[llvm-dev] Cross compiling C++ program

[Goran Mekić via llvm-dev]

Hello,

As a newcomer to the libcxx implementation, I'll probably ask a silly question, but why following example can be compiled natively, but not cross compiled?

#include <cstdlib>
using ::atof;
int main() {
return 0;
}

Native compile: clang++ -o example example.cpp
Cross compile: clang++ -fno-builtin -fno-exceptions -fcheck-new -nostdlib -ffreestanding -target arm-none-eabi -march=armv7-m -mcpu=cortex-m4 -Wall -Wshadow -Wundef -g -Os -fno-strict-aliasing -fomit-frame-pointer -mcpu=cortex-m4 -mthumb -mfloat-abi=soft -stdlib=libc++ -o example example.cpp

Error I get:
In file included from example.cpp:1:
In file included from /usr/include/c++/v1/cstdlib:85:
/usr/include/c++/v1/__config:938:4: error: "No thread API"
# error "No thread API"
^
1 error generated.


Without -stdlib=libc++ I get this error:
example.cpp:1:10: fatal error: 'cstdlib' file not found
#include <cstdlib>
^~~~~~~~~
1 error generated.

My OS is FreeBSD 11.1 and Clang version is 4.0.0. What am I not understanding well enough and could you recommend reading something on the topic? Any help is welcome! Thank you!

Regards,
meka

[Jonathan Roelofs via llvm-dev]

On 8/2/17 5:28 PM, Goran Mekić via llvm-dev wrote:
> Hello,
>
> As a newcomer to the libcxx implementation, I'll probably ask a silly question, but why following example can be compiled natively, but not cross compiled?
>
> #include <cstdlib>
> using ::atof;
> int main() {
> return 0;
> }
>
> Native compile: clang++ -o example example.cpp
> Cross compile: clang++ -fno-builtin -fno-exceptions -fcheck-new -nostdlib -ffreestanding -target arm-none-eabi -march=armv7-m -mcpu=cortex-m4 -Wall -Wshadow -Wundef -g -Os -fno-strict-aliasing -fomit-frame-pointer -mcpu=cortex-m4 -mthumb -mfloat-abi=soft -stdlib=libc++ -o example example.cpp
>
> Error I get:
> In file included from example.cpp:1:
> In file included from /usr/include/c++/v1/cstdlib:85:

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

> /usr/include/c++/v1/__config:938:4: error: "No thread API"
> # error "No thread API"
> ^
> 1 error generated.
>
>
> Without -stdlib=libc++ I get this error:
> example.cpp:1:10: fatal error: 'cstdlib' file not found
> #include <cstdlib>
> ^~~~~~~~~
> 1 error generated.
>
> My OS is FreeBSD 11.1 and Clang version is 4.0.0. What am I not understanding well enough and could you recommend reading something on the topic? Any help is welcome! Thank you!
>
> Regards,
> meka
>
>
>

> _______________________________________________
> LLVM Developers mailing list
> llvm...@lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>

--
Jon Roelofs
jona...@codesourcery.com
CodeSourcery / Mentor Embedded / Siemens
_______________________________________________
LLVM Developers mailing list
llvm...@lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

[Goran Mekić via llvm-dev]

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

[Jonathan Roelofs via llvm-dev]

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

[Goran Mekić via llvm-dev]

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)

[Jonathan Roelofs via llvm-dev]

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

[Matthias Braun via llvm-dev]

You may also take a look at the ELLCC project (www.ellcc.org). As far as I understand it they produce and package cross toolchains for a number of popular targets.

- Matthias

[Jonathan Roelofs via llvm-dev]

On 8/4/17 3:04 PM, Matthias Braun wrote:

You may also take a look at the ELLCC project (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

[Goran Mekić via llvm-dev]

On Fri, Aug 04, 2017 at 02:04:17PM -0700, 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.
>
> - Matthias

I am talking with him on IRC to try to make ELLCC run on FreeBSD via Linux emulation and maybe get started with that.

[Goran Mekić via llvm-dev]

~/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).

[Jonathan Roelofs via llvm-dev]

I think you need to copy them, not move them.

>
>> 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

Sorry, I wasn't explicit enough. You need to define all of the variables
that the cache is looking for.

> 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".

I think it found an installed llvm, and is trying to use that to build
clang against. First, the clang and llvm's versions have to match.
Second, when I mentioned building clang, I really meant building
llvm+clang... sorry.

$ cmake -G 'Unix Makefiles' -DBAREMETAL_ARMV7EM_SYSROOT=~/arm-sysroot -C
path/to/copy/of/edited/BaremetalARM.cmake path/to/llvm

Where your path/to/llvm has the symlinks:

* path/to/llvm/tools/clang -> path/to/clang
* path/to/llvm/runtimes/compiler_rt path/to/compiler_rt

And by "edited BaremetalARM.cmake" I mean you can strip out the other
targets in there that you don't need.

