LLVM: Frequently Asked Questions

1. License
   1. Why are the LLVM source code and the front-end distributed under different licenses?
   2. Does the University of Illinois Open Source License really qualify as an "open source" license?
   3. Can I modify LLVM source code and redistribute the modified source?
   4. Can I modify LLVM source code and redistribute binaries or other tools based on it, without redistributing the source?
2. Source code
   1. In what language is LLVM written?
   2. How portable is the LLVM source code?
3. Build Problems
   1. When I run configure, it finds the wrong C compiler.
   2. The `configure` script finds the right C compiler, but it uses the LLVM linker from a previous build. What do I do?
   3. When creating a dynamic library, I get a strange GLIBC error.
   4. I've updated my source tree from CVS, and now my build is trying to use a file/directory that doesn't exist.
   5. I've modified a Makefile in my source tree, but my build tree keeps using the old version. What do I do?
   6. I've upgraded to a new version of LLVM, and I get strange build errors.
   7. I've built LLVM and am testing it, but the tests freeze.
   8. Why do test results differ when I perform different types of builds?
   9. Compiling LLVM with GCC 3.3.2 fails, what should I do?
   10. When I use the test suite, all of the C Backend tests fail. What is wrong?
   11. After CVS update, rebuilding gives the error "No rule to make target".
4. Source Languages
   1. What source languages are supported?
   2. What support is there for higher level source language constructs for building a compiler?
   3. I don't understand the GetElementPtr instruction. Help!
5. Using the GCC Front End
   1. When I compile software that uses a configure script, the configure script thinks my system has all of the header files and libraries it is testing for. How do I get configure to work correctly?
   2. When I compile code using the LLVM GCC front end, it complains that it cannot find libcrtend.a.
   3. How can I disable all optimizations when compiling code using the LLVM GCC front end?
   4. Can I use LLVM to convert C++ code to C code?
6. Questions about code generated by the GCC front-end
   1. What is this `__main()` call that gets inserted into `main()`?
   2. What is this `llvm.global_ctors` and `_GLOBAL__I__tmp_webcompile...` stuff that happens when I #include <iostream>?
   3. Where did all of my code go??
   4. What is this "`undef`" thing that shows up in my code?

Written by The LLVM Team

License

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 under 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 University of Illinois Open Source License really qualify as an "open source" license?

Yes, the 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 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 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.

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=[the path without the bad program] ./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:

`gmake ENABLE_PROFILING=1`

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

`cd llvm/test
gmake ENABLE_PROFILING=1`

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.

Compiling LLVM with GCC 3.3.2 fails, what should I do?

This is a bug in GCC, and affects projects other than LLVM. Try upgrading or downgrading your GCC.

After CVS update, rebuilding gives the error "No rule to make target".

If the error is of the form:

`gmake[2]: *** No rule to make target `/path/to/somefile', needed by `/path/to/another/file.d'.
Stop.`

This may occur anytime files are moved within the CVS repository or removed entirely. In this case, the best solution is to erase all `.d` files, which list dependencies for source files, and rebuild:

    % cd $LLVM_OBJ_DIR
    % rm -f `find . -name \*\.d`
    % gmake

In other cases, it may be necessary to run `make clean` before rebuilding.

Source Languages

What source languages are supported?

LLVM currently has full support for C and C++ source languages. These are available through a special version of GCC that LLVM calls the C Front End

There is an incomplete version of a Java front end available in the `llvm-java` CVS repository. There is no documentation on this yet so you'll need to download the code, compile it, and try it.

In the `examples/BFtoLLVM` directory is a translator for the BrainF*** language (2002 Language Specification).

In the `projects/Stacker` directory is a compiler and runtime library for the Stacker language, a "toy" language loosely based on Forth.

The PyPy developers are working on integrating LLVM into the PyPy backend so that PyPy language can translate to LLVM.

What support is there for a higher level source language constructs for building a compiler?

Currently, there isn't much. LLVM supports an intermediate representation which is useful for code representation but will not support the high level (abstract syntax tree) representation needed by most compilers. There are no facilities for lexical nor semantic analysis. There is, however, a mostly implemented configuration-driven compiler driver which simplifies the task of running optimizations, linking, and executable generation.

I don't understand the GetElementPtr instruction. Help!

See The Often Misunderstood GEP Instruction.

Using the GCC Front End

When I compile software that uses a configure script, the configure script thinks my system has all of the header files and libraries it is testing for. How do I get configure to work correctly?

