Frequently Asked Questions (FAQ) — LLVM 12 documentation

# Frequently Asked Questions (FAQ)

- 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?

- 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

### Can I modify LLVM source code and redistribute the modified source?

Yes. The modified source distribution must retain the copyright notice and follow the conditions listed in the Apache License v2.0 with LLVM Exceptions.

### 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. LLVM also has excellent support on Windows 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.

### 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()`).

## Source Languages

### What source languages are supported?

LLVM currently has full support for C and C++ source languages through Clang. Many other language frontends have been written using LLVM, and an incomplete list is available at projects with 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
    }