93 lines
2.7 KiB
Text
93 lines
2.7 KiB
Text
.NH 2
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Loop detection
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.PP
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Loops are detected by using the loop construction
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algorithm of.
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.[~[
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aho compiler design
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.], section 13.1.]
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This algorithm uses \fIback edges\fR.
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A back edge is an edge from B to C in the CFG,
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whose head (C) dominates its tail (B).
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The loop associated with this back edge
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consists of C plus all nodes in the CFG
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that can reach B without going through C.
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.PP
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As an example of how the algorithm works,
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consider the piece of program of Fig. 4.1.
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First just look at the program and try to
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see what part of the code constitutes the loop.
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.DS
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loop
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if cond then 1
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-- lots of simple
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-- assignment
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-- statements 2 3
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exit; -- exit loop
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else
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S; -- one statement
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end if;
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end loop;
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Fig. 4.1 A misleading loop
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.DE
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Although a human being may be easily deceived
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by the brackets "loop" and "end loop",
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the loop detection algorithm will correctly
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reply that only the test for "cond" and
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the single statement in the false-part
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of the if statement are part of the loop!
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The statements in the true-part only get
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executed once, so there really is no reason at all
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to say they're part of the loop too.
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The CFG contains one back edge, "3->1".
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As node 3 cannot be reached from node 2,
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the latter node is not part of the loop.
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.PP
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A source of problems with the algorithm is the fact
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that different back edges may result in
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the same loop.
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Such an ill-structured loop is
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called a \fImessy\fR loop.
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After a loop has been constructed, it is checked
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if it is really a new loop.
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.PP
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Loops can partly overlap, without one being nested
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inside the other.
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This is the case in the program of Fig. 4.2.
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.DS
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1: 1
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S1;
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2:
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S2; 2
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if cond then
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goto 4;
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S3; 3 4
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goto 1;
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4:
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S4;
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goto 1;
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Fig. 4.2 Partly overlapping loops
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.DE
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There are two back edges "3->1" and "4->1",
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resulting in the loops {1,2,3} and {1,2,4}.
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With every basic block we associate a set of
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all loops it is part of.
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It is not sufficient just to record its
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most enclosing loop.
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.PP
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After all loops of a procedure are detected, we determine
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the nesting level of every loop.
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Finally, we find all strong and firm blocks of the loop.
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If the loop has only one back edge (i.e. it is not messy),
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the set of firm blocks consists of the
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head of this back edge and its dominators
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in the loop (including the loop entry block).
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A firm block is also strong if it is not a
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successor of a block that may exit the loop;
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a block may exit a loop if it has an (immediate) successor
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that is not part of the loop.
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For messy loops we do not determine the strong
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and firm blocks. These loops are expected
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to occur very rarely.
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