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