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/****************************************************************************
**
** Copyright (C) 2018 The Qt Company Ltd.
** Contact: https://www.qt.io/licensing/
**
** This file is part of the QtQml module of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and The Qt Company. For licensing terms
** and conditions see https://www.qt.io/terms-conditions. For further
** information use the contact form at https://www.qt.io/contact-us.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 3 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL3 included in the
** packaging of this file. Please review the following information to
** ensure the GNU Lesser General Public License version 3 requirements
** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
**
** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 2.0 or (at your option) the GNU General
** Public license version 3 or any later version approved by the KDE Free
** Qt Foundation. The licenses are as published by the Free Software
** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
** included in the packaging of this file. Please review the following
** information to ensure the GNU General Public License requirements will
** be met: https://www.gnu.org/licenses/gpl-2.0.html and
** https://www.gnu.org/licenses/gpl-3.0.html.
**
** $QT_END_LICENSE$
**
****************************************************************************/
#ifndef QV4BLOCKSCHEDULER_P_H
#define QV4BLOCKSCHEDULER_P_H
//
// W A R N I N G
// -------------
//
// This file is not part of the Qt API. It exists purely as an
// implementation detail. This header file may change from version to
// version without notice, or even be removed.
//
// We mean it.
//
#include <QtCore/qstack.h>
#include "qv4mi_p.h"
#include "qv4util_p.h"
QT_REQUIRE_CONFIG(qml_tracing);
QT_BEGIN_NAMESPACE
namespace QV4 {
namespace IR {
class DominatorTree;
class LoopInfo;
// High-level algorithm:
// 0. start with the first node (the start node) of a function
// 1. emit the node
// 2. add all outgoing edges that are not yet emitted to the postponed stack
// 3. When the postponed stack is empty, pop a stack from the loop stack. If that is empty too,
// we're done.
// 4. pop a node from the postponed stack, and check if it can be scheduled:
// a. if all incoming edges are scheduled, go to 4.
// b. if an incoming edge is unscheduled, but it's a back-edge (an edge in a loop that jumps
// back to the start of the loop), ignore it
// c. if there is any unscheduled edge that is not a back-edge, ignore this node, and go to 4.
// 5. if this node is the start of a loop, push the postponed stack on the loop stack.
// 6. go back to 1.
//
// The postponing action in step 2 will put the node into its containing group. The case where this
// is important is when a (labeled) continue or a (labeled) break statement occur in a loop: the
// outgoing edge points to a node that is not part of the current loop (and possibly not of the
// parent loop).
//
// Linear scan register allocation benefits greatly from short life-time intervals with few holes
// (see for example section 4 (Lifetime Analysis) of [Wimmer1]). This algorithm makes sure that the
// blocks of a group are scheduled together, with no non-loop blocks in between. This applies
// recursively for nested loops. It also schedules groups of if-then-else-endif blocks together for
// the same reason.
class BlockScheduler
{
const DominatorTree &dominatorTree;
const LoopInfo &loopInfo;
struct WorkForGroup
{
MIBlock *group;
QStack<MIBlock *> postponed;
WorkForGroup(MIBlock *group = nullptr) : group(group) {}
};
WorkForGroup currentGroup;
QStack<WorkForGroup> postponedGroups;
std::vector<MIBlock *> sequence;
class ProcessedBlocks
{
BitVector processed;
public:
ProcessedBlocks(MIFunction *function)
: processed(int(function->blockCount()), false)
{}
bool alreadyProcessed(MIBlock *bb) const
{
Q_ASSERT(bb);
return processed.at(bb->index());
}
void markAsProcessed(MIBlock *bb)
{
processed.setBit(bb->index());
}
} emitted;
QHash<MIBlock *, MIBlock *> loopsStartEnd;
bool checkCandidate(MIBlock *candidate);
MIBlock *pickNext();
void emitBlock(MIBlock *bb);
void schedule(MIBlock *functionEntryPoint);
void postpone(MIBlock *bb);
void dump() const;
public:
BlockScheduler(const DominatorTree &dominatorTree, const LoopInfo &loopInfo);
const std::vector<MIBlock *> &scheduledBlockSequence() const
{ return sequence; }
QHash<MIBlock *, MIBlock *> loopEndsByStartBlock() const
{ return loopsStartEnd; }
};
} // namespace IR
} // namespace QV4
QT_END_NAMESPACE
#endif // QV4BLOCKSCHEDULER_P_H
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