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// 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_BRANCH_CONDITION_DUPLICATOR_H_
#define V8_COMPILER_BRANCH_CONDITION_DUPLICATOR_H_
#include "src/base/macros.h"
#include "src/compiler/node-marker.h"
#include "src/compiler/node.h"
#include "src/zone/zone.h"
namespace v8 {
namespace internal {
namespace compiler {
// Forward declare.
class Graph;
// BranchConditionDuplicator makes sure that the condition nodes of branches are
// used only once. When it finds a branch node whose condition has multiples
// uses, this condition is duplicated.
//
// Doing this enables the InstructionSelector to generate more efficient code
// for branches. For instance, consider this code:
//
// if (a + b == 0) { /* some code */ }
// if (a + b == 0) { /* more code */ }
//
// Then the generated code will be something like (using registers "ra" for "a"
// and "rb" for "b", and "rt" a temporary register):
//
// add ra, rb ; a + b
// cmp ra, 0 ; (a + b) == 0
// sete rt ; rt = (a + b) == 0
// cmp rt, 0 ; rt == 0
// jz
// ...
// cmp rt, 0 ; rt == 0
// jz
//
// As you can see, TurboFan materialized the == bit into a temporary register.
// However, since the "add" instruction sets the ZF flag (on x64), it can be
// used to determine wether the jump should be taken or not. The code we'd like
// to generate instead if thus:
//
// add ra, rb
// jnz
// ...
// add ra, rb
// jnz
//
// However, this requires to generate twice the instruction "add ra, rb". Due to
// how virtual registers are assigned in TurboFan (there is a map from node ID
// to virtual registers), both "add" instructions will use the same virtual
// register as output, which will break SSA.
//
// In order to overcome this issue, BranchConditionDuplicator duplicates branch
// conditions that are used more than once, so that they can be generated right
// before each branch without worrying about breaking SSA.
class V8_EXPORT_PRIVATE BranchConditionDuplicator final {
public:
BranchConditionDuplicator(Zone* zone, Graph* graph);
~BranchConditionDuplicator() = default;
void Reduce();
Node* DuplicateNode(Node* node);
void DuplicateConditionIfNeeded(Node* node);
void Enqueue(Node* node);
void VisitNode(Node* node);
void ProcessGraph();
private:
Graph* const graph_;
ZoneQueue<Node*> to_visit_;
NodeMarker<bool> seen_;
};
} // namespace compiler
} // namespace internal
} // namespace v8
#endif // V8_COMPILER_BRANCH_CONDITION_DUPLICATOR_H_
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