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85 lines
3.5 KiB
C++
85 lines
3.5 KiB
C++
/*
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* Copyright (C) 2016 Apple Inc. All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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*
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* THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
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* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
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* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
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* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
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* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
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* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
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* OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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*/
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#pragma once
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#if ENABLE(DFG_JIT)
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#include "DFGBackwardsDominators.h"
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#include "DFGDominators.h"
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namespace JSC { namespace DFG {
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class ControlEquivalenceAnalysis {
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WTF_MAKE_NONCOPYABLE(ControlEquivalenceAnalysis);
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WTF_MAKE_FAST_ALLOCATED;
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public:
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ControlEquivalenceAnalysis(Graph& graph)
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: m_dominators(graph.ensureSSADominators())
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, m_backwardsDominators(graph.ensureBackwardsDominators())
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{
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}
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// This returns true iff:
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//
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// - If b executes then a must have executed before it (a dominates b).
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// - If a executes then b will execute after it (b backwards-dominates a).
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//
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// Note that like Dominators and BackwardsDominators, this analysis ignores OSR:
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//
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// - This may return true even if we OSR enter in beteen a and b. OSR entry would mean that b
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// could execute even if a had not executed. This is impossible in DFG SSA but it's possible
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// in DFG CPS.
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// - This may return true even if we OSR exit in between a and b. OSR exit would mean that a
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// could execute even though b will not execute. This is possible in all forms of DFG IR.
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//
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// In DFG SSA you only have to worry about the definition being weaked by exits. This is usually
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// OK, since we use this analysis to determine the cost of moving exits from one block to
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// another. If we move an exit from b to a and a equivalently dominates b then at worst we have
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// made the exit happen sooner. If we move an exit from b to a and a dominates b but not
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// equivalently then we've done something much worse: the program may now exit even if it would
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// not have ever exited before.
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bool dominatesEquivalently(BasicBlock* a, BasicBlock* b)
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{
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return m_dominators.dominates(a, b)
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&& m_backwardsDominators.dominates(b, a);
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}
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// This returns true iff the execution of a implies that b also executes and vice-versa.
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bool areEquivalent(BasicBlock* a, BasicBlock* b)
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{
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return dominatesEquivalently(a, b)
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|| dominatesEquivalently(b, a);
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}
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private:
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SSADominators& m_dominators;
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BackwardsDominators& m_backwardsDominators;
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};
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} } // namespace JSC::DFG
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#endif // ENABLE(DFG_JIT)
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