Building LLVM with LLVM

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1

Hi folks!

I'm trying to build LLVM with LLVM on Ubuntu using the commands below.
Building it with the GNU stuff (cpp, gcc, g++ and binutils packages) works,
but after removing it, moving LLVM to /usr/local and symlinking /usr/bin/ld
to ld.lld, I'm getting the following:
=== snip ===
-- The C compiler identification is Clang 8.0.1
-- The CXX compiler identification is Clang 8.0.1
-- The ASM compiler identification is Clang
-- Found assembler: /usr/local/bin/clang
-- Check for working C compiler: /usr/local/bin/clang
-- Check for working C compiler: /usr/local/bin/clang -- broken
CMake Error at /usr/share/cmake-3.5/Modules/CMakeTestCCompiler.cmake:61 (message):
  The C compiler "/usr/local/bin/clang" is not able to compile a simple test
  program.

  It fails with the following output:

   Change Dir: /home/elux/build/CMakeFiles/CMakeTmp

  Run Build Command:"/usr/bin/make" "cmTC_a9eab/fast"

  /usr/bin/make -f CMakeFiles/cmTC_a9eab.dir/build.make
  CMakeFiles/cmTC_a9eab.dir/build

  make[1]: Entering directory '/home/elux/build/CMakeFiles/CMakeTmp'

  Building C object CMakeFiles/cmTC_a9eab.dir/testCCompiler.c.o

  /usr/local/bin/clang -o CMakeFiles/cmTC_a9eab.dir/testCCompiler.c.o -c
  /home/elux/build/CMakeFiles/CMakeTmp/testCCompiler.c

  Linking C executable cmTC_a9eab

  /usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_a9eab.dir/link.txt
  --verbose=1

  /usr/local/bin/clang CMakeFiles/cmTC_a9eab.dir/testCCompiler.c.o -o
  cmTC_a9eab

  ld: error: cannot open crt1.o: No such file or directory

  ld: error: cannot open crti.o: No such file or directory

  ld: error: cannot open crtbegin.o: No such file or directory

  ld: error: unable to find library -lgcc

  ld: error: unable to find library -lgcc_s

  ld: error: unable to find library -lc

  ld: error: unable to find library -lgcc

  ld: error: unable to find library -lgcc_s

  ld: error: cannot open crtend.o: No such file or directory

  ld: error: cannot open crtn.o: No such file or directory

  clang-8: error: linker command failed with exit code 1 (use -v to see
  invocation)

  CMakeFiles/cmTC_a9eab.dir/build.make:97: recipe for target 'cmTC_a9eab'
  failed

  make[1]: *** [cmTC_a9eab] Error 1

  make[1]: Leaving directory '/home/elux/build/CMakeFiles/CMakeTmp'

  Makefile:126: recipe for target 'cmTC_a9eab/fast' failed

  make: *** [cmTC_a9eab/fast] Error 2

  CMake will not be able to correctly generate this project.
Call Stack (most recent call first):
  CMakeLists.txt:39 (project)

-- Configuring incomplete, errors occurred!
See also "/home/elux/build/CMakeFiles/CMakeOutput.log".
See also "/home/elux/build/CMakeFiles/CMakeError.log".
=== snap ===

