[llvm-dev] Cross compiling C++ program

Sat Aug 5 05:41:52 PDT 2017

On 08/04/2017 04:34 PM, Jonathan Roelofs via llvm-dev wrote:
>
>
>
> On 8/4/17 3:04 PM, Matthias Braun wrote:
>> You may also take a look at the ELLCC project (www.ellcc.org
>> <http://www.ellcc.org>). As far as I understand it they produce and
>> package cross toolchains for a number of popular targets.
>
> AFAIU, ELLCC doesn't support baremetal. It only has linux targets.
>
>
> Jon
>
ELLCC has been used for baremetal stuff, but it takes some work. I'm
looking at adding NuttX support, which will make baremetal much easier.

-Rich

>>
>> - Matthias
>>
>>> On Aug 4, 2017, at 12:54 PM, Jonathan Roelofs via llvm-dev
>>> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> 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
>>> 2) Install the libc headers there (without having built it)
>>> 3) Build clang w/ baremetal cache, pointing it at the partially
>>> built sysroot.
>>> 4) Build your libc.
>>> 4A) Test your libc.
>>> 4B) Install your libc in your sysroot.
>>> 5) Build libcxxabi. (it's build needs to know where the source for
>>> libcxx lives)
>>> 5A) Test libcxxabi. (testing it requires knowing where source for
>>> libcxx lives. use remote executors & linker scripts to run on your
>>> board, or in qemu)
>>> 5B) Install libcxxabi in your sysroot.
>>> 6) Build libunwind.
>>> 6A) Test libunwind. (testing it requires knowing where source for
>>> libcxx lives. use remote executors & linker scripts to run on your
>>> board, or in qemu)
>>> 6B) Install libunwind in your sysroot.
>>> 7) Build libcxx.
>>> 7A) Test libcxx. (use remote executors & linker scripts to run on
>>> your board, or in qemu)
>>> 7B) Install libcxx in your sysroot.
>>>
>>>
>>> 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).
>>>
>>>> If you know any doc on how to get clang++ compile for any bare
>>>> metal arm, I'll be glad to RTFM. :o)
>>>
>>> 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.
>>>
>>>
>>> Jon
>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>> -- 
>>> Jon Roelofs
>>> jonathan at codesourcery.com <mailto:jonathan at codesourcery.com>
>>> CodeSourcery / Mentor Embedded / Siemens
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
> -- 
> Jon Roelofs
> jonathan at codesourcery.com
> CodeSourcery / Mentor Embedded / Siemens
>
>
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