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![]() In. Down. Right","exit. Animation": "rotate. Out. Down. Right","timer": "","sensitivity": "2. Expire": "1","cookie. 4 Examples of Open Source Software You can find information about open source products on the Internet by using a search engine and typing the keywords “open source. Domain": "","auto. Fire": "","is. Analytics. The Architecture of Open Source Applications: LLVMThis chapter discusses some of the design decisions that shaped. LLVM, an umbrella project that hosts. Unix systems. The. LLVM" was once an acronym, but is now just a brand for the. While LLVM provides some unique capabilities, and. Clang. compiler, a C/C++/Objective- C. GCC compiler). the main thing that sets LLVM apart from other compilers is its. From its beginning in December 2. LLVM was designed as a set of. LA0. 4]. At the time, open source. For example, it was. GCC). for doing static analysis or refactoring. While scripting languages. There was no way to reuse pieces, and. Beyond the composition of the compiler itself, the communities. GCC, Free Pascal, and Free. BASIC. or it provided a runtime compiler in the form of an. Just- In- Time (JIT) compiler. It was very uncommon to. Over the last ten years, LLVM has substantially altered this. LLVM is now used as a common infrastructure to implement a. GCC, Java, . NET, Python, Ruby. Scheme, Haskell, D, as well as countless lesser known languages). It. has also replaced a broad variety of special purpose compilers, such. Apple's Open. GL stack and the. Adobe's After Effects product. Finally. LLVM has also been used to create a broad variety of new products. Open. CL GPU programming language. A Quick Introduction to Classical Compiler Design. The most popular design for a traditional static compiler (like most C. Figure 1. 1. 1). The front end parses source code, checking. Abstract Syntax Tree. AST) to represent the input code. The AST is optionally converted to. Figure 1. 1. 1: Three Major Components of a Three- Phase Compiler. The optimizer is responsible for doing a broad variety of. The back end (also known as the. In addition to making correct code, it is responsible for. Common parts of a compiler back end. This model applies equally well to interpreters and JIT compilers. The Java Virtual Machine (JVM) is also an implementation of this. Java bytecode as the interface between the front end. Implications of this Design. The most important win of this classical design comes when a compiler. If the compiler uses a common code representation in. Figure 1. 1. 2. Figure 1. Retargetablity. With this design, porting the compiler to support a new source. Algol or BASIC) requires implementing a new front end. If these parts. weren't separated, implementing a new source language would require. N targets and. M source languages would need N*M compilers. Another advantage of the three- phase design (which follows directly. For an open source project, this means that there is a. This is the reason why open source compilers that serve many. GCC) tend to generate better optimized machine code. Free. PASCAL. This isn't the case for. For example, the Intel ICC Compiler is widely known. A final major win of the three- phase design is that the skills. Separating these makes it easier for a. While this is a social issue, not a technical one, it. Existing Language Implementations. While the benefits of a three- phase design are compelling and. Looking across open source language implementations. LLVM was started), you'd find that the implementations of. Perl, Python, Ruby and Java share no code. Further, projects like the. Glasgow Haskell Compiler (GHC) and Free. BASIC are retargetable to. CPUs, but their implementations are very specific. There is also a broad. JIT compilers for image processing, regular expressions, graphics card. CPU intensive work. That said, there are three major success stories for this model, the. Java and . NET virtual machines. These systems. provide a JIT compiler, runtime support, and a very well defined. This means that any language that can compile to the. JIT as. well as the runtime. The tradeoff is that these implementations. JIT compilation, garbage collection, and the use of. This leads to suboptimal performance. C (e. g., with the LLJVM project). A second success story is perhaps the most unfortunate, but. C code (or some other language) and send it through existing. C compilers. This allows reuse of the optimizer and code generator. Unfortunately, doing this prevents efficient implementation of. C). A final successful implementation of this model is GCC. GCC. supports many front ends and back ends, and has an active and broad. GCC has a long history of being a C. As the years go by, the GCC community. As of GCC 4. 