Benefits
The primary usefulness of SSA comes from how it simultaneously simplifies and improves the results of a variety of compiler optimizations, by simplifying the properties of variables. For example, consider this piece of code:
y := 1 y := 2 x := yHumans can see that the first assignment is not necessary, and that the value of y being used in the third line comes from the second assignment of y. A program would have to perform reaching definition analysis to determine this. But if the program is in SSA form, both of these are immediate:
Compiler optimization algorithms which are either enabled or strongly enhanced by the use of SSA include:
- constant propagation
- value range propagation
- sparse conditional constant propagation
- dead code elimination
- global value numbering
- partial redundancy elimination
- strength reduction
- register allocation
Read more about this topic: Static Single Assignment Form
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