# Frequently Asked Questions (FAQ)

- License
  - Does the University of Illinois Open Source License really qualify as an “open source” license?
  - Can I modify LLVM source code and redistribute the modified source?
  - Can I modify the LLVM source code and redistribute binaries or other tools based on it, without redistributing the source?
- Source Code
  - In what language is LLVM written?
  - How portable is the LLVM source code?
  - What API do I use to store a value to one of the virtual registers in LLVM IR’s SSA representation?
- Build Problems
  - When I run configure, it finds the wrong C compiler.
  - The `configure` script finds the right C compiler, but it uses the LLVM tools from a previous build. What do I do?
  - When creating a dynamic library, I get a strange GLIBC error.
  - I’ve updated my source tree from Subversion, and now my build is trying to use a file/directory that doesn’t exist.
  - I’ve modified a Makefile in my source tree, but my build tree keeps using the old version. What do I do?
  - I’ve upgraded to a new version of LLVM, and I get strange build errors.
  - I’ve built LLVM and am testing it, but the tests freeze.
  - Why do test results differ when I perform different types of builds?
  - Compiling LLVM with GCC 3.3.2 fails, what should I do?
  - After Subversion update, rebuilding gives the error “No rule to make target”.
- Source Languages
  - What source languages are supported?
  - 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?
  - What support is there for a higher level source language constructs for building a compiler?
  - I don’t understand the `GetElementPtr` instruction. Help!
- Using the C and C++ Front Ends
  - Can I compile C or C++ code to platform-independent LLVM bitcode?
- Questions about code generated by the demo page
  - What is this `llvm.global_ctors` and `_GLOBAL__I_a...` stuff that happens when I `#include <iostream>`?
  - Where did all of my code go??
  - What is this “`undef`” thing that shows up in my code?
  - Why does instcombine + simplifycfg turn a call to a function with a mismatched calling convention into “unreachable”? Why not make the verifier reject it?

## License

### 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 the 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 autoconf/makefile 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.

### What API do I use to store a value to one of the virtual registers in LLVM IR’s SSA representation?

In short: you can’t. It’s actually kind of a silly question once you grok what’s going on. Basically, in code like:

    %result = add i32 %foo, %bar

, `%result` is just a name given to the `Value` of the `add` instruction. In other words, `%result` is the add instruction. The “assignment” doesn’t explicitly “store” anything to any “virtual register”; the “`=`” is more like the mathematical sense of equality.

Longer explanation: In order to generate a textual representation of the IR, some kind of name has to be given to each instruction so that other instructions can textually reference it. However, the isomorphic in-memory representation that you manipulate from C++ has no such restriction since instructions can simply keep pointers to any other `Value`‘s that they reference. In fact, the names of dummy numbered temporaries like `%1` are not explicitly represented in the in-memory representation at all (see `Value::getName()`).

## 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 tools 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 Bourne compatible shell, the syntax would be:

    % PATH=[the path without the bad program] $LLVM_SRC_DIR/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.

### 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.

## Source Languages

### What source languages are supported?

LLVM currently has full support for C and C++ source languages. These are available through both Clang and DragonEgg.

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:

1. 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

2. 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: it may be harder to track changes to the IR

3. 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 C and C++ Front Ends

### 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 demo page

### What is this `llvm.global_ctors` and `_GLOBAL__I_a...` 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
    }