Intermediate Representation (IR): An Easy Overview
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Wild Boar Dev

Wild Boar Dev @wildboar_developer

About: I'm a software developer who enjoys building useful things and sharing my knowledge. Just putting my thoughts, tips, and lessons out there—hoping they help someone else on their journey too.

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Intermediate Representation (IR): An Easy Overview

Publish Date: Jun 9
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Intermediate Representation (IR) is like a translation of your code into a simpler, more structured version — just for the computer.

It’s not exactly the code you wrote, and it’s not machine code either. It’s in the middle, and it helps the compiler do its job: check, analyze, and make the code faster before it runs.

In short, It is the middle layer that helps compilers understand and improve your code.

🗂️ Are there different types of IR?

Yes! Most modern compilers break IR into multiple layers, going from high-level (close to your original code) down to low-level (closer to machine code):

The IR flow

1️⃣ HIR – High-level IR

  • Still looks a lot like your original code.

  • Keeps if, &&, for, and other logic.

  • Easy for the compiler to analyze what the code means, but not ready for optimization.

Example:

if (user.isLoggedIn && user.hasRole("admin")) { ... }
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2️⃣ MIR – Mid-level IR

  • Splits big expressions into simple steps.

  • Much easier for the compiler to optimize or rearrange.

t1 = user.isLoggedIn  
t2 = user.hasRole("admin")  
t3 = t1 && t2  
if (t3) goto BLOCK_ADMIN
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3️⃣ LIR – Low-level IR

  • Now the code looks like instructions for the machine.

  • You’ll see low-level operations like LOAD, CMP, JUMP,...

LOAD R1, user.isLoggedIn  
LOAD R2, user.hasRole  
CALL R2, "admin"  
AND R3, R1, R2  
CMP R3, 0  
JZ END
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⁉️ How Compilers Understand IR?

To make sense of your program, compilers don’t just read code line by line — they look at how the logic flows. That’s where Control-Flow Graphs (CFGs) come in.

  • A CFG is like a roadmap that shows all the different routes your program might follow as it runs.
  • It breaks your code into basic blocks — small, straight pieces of code with no jumps — and connects them based on decisions like if, else, or loops.
  • This helps the compiler understand your program's logic clearly, clean up unused code, and make everything run faster.

Example:

CFG of HIR

⚒️ Tools that use IR

Tool / Compiler Note
LLVM (Clang, Swift) Uses powerful LLVM IR
Rust Uses HIR → MIR → LLVM IR
React Compiler Uses HIR and reactive scopes
V8 (Chrome JS engine) Uses internal IR for JIT compilation

🎯 Conclusion

IR is how your code becomes smarter before it runs.
It helps compilers understand, improve, and prepare your program step by step.

If you:

  • Build developer tools.
  • Work with performance-critical code.
  • Want to dive deeper into how compilers think.

🔗 References

😊 Thanks for reading!

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