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