LLVM: Frequently Asked Questions


Why are the LLVM source code and the front-end distributed under different licenses?
The C/C++ front-ends are based on GCC and must be distributed under the GPL. Our aim is to distribute LLVM source code are a much less restrictive license, in particular one that does not compel users who distribute tools based on modifying the source to redistribute the modified source code as well.
Does the Illinois Open Source License really qualify as an "open source" license?
The UI Open Source License is certified by the Open Source Initiative (OSI).
Can I modify LLVM source code and redistribute the modified source?
Yes. The modified source distribution must retain notice both the copyright notice and follow the three bulletted conditions listed in the LLVM license.
Can I modify LLVM source code and redistribute binaries or other tools based on it, without redistributing the source.
Yes, this is why we distribute LLVM under a less restrictive license than GPL, as explained in the first question above.

Source Code

In what language is LLVM written?
All of the LLVM tools and libraries are written in C++ with extensive use of the STL.

How portable is the LLVM source code?
The LLVM source code should be portable to most modern UNIX-like operating systems. Most of the code is written in standard C++ with operating system services abstracted to a support library. The tools required to build and test LLVM have been ported to a plethora of platforms.

Some porting problems may exist in the following areas:

  • The GCC front end code is not as portable as the LLVM suite, so it may not compile as well on unsupported platforms.

  • The Python test classes are more UNIX-centric than they should be, so porting to non-UNIX like platforms (i.e. Windows, MacOS 9) will require some effort.

  • The LLVM build system relies heavily on UNIX shell tools, like the Bourne Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will require more effort.

Build Problems

When I run configure, it finds the wrong C compiler.
The configure script attempts to locate first gcc and then cc, unless it finds compiler paths set in CC and CXX for the C and C++ compiler, respectively. If configure finds the wrong compiler, either adjust your PATH environment variable or set CC and CXX explicitly.

I compile the code, and I get some error about /localhome.
There are several possible causes for this. The first is that you didn't set a pathname properly when using configure, and it defaulted to a pathname that we use on our research machines.

Another possibility is that we hardcoded a path in our Makefiles. If you see this, please email the LLVM bug mailing list with the name of the offending Makefile and a description of what is wrong with it.

The configure script finds the right C compiler, but it uses the LLVM linker from a previous build. What do I do?
The configure script uses the PATH to find executables, so if it's grabbing the wrong linker/assembler/etc, there are two ways to fix it:
  1. Adjust your PATH environment variable so that the correct program appears first in the PATH. This may work, but may not be convenient when you want them first in your path for other work.

  2. Run configure with an alternative PATH that is correct. In a Borne compatible shell, the syntax would be:

    PATH= ./configure ...

    This is still somewhat inconvenient, but it allows configure to do its work without having to adjust your PATH permanently.

When creating a dynamic library, I get a strange GLIBC error.
Under some operating systems (i.e. Linux), libtool does not work correctly if GCC was compiled with the --disable-shared option. To work around this, install your own version of GCC that has shared libraries enabled by default.

I've updated my source tree from CVS, and now my build is trying to use a file/directory that doesn't exist.
You need to re-run configure in your object directory. When new Makefiles are added to the source tree, they have to be copied over to the object tree in order to be used by the build.

I've modified a Makefile in my source tree, but my build tree keeps using the old version. What do I do?
If the Makefile already exists in your object tree, you can just run the following command in the top level directory of your object tree:

./config.status <relative path to Makefile>

If the Makefile is new, you will have to modify the configure script to copy it over.

I've upgraded to a new version of LLVM, and I get strange build errors.
Sometimes changes to the LLVM source code alters how the build system works. Changes in libtool, autoconf, or header file dependencies are especially prone to this sort of problem.

The best thing to try is to remove the old files and re-build. In most cases, this takes care of the problem. To do this, just type make clean and then make in the directory that fails to build.

I've built LLVM and am testing it, but the tests freeze.
This is most likely occurring because you built a profile or release (optimized) build of LLVM and have not specified the same information on the gmake command line.

For example, if you built LLVM with the command:


...then you must run the tests with the following commands:

cd llvm/test

Why do test results differ when I perform different types of builds?
The LLVM test suite is dependent upon several features of the LLVM tools and libraries.

First, the debugging assertions in code are not enabled in optimized or profiling builds. Hence, tests that used to fail may pass.

Second, some tests may rely upon debugging options or behavior that is only available in the debug build. These tests will fail in an optimized or profile build.

The LLVM Compiler Infrastructure