cs394p p. 51

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 &rarr 6.28
Reduction in strength: x² &rarr 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.

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