58 lines
2.1 KiB
Text
58 lines
2.1 KiB
Text
.bp
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.NH 1
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Use-Definition analysis
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.NH 2
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Introduction
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.PP
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The "Use-Definition analysis" phase (UD) consists of two related optimization
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techniques that both depend on "Use-Definition" information.
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The techniques are Copy Propagation and Constant Propagation.
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They are best explained via an example (see Figs. 11.1 and 11.2).
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.DS
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(1) A := B A := B
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... --> ...
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(2) use(A) use(B)
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Fig. 11.1 An example of Copy Propagation
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.DE
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.DS
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(1) A := 12 A := 12
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... --> ...
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(2) use(A) use(12)
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Fig. 11.2 An example of Constant Propagation
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.DE
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Both optimizations have to check that the value of A at line (2)
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can only be obtained at line (1).
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Copy Propagation also has to assure that the value of B is
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the same at line (1) as at line (2).
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.PP
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One purpose of both transformations is to introduce
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opportunities for the Dead Code Elimination optimization.
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If the variable A is used nowhere else, the assignment A := B
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becomes useless and can be eliminated.
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.sp 0
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If B is less expensive to access than A (e.g. this is sometimes the case
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if A is a local variable and B is a global variable),
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Copy Propagation directly improves the code itself.
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If A is cheaper to access the transformation will not be performed.
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Likewise, a constant as operand may be cheeper than a variable.
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Having a constant as operand may also facilitate other optimizations.
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.PP
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The design of UD is based on the theory described in section
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14.1 and 14.3 of.
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.[
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aho compiler design
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.]
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As a main departure from that theory,
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we do not demand the statement A := B to become redundant after
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Copy Propagation.
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If B is cheaper to access than A, the optimization is always performed;
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if B is more expensive than A, we never do the transformation.
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If A and B are equally expensive UD uses the heuristic rule to
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replace infrequently used variables by frequently used ones.
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This rule increases the chances of the assignment to become useless.
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.PP
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In the next section we will give a brief outline of the data
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flow theory used
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for the implementation of UD.
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