# 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 Subversion, 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. Compiling LLVM with GCC succeeds, but the resulting tools do not work, what can be wrong?
   11. When I use the test suite, all of the C Backend tests fail. What is wrong?
   12. After Subversion update, rebuilding gives the error "No rule to make target".
   13. When I compile LLVM-GCC with srcdir == objdir, it fails. Why?
4. Source Languages
   1. What source languages are supported?
   2. I'd like to write a self-hosting LLVM compiler. How should I interface with the LLVM middle-end optimizers and back-end code generators?
   3. What support is there for higher level source language constructs for building a compiler?
   4. 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?
   5. Can I compile C or C++ code to platform-independent LLVM bitcode?
6. Questions about code generated by the GCC front-end
   1. What is this `llvm.global_ctors` and `_GLOBAL__I__tmp_webcompile...` stuff that happens when I #include <iostream>?
   2. Where did all of my code go??
   3. What is this "`undef`" thing that shows up in my code?
   4. Why does instcombine + simplifycfg turn a call to a function with a mismatched calling convention into "unreachable"? Why not make the verifier reject it?

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 Subversion, 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.

Compiling LLVM with GCC succeeds, but the resulting tools do not work, what can be wrong?

Several versions of GCC have shown a weakness in miscompiling the LLVM codebase. Please consult your compiler version (`gcc --version`) to find out whether it is broken. If so, your only option is to upgrade GCC to a known good version.

After Subversion 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 Subversion 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.

When I compile LLVM-GCC with srcdir == objdir, it fails. Why?

The `GNUmakefile` in the top-level directory of LLVM-GCC is a special `Makefile` used by Apple to invoke the `build_gcc` script after setting up a special environment. This has the unfortunate side-effect that trying to build LLVM-GCC with srcdir == objdir in a "non-Apple way" invokes the `GNUmakefile` instead of `Makefile`. Because the environment isn't set up correctly to do this, the build fails.

People not building LLVM-GCC the "Apple way" need to build LLVM-GCC with srcdir != objdir, or simply remove the GNUmakefile entirely.

We regret the inconvenience.

## 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 `java` module. There is no documentation on this yet so you'll need to download the code, compile it, and try it.

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

I'd like to write a self-hosting LLVM compiler. How should I interface with the LLVM middle-end optimizers and back-end code generators?

Your compiler front-end will communicate with LLVM by creating a module in the LLVM intermediate representation (IR) format. Assuming you want to write your language's compiler in the language itself (rather than C++), there are 3 major ways to tackle generating LLVM IR from a front-end:

- Call into the LLVM libraries code using your language's FFI (foreign function interface).
  - for: best tracks changes to the LLVM IR, .ll syntax, and .bc format
  - for: enables running LLVM optimization passes without a emit/parse overhead
  - for: adapts well to a JIT context
  - against: lots of ugly glue code to write
- Emit LLVM assembly from your compiler's native language.
  - for: very straightforward to get started
  - against: the .ll parser is slower than the bitcode reader when interfacing to the middle end
  - against: you'll have to re-engineer the LLVM IR object model and asm writer in your language
  - against: it may be harder to track changes to the IR
- Emit LLVM bitcode from your compiler's native language.
  - for: can use the more-efficient bitcode reader when interfacing to the middle end
  - against: you'll have to re-engineer the LLVM IR object model and bitcode writer in your language
  - against: it may be harder to track changes to the IR

If you go with the first option, the C bindings in include/llvm-c should help a lot, since most languages have strong support for interfacing with C. The most common hurdle with calling C from managed code is interfacing with the garbage collector. The C interface was designed to require very little memory management, and so is straightforward in this regard.

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.

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. Also, there are several limitations noted below.

Use commands like this:

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

       % llvm-g++ -emit-llvm x.cpp -o program.bc -c

   or:

       % llvm-g++ a.cpp -c -emit-llvm
       % llvm-g++ b.cpp -c -emit-llvm
       % llvm-ld a.o b.o -o program

   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 -lstdc++

Using LLVM does not eliminate the need for C++ library support. If you use the llvm-g++ front-end, the generated code will depend on g++'s C++ support libraries in the same way that code generated from g++ would. If you use another C++ front-end, the generated code will depend on whatever library that front-end would normally require.

If you are working on a platform that does not provide any C++ libraries, you may be able to manually compile libstdc++ to LLVM bitcode, statically link it into your program, then use the commands above to convert the whole result into C code. Alternatively, you might compile the libraries and your application into two different chunks of C code and link them.

