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+.bp
+.NH 1
+Cross jumping
+.NH 2
+Introduction
+.PP
+The "Cross Jumping" optimization technique (CJ)
+.[
+wulf design optimizing compiler
+.]
+is basically a space optimization technique. It looks for pairs of
+basic blocks (B1,B2), for which:
+.DS
+SUCC(B1) = SUCC(B2) = {S}
+.DE
+(So B1 and B2 both have one and the same successor).
+If the last few non-branch instructions are the same for B1 and B2,
+one such sequence can be eliminated.
+.DS
+Pascal:
+
+if cond then
+    S1
+    S3
+else
+    S2
+    S3
+
+(pseudo) EM:
+
+TEST COND		TEST COND
+BNE *1			BNE *1
+S1			S1
+S3	   --->		BRA *2
+BRA *2			1:
+1:			S2
+S2			2:
+S3			S3
+2:
+
+Fig. 9.1 An example of Cross Jumping
+.DE
+As the basic blocks have the same successor,
+at least one of them ends in an unconditional branch instruction (BRA).
+Hence no extra branch instruction is ever needed, just the target
+of an existing branch needs to be changed; neither the program size
+nor the execution time will ever increase.
+In general, the execution time will remain the same, unless
+further optimizations can be applied because of this optimization.
+.PP
+This optimization is particularly effective,
+because it cannot always be done by the programmer at the source level,
+as demonstrated by the Fig. 8.2.
+.DS
+	Pascal:
+
+	if cond then
+	   x := f(4)
+	else
+	   x := g(5)
+
+
+	EM:
+
+	...                     ...
+	LOC 4			LOC 5
+	CAL F			CAL G
+	ASP 2			ASP 2
+	LFR 2			LFR 2
+	STL X			STL X
+
+Fig. 9.2 Effectiveness of Cross Jumping
+.DE
+At the source level there is no common tail,
+but at the EM level there is a common tail.
+.NH 2
+Implementation
+.PP
+The implementation of cross jumping is rather straightforward.
+The technique is applied to one procedure at a time.
+The control flow graph of the procedure 
+is scanned for pairs of basic blocks
+with the same (single) successor and with common tails.
+Note that there may be more than two such blocks (e.g. as the result
+of a case statement).
+This is dealt with by repeating the entire process until no
+further optimizations can de done for the current procedure.
+.sp
+If a suitable pair of basic blocks has been found, the control flow
+graph must be altered. One of the basic
+blocks must be split into two.
+The control flow graphs before and after the optimization are shown
+in Fig. 9.3 and Fig. 9.4.
+.DS
+
+	--------				--------
+	|      |				|      |
+	| S1   |			        | S2   |
+	| S3   |   				| S3   |
+	|      |				|      |
+	--------				--------
+	   |					   |
+	   |------------------|--------------------|
+			      |
+			      v
+
+Fig. 9.3 CFG before optimization
+.DE
+.DS
+
+	--------				--------
+	|      |				|      |
+	| S1   |			        | S2   |
+	|      |				|      |
+	--------				--------
+	   |					   |
+	   |--------------------<------------------|
+	   v
+	--------
+	|      |
+	| S3   |
+	|      |
+	--------
+	   |
+	   v
+
+Fig. 9.4 CFG after optimization
+.DE
+Some attributes of the three resulting blocks (such as immediate dominator)
+are updated.
+.PP
+In some cases, cross jumping might split the computation of an expression
+into two, by inserting a branch somewhere in the middle.
+Most code generators will generate very poor assembly code when
+presented with such EM code. 
+Therefor, cross jumping is not performed in these cases.