[cfe-users] Need a GCC-free LLVM/Clang on Linux.

[Richard Smith via cfe-users]

On Thu, 23 Jan 2020 at 07:24, Christopher H Green via cfe-users <
cfe-users at lists.llvm.org> wrote:

> Hi,
>
> I've spent the last several days trying to build a fast, full-featured
> relocatable distribution of LLVM/Clang 9.0.1 on Linux RHEL7, which has an
> older native GCC (4.8.5)—I can't require the RH toolset. I have access to a
> modern version of GCC in order to make the stage-1 build, but I need the
> final product not only to be relocatable, but also to be free of any
> dependence on that GCC or its libraries. In other words, I want to be able
> to distribute an LLVM/CLang which defaults to compiling using libc++,
> libcxxabi, compiler-rt, libunwind, and ldd, and which itself depends
> neither on libstdc++ or  libgcc_s. I'd also like to be able to to provide
> LTO, but whether the compiler is itself the beneficiary of LTO is optional.
> As a final wish, I'd like the distribution as installed to serve as an SDK
> against which to build other components such as separately packaged lldb
> and f18. Oh, and I want a pony.
>
> Here's what I have so far (CMake cache files attached for stages 1 and 2).
> Invoked  in a separate build directory with:
>
> env CC=gcc-9 CXX=g++-9 \
> 'CXXFLAGS=-Wno-cast-function-type -Wno-deprecated-copy -Wno-init-list-lifetime -Wno-pessimizing-move -Wno-redundant-move -Wno-uninitialized -Wno-unused-function -Wno-unused-variable' \
> cmake -GNinja -C FNAL.cmake -DCMAKE_INSTALL_PREFIX=<install-path> \
> -DBOOTSTRAP_LLVM_CXX_STD=c++17 <src-path>
> ninja
> ninja install
>
> (The CXXFLAGS are just to keep the noise down during the stage-1 build).
>
> I appear (finally) to have a mostly functioning build *in situ*, but the
> installed compiler depends upon libstdc++ to be able to run, and I should
> have put --rtlib=compiler-rt in LINKER_FLAGS rather than CXX_FLAGS.
> Additionally, I haven't been able to come up with a good set of targets for
> LLVM_DISTRIBUTION_COMPONENTS, but when I do I think I need to exclude
> static libraries if I'm doing LTO, no?
>
> I suspect I need a three stage build—well, two stage 2s, at least—but any
> advice would be appreciated as I'm wandering around in the dark at this
> point.
>
I have a two-stage setup that builds a GCC-free Clang except for libgcc_s
(I've not looked at what's pulling that in yet, but I expect that's
resolvable).

My stage1 cmake setup has: -DCLANG_DEFAULT_CXX_STDLIB=libc++
-DCLANG_DEFAULT_RTLIB=compiler-rt
My stage2 cmake setup additionally has: -DLIBCXXABI_USE_LLVM_UNWINDER=ON
-DLIBCXXABI_USE_COMPILER_RT=ON -DLIBCXX_USE_COMPILER_RT=ON
(and points CMAKE_C_COMPILER and CMAKE_CXX_COMPILER at the stage1 clang).

There might be some other things you need, I'm not sure. But it should be
doable in two stages.

> Many thanks for any help,
>
> Chris.
>
>
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