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