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(B3_JIT)
29
30#include "B3HeapRange.h"
31#include "B3Value.h"
32
33namespace JSC { namespace B3 {
34
35class JS_EXPORT_PRIVATE FenceValue : public Value {
36public:
37 static bool accepts(Kind kind) { return kind == Fence; }
38
39 ~FenceValue();
40
41 // The read/write heaps are reflected in the effects() of this value. The compiler may change
42 // the lowering of a Fence based on the heaps. For example, if a fence does not write anything
43 // then it is understood to be a store-store fence. On x86, this may lead us to not emit any
44 // code, while on ARM we may emit a cheaper fence (dmb ishst instead of dmb ish). We will do
45 // the same optimization for load-load fences, which are expressed as a Fence that writes but
46 // does not read.
47 //
48 // This abstraction allows us to cover all of the fences on x86 and all of the standalone fences
49 // on ARM. X86 really just has one fence: mfence. This fence should be used to protect stores
50 // from being sunk below loads. WTF calls it the storeLoadFence. A classic example is the Steele
51 // barrier:
52 //
53 // o.f = v => o.f = v
54 // if (color(o) == black)
55 // log(o)
56 //
57 // We are trying to ensure that if the store to o.f occurs after the collector has started
58 // visiting o, then we will log o. Under sequential consistency, this would work. The collector
59 // would set color(o) to black just before it started visiting. But x86's illusion of sequential
60 // consistency is broken in exactly just this store->load ordering case. The store to o.f may
61 // get buffered, and it may occur some time after we have loaded and checked color(o). As well,
62 // the collector's store to set color(o) to black may get buffered and it may occur some time
63 // after the collector has finished visiting o. Therefore, we need mfences. In B3 we model this
64 // as a Fence that reads and writes some heaps. Setting writes to the empty set will cause B3 to
65 // not emit any barrier on x86.
66 //
67 // On ARM there are many more fences. The Fence instruction is meant to model just two of them:
68 // dmb ish and dmb ishst. You can emit a dmb ishst by using a Fence with an empty write heap.
69 // Otherwise, you will get a dmb ish.
70 // FIXME: Add fenced memory accesses. https://bugs.webkit.org/show_bug.cgi?id=162349
71 // FIXME: Add a Depend operation. https://bugs.webkit.org/show_bug.cgi?id=162350
72 HeapRange read { HeapRange::top() };
73 HeapRange write { HeapRange::top() };
74
75 B3_SPECIALIZE_VALUE_FOR_NO_CHILDREN
76
77private:
78 friend class Procedure;
79 friend class Value;
80
81 static Opcode opcodeFromConstructor(Origin, HeapRange = HeapRange(), HeapRange = HeapRange()) { return Fence; }
82 FenceValue(Origin origin, HeapRange read, HeapRange write);
83 FenceValue(Origin origin);
84};
85
86} } // namespace JSC::B3
87
88#endif // ENABLE(B3_JIT)
89
90