42 lines
1.3 KiB
Text
42 lines
1.3 KiB
Text
.bp
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.NH 1
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Common subexpression elimination
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.NH 2
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Introduction
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.PP
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The Common Subexpression Elimination optimization technique (CS)
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tries to eliminate multiple computations of EM expressions
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that yield the same result.
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It places the result of one such computation
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in a temporary variable,
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and replaces the other computations by a reference
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to this temporary variable.
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The primary goal of this technique is to decrease
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the execution time of the program,
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but in general it will save space too.
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.PP
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As an example of the application of Common Subexpression Elimination,
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consider the piece of program in Fig. 7.1(a).
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.DS
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x := a * b; TMP := a * b; x := a * b;
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CODE; x := TMP; CODE
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y := c + a * b; CODE y := x;
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y := c + TMP;
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(a) (b) (c)
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Fig. 7.1 Examples of Common Subexpression Elimination
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.DE
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If neither a nor b is changed in CODE,
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the instructions can be replaced by those of Fig. 7.1(b),
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which saves one multiplication,
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but costs an extra store instruction.
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If the value of x is not changed in CODE either,
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the instructions can be replaced by those of Fig. 7.1(c).
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In this case
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the extra store is not needed.
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.PP
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In the following sections we will describe
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which transformations are done
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by CS and how this phase
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was implemented.
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