Semantic Analysis - Crafting a Compiler - page 364

Switching

control cases

CaseItem

labelList

stmts more

Expr

LabelList

caseLabel isDefault more

caseExp

CaseItem

. . .

StmtList

LabelList

Expr

Figure 9.20: Abstract Syntax Tree for a Switch Statement

Finally, some programming languages forbid gotos into a conditional or
iterative statement from outside. Thatis,evenifthescopeofalabelisan
entire subprogram, a goto into a loop body, or from a then part to an else
part, is forbidden. Such restrictions can be enforced by marking each label in
declaredLabels as either active or inactive . Gotos are allowed only to active labels,
and a label within a conditional or iterative statement is active only while the
AST that contains the label is being processed. Thus, a label L within a while
loop becomes activewhen the loop body's AST is checked and is inactive when
statements outside the loop body are checked.

9.1.6 Switch and Case Statements

Java, C, C contain a switch statement that allows the selection of

one of a number of statements based on the value of a control expression.
ML, Ada, and a number of older languages contain a case statement that is
equivalent. We shall focus on translating switch statements, but our discussion
applies equally to case statements.
The AST for a switch statement rooted at Switchingis shown in Figure 9.20
(fields not needed for semantic analysis are omitted for clarity). In the AST,
control represents an integer-valued expression; cases is a CaseItem,repre-
senting the cases in the switch. Each CaseItemhas three fields. labelList is a
LabelList that represents one or more case labels. stmts is an AST node repre-
senting the statements following a case constant in the switch. more is either
null or another CaseItem, representing the remaining cases in the switch.
A LabelList contains an integer field caseLabel ,anAST caseExp , a Boolean
field isDe f ault

,andC ++

(representing the default case label), and more , a field that is