[llvm-dev] Cross compiling C++ program

[Goran Mekić via llvm-dev]

On Fri, Aug 04, 2017 at 01:54:33PM -0600, Jonathan Roelofs wrote:
>
>
> On 8/4/17 1:14 PM, Goran Mekić via llvm-dev wrote:
> > On Thu, Aug 03, 2017 at 08:22:24AM -0600, Jonathan Roelofs wrote:
> > > IIUC, you don't want to cross compile llvm itself (which is what those
> > > instructions are for), but instead you want to *use* llvm to cross compile
> > > things.
> > >
> > > To build your sysroot, you'll need to cross-build:
> > >
> > >    1) A libc. Good choices for that for baremetal are: newlib or musl.
> > >    2) Builtins. In llvm, that's provided in the compiler-rt repo. Otherwise
> > > you can use libgcc from an arm-none-eabi-gcc toolchain.
> > >    3) If you want c++, you'll also need:
> > >       3A) c++ abi library. llvm provides this as libcxxabi. There's also
> > > libsupc++, and libcxxrt, depending on what licenses you're comfortable with.
> > >       3B) An unwinder. llvm provides libunwind. There's also one packaged in
> > > libgcc_s (depending on how it was built), and another from the old HP
> > > libunwind project (different codebase, but same name as the llvm one).
> > >       3C) A c++ standard library. llvm provides this as libcxx. There's also
> > > libstdc++.
> > >
> > >
> > > To start with, I recommend using the cmake cache in:
> > > $clang_src/cmake/caches/BaremetalARM.cmake. You'll want the stuff in it that
> > > references the armv7em-none-eabi triple. To use it, do something like:
> > >
> > > $  cmake -G <build system>
> > > -DBAREMETAL_ARMV7EM_SYSROOT=path/to/your/v7em/sysroot -C
> > > path/to/BaremetalARM.cmake [other CMake Options]
> > >
> > > Assuming your sysroot has libc headers in it, that should get you a clang
> > > that's capable of compiling basic things, which you can use to build all the
> > > libraries above.
> > >
> > >
> > > Jon
> >
> > Hello,
> >
> > Your input was very valuable, but I must be too noob so I have to ask. I tried to read as much as I could about musl, compiler-rt and libunwind to understand what are those libs and I decided to go with a "relexed" licenced alternatives. :o) If I got it right, the order of repos I need to compile is:
> >
> > - musl
> > - compiler-rt
> > - libcxxabi
> > - libunwind
> > - libcxx
> > - clang (with the suggestion about baremetal cache)
>
>
> Not quite. You'll need clang built in order to build the other things. (I
> guess you could use your existing toolchain to do that, but IIUC, you want
> to use clang for all of it & bootstrap a full clang-based toolchain). For
> that, the usual order is to:
>
> 1) Pick a directory as your sysroot
~/arm-sysroot

> 2) Install the libc headers there (without having built it)
What I did was mv musl-*/include ~/arm-sysroot/usr/include.

> 3) Build clang w/ baremetal cache, pointing it at the partially built
> sysroot.
 cmake -G 'Unix Makefiles' -DBAREMETAL_ARMV7EM_SYSROOT=~/arm-sysroot -C ../../code/clang/cmake/caches/BaremetalARM.cmake ../../code/clang
loading initial cache file ../../code/clang/cmake/caches/BaremetalARM.cmake
-- Found LLVM_CONFIG as /usr/local/bin/llvm-config
-- Building with -fPIC
-- Could NOT find Z3 (missing:  Z3_LIBRARIES Z3_INCLUDE_DIR) (Required is at least version "4.5")
-- Clang version: 4.0.1
CMake Error at /usr/local/llvm40/lib/cmake/llvm/LLVM-Config.cmake:203 (message):
  Library `BinaryFormat' not found in list of llvm libraries.
Call Stack (most recent call first):
  /usr/local/llvm40/lib/cmake/llvm/AddLLVM.cmake:508 (llvm_map_components_to_libnames)
  cmake/modules/AddClang.cmake:86 (llvm_add_library)
  lib/AST/CMakeLists.txt:6 (add_clang_library)

-- Configuring incomplete, errors occurred!
See also "/usr/home/meka/repos/clang/build/clang/CMakeFiles/CMakeOutput.log".



> A note on linker scripts: support for them in lld is still a bit rough
> around the edges, so you may want to use the linker that comes with your
> board (presumably binutils ld).
Thank you!

> Sorry, I don't have a doc for that. We're getting closer to making it work
> "out of the tin", but it's not there yet. We (wearing my CodeSourcery hat)
> are working on it, but it's going to take some time before it "just works"
> upstream.
I will test what ever you throw my way regarding this (or write a doc).
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