Commands used:
=== snip ===
VERSION=8.0.1
DESTDIR=/tmp/llvm
PREFIX=/usr/local

#wget https://github.com/llvm/llvm-project/releases/download/llvmorg-$\{VERSION\}/llvm\-$\{VERSION\}\.src\.tar\.xz
#wget https://github.com/llvm/llvm-project/releases/download/llvmorg-$\{VERSION\}/cfe\-$\{VERSION\}\.src\.tar\.xz
#wget https://github.com/llvm/llvm-project/releases/download/llvmorg-$\{VERSION\}/libcxx\-$\{VERSION\}\.src\.tar\.xz
#wget https://github.com/llvm/llvm-project/releases/download/llvmorg-$\{VERSION\}/libcxxabi\-$\{VERSION\}\.src\.tar\.xz
#wget https://github.com/llvm/llvm-project/releases/download/llvmorg-$\{VERSION\}/lld\-$\{VERSION\}\.src\.tar\.xz
#wget https://github.com/llvm/llvm-project/releases/download/llvmorg-$\{VERSION\}/compiler\-rt\-$\{VERSION\}\.src\.tar\.xz
tar xf llvm-${VERSION}.src.tar.xz
tar xf cfe-${VERSION}.src.tar.xz
tar xf libcxx-${VERSION}.src.tar.xz
tar xf libcxxabi-${VERSION}.src.tar.xz
tar xf lld-${VERSION}.src.tar.xz
tar xf compiler-rt-${VERSION}.src.tar.xz
mv cfe-${VERSION}.src llvm-${VERSION}.src/projects/clang
mv libcxx-${VERSION}.src llvm-${VERSION}.src/projects/libcxx
mv libcxxabi-${VERSION}.src llvm-${VERSION}.src/projects/libcxxabi
mv lld-${VERSION}.src llvm-${VERSION}.src/projects/lld
mv compiler-rt-${VERSION}.src llvm-${VERSION}.src/projects/compiler-rt
mkdir build
cd build
cmake \
  -DLLVM_ENABLE_FFI=ON \
  -DCMAKE_BUILD_TYPE=Release \
  -DLLVM_ENABLE_RTTI=ON \
  -DLLVM_TARGETS_TO_BUILD="host" \
  -DBUILD_SHARED_LIBS=ON \
  ../llvm-${VERSION}.src
cmake --build .
cmake -DCMAKE_INSTALL_PREFIX=${DESTDIR}${PREFIX} -P cmake_install.cmake
=== snap ===

Any hints?

Greetings,
Fabiano

2

If you removed your entire GCC installation (including the runtime libraries and standard library), yeah, I’m not sure that’ll work out of the box with Clang - clang does have implementations of some pieces (libc++, for instance - though clang doesn’t default to using it (not sure if it’ll use it in favor if libstdc++ is missing, or if it’ll fail)) I’m not sure all the pieces are there. (I don’t know the nuance of the various libgcc_s, compiler-rt, etc, etc)

Long story short - keep your GCC installation, install clang, use the CMAKE_C_COMPILER, CMAKE_CXX_COMPILER, etc, flags to specify that you want to use clang instead of gcc as your compiler.

3

If you removed your entire GCC installation (including the runtime libraries and standard library), yeah, I’m not sure that’ll work out of the box with Clang - clang does have implementations of some pieces (libc++, for instance - though clang doesn’t default to using it (not sure if it’ll use it in favor if libstdc++ is missing, or if it’ll fail)) I’m not sure all the pieces are there. (I don’t know the nuance of the various libgcc_s, compiler-rt, etc, etc)

Specifically the llvm-project does not yet have a libc, so it doesn’t provide crt1.o and friends. To use clang without any gnu facilities you’ll need a libc (musl is pretty good) and you’ll need to change the driver to use compiler-rt. I think you can look at the freebsd driver for a platform that uses as much llvm as possible.

  • Michael Spencer
4

Thus wrote Michael Spencer:

Specifically the llvm-project does not yet have a libc, so it doesn't
provide crt1.o and friends. To use clang without any gnu facilities you'll
need a libc (musl is pretty good) and you'll need to change the driver to
use compiler-rt. I think you can look at the freebsd driver for a platform
that uses as much llvm as possible.

Thanks for the recommendation, but using the default system libc is fine for me.

Greetings,
Fabiano

5

Thus wrote David Blaikie:

If you removed your entire GCC installation (including the runtime
libraries and standard library), yeah, I'm not sure that'll work out of the
box with Clang - clang does have implementations of some pieces (libc++,
for instance - though clang doesn't default to using it (not sure if it'll
use it in favor if libstdc++ is missing, or if it'll fail)) I'm not sure
all the pieces are there. (I don't know the nuance of the various libgcc_s,
compiler-rt, etc, etc)

I'm at that point now. I'v successfully built LLVM9 (including compiler-rt
and libcxx{,abi}, but I still can't build LLVM9 without GNU's libstdc++
because configuring without it fails with:
ld: error: unable to find library -lstdc++
Anyone knows how to enforce the use of LLVM's libcxx?

Greetings
Fabiano

6

Thus wrote Olivier Galibert:

7

you'd need to add -stdlib=libc++ to your C++ compiler flags, I believe