18 01 compile time known and local compile time known values

Certain expressions in a P4 program have the property that their value can be determined at compile time. Moreover, for some of these expressions, their value can be determined only using information in the current scope. We call these compile-time known values and local compile-time known values respectively.

The following are local compile-time known values:

• Integer literals, Boolean literals, and string literals.
• Identifiers declared as constants using the const keyword.
• Identifiers declared in an error, enum, or match_kind declaration.
• The default identifier.
• The size field of a value with type header stack.
• The _ identifier when used as a select expression label
• The expression {#} representing an invalid header or header union value.
• Instances constructed by instance declarations (Section Instantiations) and constructor invocations.
• Identifiers that represent declared types, actions, functions, tables, parsers, controls, or packages.
• Tuple expression where all components are local compile-time known values.
• Structure-valued expressions, where all fields are local compile-time known values.
• Expressions evaluating to a list type, where all elements are local compile-time known values.
• Legal casts applied to local compile-time known values.
• The following expressions (+, -, |+|, |-|, *, /, %, !, &, |, ^, &&, ||, <<, >>, ~, /, >, <, ==, !=, <=, >=, ++, [:], ?:) when their operands are all local compile-time known values.
• Expressions of the form e.minSizeInBits(), e.minSizeInBytes(), e.maxSizeInBits() and e.maxSizeInBytes() where the type of e is not generic.

The following are compile-time known values:

• All local compile-time known values.
• Constructor parameters (i.e., the declared parameters for a parser, control, etc.)
• Tuple expression where all components are compile-time known values.
• Expressions evaluating to a list type, where all elements are compile-time known values.
• Structure-valued expressions, where all fields are compile-time known values.
• Expressions evaluating to a list type, where all elements are compile-time known values.
• Legal casts applied to compile-time known values.
• The following expressions (+, -, |+|, |-|, *, /, %, cast, !, &, |, ^, &&, ||, <<, >>, ~, /, >, <, ==, !=, <=, >=, ++, [:], ?:) when their operands are all compile-time known values.
• Expressions of the form e.minSizeInBits(), e.minSizeInBytes(), e.maxSizeInBits() and e.maxSizeInBytes() where the the type of e is generic.

Intuitively, the main difference between compile-time known values and local compile-time known values is that the former also contains constructor parameters. The distinction is important when it comes to defining the meaning of features like types. For example, in the type bit<e>, the expression e must be a local compile-time known value. Suppose instead that e were a constructor parameter—i.e., merely a compile-time known value. In this situation, it would be impossible to resolve bit<e> to a concrete type using purely local information—we would have to wait until the constructor was instantiated and the value of e known.