variable-reducer.h | Explore in Territory

// Copyright 2022 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef V8_COMPILER_TURBOSHAFT_VARIABLE_REDUCER_H_
#define V8_COMPILER_TURBOSHAFT_VARIABLE_REDUCER_H_

#include <algorithm>
#include <optional>

#include "src/base/logging.h"
#include "src/codegen/machine-type.h"
#include "src/compiler/turboshaft/assembler.h"
#include "src/compiler/turboshaft/graph.h"
#include "src/compiler/turboshaft/operations.h"
#include "src/compiler/turboshaft/representations.h"
#include "src/compiler/turboshaft/required-optimization-reducer.h"
#include "src/compiler/turboshaft/snapshot-table.h"
#include "src/zone/zone-containers.h"

namespace v8::internal::compiler::turboshaft {

#include "src/compiler/turboshaft/define-assembler-macros.inc"

// When cloning a Block or duplicating an Operation, we end up with some
// Operations of the old graph mapping to multiple Operations in the new graph.
// When using those Operations in subsequent Operations, we need to know which
// of the new-Operation to use, and, in particular, if a Block has 2
// predecessors that have a mapping for the same old-Operation, we need to
// merge them in a Phi node. All of this is handled by the VariableAssembler.
//
// The typical workflow when working with the VariableAssembler would be:
//    - At some point, you need to introduce a Variable (for instance
//      because you cloned a block or an Operation) and call NewVariable or
//      NewLoopInvariantVariable to get a fresh Variable. A loop invariant
//      variable must not need loop phis, that is, not change its value
//      depending on loop iteration while being visible across loop iterations.
//    - You can then Set the new-OpIndex associated with this Variable in the
//      current Block with the Set method.
//    - If you later need to set an OpIndex for this Variable in another Block,
//      call Set again.
//    - At any time, you can call Get to get the new-Operation associated to
//      this Variable. Get will return:
//         * if the current block is dominated by a block who did a Set on the
//           Variable, then the Operation that was Set then.
//         * otherwise, the current block must be dominated by a Merge whose
//           predecessors have all Set this Variable. In that case, the
//           VariableAssembler introduced a Phi in this merge, and will return
//           this Phi.
//
// Note that the VariableAssembler does not do "old-OpIndex => Variable"
// book-keeping: the users of the Variable should do that themselves (which
// is what CopyingPhase does for instance).

// VariableReducer always adds a RequiredOptimizationReducer, because phis
// with constant inputs introduced by `VariableReducer` need to be eliminated.
template <class AfterNext>
class VariableReducer : public RequiredOptimizationReducer<AfterNext> { … };

#include "src/compiler/turboshaft/undef-assembler-macros.inc"

}  // namespace v8::internal::compiler::turboshaft

#endif  // V8_COMPILER_TURBOSHAFT_VARIABLE_REDUCER_H_
chromium/v8/src/compiler/turboshaft/variable-reducer.h
