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...SSA ( Static Single Assignment ) on compilers

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Question by Ganesan
Submitted on 12/24/2003
Related FAQ: N/A
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What is SSA ( Static Single Assignment ) on compilers


Answer by mani
Submitted on 3/23/2004
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In compilers, static single assignment form, more often abbreviated SSA form or just SSA, is an intermediate representation in which every variable is assigned exactly once.
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 := y


As humans, we can see that the first assignment isn't 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:

y1 := 1
y2 := 2
x1 := y2


Compiler optimization algorithms are  strongly enhanced by the use of SSA. I will not go into more detail on this now.

 

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