INSTALL_PREFIX}/{include,lib}/<target>/...`. There are two issues:
> 1. `CMAKE_INSTALL_PREFIX` only comes into play during install step, but we want the build layout to match the installation layout so we need a different variable to control the output directory.
> 2. We already use `CMAKE_INSTALL_PREFIX` for the toolchain itself and there's no way to use it hierarchically, that is you cannot do something like `${SUPER_CMAKE_INSTALL_PREFIX}/{include,lib}/${CMAKE_INSTALL_PREFIX}/...` which is why we need a separate variable to control the installation directory.
> 
> Regarding `LIBCXX_INSTALL_HEADER_PREFIX`, I haven't found any current uses of that variable. I introduced it in D59168 which was an earlier attempt at having per-target headers, but D89013 is strictly better for the reasons I described in https://reviews.llvm.org/D89013#2662578 (no duplicate headers) so I think we can just remove this variable.
I have sent D99697 for review which removes these variables.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D99484/new/

https://reviews.llvm.org/D99484