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. Can I use AI coding tools, such as GitHub co-pilot, to write LLVM patches? # Yes, as long as the resulting work can be licensed under the project license, as covered in the LLVM Developer Policy . Using an AI tool to reproduce copyrighted work does not rinse it of copyright and grant you the right to relicense it. 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 understand what’s going on. Basically, in code like: , %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; 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: Call into the LLVM libraries 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 an emit/parse overhead for: adapts well to a JIT context against: lots of ugly glue code to write Call into the LLVM libraries using your language’s FFI (foreign function interface). for: best tracks changes to the LLVM IR, .ll syntax, and .bc format for: best tracks changes to the LLVM IR, .ll syntax, and .bc format for: enables running LLVM optimization passes without an emit/parse overhead for: enables running LLVM optimization passes without an emit/parse overhead for: adapts well to a JIT context for: adapts well to a JIT context against: lots of ugly glue code to write 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: it may be harder to track changes to the IR Emit LLVM assembly from your compiler’s native language. for: very straightforward to get started for: very straightforward to get started against: the .ll parser is slower than the bitcode reader when interfacing to the middle end 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 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 Emit LLVM bitcode from your compiler’s native language. for: can use the more-efficient bitcode reader when interfacing to the middle end 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: 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 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: 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 encountered 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: Is optimized to: … 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: In this example, “test” always passes @foo / false into bar , which ensures that it is dynamically called with the right calling convention (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): 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: 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: