NormalizeHashList Class Reference

Normalize field lists in hashing expression (Hash.get primitive) More...

#include <normalize_hash_list.h>

Inheritance diagram for NormalizeHashList:

Public Member Functions
	NormalizeHashList (::P4::ReferenceMap *ref_map_, ::P4::TypeMap *type_map_, ::P4::TypeChecking *type_checking_)
NormalizeHashList &	operator= (NormalizeHashList &&)=delete

Static Public Member Functions
static bool	isHasher (const IR::MethodCallExpression *mc)
	Check whether a method call is the hasher call.

Detailed Description

Normalize field lists in hashing expression (Hash.get primitive)

This visitor replaces complex expressions in hashing field lists by temporary variables. The variable is placed in the scope of current control and its initialization is moved just before the statement in the apply block, which invokes the action the hash calculation belongs to (both direct invocation and table application are supported).

The temporary variable is needed as the hashing engine takes inputs from the PHV crossbar. There are some exceptions:

• compile-time constants are computed into the hashing seeds
• concatenation operator is ignored as it's equivalent to listing its operands in the hash field list and it's processed later in the same way.

This pass expects that the local copy propagation has been finished (the P4::LocalCopyPropagation pass) and that all local variables are moved into the control scope (the P4::MoveDeclarations pass). Otherwise, the initialization code placed into the apply block could contain some local variables out of the scope.

Example:

control Ingress(...) {
  Hash<...>(...) hasher;
 
  action awesomeHashing() {
    ... = hasher.get({hdr.field1 & hdr.field2});
  }
 
  apply {
    ...
    awesomeHashing();
  }
}

is transformed to:

control Ingress(...) {
  bits<...> $hash_field_argument0;
  Hash<...>(...) hasher;
 
  action awesomeHashing() {
    ... = hasher.get({$hash_field_argument0});
  }
 
  apply {
    ...
    $hash_field_argument = hdr.field1 & hdr.field2;
    awesomeHashing();
  }
}