[llvm-dev] ARM baremetal linking

[Goran Mekić via llvm-dev]

On Mon, Oct 08, 2018 at 10:23:07AM +0100, Peter Smith wrote:
> For building compiler-rt standalone for Arm there is the docs page
> https://llvm.org/docs/HowToCrossCompileBuiltinsOnArm.html . That will
> be a good place to start. The hardest part is usually fighting your
> way past the CMAKE try compile step. I strongly recommend using a
> version of cmake >= 3.6 so that you can use the
> -DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY .
>
> Hope that this gets you a little bit further.
>
> Peter

I chose to go with this option, and these are the steps

Remove arm_Thumb1_VFPv2_SOURCES from arm_Thumb1_SOURCES because
Cortext-M4 is Armv7E-M according to https://en.wikipedia.org/wiki/ARM_Cortex-M#Cortex-M4

cmake ../compiler-rt \
  -DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY \
  -DCOMPILER_RT_OS_DIR="baremetal" \
  -DCOMPILER_RT_BUILD_BUILTINS=ON \
  -DCOMPILER_RT_BUILD_SANITIZERS=OFF \
  -DCOMPILER_RT_BUILD_XRAY=OFF \
  -DCOMPILER_RT_BUILD_LIBFUZZER=OFF \
  -DCOMPILER_RT_BUILD_PROFILE=OFF \
  -DCMAKE_C_COMPILER=/usr/local/libexec/ccache/clang60 \
  -DCMAKE_C_COMPILER_TARGET="arm-none-eabi" \
  -DCMAKE_AR=/usr/local/bin/llvm-ar60 \
  -DCMAKE_NM=/usr/local/bin/llvm-nm60 \
  -DCMAKE_RANLIB=/usr/local/bin/llvm-ranlib60 \
  -DCOMPILER_RT_BAREMETAL_BUILD=ON \
  -DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON \
  -DLLVM_CONFIG_PATH=/usr/local/bin/llvm-config60 \
  -DCMAKE_C_FLAGS="-target arm-none-eabi -march=armv7e-m -mcpu=cortex-m4 -isystem /usr/local/include -isystem /usr/include" \
  -DCMAKE_ASM_FLAGS="-target arm-none-eabi -march=armv7e-m -mcpu=cortex-m4" \
  -DCOMPILER_RT_INCLUDE_TESTS=OFF

After "make && sudo make install" I now have
/usr/local/lib/baremetal/libclang_rt.builtins-arm.a. I'm wondering why
that location, and not what clang60 returns for libgcc which is

clang60 -target arm-none-eabi -march=armv7-m -mcpu=cortex-m4 -print-libgcc-file-name
/usr/local/llvm60/lib/clang/6.0.1/lib/libclang_rt.builtins-arm.a

Anyway, I linked manually using:

ld.lld60 -m armelf --entry=__start -T/usr/home/meka/repos/nuttx/nuttx/configs/nucleo-f4x1re/scripts/f401re.ld -L"/usr/home/meka/repos/nuttx/nuttx/staging" -L"/usr/home/meka/repos/nuttx/nuttx/arch/arm/src/board" -o "/usr/home/meka/repos/nuttx/nuttx/nuttx" --start-group -lsched -ldrivers -lconfigs -lc -lmm -larch -lxx -lapps -lfs -lbinfmt -lxx -lboard /usr/local/lib/baremetal/libclang_rt.builtins-arm.a --end-group

It seams to almost work as I don't get an error imediatly on entrypoint,
which is good (but not much beyond entrypoint either):

lldb nuttx
(lldb) target create "nuttx"
Current executable set to 'nuttx' (arm).
(lldb) gdb-remote 3333
Process 1 stopped
* thread #1, stop reason = signal SIGINT
    frame #0: 0x08000188 nuttx`__start
nuttx`__start:
->  0x8000188 <+0>:  push   {r7, lr}
    0x800018a <+2>:  bl     0x800bb30                 ; stm32_clockconfig
    0x800018e <+6>:  bl     0x8003bd0                 ; stm32_lowsetup
    0x8000192 <+10>: bl     0x80009f8                 ; stm32_gpioinit
(lldb)

On the WikiPedia link I read that M4 doesn't have FPU, only M4F has it,
so I was wondering if that's the cause of this error. Even if it isn't
the cause of error, how would I build compiler-rt for this chip and
explicitely state that it doesn't have FPU? Also, I obviously need to
build with debug symbols (which I forgot this time), but that should
be easy. Any other tip on how to make more progress is welcome!

Regards,
meka
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 833 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20181008/ab8305de/attachment.sig>