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

Petr Hosek via Phabricator via lldb-commits lldb-commits at lists.llvm.org
Wed Mar 31 17:17:58 PDT 2021

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}
             COMPONENT cxx-headers
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:
> ```
> ```
> (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.

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