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. ``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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24 */
25
26#pragma once
27
28#if ENABLE(DFG_JIT)
29
30#include "DFGBackwardsDominators.h"
31#include "DFGDominators.h"
32
33namespace JSC { namespace DFG {
34
35class ControlEquivalenceAnalysis {
36 WTF_MAKE_NONCOPYABLE(ControlEquivalenceAnalysis);
37 WTF_MAKE_FAST_ALLOCATED;
38public:
39 ControlEquivalenceAnalysis(Graph& graph)
40 : m_dominators(graph.ensureSSADominators())
41 , m_backwardsDominators(graph.ensureBackwardsDominators())
42 {
43 }
44
45 // This returns true iff:
46 //
47 // - If b executes then a must have executed before it (a dominates b).
48 // - If a executes then b will execute after it (b backwards-dominates a).
49 //
50 // Note that like Dominators and BackwardsDominators, this analysis ignores OSR:
51 //
52 // - This may return true even if we OSR enter in beteen a and b. OSR entry would mean that b
53 // could execute even if a had not executed. This is impossible in DFG SSA but it's possible
54 // in DFG CPS.
55 // - This may return true even if we OSR exit in between a and b. OSR exit would mean that a
56 // could execute even though b will not execute. This is possible in all forms of DFG IR.
57 //
58 // In DFG SSA you only have to worry about the definition being weaked by exits. This is usually
59 // OK, since we use this analysis to determine the cost of moving exits from one block to
60 // another. If we move an exit from b to a and a equivalently dominates b then at worst we have
61 // made the exit happen sooner. If we move an exit from b to a and a dominates b but not
62 // equivalently then we've done something much worse: the program may now exit even if it would
63 // not have ever exited before.
64 bool dominatesEquivalently(BasicBlock* a, BasicBlock* b)
65 {
66 return m_dominators.dominates(a, b)
67 && m_backwardsDominators.dominates(b, a);
68 }
69
70 // This returns true iff the execution of a implies that b also executes and vice-versa.
71 bool areEquivalent(BasicBlock* a, BasicBlock* b)
72 {
73 return dominatesEquivalently(a, b)
74 || dominatesEquivalently(b, a);
75 }
76
77private:
78 SSADominators& m_dominators;
79 BackwardsDominators& m_backwardsDominators;
80};
81
82} } // namespace JSC::DFG
83
84#endif // ENABLE(DFG_JIT)
85