1/*
2 * Copyright (C) 2016 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. AND ITS CONTRIBUTORS ``AS IS''
14 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
15 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
16 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
17 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
18 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
19 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
20 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
21 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
22 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
23 * THE POSSIBILITY OF SUCH DAMAGE.
24 */
25
26#pragma once
27
28#if ENABLE(DFG_JIT)
29
30namespace JSC { namespace DFG {
31
32class Graph;
33
34// Picks up groups of barriers that could be executed in any order with respect to each other and
35// places then at the earliest point in the program where the cluster would be correct. This phase
36// makes only the first of the cluster be a FencedStoreBarrier while the rest are normal
37// StoreBarriers. This phase also removes redundant barriers - for example, the cluster may end up
38// with two or more barriers on the same object, in which case it is totally safe for us to drop
39// one of them. The reason why this is sound hinges on the "earliest point where the cluster would
40// be correct" property. For example, take this input:
41//
42// a: Call()
43// b: PutByOffset(@o, @o, @x)
44// c: FencedStoreBarrier(@o)
45// d: PutByOffset(@o, @o, @y)
46// e: FencedStoreBarrier(@o)
47// f: PutByOffset(@p, @p, @z)
48// g: FencedStoreBarrier(@p)
49// h: GetByOffset(@q)
50// i: Call()
51//
52// The cluster of barriers is @c, @e, and @g. All of the barriers are between two doesGC effects:
53// the calls at @a and @i. Because there are no doesGC effects between @a and @i and there is no
54// possible control flow entry into this sequence between @ and @i, we could could just execute all
55// of the barriers just before @i in any order. The earliest point where the cluster would be
56// correct is just after @f, since that's the last operation that needs a barrier. We use the
57// earliest to reduce register pressure. When the barriers are clustered just after @f, we get:
58//
59// a: Call()
60// b: PutByOffset(@o, @o, @x)
61// d: PutByOffset(@o, @o, @y)
62// f: PutByOffset(@p, @p, @z)
63// c: FencedStoreBarrier(@o)
64// e: FencedStoreBarrier(@o)
65// g: FencedStoreBarrier(@p)
66// h: GetByOffset(@q)
67// i: Call()
68//
69// This phase does more. It takes advantage of the clustering to remove fences and remove redundant
70// barriers. So this phase will output this:
71//
72// a: Call()
73// b: PutByOffset(@o, @o, @x)
74// d: PutByOffset(@o, @o, @y)
75// f: PutByOffset(@p, @p, @z)
76// c: FencedStoreBarrier(@o)
77// g: StoreBarrier(@p)
78// h: GetByOffset(@q)
79// i: Call()
80//
81// This optimization improves both overall throughput and the throughput while the concurrent GC is
82// running. In the former, we are simplifying instruction selection for all but the first fence. In
83// the latter, we are reducing the cost of all but the first barrier. The first barrier will awlays
84// take slow path when there is concurrent GC activity, since the slow path contains the fence. But
85// all of the other barriers will only take slow path if they really need to remember the object.
86bool performStoreBarrierClustering(Graph&);
87
88} } // namespace JSC::DFG
89
90#endif // ENABLE(DFG_JIT)
91
92