llvm dev - Aug 2017 - Cross compiling C++ program

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!

Regards,
meka
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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

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)

If you know any doc on how to get clang++ compile for any bare metal arm,
I'll be glad to RTFM. :o)
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