[llvm-dev] Cross compiling C++ program

[Jonathan Roelofs via llvm-dev]

On 8/2/17 5:57 PM, Goran Mekić wrote:
> On Wed, Aug 02, 2017 at 05:48:20PM -0600, Jonathan Roelofs wrote:
>> I strongly recommend against using the host's headers when cross compiling.
>> You need to either find or build an arm-none-eabi sysroot, and use the
>> --sysroot= flag.
>>
>>
>> Jon
> OK, that's a start. I found https://www.llvm.org/docs/HowToCrossCompileLLVM.html. Is it enough for --sysroot? In case it is, what should be the value of -DLLVM_DEFAULT_TARGET_TRIPLE if I want to use FreeBSD as host and Cortex-M4 as sysroot? Thanx!

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


>
> Regards,
> meka

-- 
Jon Roelofs
jonathan at codesourcery.com
CodeSourcery / Mentor Embedded / Siemens