Fix for issue #542

p4-16/spec/P4-16-spec.mdk

@@ -2982,6 +2982,10 @@ Variable-length bit-strings support a limited set of operations:
 - The `emit` method of a `packet_out` extern object, which
   inserts a variable-sized bit-string with a known dynamic width into
   the packet being constructed (see Section [#sec-deparse]).
+- 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.
 
 ## Casts { #sec-casts }
 
@@ -3281,6 +3285,11 @@ copying `struct`s using assignment when the source and target of the
 assignment have the same type. Finally, `struct`s can be initialized
 with a list expression, as discussed in Section [#sec-list-exprs].
 
+Two structs can be compared for equality (==) or inequality (!=) only
+if they have the same type and all of their fields can be recursively
+compared for equality.  Two structures are equal if and only if all
+their corresponding fields are equal.
+
 ## Operations on headers { #sec-ops-on-hdrs }
 
 Headers provide the same operations as `struct`s. Assignment between
@@ -3352,6 +3361,11 @@ H h;
 h = { 10, 12 };  // This also makes the header h valid
 ~ End P4Example
 
+Two headers can be compared for equality (==) or inequality (!=) only
+if they have the same type.  Two headers are equal if and only if they
+are both invalid, or they are both valid and all their corresponding
+fields are equal.
+
 ## Operations on header stacks { #sec-expr-hs }
 
 A header stack is a fixed-size array of headers with the same
@@ -3471,6 +3485,11 @@ void pop_front(int count) {
 }
 ~ End P4Pseudo
 
+Two header stacks can be compared for equality (==) or inequality (!=)
+only if they have the same element type and the same length.  Two
+stacks are equal if and only if all their corresponding elements are
+equal.  Note that the `nextIndex` value is not used in the equality comparison.
+
 ## Operations on header unions { #sec-expr-hu }
 
 A variable declared with a union type is initially invalid. For example:
@@ -3673,6 +3692,12 @@ parser Tcp_option_parser(packet_in b, out Tcp_option_stack vec) {
 }
 ~End P4Example
 
+Two header unions can be compared for equality (==) or inequality (!=)
+if they have the same type.  The unions are equal if and only if all
+their corresponding fields are equal (i.e., either all fields are
+invalid in both unions, or in both unions the same field is valid, and
+the values of the valid fields are equal as headers).
+
 ## Method invocations and function calls { #sec-functions }
 
 Method invocations and function calls can be invoked using standard