Bootstrapping the C/C++ Front-End

This document is intended to explain the process of building the LLVM C/C++ front-end, based on GCC 3.4, from source.

NOTE: This is currently a somewhat fragile, error-prone process, and you should only try to do it if (A) you really, really, really can't use the binaries we distribute, and (B) you are a wicked good GCC hacker.

We welcome patches to help make this process simpler.


  1. Configure and build the LLVM libraries and tools using:

     % cd llvm
     % ./configure [options...]
     % gmake tools-only

    The use of the non-default target "tools-only" means that the LLVM tools and libraries will build, and the binaries will be deposited in llvm/tools/Debug, but the runtime (bytecode) libraries will not build.

  2. Add the directory containing the tools to your PATH.

     % set path = ( `cd llvm/tools/Debug && pwd` $path )
  3. Unpack the C/C++ front-end source into cfrontend/src.

  4. Edit src/configure. Change the first line (starting w/ #!) to contain the correct full pathname of sh.

  5. Make "build" and "install" directories as siblings of the "src" tree.

     % pwd
     % cd ..
     % mkdir build install
     % set CFEINSTALL = `pwd`/install
  6. Configure, build and install the C front-end:

     % cd build
     % ../src/configure --prefix=$CFEINSTALL --disable-nls --disable-shared \
     % gmake all-gcc
     % setenv LLVM_LIB_SEARCH_PATH `pwd`/gcc 
     % gmake all; gmake install

    Common Problem: You may get error messages regarding the fact that LLVM does not support inline assembly. Here are two common fixes:

    • Fix 1: If you have system header files that include inline assembly, you may have to modify them to remove the inline assembly, and install the modified versions in $CFEINSTALL/target-triplet/sys-include.

    • Fix 2: If you are building the C++ front-end on a CPU we haven't tried yet, you will probably have to edit the appropriate version of atomicity.h under src/libstdc++-v3/config/cpu/name-of-cpu/atomicity.h and apply a patch so that it does not use inline assembly.

    Porting to a new architecture: If you are porting the new front-end to a new architecture, or compiling in a different configuration that we have previously, there are probably several changes you will have to make to the GCC target to get it to work correctly. These include:

    • Often targets include special or assembler linker flags which gccas/gccld does not understand. In general, these can just be removed.
    • LLVM currently does not support any floating point values other than 32-bit and 64-bit IEEE floating point. The primary effect of this is that you may have to map "long double" onto "double".
    • The profiling hooks in GCC do not apply at all to the LLVM front-end. These may need to be disabled.
    • No inline assembly for position independent code. At the LLVM level, everything is position independent.
    • We handle .init and .fini differently.
    • You may have to disable multilib support in your target. Using multilib support causes the GCC compiler driver to add a lot of "-L" options to the link line, which do not relate to LLVM and confuse gccld. To disable multilibs, delete any MULTILIB_OPTIONS lines from your target files.
    • Did we mention that we don't support inline assembly? You'll probably have to add some fixinclude hacks to disable it in the system headers.
  7. Go back into the LLVM source tree proper. Edit Makefile.config to redefine LLVMGCCDIR to the full pathname of the $CFEINSTALL directory, which is the directory you just installed the C front-end into. (The ./configure script is likely to have set this to a directory which does not exist on your system.)

  8. If you edited header files during the C/C++ front-end build as described in "Fix 1" above, you must now copy those header files from $CFEINSTALL/target-triplet/sys-include to $CFEINSTALL/lib/gcc/target-triplet/3.4-llvm/include. (This should be the "include" directory in the same directory as the libgcc.a library, which you can find by running $CFEINSTALL/bin/gcc --print-libgcc-file-name.)

  9. Build and install the runtime (bytecode) libraries by running:

     % gmake -C runtime
     % mkdir $CFEINSTALL/bytecode-libs
     % gmake -C runtime install
     % setenv LLVM_LIB_SEARCH_PATH $CFEINSTALL/bytecode-libs
  10. Test the newly-installed C frontend by one or more of the following means:

    • compiling and running a "hello, LLVM" program in C and C++.
    • running the tests under test/Programs using gmake -C test/Programs

Brian Gaeke
Last modified: $Date$