1/*
2 * Copyright (C) 2013, 2015 Apple Inc. All rights reserved.
3 *
4 * Redistribution and use in source and binary forms, with or without
5 * modification, are permitted provided that the following conditions
6 * are met:
7 * 1. Redistributions of source code must retain the above copyright
8 * notice, this list of conditions and the following disclaimer.
9 * 2. Redistributions in binary form must reproduce the above copyright
10 * notice, this list of conditions and the following disclaimer in the
11 * documentation and/or other materials provided with the distribution.
12 *
13 * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
14 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
15 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
16 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
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18 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
19 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
20 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
21 * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
22 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
23 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
24 */
25
26#include "config.h"
27#include "DFGLoopPreHeaderCreationPhase.h"
28
29#if ENABLE(DFG_JIT)
30
31#include "DFGBasicBlockInlines.h"
32#include "DFGBlockInsertionSet.h"
33#include "DFGDominators.h"
34#include "DFGGraph.h"
35#include "DFGNaturalLoops.h"
36#include "DFGPhase.h"
37#include "JSCInlines.h"
38#include <wtf/HashMap.h>
39
40namespace JSC { namespace DFG {
41
42BasicBlock* createPreHeader(Graph& graph, BlockInsertionSet& insertionSet, BasicBlock* block)
43{
44 ASSERT_WITH_MESSAGE(!graph.isRoot(block), "A CFG root should not be in a loop");
45
46 // FIXME: If we run this utility on SSA IR, then we may end up with a bizarre arrangement of
47 // Upsilons and Phis, like:
48 //
49 // BB#1:
50 // Upsilon(@a, ^p)
51 // Jump(#3)
52 //
53 // BB#2:
54 // Upsilon(@b, ^p)
55 // Jump(#3)
56 //
57 // BB#3:
58 // Jump(#4)
59 //
60 // BB#4:
61 // p: Phi()
62 //
63 // Notice how the Upsilons are not in the predecessor of the Phi anymore. It's not clear if this
64 // would be bad. Probably not, but it's weird anyway. We should add a validation rule, and we
65 // should implement a Upsilon/Phi canonicalization that handles this by calling into the
66 // SSACalculator and treating the Upsilons as Defs and rebuilding the Phis from scratch.
67 //
68 // https://bugs.webkit.org/show_bug.cgi?id=148587
69
70 // Determine a good frequency for the pre-header. It's definitely not the frequency of the loop body.
71 // Instead, we use the max of the frequencies of the loop body's non-loop predecessors.
72 float frequency = 0;
73 for (BasicBlock* predecessor : block->predecessors) {
74 ASSERT(graph.m_form != SSA);
75 if (graph.m_cpsDominators->dominates(block, predecessor))
76 continue;
77 frequency = std::max(frequency, predecessor->executionCount);
78 }
79 BasicBlock* preHeader = insertionSet.insertBefore(block, frequency);
80
81 // FIXME: It would be great if we put some effort into enabling exitOK at this origin, if it
82 // happens to be unset. It might not be set because the loop header (i.e. "block") has Phis in it.
83 // Phis have to have exitOK=false. There are a few ways to try to set exitOK:
84 //
85 // - Regenerate an exit origin by proving that we are at an exit origin boundary. If all of the
86 // predecessors' terminals have different exit origins than the exit origin of head of block,
87 // then we can leverage the assumption that exit origin boundaries can always exit. We could
88 // extend this further, and say that we will set exitOK even if a predecessor's terminal has the
89 // same exit origin, but so long it hadn't done anything that clobbers exit since the start of
90 // the origin.
91 //
92 // - Analyze the Phi's and MovHint's at the head of block. If prior to the ExitOK there are only
93 // Phi's and MovHint's, we could "roll them back" by proving that for each of the MovHints, the
94 // referenced Phi has a child that dominates the pre-header, and that child is the node that is
95 // OSR-available at the local being MovHinted.
96 //
97 // Note that there are some obviously wrong ways to try to set exitOK. For example, we cannot
98 // simply use the origin of our predecessors, since in bytecode that could be *any* kind of
99 // instruction. It may not even be a control flow construct, if we had lowered some non-control
100 // bytecode operation into DFG IR that has control flow. Hence, we really do need to try to use the
101 // origin of the head of the loop header.
