Motivation for Dataflow Analysis

A compiler can perform some optimizations based only on local information. For example, consider the following code:

x = a + b;
x = 5 * 2;

It is easy for an optimizer to recognize that:

The first assignment to x is a "useless" assignment, since the value computed for x is never used (and thus the first statement can be eliminated from the program).
The expression 5 * 2 can be computed at compile time, simplifying the second assignment statement to x = 10;

Some optimizations, however, require more "global" information. For example, consider the following code:

a = 1;
b = 2;
c = 3;
if (...) x = a + 5;
else x = b + 4;
c = x + 1;

In this example, the initial assignment to c (at line 3) is useless, and the expression x + 1 can be simplified to 7, but it is less obvious how a compiler can discover these facts since they cannot be discovered by looking only at one or two consecutive statements. A more global analysis is needed so that the compiler knows at each point in the program:

which variables are guaranteed to have constant values, and
which variables will be used before being redefined.

To discover these kinds of properties, we use dataflow analysis. Dataflow analysis is usually performed on the program's control-flow graph (CFG); the goal is to associate with each program component (each node of the CFG) information that is guaranteed to hold at that point on all executions.

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