.bp
.NH 1
Use-Definition analysis
.NH 2
Introduction
.PP
The "Use-Definition analysis" phase (UD) consists of two related optimization
techniques that both depend on "Use-Definition" information.
The techniques are Copy Propagation and Constant Propagation.
They are best explained via an example (see Figs. 11.1 and 11.2).
.DS
   (1)  A := B                  A := B
	 ...          -->        ...
   (2)  use(A)                  use(B)

Fig. 11.1 An example of Copy Propagation
.DE
.DS
   (1)  A := 12                  A := 12
	 ...          -->        ...
   (2)  use(A)                  use(12)

Fig. 11.2 An example of Constant Propagation
.DE
Both optimizations have to check that the value of A at line (2)
can only be obtained at line (1).
Copy Propagation also has to assure that the value of B is
the same at line (1) as at line (2).
.PP
One purpose of both transformations is to introduce
opportunities for the Dead Code Elimination optimization.
If the variable A is used nowhere else, the assignment A := B
becomes useless and can be eliminated.
.sp 0
If B is less expensive to access than A (e.g. this is sometimes the case
if A is a local variable and B is a global variable),
Copy Propagation directly improves the code itself.
If A is cheaper to access the transformation will not be performed.
Likewise, a constant as operand may be cheeper than a variable.
Having a constant as operand may also facilitate other optimizations.
.PP
The design of UD is based on the theory described in section
14.1 and 14.3 of.
.[
aho compiler design
.]
As a main departure from that theory,
we do not demand the statement A := B to become redundant after
Copy Propagation.
If B is cheaper to access than A, the optimization is always performed;
if B is more expensive than A, we never do the transformation.
If A and B are equally expensive UD uses the heuristic rule to
replace infrequently used variables by frequently used ones.
This rule increases the chances of the assignment to become useless.
.PP
In the next section we will give a brief outline of the data
flow theory used
for the implementation of UD.