GNU Compiler Collection. Includes C Compiler.
Optimization
We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil. Yet we shuold not pass up our opportunities in that critical 3% - Knuth
Basic Optimization
Constant Folding
Evaluates expressions at compile time and replace them with their constant values
Constant Propagation
Replaces variables with their constant values wherever possible
Common Subexpression Elimination
Identifies and elimminates redundant calculations by resuing the result of a previous calculation
Dead Code Elimination
Removes code that has no effect on the program’s output
Loop Optimizations
Loop Unrolling
Copies the loop body multiple times to reduce loop overhead and improve performance
Loop Invariant Code Motion
Move calculations that are independent of the loop counter outside the loop
Loop Fusion
combines adjacent loops that operate on the same data
Loop Fission
Splits a loop into multiple loops to enable further optimizations
Function Optimizations
Function Inlining
See [[C Function#inline|inline]]
Tail Call Optimization
Optimizes tail recursive calls to avoid stack frame allocation
Advanced Optimizations
Instruction Scheduling
Strength Reduction
Replaces rexpensive operations with cheaper equivalents
Vectorization
Uses SIMD instructions to perform operations on multiple data elements simulatenously
Profile Guided Optimization
Uses runtime profiling data to optimize code for real-world execution scenarios
Interprocedural Optimization
Optimizes across functionboundaries such as inlining across files or optimizing based on the call graph
Flags
-O{level}for optimization- Levels
- 0 - Don’t move code around
- 1 - Quick optimizations that don’t take much compilation time
- 2 - all optimizations that don’t involve speed/space tradeoff
- 3 - As much as possible
- Use 0 or 1 with a debugger
- Levels