basic_llvm.md

Addition

Setup

The code is available in calculator/examples/llvm/src/main.rs. Because my llvm-config --version shows 10.0.0 so I'm using features = ["llvm10-0"] in inkwell

inkwell = { git = "https://github.com/TheDan64/inkwell", branch = "master", features = ["llvm10-0"] }

Go to calculator/examples/llvm sub-crate and cargo run.

Add Function

We want to define an add function like

add(x: i32, y: i32) -> i32 { x + y }

but using the LLVM language and JIT it. For that, we need to define every bit of what makes up a function through LLVM basic constructs such as context, module, function signature setups, argument types, basic block, etc.

Here is how to stitch our add function in LLVM

1. We start by creating a context, adding the addition module and setting up the data type we want to use i32_type of type IntType

{{#include ../../../calculator/examples/llvm/src/main.rs:first}}

2. We define the signature of add(i32, i32) -> i32, add the function to our module, create a basic block entry point and a builder to add later parts

{{#include ../../../calculator/examples/llvm/src/main.rs:second}}

3. We create the arguments x and y and add them to the builder to make up the return instruction

{{#include ../../../calculator/examples/llvm/src/main.rs:third}}

4. Finally, we create a JIT execution engine (with no optimization for now) and let LLVM handle rest of the work for us

{{#include ../../../calculator/examples/llvm/src/main.rs:fourth}}

Yes! all of this just to add two integers.