50 lines
1.8 KiB
Text
50 lines
1.8 KiB
Text
.NH 2
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Control Flow
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.PP
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A \fIsuccessor\fR of a basic block B is a block C
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that can be executed immediately after B.
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C is said to be a \fIpredecessor\fR of B.
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A block ending with a RET instruction
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has no successors.
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Such a block is called a \fIreturn block\fR.
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Any block that has no predecessors cannot be
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executed at all (i.e. it is unreachable),
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unless it is the first block of a procedure,
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called the \fIprocedure entry block\fR.
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.PP
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Internally, the successor and predecessor
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attributes of a basic block are stored as \fIsets\fR.
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Alternatively, one may regard all these
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sets of all basic blocks as a conceptual \fIgraph\fR,
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in which there is an edge from B to C if C
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is in the successor set of B.
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We call this conceptual graph
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the \fIControl Flow Graph\fR.
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.PP
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The only successor of a basic block ending on an
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unconditional branch instruction is the block that
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contains the label definition of the target of the jump.
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The target instruction can be found via the LAB_ID
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that is the operand of the jump instruction,
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by using the label-map table mentioned
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above.
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If the last instruction of a block is a
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conditional jump,
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the successors are the target block and the textually
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next block.
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The last instruction can also be a case jump
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instruction (CSA or CSB).
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We then analyze the case descriptor,
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to find all possible target instructions
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and their associated blocks.
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We require the case descriptor to be allocated in
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a ROM, so it cannot be changed dynamically.
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A case jump via an alterable descriptor could in principle
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go to any label in the program.
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In the presence of such an uncontrolled jump,
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hardly any optimization can be done.
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We do not expect any front end to generate such a descriptor,
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however, because of the controlled nature
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of case statements in high level languages.
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If the basic block does not end in a jump instruction,
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its only successor is the textually next block.
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