Compiler Optimization Techniques

1
Compiler Optimization Techniques
2
How Do Compilers Work?

• A compiler has roughly 4 phases
• Lexing (or scanning)
• Parsing
• Semantic Analysis / Code Generation
• Code Optimization

3
Optimization

• Often the code generation phase will produce code
  which is correct, but not optimal.
• The goal of the optimization phase is to produce
  code which is equivalent, but more efficient.
• Efficient how?
• Let's look at a few examples...

4
Optimization Types

• There are two main types or categories of
  optimizations
• Local (sometimes called peephole)
• Global

5
Local Optimizations

• Sometimes called peephole optimizations because
  it is like a small peephole is put overtop of the
  code
• Examines small portions of code at a time

6
Local Optimizations

• Strength Reduction
• Replace slow operations with faster ones
• Constant Evaluation
• Evaluate mathematical expressions at compile time
  if possible
• Dead Code Removal (eliminating unnecessary
  operations)

7
Global Optimizations

• Look at large segments of code, function calls,
  if statements, while loops, etc for optimization
  opportunities

8
Peephole Optimization Tradeoffs

• How big should the peephole be?
• That is, how many instructions should we look at
  at once?
• Too many optimization becomes very complicated
• Too few miss out on optimization opportunities