cs375 p. 257

Contents Page-10 Prev Next Page+10 Index

Local and Global Optimization

Local optimization is that which can be done correctly based on analysis of a small part of the program.
Examples:

Constant folding: 2 * 3.14 → 6.28
Reduction in strength: x * 2 → x + x
Removing branches to branches:
```
   L1:   Goto  L2
```

Global optimization requires information about the whole program to be done correctly.

Example:


   I * 8           ==>       R1 = I * 8

   ...                       ...

   I * 8           ==>       R1

This is correct only if I is not redefined between the two points. Doing optimization correctly requires program analysis: a special-purpose proof that program P' produces the same output values as P.