Editing Static single-assignment form (section)

==Benefits==
The primary usefulness of SSA comes from how it simultaneously simplifies and improves the results of a variety of [[compiler optimization]]s, by simplifying the properties of variables. For example, consider this piece of code:

 y := 1
 y := 2
 x := y

Humans can see that the first assignment is not necessary, and that the value of <code>y</code> being used in the third line comes from the second assignment of <code>y</code>. A program would have to perform [[reaching definition|reaching definition analysis]] to determine this. But if the program is in SSA form, both of these are immediate:

 y<sub>1</sub> := 1
 y<sub>2</sub> := 2
 x<sub>1</sub> := y<sub>2</sub>

[[Compiler optimization]] algorithms that are either enabled or strongly enhanced by the use of SSA include:
* [[Constant propagation]] – conversion of computations from runtime to compile time, e.g. treat the instruction <code>a=3*4+5;</code> as if it were <code>a=17;</code> 
* [[Value range propagation]]<ref>[http://llvm.org/devmtg/2007-05/05-Lewycky-Predsimplify.pdf value range propagation]</ref> – precompute the potential ranges a calculation could be, allowing for the creation of branch predictions in advance
* [[Sparse conditional constant propagation]] – range-check some values, allowing tests to predict the most likely branch
* [[Dead-code elimination]] – remove code that will have no effect on the results
* [[Global value numbering]] – replace duplicate calculations producing the same result
* [[Partial-redundancy elimination]] – removing duplicate calculations previously performed in some branches of the program
* [[Strength reduction]] – replacing expensive operations by less expensive but equivalent ones, e.g. replace integer multiply or divide by powers of 2 with the potentially less expensive shift left (for multiply) or shift right (for divide).
* [[Register allocation]] – optimize how the limited number of machine registers may be used for calculations