12 07 switch statement

The switch statement can only be used within control blocks.

switchStatement
: SWITCH "(" expression ")" "{" switchCases "}"
;

switchCases
: /* empty */
| switchCases switchCase
;

switchCase
: switchLabel ":" blockStatement
| switchLabel ":"  // fall-through
;

switchLabel
: DEFAULT
| nonBraceExpression
;

nonBraceExpression
: INTEGER
| STRING_LITERAL
| TRUE
| FALSE
| THIS
| prefixedNonTypeName
| nonBraceExpression "[" expression "]"
| nonBraceExpression "[" expression ":" expression "]"
| nonBraceExpression "[" expression "+" ":" expression "]"
| "(" expression ")"
| "!" expression %prec PREFIX
| "~" expression %prec PREFIX
| "-" expression %prec PREFIX
| "+" expression %prec PREFIX
| typeName "." member
| ERROR "." member
| nonBraceExpression "." member
| nonBraceExpression "*" expression
| nonBraceExpression "/" expression
| nonBraceExpression "%" expression
| nonBraceExpression "+" expression
| nonBraceExpression "-" expression
| nonBraceExpression "|+|" expression
| nonBraceExpression "|-|" expression
| nonBraceExpression "<<" expression
| nonBraceExpression ">>" expression
| nonBraceExpression "<=" expression
| nonBraceExpression ">=" expression
| nonBraceExpression "<" expression
| nonBraceExpression ">" expression
| nonBraceExpression "!=" expression
| nonBraceExpression "==" expression
| nonBraceExpression "&" expression
| nonBraceExpression "^" expression
| nonBraceExpression "|" expression
| nonBraceExpression "++" expression
| nonBraceExpression "&&" expression
| nonBraceExpression "||" expression
| nonBraceExpression "?" expression ":" expression
| nonBraceExpression "<" realTypeArgumentList ">" "(" argumentList ")"
| nonBraceExpression "(" argumentList ")"
| namedType "(" argumentList ")"
| "(" typeRef ")" expression
;

The nonBraceExpression is the same as expression as defined in Section Expressions, except it does not include any cases that can begin with a left brace { character, to avoid syntactic ambiguity with a block statement.

There are two kinds of switch expressions allowed, described separately in the following two subsections.

Switch statement with action_run expression

For this variant of switch statement, the expression must be of the form t.apply().action_run, where t is the name of a table (see Section Match-action unit invocation). All switch labels must be names of actions of the table t, or default.

switch (t.apply().action_run) {
   action1:          // fall-through to action2:
   action2: { /* body omitted */ }
   action3: { /* body omitted */ }  // no fall-through from action2 to action3 labels
   default: { /* body omitted */ }
}

Note that the default label of the switch statement is used to match on the kind of action executed, no matter whether there was a table hit or miss. The default label does not indicate that the table missed and the default_action was executed.

Switch statement with integer or enumerated type expression

For this variant of switch statement, the expression must evaluate to a result with one of these types:

• bit<W>
• int<W>
• enum, either with or without an underlying representation specified
• error

All switch labels must be expressions with compile-time known values, and must have a type that can be implicitly cast to the type of the switch expression (see Section Implicit casts). Switch labels must not begin with a left brace character {, to avoid ambiguity with a block statement.

// Assume the expression hdr.ethernet.etherType has type bit<16>.
switch (hdr.ethernet.etherType) {
   0x86dd: { /* body omitted */ }
   0x0800:          // fall-through to the next body
   0x0802: { /* body omitted */ }
   0xcafe: { /* body omitted */ }
   default: { /* body omitted */ }
}

Notes common to all switch statements

It is a compile-time error if two labels of a switch statement equal each other. The switch label values need not include all possible values of the switch expression. It is optional to have a switch case with the default label, but if one is present, it must be the last one in the switch statement.

If a switch label is not followed by a block statement, it falls through to the next label. However, if a block statement is present, it does not fall through. Note that this is different from C-style switch statements, where a break is needed to prevent fall-through. If the last switch label is not followed by a block statement, the behavior is the same as if the last switch label were followed by an empty block statement { }.

When a switch statement is executed, first the switch expression is evaluated, and any side effects from evaluating this expression are visible to any switch case that is executed. Among switch labels that are not default, at most one of them can equal the value of the switch expression. If one is equal, that switch case is executed.

If no labels are equal to the switch expression, then:

• if there is a default label, the case with the default label is executed.
• if there is no default label, then no switch case is executed, and execution continues after the end of the switch statement, with no side effects (except any that were caused by evaluating the switch expression).

See “Implementing generalized P4_16 switch statements” GeneralizedSwitchStatements for possible techniques that one might use to implement generalized switch statements.