Optimization
- C++ course
- Expert
- Lesson 41/47
- 5 min
Learn compiler optimizations.
Optimization
Optimization is the compiler transforming generated code to be faster or smaller. In GCC and Clang, the -O1, -O2, -O3 flags set the level; -O0 disables optimization (for debugging).
Common optimizations: constant folding (compute constant expressions at compile time), dead code elimination (remove unused code), inlining (put a function body in place of a call), loop unrolling (expand a loop to reduce iteration count). Optimization lengthens compile time but speeds up run time.
Constant folding
Constant folding is the compiler computing constant expressions at compile time. Example: when you write constexpr int a = 5; constexpr int b = a * 2;, the compiler computes b as 10 at compile time; the executable has b directly as 10.
This optimization speeds up the program; the computation finishes at compile time. The constexpr keyword gives this guarantee: the compiler computes these values at compile time. There is no run-time read of a; only the b constant is used.
Practice Write a constexpr constant folding example
Define constexpr int a = 5; and constexpr int b = a * 2;. In main write std::cout << b;.
Starter code
#include <iostream>
// Define the a and b constexpr constants here
int main() {
// Print b
return 0;
}Sample solution
#include <iostream>
constexpr int a = 5;
constexpr int b = a * 2;
int main() {
std::cout << b;
return 0;
}Expected output
10
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the array size
int data[____] = {1, 2, 3};int data[3] = {1, 2, 3};Explanation: For a three-element array, [3].
2 Match each flag to its level
| Term | Meaning |
|---|---|
-O0 |
No optimization |
-O1 |
Basic optimization |
-O2 |
Standard optimization |
-O3 |
Aggressive optimization |
3 Which optimization folds `int x = 2 * 3;` into `int x = 6;`?
int x = 2 * 3; // computable at compile time- Constant folding Correct answer
- Loop unrolling
- Inlining
- Vectorization
Explanation: Constant folding computes constant expressions at compile time.