Jon

>
>
>
>> 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).
>
>
>

[Goran Mekić via llvm-dev]

On Fri, Aug 04, 2017 at 04:03:35PM -0600, Jonathan Roelofs wrote:
> > > 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.
>
> I think you need to copy them, not move them.

On a new directories and fresh untar of musl, I don't think there's a difference

ln -s `which clang40` path/to/llvm/tools/clang
> * path/to/llvm/runtimes/compiler_rt path/to/compiler_rt
ln -s path/to/llvm/projects/compiler-rt path/to/llvm/runtimes/compiler_rt

Is that right?

>
> And by "edited BaremetalARM.cmake" I mean you can strip out the other
> targets in there that you don't need.

So I have copied BaremetalARM.cmake into llvm/cmake/caches/BaremetalARM.cmake to be able to alter it, I've put all other repos in llvm/projects and the command is:

# cmake -G 'Unix Makefiles' -DBAREMETAL_ARMV7EM_SYSROOT=~/arm-sysroot -DBAREMETAL_ARMV6M_SYSROOT=~/arm-sysroot -DBAREMETAL_ARMV7M_SYSROOT=~/arm-sysroot -C ../../code/llvm/cmake/caches/BaremetalARM.cmake ../../code/llvm

Before adding all repos into llvm/projects, it complained about 'Specified distribution component 'clang' doesn't have a target' (just an example). Now I have 'Specified distribution component 'builtins-armv6m-none-eabi' doesn't have a target' as error.

[Jonathan Roelofs via llvm-dev]

On 8/4/17 4:29 PM, Goran Mekić wrote:
> On Fri, Aug 04, 2017 at 04:03:35PM -0600, Jonathan Roelofs wrote:
>>>> 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.
>> I think you need to copy them, not move them.
> On a new directories and fresh untar of musl, I don't think there's a difference

I was assuming you'd want to build it from the same sources later...

No, not the binary, the source.

The closer to top of trunk you pull the source, the easier it'll be to
get it working. A lot of the upstreaming of baremetal support has been
*very* recent.... "bleeding edge" recent.

>> * path/to/llvm/runtimes/compiler_rt path/to/compiler_rt
> ln -s path/to/llvm/projects/compiler-rt path/to/llvm/runtimes/compiler_rt
>
> Is that right?

Sorry, it should be spelled `compiler-rt`, not `compiler_rt`.

>> And by "edited BaremetalARM.cmake" I mean you can strip out the other
>> targets in there that you don't need.
> So I have copied BaremetalARM.cmake into llvm/cmake/caches/BaremetalARM.cmake to be able to alter it, I've put all other repos in llvm/projects and the command is:

Leave all the others out for now. An in-tree build of them is just going
to give you host binaries, which isn't helpful in your case. You'll have
to build them "out of tree".

Might be a helpful exercise for you to try building vanilla clang with
runtimes for the host, before worrying about how to build everything for
your baremetal case:
http://llvm.org/docs/GettingStarted.html#getting-started-quickly-a-summary

>
> # cmake -G 'Unix Makefiles' -DBAREMETAL_ARMV7EM_SYSROOT=~/arm-sysroot -DBAREMETAL_ARMV6M_SYSROOT=~/arm-sysroot -DBAREMETAL_ARMV7M_SYSROOT=~/arm-sysroot -C ../../code/llvm/cmake/caches/BaremetalARM.cmake ../../code/llvm
>
> Before adding all repos into llvm/projects, it complained about 'Specified distribution component 'clang' doesn't have a target' (just an example). Now I have 'Specified distribution component 'builtins-armv6m-none-eabi' doesn't have a target' as error.

I think that error happened because you symlinked the clang binary in,
not the source.

Jon

[Goran Mekić via llvm-dev]

On Fri, Aug 04, 2017 at 04:40:44PM -0600, Jonathan Roelofs wrote:
> Might be a helpful exercise for you to try building vanilla clang with
> runtimes for the host, before worrying about how to build everything for
> your baremetal case:
> http://llvm.org/docs/GettingStarted.html#getting-started-quickly-a-summary

I think I'll start with that, first :o)

[Richard Pennington via llvm-dev]

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). 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

[Goran Mekić via llvm-dev]

On Fri, Aug 04, 2017 at 04:40:44PM -0600, Jonathan Roelofs wrote:

> Might be a helpful exercise for you to try building vanilla clang with
> runtimes for the host, before worrying about how to build everything for
> your baremetal case:
> http://llvm.org/docs/GettingStarted.html#getting-started-quickly-a-summary

So I did. :o) It was fun realizing how much Release type reduces LD memory usage :o)