The configure script is getting things wrong because the LLVM linker allows symbols to be undefined at link time (so that they can be resolved during JIT or translation to the C back end). That is why configure thinks your system "has everything."

To work around this, perform the following steps:

1. Make sure the CC and CXX environment variables contains the full path to the LLVM GCC front end.
2. Make sure that the regular C compiler is first in your PATH.
3. Add the string "-Wl,-native" to your CFLAGS environment variable.

This will allow the `llvm-ld` linker to create a native code executable instead of shell script that runs the JIT. Creating native code requires standard linkage, which in turn will allow the configure script to find out if code is not linking on your system because the feature isn't available on your system.

When I compile code using the LLVM GCC front end, it complains that it cannot find libcrtend.a.

The only way this can happen is if you haven't installed the runtime library. To correct this, do:

      % cd llvm/runtime
      % make clean ; make install-bytecode

How can I disable all optimizations when compiling code using the LLVM GCC front end?

Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and optimizations done at the llvm level, leaving you with the truly horrible code that you desire.

Can I use LLVM to convert C++ code to C code?

Yes, you can use LLVM to convert code from any language LLVM supports to C. Note that the generated C code will be very low level (all loops are lowered to gotos, etc) and not very pretty (comments are stripped, original source formatting is totally lost, variables are renamed, expressions are regrouped), so this may not be what you're looking for. However, this is a good way to add C++ support for a processor that does not otherwise have a C++ compiler.

Use commands like this:

1. Compile your program as normal with llvm-g++:

$ llvm-g++ x.cpp -o program

or:

llvm-g++ a.cpp -c
llvm-g++ b.cpp -c
llvm-g++ a.o b.o -o program

With llvm-gcc3, this will generate program and program.bc. The .bc file is the LLVM version of the program all linked together.

2. Convert the LLVM code to C code, using the LLC tool with the C backend:

$ llc -march=c program.bc -o program.c

3. Finally, compile the c file:

$ cc x.c

Note that, by default, the C backend does not support exception handling. If you want/need it for a certain program, you can enable it by passing "-enable-correct-eh-support" to the llc program. The resultant code will use setjmp/longjmp to implement exception support that is correct but relatively slow.

Also note: this specific sequence of commands won't work if you use a function defined in the C++ runtime library (or any other C++ library). To access an external C++ library, you must manually compile libstdc++ to LLVM bytecode, statically link it into your program, then use the commands above to convert the whole result into C code. Alternatively, you can compile the libraries and your application into two different chunks of C code and link them.

Questions about code generated by the GCC front-end

What is this `__main()` call that gets inserted into `main()`?

The `__main` call is inserted by the C/C++ compiler in order to guarantee that static constructors and destructors are called when the program starts up and shuts down. In C, you can create static constructors and destructors by using GCC extensions, and in C++ you can do so by creating a global variable whose class has a ctor or dtor.

The actual implementation of `__main` lives in the `llvm/runtime/GCCLibraries/crtend/` directory in the source-base, and is linked in automatically when you link the program.

What is this `llvm.global_ctors` and `_GLOBAL__I__tmp_webcompile...` stuff that happens when I #include <iostream>?

If you #include the <iostream> header into a C++ translation unit, the file will probably use the `std::cin`/`std::cout`/... global objects. However, C++ does not guarantee an order of initialization between static objects in different translation units, so if a static ctor/dtor in your .cpp file used `std::cout`, for example, the object would not necessarily be automatically initialized before your use.

To make `std::cout` and friends work correctly in these scenarios, the STL that we use declares a static object that gets created in every translation unit that includes `<iostream>`. This object has a static constructor and destructor that initializes and destroys the global iostream objects before they could possibly be used in the file. The code that you see in the .ll file corresponds to the constructor and destructor registration code.

If you would like to make it easier to understand the LLVM code generated by the compiler in the demo page, consider using `printf()` instead of `iostream`s to print values.

Where did all of my code go??

If you are using the LLVM demo page, you may often wonder what happened to all of the code that you typed in. Remember that the demo script is running the code through the LLVM optimizers, so if your code doesn't actually do anything useful, it might all be deleted.

To prevent this, make sure that the code is actually needed. For example, if you are computing some expression, return the value from the function instead of leaving it in a local variable. If you really want to constrain the optimizer, you can read from and assign to `volatile` global variables.

What is this "`undef`" thing that shows up in my code?

`undef` is the LLVM way of representing a value that is not defined. You can get these if you do not initialize a variable before you use it. For example, the C function:

`int X() { int i; return i; }`

Is compiled to "`ret int undef`" because "i" never has a value specified for it.