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+.bp
+.NH 1
+Branch Optimization
+.NH 2
+Introduction
+.PP
+The Branch Optimization phase (BO) performs two related
+(branch) optimizations.
+.NH 3
+Fusion of basic blocks
+.PP
+If two basic blocks B1 and B2 have the following properties:
+.DS
+SUCC(B1) = {B2}
+PRED(B2) = {B1}
+.DE
+then B1 and B2 can be combined into one basic block.
+If B1 ends in an unconditional jump to the beginning of B2, this
+jump can be eliminated,
+hence saving a little execution time and object code size.
+This technique can be used to eliminate some deficiencies
+introduced by the front ends (for example, the "C" front end
+translates switch statements inefficiently due to its one pass nature).
+.NH 3
+While-loop optimization
+.PP
+The straightforward way to translate a while loop is to
+put the test for loop termination at the beginning of the loop.
+.DS
+while cond loop                  LAB1: Test cond
+   body of the loop     --->           Branch On False To LAB2
+end loop                               code for body of loop
+				       Branch To LAB1
+				 LAB2:
+
+Fig. 10.1 Example of Branch Optimization
+.DE
+If the condition fails at the Nth iteration, the following code
+gets executed (dynamically):
+.DS
+N   *  conditional branch (which fails N-1 times)
+N-1 *  unconditional branch
+N-1 *  body of the loop
+.DE
+An alternative translation is:
+.DS
+     Branch To LAB2
+LAB1:
+     code for body of loop
+LAB2:
+     Test cond
+     Branch On True To LAB1
+.DE
+This translation results in the following profile:
+.DS
+N   *  conditional branch (which succeeds N-1 times)
+1   *  unconditional branch
+N-1 *  body of the loop
+.DE
+So the second translation will be significantly faster if N >> 2.
+If N=2, execution time will be slightly increased.
+On the average, the program will be speeded up.
+Note that the code sizes of the two translations will be the same.
+.NH 2
+Implementation
+.PP
+The basic block fusion technique is implemented
+by traversing the control flow graph of a procedure,
+looking for basic blocks B with only one successor (S).
+If one is found, it is checked if S has only one predecessor
+(which has to be B).
+If so, the two basic blocks can in principle be combined.
+However, as one basic block will have to be moved,
+the textual order of the basic blocks will be altered.
+This reordering causes severe problems in the presence
+of conditional jumps.
+For example, if S ends in a conditional branch,
+the basic block that comes textually next to S must stay
+in that position.
+So the transformation in Fig. 10.2 is illegal.
+.DS
+LAB1: S1              LAB1: S1
+      BRA LAB2        S2
+      ...       -->   BEQ LAB3
+LAB2: S2              ...
+      BEQ LAB3        S3
+      S3
+
+Fig. 10.2 An illegal transformation of Branch Optimization
+.DE
+If B is moved towards S the same problem occurs if the block before B
+ends in a conditional jump.
+The problem could be solved by adding one extra branch,
+but this would reduce the gains of the optimization to zero.
+Hence the optimization will only be done if the block that
+follows S (in the textual order) is not a successor of S.
+This condition assures that S does not end in a conditional branch.
+The condition always holds for the code generated by the "C"
+front end for a switch statement.
+.PP
+After the transformation has been performed,
+some attributes of the basic blocks involved (such as successor and
+predecessor sets and immediate dominator) must be recomputed.
+.PP
+The while-loop technique is applied to one loop at a time.
+The list of basic blocks of the loop is traversed to find
+a block B that satisfies the following conditions:
+.IP 1.
+the textually next block to B is not part of the loop
+.IP 2.
+the last instruction of B is an unconditional branch;
+hence B has only one successor, say S
+.IP 3.
+the textually next block of B is a successor of S
+.IP 4.
+the last instruction of S is a conditional branch
+.LP
+If such a block B is found, the control flow graph is changed
+as depicted in Fig. 10.3.
+.DS
+       |				    |
+       |				    v
+       v				    |
+       |-----<------|			    ----->-----|
+   ____|____	    |				       |
+   |	   |	    |		    |-------|	       |
+   |  S1   |	    |		    |	    v	       |
+   |  Bcc  |	    |		    |	  ....	       |
+|--|	   |	    |		    |		       |
+|  ---------	    |		    |	----|----      |
+|		    |		    |	|	|      |
+|     ....	    ^		    |	|  S2	|      |
+|		    |		    |	|	|      |
+|   ---------	    |		    |	|	|      |
+v   |	    |	    |		    ^	---------      |
+|   |  S2   |	    |		    |	    |	       |
+|   | BRA   |	    |		    |	    |-----<-----
+|   |	    |	    |		    |	    v
+|   ---------	    |		    |	____|____
+|	|	    |		    |	|	|
+|	------>------		    |	|  S1	|
+|				    |	|  Bnn  |
+|-------|			    |	|	|
+	|			    |	----|----
+	v			    |	    |
+				    |----<--|
+					    |
+					    v
+
+Fig. 10.3 Transformation of the CFG by Branch Optimization
+.DE