Anyway, just to be clear what "I did" means:
- llvm sources from trunk and projects/tools repos checkout under llvm
- clang-6.0 compiles "Hello World"
- make check-all fails (not sure I'm not causing it)
- make install creates everything under PREFIX

# cmake -G 'Unix Makefiles' -DCMAKE_BUILD_TYPE=Release -DLLVM_TARGETS_TO_BUILD=X86 -DCMAKE_INSTALL_PREFIX=/tmp/x86-sysroot ../../source/llvm
# make -j4
# make install

It was fun learning how to manualy compile compiler for the first time. Now how to cross compile for bare? Given that I have musl-1.1.16 directory and that I'm preseeding sysroot with:

# rm -rf /tmp/barearm-sysroot
# mkdir -p /tmp/barearm-sysroot
# cp -r musl-1.1.16/include /tmp/barearm-sysroot/ # Thanx for the tip!

So I tried to use it, and clang/llvm build, but compiler-rt fails:

# cmake -G 'Unix Makefiles' -DCMAKE_BUILD_TYPE=Release -DBAREMETAL_ARMV7EM_SYSROOT=/tmp/barearm-sysroot -DCMAKE_INSTALL_PREFIX=/tmp/barearm-sysroot -C ../../source/llvm/cmake/caches/BaremetalARM.cmake ../../source/llvm
# make -j4
https://gist.github.com/anonymous/85cd5081a6d46a63731d970ee95b20b4

This is the BaremetalARM.cmake:
https://gist.github.com/anonymous/ff3248f08ddc99361dcc009e9f4c1020

[Richard Pennington via llvm-dev]

On 08/05/2017 08:41 AM, Goran Mekić via llvm-dev wrote:

>
> So I tried to use it, and clang/llvm build, but compiler-rt fails:
>
> # cmake -G 'Unix Makefiles' -DCMAKE_BUILD_TYPE=Release -DBAREMETAL_ARMV7EM_SYSROOT=/tmp/barearm-sysroot -DCMAKE_INSTALL_PREFIX=/tmp/barearm-sysroot -C ../../source/llvm/cmake/caches/BaremetalARM.cmake ../../source/llvm
> # make -j4
> https://gist.github.com/anonymous/85cd5081a6d46a63731d970ee95b20b4
>
> This is the BaremetalARM.cmake:
> https://gist.github.com/anonymous/ff3248f08ddc99361dcc009e9f4c1020

It is slightly tricky to compile compile musl and compiler-rt. The bits
directory is populated when you install musl, but musl needs compiler-rt
built if you want to build libc.so.
The solution that works is to first do a "make install-headers" on musl,
then build compiler-rt, then build musl. If you look at the ELLCC
make-libraries and build-musl scripts, you'll see how ELLCC does it. Or
you can disable building the musl shared library.

The other problem you'll run into is that musl is nowhere near bare
metal. It's whole purpose in life is to turn the C standard library
calls into Linux system calls. While a baremetal version of musl would
be very nice to have, it is a non-trivial project. Baremetal is what
something like NuttX's C library provides.

-Rich

[Goran Mekić via llvm-dev]

On Sat, Aug 05, 2017 at 09:19:57AM -0500, Richard Pennington via llvm-dev wrote:
> On 08/05/2017 08:41 AM, Goran Mekić via llvm-dev wrote:
>
> >
> > So I tried to use it, and clang/llvm build, but compiler-rt fails:
> >
> > # cmake -G 'Unix Makefiles' -DCMAKE_BUILD_TYPE=Release -DBAREMETAL_ARMV7EM_SYSROOT=/tmp/barearm-sysroot -DCMAKE_INSTALL_PREFIX=/tmp/barearm-sysroot -C ../../source/llvm/cmake/caches/BaremetalARM.cmake ../../source/llvm
> > # make -j4
> > https://gist.github.com/anonymous/85cd5081a6d46a63731d970ee95b20b4
> >
> > This is the BaremetalARM.cmake:
> > https://gist.github.com/anonymous/ff3248f08ddc99361dcc009e9f4c1020
> It is slightly tricky to compile compile musl and compiler-rt. The bits
> directory is populated when you install musl, but musl needs compiler-rt
> built if you want to build libc.so.
> The solution that works is to first do a "make install-headers" on musl,
> then build compiler-rt, then build musl. If you look at the ELLCC
> make-libraries and build-musl scripts, you'll see how ELLCC does it. Or
> you can disable building the musl shared library.

