code optimization分成兩種 local和global
local code optimization在section 8.5
大多數的global optimization是data-flow analysis
9.1 The Principal Sources of Optimization
Semantics-Preserving Transformations
1)common-subexpression elimation
2)copy propagation
3)dead-code elimination
4)constant folding
5)code motion
6)induction variables and reduction in strength
