Compiler-Automated Optimizations

Compiler-Automated Optimizations

What does the optimizer do? In order to optimize code, it's important to know what sorts of optimizations your compiler is doing automatically. Compilers have been doing optimizations for literally 50 years, and the state-of-the-art is quite amazing, with an extensive body of research theory. Some of the main automated compiler optimizations include:

• Constant folding/propagation
• Constant expression evaluation
• Common subexpression elimination
• Redundant assignment removal
• Strength reduction
• Algebraic optimizations
• Register allocation
• Loop optimizations (e.g. unrolling)
• Auto-vectorization

If you make simple changes to your code with some of the obvious things above, it's not going to give you a speedup. The compiler has already done it for you.

However, there's a limit to what compilers can do. They certainly can't make architectural changes, and there's also many mid-level algorithmic changes that cannot be automated.

Function calls inside expressions are a good example of code changes that might need to be manually optimized. When the compiler sees a function call used in arithmetic, it isn't always able to know what that function is going to do, and has to be conservative by avoiding possibly incorrect optimizations.

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