03_optimizer

IR Optimizer

The IR Optimizer

Purpose

The ARES IR Optimizer is the tool that makes your final program faster and smaller. It looks at the simplified list of instructions (IR) and finds ways to remove unnecessary work before the system writes the final code. It transforms the instructions into an equivalent but more efficient version.

Why it exists

When a computer translates code, it often creates steps that aren't strictly necessary. For example, if you write x = 10 + 20, there is no reason for the final program to do that math every time it runs. The optimizer exists to handle these simple tasks at compile time, so the final program can focus on more difficult work. It ensures that the computer does as little work as possible when your program is actually running.

How it works

The optimizer uses two primary techniques to clean up your code.

  1. Constant Folding. If a math step only uses numbers that never change (like 10 + 20), the optimizer does the math itself and replaces the whole step with the final result (30). This is called "folding" because it collapses multiple steps into one.
  2. Dead Code Elimination. The system looks for any instructions that don't actually do anything. If you calculate a number but never use it, or if you write an "if" statement that can never be true, the optimizer simply removes those lines. This is called "dead code" because it is code that will never be executed.
  3. Repeating the check. The system runs these checks several times in a loop. This is because removing one piece of code might reveal that another piece of code is now also unnecessary.

Intuition

Think of the optimizer like a careful book editor. The author (the IR Generator) might write a long, repetitive first draft. The editor goes through and finds sentences that say the same thing and combines them. They also find whole chapters that don't add anything to the story and remove them entirely. The result is a book that tells the same story but is much shorter and easier to read.

Implementation details

The core logic for the optimizer is in src/ir/optimizer.ts. It uses a simple loop to keep cleaning the code until no more changes can be made.

  • Folding: constantFold(ir) resolves binary operations like addition or multiplication.
  • Elimination: deadCodeElimination(ir) tracks which variables are actually used and removes the ones that are not.

Complexity

The optimizer is very fast. It typically only needs to look at your code 2 or 3 times to find all the possible improvements. This adds almost no time to the compilation process but can make your final program significantly faster.

Trace example

This is what happens when the optimizer sees this code:

  1. x = 10
  2. y = 20
  3. z = x + y
  4. print z

Step 1 (Folding): The system sees x + y is really 10 + 20. It changes line 3 to z = 30. Step 2 (Folding): The system sees print z is really print 30. It changes line 4 to print 30. Step 3 (Elimination): The system sees that x, y, and z are no longer needed for anything else. It removes lines 1, 2, and 3. Result: The final program only has one instruction: print 30.

Related Entities

  • 02_ir_design.md: Explains the simplified instructions that the optimizer helps to clean.
  • src/ir/optimizer.ts: The actual code that performs these optimizations.
ARES