44 lines
1.3 KiB
Text
44 lines
1.3 KiB
Text
.bp
|
|
.NH 1
|
|
Strength reduction
|
|
.NH 2
|
|
Introduction
|
|
.PP
|
|
The Strength Reduction optimization technique (SR)
|
|
tries to replace expensive operators
|
|
by cheaper ones,
|
|
in order to decrease the execution time
|
|
of the program.
|
|
A classical example is replacing a 'multiplication by 2'
|
|
by an addition or a shift instruction.
|
|
These kinds of local transformations are already
|
|
done by the EM Peephole Optimizer.
|
|
Strength reduction can also be applied
|
|
more generally to operators used in a loop.
|
|
.DS
|
|
i := 1; i := 1;
|
|
while i < 100 loop --> TMP := i * 118;
|
|
put(i * 118); while i < 100 loop
|
|
i := i + 1; put(TMP);
|
|
end loop; i := i + 1;
|
|
TMP := TMP + 118;
|
|
end loop;
|
|
|
|
Fig. 6.1 An example of Strenght Reduction
|
|
.DE
|
|
In Fig. 6.1, a multiplication inside a loop is
|
|
replaced by an addition inside the loop and a multiplication
|
|
outside the loop.
|
|
Clearly, this is a global optimization; it cannot
|
|
be done by a peephole optimizer.
|
|
.PP
|
|
In some cases a related technique, \fItest replacement\fR,
|
|
can be used to eliminate the
|
|
loop variable i.
|
|
This technique will not be discussed in this report.
|
|
.sp 0
|
|
In the example above, the resulting code
|
|
can be further optimized by using
|
|
constant propagation.
|
|
Obviously, this is not the task of the
|
|
Strength Reduction phase.
|