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+.NH 2
+Immediate dominators
+.PP
+A basic block B dominates a block C if every path
+in the control flow graph from the procedure entry block
+to C goes through B.
+The immediate dominator of C is the closest dominator
+of C on any path from the entry block.
+See also
+.[~[
+aho compiler design
+.], section 13.1.]
+.PP
+There are a number of algorithms to compute
+the immediate dominator relation.
+.IP 1.
+Purdom and Moore give an algorithm that is
+easy to program and easy to describe (although the
+description they give is unreadable;
+it is given in a very messy Algol60 program full of gotos).
+.[
+predominators 
+.]
+.IP 2.
+Aho and Ullman present a bitvector algorithm, which is also
+easy to program and to understand.
+(See 
+.[~[
+aho compiler design
+.], section 13.1.]).
+.IP 3
+Lengauer and Tarjan introduce a fast algorithm that is
+hard to understand, yet remarkably easy to implement.
+.[
+lengauer dominators
+.]
+.LP
+The Purdom-Moore algorithm is very slow if the
+number of basic blocks in the flow graph is large.
+The Aho-Ullman algorithm in fact computes the
+dominator relation,
+from which the immediate dominator relation can be computed
+in time quadratic to the number of basic blocks, worst case.
+The storage requirement is also quadratic to the number
+of blocks.
+The running time of the third algorithm is proportional
+to:
+.DS
+(number of edges in the graph) * log(number of blocks).
+.DE
+We have chosen this algorithm because it is fast
+(as shown by experiments done by Lengauer and Tarjan),
+it is easy to program and requires little data space.