Thanx for this. I got to libunwind (libcxxabi expects it as dependency) and this is the error I get:

# set -xg CC /tmp/barearm-sysroot/bin/clang-6.0
# set -xg CXX /tmp/barearm-sysroot/bin/clang-6.0
# cmake -G 'Unix Makefiles' -DCMAKE_BUILD_TYPE=Release -DBAREMETAL_ARMV7EM_SYSROOT=/tmp/barearm-sysroot -DCMAKE_INSTALL_PREFIX=/tmp/barearm-sysroot -C ../../source/llvm/cmake/caches/BaremetalARM.cmake ../../source/libunwind
# make -j4 VERBOSE=1
https://gist.github.com/anonymous/b3a211e90808058a2c0b083154ddf5c7

> The other problem you'll run into is that musl is nowhere near bare
> metal. It's whole purpose in life is to turn the C standard library
> calls into Linux system calls. While a baremetal version of musl would
> be very nice to have, it is a non-trivial project. Baremetal is what
> something like NuttX's C library provides.

I know, but I though that this is how ELLCC is built, too. Baremetal clang build with musl not having much to with libc that NuttX uses. Also, I though this is a way to create sysroot. Am I wrong?

[Jonathan Roelofs via llvm-dev]

On 8/5/17 7:41 AM, Goran Mekić wrote:
> On Fri, Aug 04, 2017 at 04:40:44PM -0600, Jonathan Roelofs wrote:
>> Might be a helpful exercise for you to try building vanilla clang with
>> runtimes for the host, before worrying about how to build everything for
>> your baremetal case:
>> http://llvm.org/docs/GettingStarted.html#getting-started-quickly-a-summary
> So I did. :o) It was fun realizing how much Release type reduces LD memory usage :o)
>
> Anyway, just to be clear what "I did" means:
> - llvm sources from trunk and projects/tools repos checkout under llvm
> - clang-6.0 compiles "Hello World"
> - make check-all fails (not sure I'm not causing it)
> - make install creates everything under PREFIX
>
> # cmake -G 'Unix Makefiles' -DCMAKE_BUILD_TYPE=Release -DLLVM_TARGETS_TO_BUILD=X86 -DCMAKE_INSTALL_PREFIX=/tmp/x86-sysroot ../../source/llvm
> # make -j4
> # make install
>
> It was fun learning how to manualy compile compiler for the first time. Now how to cross compile for bare? Given that I have musl-1.1.16 directory and that I'm preseeding sysroot with:
>
> # rm -rf /tmp/barearm-sysroot
> # mkdir -p /tmp/barearm-sysroot
> # cp -r musl-1.1.16/include /tmp/barearm-sysroot/ # Thanx for the tip!

Your sysroot needs to be laid out like:

$sysroot/usr/include/<libc headers here>
$sysroot/usr/include/c++/v1/<libcxx headers here>
$sysroot/usr/lib/<all target *.a's here, except for the builtins>

with the builtins placed in:

$PREFIX/lib/clang/$version/lib/baremetal/libclang_rt.builtins-armv7em.a
$BUILD/lib/clang/$version/lib/baremetal/libclang_rt.builtins-armv7em.a

Jon

>
> So I tried to use it, and clang/llvm build, but compiler-rt fails:
>
> # cmake -G 'Unix Makefiles' -DCMAKE_BUILD_TYPE=Release -DBAREMETAL_ARMV7EM_SYSROOT=/tmp/barearm-sysroot -DCMAKE_INSTALL_PREFIX=/tmp/barearm-sysroot -C ../../source/llvm/cmake/caches/BaremetalARM.cmake ../../source/llvm
> # make -j4
> https://gist.github.com/anonymous/85cd5081a6d46a63731d970ee95b20b4
>
> This is the BaremetalARM.cmake:
> https://gist.github.com/anonymous/ff3248f08ddc99361dcc009e9f4c1020

--

[Jonathan Roelofs via llvm-dev]

On 8/5/17 3:47 PM, Goran Mekić via llvm-dev wrote:

>
> # set -xg CC /tmp/barearm-sysroot/bin/clang-6.0
> # set -xg CXX /tmp/barearm-sysroot/bin/clang-6.0

So I think I need to clarify what a sysroot actually is, since that ^
suggests you think it's like $prefix, which it's not.

A sysroot is a collection of headers, and runtime bits, all of which are
designed to run on a $target. Your clang however, was built to run on
$host. For non-baremetal targets, the headers & libraries are a snapshot
of how they'd be laid out on the actual target, i.e. if your target has
`/usr/lib/libc.a`, then your sysroot should have
`$sysroot/usr/lib/libc.a`. Likewise for all the relevant headers, and
$target binaries.

Usually, when you build a packaged toolchain, the sysroots live inside
$prefix next to the compiler installation like so:

$prefix/armv7em-none-eabi/usr/include/stdio.h
$prefix/armv7em-none-eabi/usr/lib/libc.a
$prefix/bin/clang

Then when compiling things with that clang, you do:

> $prefix/bin/clang++ -target armv7em-none-eabi
--sysroot=$prefix/armv7em-none-eabi -T path/to/some/ldscript.ld
hello_world.cpp -o hw.out

Jon

--
Jon Roelofs
jona...@codesourcery.com
CodeSourcery / Mentor Embedded / Siemens