08 10 operations on variable size bit types

To support parsing headers with variable-length fields, P4 offers a type varbit. Each occurrence of the type varbit has a statically-declared maximum width, as well as a dynamic width, which must not exceed the static bound. Prior to initialization a variable-size bit-string has an unknown dynamic width.

Variable-length bit-strings support a limited set of operations:

• Assignment to another variable-sized bit-string. The target of the assignment must have the same static width as the source. When executed, the assignment sets the dynamic width of the target to the dynamic width of the source.
• Comparison for equality or inequality with another varbit field. Two varbit fields can be compared only if they have the same type. Two varbits are equal if they have the same dynamic width and all the bits up to the dynamic width are the same.

The following operations are not supported directly on a value of type varbit, but instead on any type for which extract and emit operations are supported (e.g. a value with type header) that may contain a field of type varbit. They are mentioned here only to ease finding this information in a section dedicated to type varbit.

• Parser extraction into a header containing a variable-sized bit-string using the two-argument extract method of a packet_in extern object (see Section Variable-width extraction). This operation sets the dynamic width of the field.
• The emit method of a packet_out extern object can be performed on a header and a few other types (see Section Deparsing) that contain a field with type varbit. Such an emit method call inserts a variable-sized bit-string with a known dynamic width into the packet being constructed.

Additionally, the maximum size of a variable-length bit-string can be determined at compile-time (Section Compile-time size determination).