10.1 Local Optimizationss

10.2 Loop Optimizations

10.3 Global Optimizations

10.4 An Example

10.5 High-Level Optimizations

10.6 Optimization Strategies

10.7 Interprocedural Optimization

10.8 Summary

Web References

Exercises

This chapter has discussed a large number of optimizations, most of which are found on the control flow graph. The reader may be wondering what the improvements really are. Is it worth the time and effort at compile time -- not to mention at compiler creation time -- to perform such aggressive optimizations?

The answer is -- it depends. Statistics have shown that there is a 5-25% improvement in running time for the loop optimizations:

• Code motion of loop invariabnt statements
• Induction variable elimination

There is a 1-5% improvement for:

• Global common subexpression elimination
• Dead varible detection
• Use of algebraic laws
• Constant propagation

Interestingly, the topic of the next chapter, register allocation, has been shown to improve code more than the optimizations above. Because good register alloction does improve code, it is often discussed in chapters on optimization.