79 lines
2.9 KiB
Text
79 lines
2.9 KiB
Text
.NH 2
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Interprocedural analysis
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.PP
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It is often desirable to know the effects
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a procedure call may have.
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The optimization below is only possible if
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we know for sure that the call to P cannot
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change A.
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.DS
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A := 10; A:= 10;
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P; -- procedure call --> P;
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B := A + 2; B := 12;
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.DE
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Although it is not possible to predict exactly
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all the effects a procedure call has, we may
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determine a kind of upper bound for it.
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So we compute all variables that may be
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changed by P, although they need not be
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changed at every invocation of P.
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We can get hold of this set by just looking
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at all assignment (store) instructions
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in the body of P.
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EM also has a set of \fIindirect\fR assignment
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instructions,
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i.e. assignment through a pointer variable.
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In general, it is not possible to determine
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which variable is affected by such an assignment.
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In these cases, we just record the fact that P
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does an indirect assignment.
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Note that this does not mean that all variables
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are potentially affected, as the front ends
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may generate messages telling that certain
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variables can never be accessed indirectly.
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We also set a flag if P does a use (load) indirect.
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Note that we only have to look at \fIglobal\fR
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variables.
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If P changes or uses any of its locals,
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this has no effect on its environment.
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Local variables of a lexically enclosing
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procedure can only be accessed indirectly.
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.PP
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A procedure P may of course call another procedure.
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To determine the effects of a call to P,
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we also must know the effects of a call to the second procedure.
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This second one may call a third one, and so on.
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Effectively, we need to compute the \fItransitive closure\fR
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of the effects.
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To do this, we determine for every procedure
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which other procedures it calls.
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This set is the "calling" attribute of a procedure.
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One may regard all these sets as a conceptual graph,
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in which there is an edge from P to Q
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if Q is in the calling set of P. This graph will
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be referred to as the \fIcall graph\fR.
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(Note the resemblance with the control flow graph).
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.PP
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We can detect which procedures are called by P
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by looking at all CAL instructions in its body.
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Unfortunately, a procedure may also be
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called indirectly, via a CAI instruction.
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Yet, only procedures that are used as operand of an LPI
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instruction can be called indirect,
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because this is the only way to take the address of a procedure.
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We determine for every procedure whether it does
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a CAI instruction.
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We also build a set of all procedures used as
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operand of an LPI.
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.sp
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After all procedures have been processed (i.e. all CFGs
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are constructed, all loops are detected,
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all procedures are analyzed to see which variables
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they may change, which procedures they call,
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whether they do a CAI or are used in an LPI) the
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transitive closure of all interprocedural
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information is computed.
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During the same process,
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the calling set of every procedure that uses a CAI
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is extended with the above mentioned set of all
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procedures that can be called indirect.
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