9. Compile-time size determination

The method calls minSizeInBits, minSizeInBytes, maxSizeInBits, and maxSizeInBytes can be applied to certain expressions. These method calls return the minimum (or maximum) size in bits (or bytes) required to store the expression. Thus, the result type of these methods has type int. Except in certain situations involving type variables, discussed below, these method calls produce local compile-time known values; otherwise they produce compile-time known values. None of these methods evaluate the expression that is the receiver of the method call, so it may be invalid (e.g., an out-of-bounds header stack access).

The method minSizeInBytes returns the result of minSizeInBits rounded up to the next whole number of bytes. In other words, for any expression e, e.minSizeInBytes() is equal to (e.minSizeInBits() + 7) >> 3.

The method maxSizeInBytes always returns the result of maxSizeInBits rounded up to the next whole number of bytes. In other words, for any expression e, e.maxSizeInBytes() is equal to (e.maxSizeInBits() + 7) >> 3.

The definition of e.minSizeInBits() and e.maxSizeInBits() is given recursively on the type of e as described in the following table:

|—–|—————|—————| |Type | minSizeInBits | maxSizeInBits | +—–+:————-:+:————-:+ | bit<N> | N | N | +—–+—————+—————+ | int<N> | N | N | +—–+—————+—————+ | bool | 1 | 1 | +—–+—————+—————+ | enum bit<N> | N | N | +—–+—————+—————+ | enum int<N> | N | N | +—–+—————+—————+ | tuple | foreach field(tuple) sum of | foreach field(tuple) sum of | | | field.minSizeInBits() | field.maxSizeInBits() | +—–+—————+—————+ | varbit<N> | 0 | N | +—–+—————+—————+ | struct | foreach field(struct) sum of | foreach field(struct) sum of | | | field.minSizeInBits() | field.maxSizeInBits() | +—–+—————+—————+ | header | foreach field(header) sum of | foreach field(header) sum of | | | field.minSizeInBits() | field.maxSizeInBits() | +—–+—————+—————+ | H[N] | N * H.minSizeInBits() | N * H.maxSizeInBits() | +—–+—————+—————+ | header_union | max(foreach field(header_union) | max(foreach field(header_union) | | | field.minSizeInBits()) | field.maxSizeInBits()) | |—–|—————|—————|

The methods can also be applied to type name expressions e:

• if the type of e is a type introduced by type, the result is the application of the method to the underlying type
• if e is the name of a type (e.g., introduced by a typedef declaration), where the type given a name is one of the above, then the result is obtained by applying the method to the underlying type.

These methods are defined for:

• all serializable types
• for a type that does not contain varbit fields, both methods return the same result
• for a type that does contain varbit fields, maxSizeInBits is the worst-case size of the serialized representation of the data and minSizeInBits is the “best” case.

Every other case is undefined and will produce a compile-time error. In particular, cases involving type variables produce a compile-time error.