[llvm-dev] Build Clang/LLVM for AVR

[Alexander Entinger via llvm-dev]

Hi everyone,

I've been wondering how to correctly build clang/LLVM for the AVR target architecture. Unfortunately documentation is very scarce (or outdated or I didn't find it) and while I've been able to build clang/LLVM for AVR I'm still falling short of compiling an actual binary for the MCU. Here are the steps I've undertaken so far:

  git clone https://github.com/llvm/llvm-project
  cd llvm-project
  mkdir build && cd build

  cmake -G "Unix Makefiles" -DLLVM_ENABLE_PROJECTS="clang;libcxx;libcxxabi" -DLLVM_TARGETS_TO_BUILD="AVR" -DCMAKE_BUILD_TYPE=Release ../llvm
  make -j8
  sudo make install

Next I created a pretty empty main.cpp and tried to compile that:

  #include <stdint.h>
  int main()
  {
    for(;;) { }
    return 0;
  }

Here's the result of attempting the compilation ...

  $ clang++ --target=avr -mmcu=atmega328p -c main.cpp -o main.o -v
  clang version 11.0.0 (https://github.com/llvm/llvm-project 177dd63c8d742250dac6ea365e7c30f0fbab3257)
  Target: avr
  Thread model: posix
  InstalledDir: /usr/local/bin
  Found candidate GCC installation: /usr/lib/gcc/avr/5.4.0
  Selected GCC installation: /usr/lib/gcc/avr/5.4.0
  (in-process)
   "/usr/local/bin/clang-11" -cc1 -triple avr -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -discard-value-names -main-file-name main.cpp -mrelocation-model static -mthread-model posix -mframe-pointer=all -fmath-errno -fno-rounding-math -masm-verbose -mconstructor-aliases -target-cpu atmega328p -dwarf-column-info -fno-split-dwarf-inlining -debugger-tuning=gdb -v -resource-dir /usr/local/lib/clang/11.0.0 -fdeprecated-macro -fdebug-compilation-dir /home/alex/tmp/clang-avr -ferror-limit 19 -fmessage-length 0 -fgnuc-version=4.2.1 -fobjc-runtime=gcc -fcxx-exceptions -fexceptions -fdiagnostics-show-option -fcolor-diagnostics -faddrsig -o main.o -x c++ main.cpp
  clang -cc1 version 11.0.0 based upon LLVM 11.0.0git default target x86_64-unknown-linux-gnu
  #include "..." search starts here:
  #include <...> search starts here:
   /usr/local/include
   /usr/local/lib/clang/11.0.0/include
   /usr/include
  End of search list.
  In file included from main.cpp:1:
  In file included from /usr/local/lib/clang/11.0.0/include/stdint.h:52:
  /usr/include/stdint.h:26:10: fatal error: 'bits/libc-header-start.h' file not found
  #include <bits/libc-header-start.h>
           ^~~~~~~~~~~~~~~~~~~~~~~~~~
  1 error generated.

Looking for that file I can find it in the system

$ find /usr -iname libc-header-start.h
/usr/include/x86_64-linux-gnu/bits/libc-header-start.h

But adding it to the clang call via -isystem or -I does not help either.

  $ clang --target=avr -mmcu=atmega328p -c main.cpp -o main.o -v -I/usr/include/x86_64-linux-gnu
    clang version 11.0.0 (https://github.com/llvm/llvm-project 177dd63c8d742250dac6ea365e7c30f0fbab3257)
  Target: avr
  Thread model: posix
  InstalledDir: /usr/local/bin
   (in-process)
   "/usr/local/bin/clang-11" -cc1 -triple avr -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -discard-value-names -main-file-name main.cpp -mrelocation-model static -mthread-model posix -mframe-pointer=all -fmath-errno -fno-rounding-math -masm-verbose -mconstructor aliases -target-cpu atmega328p -dwarf-column-info -fno-split-dwarf-inlining -debugger-tuning=gdb -v -resource-dir /usr/local/lib/clang/11.0.0 -I /usr/include/x86_64-linux-gnu -fdeprecated-macro -fdebug-compilation-dir /home/alex/tmp/clang-avr -ferror-limit 19 -fmessage-length 0 -fgnuc-version=4.2.1 -fobjc-runtime=gcc -fcxx-exceptions -fexceptions -fdiagnostics-show-option -fcolor-diagnostics -faddrsig -o main.o -x c++ main.cpp
  clang -cc1 version 11.0.0 based upon LLVM 11.0.0git default target x86_64-unknown-linux-gnu
  #include "..." search starts here:
  #include <...> search starts here:
   /usr/include/x86_64-linux-gnu
   /usr/local/include
   /usr/local/lib/clang/11.0.0/include
   /usr/include
  End of search list.
  In file included from main.cpp:1:
  In file included from /usr/local/lib/clang/11.0.0/include/stdint.h:52:
  In file included from /usr/include/stdint.h:26:
  In file included from /usr/include/x86_64-linux-gnu/bits/libc-header-start.h:33:
  In file included from /usr/include/features.h:448:
  /usr/include/x86_64-linux-gnu/gnu/stubs.h:7:11: fatal error: 'gnu/stubs-32.h' file not found
  # include <gnu/stubs-32.h>
            ^~~~~~~~~~~~~~~~
  1 error generated.

Thinking that maybe the old version (5.4.0) of avr-gcc might be a problem I've built a 9.2.0 version from sources

  $ avr-gcc -v
  Using built-in specs.
  Reading specs from /opt/avr/avr-gcc/lib/gcc/avr/9.2.0/device-specs/specs-avr2
  COLLECT_GCC=avr-gcc
  COLLECT_LTO_WRAPPER=/opt/avr/avr-gcc/libexec/gcc/avr/9.2.0/lto-wrapper
  Target: avr
  Configured with: ../configure --prefix=/opt/avr/avr-gcc --target=avr --enable-languages=c,c++ --disable-nls --disable-libssp --disable-libada --with-dwarf2 --disable-shared --enable-static --enable-mingw-wildcard
  Thread model: single
  gcc version 9.2.0 (GCC) 

But this didn't change anything, except that when running clang without file arguments it does indicate that there is no avr-gcc in the system (although there is, but probably not spliced in there where the clang/LLVM build process expects it).

  $ clang --target=avr -mmcu=atmega328p 
  clang-11: warning: no avr-gcc installation can be found on the system, cannot link standard libraries [-Wavr-rtlib-linking-quirks]
  clang-11: warning: standard library not linked and so no interrupt vector table or compiler runtime routines will be linked [-Wavr-rtlib-linking-quirks]
  clang-11: error: no input files

I'd greatly appreciate any input helping to solve these problems or a pointer to working documentation.

With kind regards, Alex