102 //
103 // https://bugs.webkit.org/show_bug.cgi?id=148586
104 preHeader->appendNode(
105 graph, SpecNone, Jump, block->at(0)->origin, OpInfo(block));
106
107 for (unsigned predecessorIndex = 0; predecessorIndex < block->predecessors.size(); predecessorIndex++) {
108 BasicBlock* predecessor = block->predecessors[predecessorIndex];
109 if (graph.m_cpsDominators->dominates(block, predecessor))
110 continue;
111 block->predecessors[predecessorIndex--] = block->predecessors.last();
112 block->predecessors.removeLast();
113 for (unsigned successorIndex = predecessor->numSuccessors(); successorIndex--;) {
114 BasicBlock*& successor = predecessor->successor(successorIndex);
115 if (successor != block)
116 continue;
117 successor = preHeader;
118 preHeader->predecessors.append(predecessor);
119 }
120 }
121
122 block->predecessors.append(preHeader);
123 return preHeader;
124}
125
126class LoopPreHeaderCreationPhase : public Phase {
127public:
128 LoopPreHeaderCreationPhase(Graph& graph)
129 : Phase(graph, "loop pre-header creation")
130 , m_insertionSet(graph)
131 {
132 }
133
134 bool run()
135 {
136 m_graph.ensureCPSDominators();
137 m_graph.ensureCPSNaturalLoops();
138
139 for (unsigned loopIndex = m_graph.m_cpsNaturalLoops->numLoops(); loopIndex--;) {
140 const CPSNaturalLoop& loop = m_graph.m_cpsNaturalLoops->loop(loopIndex);
141 BasicBlock* existingPreHeader = nullptr;
142 bool needsNewPreHeader = false;
143 for (unsigned predecessorIndex = loop.header().node()->predecessors.size(); predecessorIndex--;) {
144 BasicBlock* predecessor = loop.header().node()->predecessors[predecessorIndex];
145 if (m_graph.m_cpsDominators->dominates(loop.header().node(), predecessor))
146 continue;
147 if (!existingPreHeader) {
148 existingPreHeader = predecessor;
149 continue;
150 }
151 // We won't have duplicate entries in the predecessors list.
152 DFG_ASSERT(m_graph, nullptr, existingPreHeader != predecessor);
153 needsNewPreHeader = true;
154 break;
155 }
156
157 // This phase should only be run on a DFG where unreachable blocks have been pruned.
158 // We also don't allow loops back to root. This means that every loop header has got
159 // to have a pre-header.
160 DFG_ASSERT(m_graph, nullptr, existingPreHeader);
161
162 // We are looking at the predecessors of a loop header. A loop header has to have
163 // some predecessor other than the pre-header. We must have broken critical edges
164 // because that is the DFG SSA convention. Therefore, each predecessor of the loop
165 // header must have only one successor.
166 DFG_ASSERT(m_graph, nullptr, existingPreHeader->terminal()->op() == Jump, existingPreHeader->terminal()->op());
167
168 // A pre-header is most useful if it's possible to exit from its terminal. Hence
169 // if the terminal of the existing pre-header doesn't allow for exit, but the first
170 // origin of the loop header does, then we should create a new pre-header.
171 if (!needsNewPreHeader && loop.header().node()->at(0)->origin.exitOK
172 && !existingPreHeader->terminal()->origin.exitOK)
173 needsNewPreHeader = true;
174
175 if (!needsNewPreHeader)
176 continue;
177
178 createPreHeader(m_graph, m_insertionSet, loop.header().node());
179 }
180
181 return m_insertionSet.execute();
182 }
183
184 BlockInsertionSet m_insertionSet;
185};
186
187bool performLoopPreHeaderCreation(Graph& graph)
188{
189 return runPhase<LoopPreHeaderCreationPhase>(graph);
190}
191
192} } // namespace JSC::DFG
193
194#endif // ENABLE(DFG_JIT)
195
196
197