Understanding the Compiler Optimization Flags
Here is a quick write up on the Compiler Optimization flags we use.
O0: The Bare Bones
At O0, the compiler’s role is straightforward: translate the source code into machine code with minimal intervention. This level avoids any changes that could alter the code’s behavior, making it an ideal setting for debugging.
O1: The Initial Enhancements
- Dead Code Elimination: Removing parts of the code that never execute.
- Basic Loop Optimization: Simplifying loops where the number of iterations is known beforehand.
- Function Inlining for Small Functions: Replacing function calls with the function's actual code when the function is small, reducing call overhead.
O2: The Performance Gear
O2 is where significant performance enhancements kick in, including:
- Advanced Loop Optimizations: Such as loop unrolling (expanding loops to reduce the number of iterations) and loop fusion (combining similar loops).
- Instruction Scheduling: Rearranging instructions to avoid CPU stalls and make better use of instruction pipelines.
- Constant Propagation: Replacing variables that act as constants with their actual values to reduce computation.
O3: Maximum Throttle
O3 extends O2 optimizations with a focus on speed, employing strategies like:
- Aggressive Loop Unrolling and Vectorization: Further enhancing loop processing efficiency by using vector operations.
- Cross-Module Inlining: Inlining functions across different modules or files.
- Advanced Instruction Scheduling: Further optimization for CPU pipeline efficiency.
Ofast: Breaking the Boundaries
Ofast disregards standard compliance for speed. It includes all O3 optimizations plus:
- Ignoring Strict Overflow Rules: This allows certain assumptions for integer overflows, which can speed up arithmetic operations but might lead to undefined behaviors.
- Fast Math Operations: Assumes associative and commutative properties in floating-point operations, which isn't always accurate but faster.
O0: The Bare Bones
At O0, the compiler’s role is straightforward: translate the source code into machine code with minimal intervention. This level avoids any changes that could alter the code’s behavior, making it an ideal setting for debugging.
O1: The Initial Enhancements
- Dead Code Elimination: Removing parts of the code that never execute.
- Basic Loop Optimization: Simplifying loops where the number of iterations is known beforehand.
- Function Inlining for Small Functions: Replacing function calls with the function's actual code when the function is small, reducing call overhead.
O2: The Performance Gear
O2 is where significant performance enhancements kick in, including:
- Advanced Loop Optimizations: Such as loop unrolling (expanding loops to reduce the number of iterations) and loop fusion (combining similar loops).
- Instruction Scheduling: Rearranging instructions to avoid CPU stalls and make better use of instruction pipelines.
- Constant Propagation: Replacing variables that act as constants with their actual values to reduce computation.
O3: Maximum Throttle
O3 extends O2 optimizations with a focus on speed, employing strategies like:
- Aggressive Loop Unrolling and Vectorization: Further enhancing loop processing efficiency by using vector operations.
- Cross-Module Inlining: Inlining functions across different modules or files.
- Advanced Instruction Scheduling: Further optimization for CPU pipeline efficiency.
Ofast: Breaking the Boundaries
Ofast disregards standard compliance for speed. It includes all O3 optimizations plus:
- Ignoring Strict Overflow Rules: This allows certain assumptions for integer overflows, which can speed up arithmetic operations but might lead to undefined behaviors.
- Fast Math Operations: Assumes associative and commutative properties in floating-point operations, which isn't always accurate but faster.
