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+.bp
+.NH
+The Control Flow Phase
+.PP
+In the previous chapter we described the intermediate
+code of the global optimizer.
+We also specified which part of this code
+was constructed by the IC phase of the optimizer.
+The Control Flow Phase (\fICF\fR) does
+the remainder of the job,
+i.e. it determines:
+.IP -
+the control flow graphs
+.IP -
+the loop tables
+.IP -
+the calling, change and use attributes of
+the procedure table entries
+.LP
+CF operates on one procedure at a time.
+For every procedure it first reads the EM instructions
+from the EM-text file and groups them into basic blocks.
+For every basic block, its successors and
+predecessors are determined,
+resulting in the control flow graph.
+Next, the immediate dominator of every basic block
+is computed.
+Using these dominators, any loop in the
+procedure is detected.
+Finally, interprocedural analysis is done,
+after which we will know the global effects of
+every procedure call on its environment.
+.sp
+CF uses the same internal data structures
+for the procedure table and object table as IC.
+.NH 2
+Partitioning into basic blocks
+.PP
+With regard to flow of control, we distinguish
+three kinds of EM instructions:
+jump instructions, instruction label definitions and
+normal instructions.
+Jump instructions are all conditional or unconditional
+branch instructions,
+the case instructions (CSA/CSB)
+and the RET (return) instruction.
+A procedure call (CAL) is not considered to be a jump.
+A defining occurrence of an instruction label
+is regarded as an EM instruction.
+.PP
+An instruction starts
+a new basic block, in any of the following cases:
+.IP 1.
+It is the first instruction of a procedure
+.IP 2.
+It is the first of a list of instruction label
+defining occurrences
+.IP 3.
+It follows a jump
+.LP
+If there are several consecutive instruction labels
+(which is highly unusual),
+all of them are put in the same basic block.
+Note that several cases may overlap,
+e.g. a label definition at the beginning of a procedure
+or a label following a jump.
+.PP
+A simple Finite State Machine is used to model
+the above rules.
+It also recognizes the end of a procedure,
+marked by an END pseudo.
+The basic blocks are stored internally as a doubly linked
+linear list.
+The blocks are linked in textual order.
+Every node of this list has the attributes described
+in the previous chapter (see syntax rule for
+basic_block).
+Furthermore, every node contains a pointer to its
+EM instructions,
+which are represented internally
+as a linear, doubly linked list,
+just as in the IC phase.
+However, instead of one list per procedure (as in IC)
+there is now one list per basic block.
+.PP
+On the fly, a table is build that maps
+every label identifier to the label definition
+instruction.
+This table is used for computing the control flow.
+The table is stored as a dynamically allocated array.
+The length of the array is the number of labels
+of the current procedure;
+this value can be found in the procedure table,
+where it was stored by IC.