64 lines
2 KiB
Text
64 lines
2 KiB
Text
.NH 2
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Data flow information
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.PP
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.NH 3
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Use-Definition information
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.PP
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A \fIdefinition\fR of a variable A is an assignment to A.
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A definition is said to \fIreach\fR a point p if there is a
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path in the control flow graph from the definition to p, such that
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A is not redefined on that path.
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.PP
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For every basic block B, we define the following sets:
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.IP GEN[b] 9
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the set of definitions in b that reach the end of b.
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.IP KILL[b]
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the set of definitions outside b that define a variable that
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is changed in b.
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.IP IN[b]
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the set of all definitions reaching the beginning of b.
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.IP OUT[b]
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the set of all definitions reaching the end of b.
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.LP
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GEN and KILL can be determined by inspecting the code of the procedure.
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IN and OUT are computed by solving the following data flow equations:
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.DS
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(1) OUT[b] = IN[b] - KILL[b] + GEN[b]
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(2) IN[b] = OUT[p1] + ... + OUT[pn],
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where PRED(b) = {p1, ... , pn}
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.DE
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.NH 3
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Copy information
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.PP
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A \fIcopy\fR is a definition of the form "A := B".
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A copy is said to be \fIgenerated\fR in a basic block n if
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it occurs in n and there is no subsequent assignment to B in n.
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A copy is said to be \fIkilled\fR in n if:
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.IP (i)
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it occurs in n and there is a subsequent assignment to B within n, or
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.IP (ii)
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it occurs outside n, the definition A := B reaches the beginning of n
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and B is changed in n (note that a copy also is a definition).
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.LP
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A copy \fIreaches\fR a point p, if there are no assignments to B
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on any path in the control flow graph from the copy to p.
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.PP
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We define the following sets:
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.IP C_GEN[b] 11
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the set of all copies in b generated in b.
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.IP C_KILL[b]
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the set of all copies killed in b.
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.IP C_IN[b]
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the set of all copies reaching the beginning of b.
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.IP C_OUT[b]
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the set of all copies reaching the end of b.
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.LP
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C_IN and C_OUT are computed by solving the following equations:
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(root is the entry node of the current procedure; '*' denotes
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set intersection)
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.DS
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(1) C_OUT[b] = C_IN[b] - C_KILL[b] + C_GEN[b]
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(2) C_IN[b] = C_OUT[p1] * ... * C_OUT[pn],
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where PRED(b) = {p1, ... , pn} and b /= root
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C_IN[root] = {all copies}
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.DE
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