4, it has a new. GIMPLE Tuples") which is. Also, its Fortran and Ada front ends use a clean AST. While very successful, these three approaches have strong limitations. As one example, it is not realistically possible to. GCC into other applications, to use GCC as a runtime/JIT. GCC without pulling in most. People who have wanted to use GCC's C++ front end for. GCC as a monolithic application that. XML, or write plugins to inject. GCC process. There are multiple reasons why pieces of GCC cannot be reused as. The hardest. problems to fix, though, are the inherent architectural problems that. Specifically, GCC suffers from. ASTs to generate debug info, the front ends generate back- end data. LLVM's Code Representation: LLVM IRWith the historical background and context out of the way, let's dive. LLVM: The most important aspect of its design is the LLVM. Intermediate Representation (IR), which is the form it uses to. LLVM IR is designed to host mid- level. It was designed with many specific goals in mind. The most important. To make this concrete, here is a. This LLVM IR corresponds to this C code, which provides two different. Perhaps not the most efficient way to add two numbers. As you can see from this example, LLVM IR is a low- level RISC- like. Like a real RISC instruction set, it. These instructions are in three address form. LLVM IR supports labels and generally looks like a weird. Unlike most RISC instruction sets, LLVM is strongly typed with a. For example, the calling convention is. Another significant difference from machine code. LLVM IR doesn't use a fixed set of named registers, it. Beyond being implemented as a language, LLVM IR is actually defined in. The LLVM. Project also provides tools to convert the on- disk format from text to. The intermediate representation of a compiler is interesting because. On. the other hand, it has to serve both well: it has to be designed to be. Writing an LLVM IR Optimization. To give some intuition for how optimizations work, it is useful to. There are lots of different kinds of. That said, most optimizations follow a. Look for a pattern to be transformed. Verify that the transformation is safe/correct for the matched. Do the transformation, updating the code. The most trivial optimization is pattern matching on arithmetic. X, X- X is 0. X- 0 is X, (X*2)- X is X. The first. question is what these look like in LLVM IR. Some examples are. For these sorts of "peephole" transformations, LLVM provides an. These particular. Simplify. Sub. Inst function and look. X - 0 - > X. if (match(Op. Zero())). // X - X - > 0. Op. 0 == Op. 1). return Constant: :get. Null. Value(Op. 0- > get. Type()). // (X*2) - X - > X. Op. 0, m_Mul(m_Specific(Op. Constant. Int< 2> ()))). Nothing matched, return null to indicate no transformation. In this code, Op. Op. 1 are bound to the left and right operands of. IEEE floating point!). LLVM is implemented in. C++, which isn't well known for its pattern matching capabilities. Objective Caml), but it does. The match function and the m_. LLVM IR code. For example, the m_Specific predicate only. Op. 1. Together, these three cases are all pattern matched and the function. The caller of this function (Simplify. Instruction). is a dispatcher that does a switch on the instruction opcode. It is called from. A simple driver looks like this. Basic. Block: :iterator I = BB- > begin(), E = BB- > end(); I != E; ++I). Value *V = Simplify. Instruction(I)). I- > replace. All. Uses. With(V). This code simply loops over each instruction in a block, checking to. If so (because. Simplify. Instruction returns non- null), it uses the. All. Uses. With method to update anything in the code using. LLVM's Implementation of Three- Phase Design. In an LLVM- based compiler, a front end is responsible for parsing. LLVM IR (usually, but not always, by building an. AST and then converting the AST to LLVM IR). This IR is optionally. Figure 1. 1. 3. This is a very. LLVM architecture derives from LLVM IR. Figure 1. 1. 3: LLVM's Implementation of the Three- Phase Design. LLVM IR is a Complete Code Representation. In particular, LLVM IR is both well specified and the only. This property means that all you need to.
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