Note that, by default, the C back end 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 relatively slow, and not C++-ABI-conforming on most platforms, but otherwise correct.

Also, there are a number of other limitations of the C backend that cause it to produce code that does not fully conform to the C++ ABI on most platforms. Some of the C++ programs in LLVM's test suite are known to fail when compiled with the C back end because of ABI incompatibilities with standard C++ libraries.

Can I compile C or C++ code to platform-independent LLVM bitcode?

No. C and C++ are inherently platform-dependent languages. The most obvious example of this is the preprocessor. A very common way that C code is made portable is by using the preprocessor to include platform-specific code. In practice, information about other platforms is lost after preprocessing, so the result is inherently dependent on the platform that the preprocessing was targeting.

Another example is `sizeof`. It's common for `sizeof(long)` to vary between platforms. In most C front-ends, `sizeof` is expanded to a constant immediately, thus hard-wiring a platform-specific detail.

Also, since many platforms define their ABIs in terms of C, and since LLVM is lower-level than C, front-ends currently must emit platform-specific IR in order to have the result conform to the platform ABI.

## Questions about code generated by the GCC front-end

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 i32 undef`" because "`i`" never has a value specified for it.

Why does instcombine + simplifycfg turn a call to a function with a mismatched calling convention into "unreachable"? Why not make the verifier reject it?

This is a common problem run into by authors of front-ends that are using custom calling conventions: you need to make sure to set the right calling convention on both the function and on each call to the function. For example, this code:

    define fastcc void @foo() {
            ret void
    }
    define void @bar() {
            call void @foo()
            ret void
    }

Is optimized to:

    define fastcc void @foo() {
    	ret void
    }
    define void @bar() {
    	unreachable
    }

... with "opt -instcombine -simplifycfg". This often bites people because "all their code disappears". Setting the calling convention on the caller and callee is required for indirect calls to work, so people often ask why not make the verifier reject this sort of thing.

The answer is that this code has undefined behavior, but it is not illegal. If we made it illegal, then every transformation that could potentially create this would have to ensure that it doesn't, and there is valid code that can create this sort of construct (in dead code). The sorts of things that can cause this to happen are fairly contrived, but we still need to accept them. Here's an example:

    define fastcc void @foo() {
            ret void
    }
    define internal void @bar(void()* %FP, i1 %cond) {
            br i1 %cond, label %T, label %F
    T:
            call void %FP()
            ret void
    F:
            call fastcc void %FP()
            ret void
    }
    define void @test() {
            %X = or i1 false, false
            call void @bar(void()* @foo, i1 %X)
            ret void
    }

In this example, "test" always passes @foo/false into bar, which ensures that it is dynamically called with the right calling conv (thus, the code is perfectly well defined). If you run this through the inliner, you get this (the explicit "or" is there so that the inliner doesn't dead code eliminate a bunch of stuff):

    define fastcc void @foo() {
    	ret void
    }
    define void @test() {
    	%X = or i1 false, false
    	br i1 %X, label %T.i, label %F.i
    T.i:
    	call void @foo()
    	br label %bar.exit
    F.i:
    	call fastcc void @foo()
    	br label %bar.exit
    bar.exit:
    	ret void
    }

Here you can see that the inlining pass made an undefined call to @foo with the wrong calling convention. We really don't want to make the inliner have to know about this sort of thing, so it needs to be valid code. In this case, dead code elimination can trivially remove the undefined code. However, if %X was an input argument to @test, the inliner would produce this:

    define fastcc void @foo() {
    	ret void
    }

    define void @test(i1 %X) {
    	br i1 %X, label %T.i, label %F.i
    T.i:
    	call void @foo()
    	br label %bar.exit
    F.i:
    	call fastcc void @foo()
    	br label %bar.exit
    bar.exit:
    	ret void
    }

The interesting thing about this is that %X must be false for the code to be well-defined, but no amount of dead code elimination will be able to delete the broken call as unreachable. However, since instcombine/simplifycfg turns the undefined call into unreachable, we end up with a branch on a condition that goes to unreachable: a branch to unreachable can never happen, so "-inline -instcombine -simplifycfg" is able to produce:

    define fastcc void @foo() {
    	ret void
    }
    define void @test(i1 %X) {
    F.i:
    	call fastcc void @foo()
    	ret void
    }