Author: Arsh Sharma
December 22nd, 2020
In computer programming, as in real life, names are useful handles for concrete entities. The key point about SSA is that having unique names for distinct entities reduces uncertainty and imprecision.
For example, Consider overhearing a conversation about ‘Apple.’ Without any more contextual clues, you cannot disambiguate between the fruit Apple and Apple the American multinational technology company. As soon as the conversation mentions iPhone (rather than the fruit properties), you are fairly sure that the company Apple(rather than the fruit) is the subject. On the other hand, if everyone had a unique name, then there would be no possibility of confusing one of the largest brand on the planet with a fruit that grows in hilly areas.
The Static Single Assignment form(SSA), is a naming convention for storage locations(variables) in low-level representations of computer programs. The term static indicates that SSA relates to properties and analysis of program text (code). The terms in general refer to the uniqueness property of variable names that SSA imposes. As from the example above, this enables a greater degree of precision. The term assignment means variable definitions. Like, in the code
x is being assigned the value of expression
(y+1). This is a definition,or assignment statement, for
x. A compiler engineer would interpret the above assignment statement to mean that the value of
x(i.e., the memory location labeled as x) should be modified to store the value
A program is defined to be in SSA form if each variable is a target of exactly one assignment statement in the program text.
However there are various, more specialized, varieties of SSA, which impose further constraints on programs. Such constraints may relate to graph-theoretic properties of variable definitions and uses, or the encapsulation of specific control-flow or data-flow information. Each distinct SSA variety has specific characteristics.
One important property that holds for all varieties of SSA, including the simplest definition above, is referential transparency. i.e., since there is only a single definition for each variable in the program text, a variable’s value is independent of its position in the program. We may refine our knowledge about a particular variable based on branching conditions, e.g. we know the value of
x in the conditionally executed block following an if-statement that begins with
however the underlying value of
x does not change at this
if statement. Programs written in pure functional languages are referentially transparent. Such referentially transparent programs are more amenable to formal methods and mathematical reasoning, since the meaning of an expression depends only on the meaning of its sub-expressions and not on the order of evaluation or side effects of other expressions.