54 lines
2.4 KiB
Markdown
54 lines
2.4 KiB
Markdown
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---
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title: Compiling a Functional Language Using C++, Part 8 - LLVM
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date: 2019-10-30T22:16:22-07:00
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draft: true
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tags: ["C and C++", "Functional Languages", "Compilers"]
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---
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We don't want a compiler that can only generate code for a single
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platform. Our language should work on macOS, Windows, and Linux,
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on x86\_64, ARM, and maybe some other architectures. We also
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don't want to manually implement the compiler for each platform,
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dealing with the specifics of each architecture and operating
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system.
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This is where LLVM comes in. LLVM (which stands for __Low Level Virtual Machine__),
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is a project which presents us with a kind of generic assembly language,
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an __Intermediate Representation__ (IR). It also provides tooling to compile the
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IR into platform-specific instructions, as well as to apply a host of various
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optimizations. We can thus translate our G-machine instructions to LLVM,
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and then use LLVM to generate machine code, which gets us to our ultimate
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goal of compiling our language.
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We start with adding LLVM to our CMake project.
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{{< codelines "CMake" "compiler/08/CMakeLists.txt" 7 7 >}}
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LLVM is a huge project, and has many components. We don't need
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most of them. We do need the core libraries, the x86 assembly
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generator, and x86 assembly parser. I'm
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not sure why we need the last one, but I ran into linking
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errors without them. We find the required link targets
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for these components using this CMake command:
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{{< codelines "CMake" "compiler/08/CMakeLists.txt" 19 20 >}}
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Finally, we add the new include directories, link targets,
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and definitions to our compiler executable:
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{{< codelines "CMake" "compiler/08/CMakeLists.txt" 39 41 >}}
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Great, we have the infrastructure updated to work with LLVM. It's
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now time to start using the LLVM API to compile our G-machine instructions
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into assembly. We start with `LLVMContext`. The LLVM documentation states:
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> This is an important class for using LLVM in a threaded context.
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> It (opaquely) owns and manages the core "global" data of LLVM's core infrastructure, including the type and constant uniquing tables.
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We will have exactly one instance of such a class in our program.
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Additionally, we want an `IRBuilder`, which will help us generate IR instructions,
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placing them into basic blocks (more on that in a bit). Also, we want
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a `Module` object, which represents some collection of code and declarations
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(perhaps like a C++ source file). Let's keep these things in our own
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`llvm_state` class.
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