[CMake] cmake build of LLVM: HAVE_CXX_ATOMICS_WITHOUT_LIB test failure

[SC Warrior]

Thanks for your response Shoaib.  I've got it building now.  It required
modifying the LLVM cmake scripts to include the libraries.  3.7 is a hard
requirement.  Cache file, great.  Thanks!

On Thu, Apr 11, 2019 at 2:09 PM Shoaib Meenai <smeenai at fb.com> wrote:

> llvm-dev at lists.llvm.org would also be a good place to ask questions about
> the LLVM build system :)
>
>
>
>    1. I believe it’s just called the configure phase. (There’s also the
>    generation phase, where CMake actually generates the build system.)
>    2. How have you set up your cross-compilation? LLVM’s build system has
>    logic to link pthread if needed, so I’m wondering why that’s failing.
>    3. LLVM’s cross-compilation runs a sub-configure for some native build
>    components (e.g. TableGen) – perhaps one of those sub-configures is
>    succeeding but the main configure is failing? I’ve never seen the “Build
>    files have been written to” and “Configuring incomplete, errors occurred”
>    messages from the same configure.
>    4. You can use a CMake cache file – see “-C <initial-cache>” in
>    https://cmake.org/cmake/help/latest/manual/cmake.1.html#options
>
>
>
> How come you’re working with LLVM 3.7, btw? That’s pretty old at this
> point, and newer versions will have improved the build system (and it’s
> also a lot easier to provide assistance for them).
>
>
>
> *From: *CMake <cmake-bounces at cmake.org> on behalf of SC Warrior <
> bhhjkl at gmail.com>
> *Date: *Wednesday, April 10, 2019 at 5:09 PM
> *To: *"cmake at cmake.org" <cmake at cmake.org>
> *Subject: *[CMake] cmake build of LLVM: HAVE_CXX_ATOMICS_WITHOUT_LIB test
> failure
>
>
>
> I watched Chris Bieneman's talk
> <https://www.youtube.com/watch?v=StF77Cx7pz8> about the transition of
> LLVM from autoconf to cmake and he raved about how friendly / vibrant the
> cmake community is so I thought I'd ask this here.  I posted it to stack
> overflow but it's fairly involved so I hope it's cool that I post (my
> first) in this forum.
>
>
>
> I'm getting this failure trying to cross compile LLVM v3.7 (linux x64 ->
> Aarch64) w/ cmake:
>
> <path to clang>/clang++  -DHAVE_CXX_ATOMICS_WITHOUT_LIB -std=c++11  -rdynamic CMakeFiles/cmTC_82a21.dir/src.cxx.o  -o cmTC_82a21  -lm -ldl && :
>
> ld.lld: error: undefined symbol: pthread_once
>
> referenced by cxa_exception_storage.cpp
>
> this is then followed by numerous other "undefined symbol: pthread_blah"
> errors in the CMakeError.log file.
>
> I have four questions:
>
> (1) What is this phase of operation that cmake is performing to determine
> the capabilities of the build environment / platform called ? (i.e. the
> configure-like phase that performs numerous small compilation tests to see
> if they succeed, looks to see if it can find header files in expected
> paths, etc).
>
> (2) It looks to me that including -lpthread as part of the test
> compilation line would fix this problem. (a) Is there a way to tell cmake
> to do that and (b) is that a smart approach to solving this problem ? Seems
> like if I could tell cmake to do this it would pass these tests.
>
> (3) The cmake command *seems* to succeed as it finishes with "Build files
> have been written to ...". What should I make of that when one of these
> pre-tests has a failure condition as cited above and reports "Configuring
> incomplete, errors occurred!".
>
> (4) I'm trying to get this cmake cross compile build to work with the
> stock LLVM v3.7 cmake files so I'm trying to control cmake using command
> line options only. This gets a little ugly and I'm wondering if there's a
> simple way to put all of these settings in a cmake file instead. One
> problem with that approach is that I'm not familiar with all of the targets
> in LLVM so if I were, to say, attempt to add libraries to targets to fix a
> linking problem, I wouldn't know offhand what targets to add them to. I'm
> hoping that in this case I can hit it with a bigger hammer and have them
> added everywhere with hopes of them being ignored if not used.
>
> I gratefully thank you for your patience and time reading all of that ^^^.
>
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