synthesizeValidField.h

/*
Copyright 2013-present Barefoot Networks, Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

#ifndef _BACKENDS_BMV2_SYNTHESIZEVALIDFIELD_H_
#define _BACKENDS_BMV2_SYNTHESIZEVALIDFIELD_H_

#include "ir/ir.h"
#include "ir/pass_manager.h"

namespace P4 {
class ReferenceMap;
class TypeMap;
}  // namespace P4

namespace BMV2 {

/**
 * Adds an explicit valid bit to each header type and replaces calls to
 * isValid() with references to the new field. This better reflects how header
 * validity is actually implemented on BMV2, which simplifies later passes.
 *
 * In most situations, `foo.isValid()` is rewritten to `foo.$valid$ == 1`.
 * However, when `foo.isValid()` is a table key element (and isn't part of a
 * larger expression), it's replaced with a simple match against `foo.$valid$`
 * itself. This changes the type of the key element from boolean to bit<1>;
 * to accomodate that change, the corresponding constant table entry keys are
 * also rewritten to use bit<1> values instead of booleans.
 *
 * Calls to isValid() on header unions aren't rewritten; they can't be desugared
 * to a simple read of a hidden field.
 *
 * XXX(seth): It would be cleaner to rewrite setValid() and setInvalid() too,
 * but we currently can't. Those compile down into special BMV2 primitives that
 * are needed for certain features (e.g. header unions) to work correctly.
 *
 * Example:
 *
 *   header h {
 *     bit<32> field;
 *   }
 *   control c(h h0, h h1) {
 *     table t {
 *       key { h0.isValid() : exact }
 *     }
 *     apply {
 *       if (h1.isValid()) t.apply();
 *     }
 *   }
 *
 * is replaced by:
 *
 *   header h {
 *     bit<32> field;
 *     bit<1> $valid$;
 *   }
 *   control c(h h0, h h1) {
 *     table t {
 *       key { h0.$valid$ : exact }
 *     }
 *     apply {
 *       if (h1.$valid$ == 1) t.apply();
 *     }
 *   }
 *
 *   @pre None.
 *   @post All header types have a new field which serves as a valid bit, and
 *         all calls to isValid() on headers are replaced with expressions
 *         involving the new field. When constant table entry keys use boolean
 *         values to match against the result of isValid(), they're rewritten to
 *         a constant bit<1> value. Afterwards, the ReferenceMap and TypeMap
 *         may be out of date.
 */
class SynthesizeValidField final : public PassManager {
 public:
    SynthesizeValidField(P4::ReferenceMap* refMap, P4::TypeMap* typeMap);
};

}  // namespace BMV2

#endif  /* _BACKENDS_BMV2_SYNTHESIZEVALIDFIELD_H_ */