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/*
* Copyright (C) 2013, 2015 Apple Inc. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
* OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#include "config.h"
#include "DFGLoopPreHeaderCreationPhase.h"
#if ENABLE(DFG_JIT)
#include "DFGBasicBlockInlines.h"
#include "DFGBlockInsertionSet.h"
#include "DFGDominators.h"
#include "DFGGraph.h"
#include "DFGNaturalLoops.h"
#include "DFGPhase.h"
#include "JSCJSValueInlines.h"
namespace JSC { namespace DFG {
BasicBlock* createPreHeader(Graph& graph, BlockInsertionSet& insertionSet, BasicBlock* block)
{
ASSERT_WITH_MESSAGE(!graph.isRoot(block), "A CFG root should not be in a loop");
// FIXME: If we run this utility on SSA IR, then we may end up with a bizarre arrangement of
// Upsilons and Phis, like:
//
// BB#1:
// Upsilon(@a, ^p)
// Jump(#3)
//
// BB#2:
// Upsilon(@b, ^p)
// Jump(#3)
//
// BB#3:
// Jump(#4)
//
// BB#4:
// p: Phi()
//
// Notice how the Upsilons are not in the predecessor of the Phi anymore. It's not clear if this
// would be bad. Probably not, but it's weird anyway. We should add a validation rule, and we
// should implement a Upsilon/Phi canonicalization that handles this by calling into the
// SSACalculator and treating the Upsilons as Defs and rebuilding the Phis from scratch.
//
// https://bugs.webkit.org/show_bug.cgi?id=148587
// Determine a good frequency for the pre-header. It's definitely not the frequency of the loop body.
// Instead, we use the max of the frequencies of the loop body's non-loop predecessors.
float frequency = 0;
for (BasicBlock* predecessor : block->predecessors) {
ASSERT(graph.m_form != SSA);
if (graph.m_cpsDominators->dominates(block, predecessor))
continue;
frequency = std::max(frequency, predecessor->executionCount);
}
BasicBlock* preHeader = insertionSet.insertBefore(block, frequency);
// FIXME: It would be great if we put some effort into enabling exitOK at this origin, if it
// happens to be unset. It might not be set because the loop header (i.e. "block") has Phis in it.
// Phis have to have exitOK=false. There are a few ways to try to set exitOK:
//
// - Regenerate an exit origin by proving that we are at an exit origin boundary. If all of the
// predecessors' terminals have different exit origins than the exit origin of head of block,
// then we can leverage the assumption that exit origin boundaries can always exit. We could
// extend this further, and say that we will set exitOK even if a predecessor's terminal has the
// same exit origin, but so long it hadn't done anything that clobbers exit since the start of
// the origin.
//
// - Analyze the Phi's and MovHint's at the head of block. If prior to the ExitOK there are only
// Phi's and MovHint's, we could "roll them back" by proving that for each of the MovHints, the
// referenced Phi has a child that dominates the pre-header, and that child is the node that is
// OSR-available at the local being MovHinted.
//
// Note that there are some obviously wrong ways to try to set exitOK. For example, we cannot
// simply use the origin of our predecessors, since in bytecode that could be *any* kind of
// instruction. It may not even be a control flow construct, if we had lowered some non-control
// bytecode operation into DFG IR that has control flow. Hence, we really do need to try to use the
// origin of the head of the loop header.
//
// https://bugs.webkit.org/show_bug.cgi?id=148586
preHeader->appendNode(
graph, SpecNone, Jump, block->at(0)->origin, OpInfo(block));
for (unsigned predecessorIndex = 0; predecessorIndex < block->predecessors.size(); predecessorIndex++) {
BasicBlock* predecessor = block->predecessors[predecessorIndex];
if (graph.m_cpsDominators->dominates(block, predecessor))
continue;
block->predecessors[predecessorIndex--] = block->predecessors.last();
block->predecessors.removeLast();
for (unsigned successorIndex = predecessor->numSuccessors(); successorIndex--;) {
BasicBlock*& successor = predecessor->successor(successorIndex);
if (successor != block)
continue;
successor = preHeader;
preHeader->predecessors.append(predecessor);
}
}
block->predecessors.append(preHeader);
return preHeader;
}
class LoopPreHeaderCreationPhase : public Phase {
public:
LoopPreHeaderCreationPhase(Graph& graph)
: Phase(graph, "loop pre-header creation"_s)
, m_insertionSet(graph)
{
}
bool run()
{
m_graph.ensureCPSDominators();
m_graph.ensureCPSNaturalLoops();
for (unsigned loopIndex = m_graph.m_cpsNaturalLoops->numLoops(); loopIndex--;) {
const CPSNaturalLoop& loop = m_graph.m_cpsNaturalLoops->loop(loopIndex);
BasicBlock* existingPreHeader = nullptr;
bool needsNewPreHeader = false;
for (unsigned predecessorIndex = loop.header().node()->predecessors.size(); predecessorIndex--;) {
BasicBlock* predecessor = loop.header().node()->predecessors[predecessorIndex];
if (m_graph.m_cpsDominators->dominates(loop.header().node(), predecessor))
continue;
if (!existingPreHeader) {
existingPreHeader = predecessor;
continue;
}
// We won't have duplicate entries in the predecessors list.
DFG_ASSERT(m_graph, nullptr, existingPreHeader != predecessor);
needsNewPreHeader = true;
break;
}
// This phase should only be run on a DFG where unreachable blocks have been pruned.
// We also don't allow loops back to root. This means that every loop header has got
// to have a pre-header.
DFG_ASSERT(m_graph, nullptr, existingPreHeader);
// We are looking at the predecessors of a loop header. A loop header has to have
// some predecessor other than the pre-header. We must have broken critical edges
// because that is the DFG SSA convention. Therefore, each predecessor of the loop
// header must have only one successor.
DFG_ASSERT(m_graph, nullptr, existingPreHeader->terminal()->op() == Jump, existingPreHeader->terminal()->op());
// A pre-header is most useful if it's possible to exit from its terminal. Hence
// if the terminal of the existing pre-header doesn't allow for exit, but the first
// origin of the loop header does, then we should create a new pre-header.
if (!needsNewPreHeader && loop.header().node()->at(0)->origin.exitOK
&& !existingPreHeader->terminal()->origin.exitOK)
needsNewPreHeader = true;
if (!needsNewPreHeader)
continue;
createPreHeader(m_graph, m_insertionSet, loop.header().node());
}
return m_insertionSet.execute();
}
BlockInsertionSet m_insertionSet;
};
bool performLoopPreHeaderCreation(Graph& graph)
{
return runPhase<LoopPreHeaderCreationPhase>(graph);
}
} } // namespace JSC::DFG
#endif // ENABLE(DFG_JIT)
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